NUREG/CR-6424

ORNL/TM-13148

Report on Aging of

Nuclear Power Plant

Reinforced Concrete Structures

Prepared by

D. J. Naus, C. B. Oland, ORNL

B. R. Ellingwood, JHU

Oak Ridge National Laboratory

The Johns Hopkins University

Prepared for

U.S. Nuclear Regulatory Commission
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NUREG/CR-6424

BNL-NUREG-13148

Report on Aging of

Nuclear Power Plant

Reinforced Concrete Structures

Manuscript Completed: March 1996

Date Published: March 1996

Prepared by

D. J. Naus, C. B. Oland, ORNL

B. R. Ellingwood, JHU

Oak Ridge National Laboratory

Operated by Martin Marietta Energy Systems, Inc.

Oak Ridge National Laboratory

Oak Ridge, TN 37831

Subcontractor:

Department of Civil Engineering

The Johns Hopkins University

3400 North Charles Street

Baltimore, MD 21218-2699

H. L. Graves, NRC Project Manager

Prepared for

Division of Engineering Technology

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

NRC Job Code B0845

ABSTRACT

The Structural Aging Program provides the United States Nuclear Regulatory Commission with

potential structural safety issues and acceptance criteria for use in continued service assessments of nuclear

power plant safety-related concrete structures. The program was organized under four task areas - Program Management, Materials Property Data Base, Structural Component Assessment/Repair

Technology, and Quantitative Methodology for Continued Service Determinations. Under these tasks,

over 90 papers and reports were prepared addressing pertinent aspects associated with aging management

of nuclear power plant reinforced concrete structures. Contained in this report is a summary of program

results in the form of information related to longevity of nuclear power plant reinforced concrete

structures, a Structural Materials Information Center presenting data and information on the time variation

of concrete materials under the influence of environmental stressors and aging factors, in-service

inspection and condition assessments techniques, repair materials and methods, evaluation of nuclear

power plant reinforced concrete structures, and a reliability-based methodology for current and future

condition assessments. Recommendations for future activities are also provided.

iii lii NUREG/CR-6424

TABLE OF CONTENTS

Page

ABSTRACT.................................................................................. iii

LIST OF FIGURES........................................................................... ix

LIST OF TABLES ............................................................................ xiii

ACKNOWLEDGEMENTS....................................................................xvii

EXECUTIVE SUMMARY .................................................................... xix

1. INTRODUCTION........................................................................1

1.1 BACKGROUND ................................................................... 1

1.2 U.S. UTILITY AND NUCLEAR REGULATORY COMMISSION (USNRC)

AGING AND LICENSE RENEWAL ACTIVITIES................................... 1

1.2.1 U.S. Utilities ................................................................ 1

1.2.2 U.S. Nuclear Regulatory Comm-ission (USNRC) .............................. 2

1.2.2.1 Rulemaking activities................................................2

1.2.2.2 Research activities...................................................4

1.3 OBJECTIVE AND APPLICATION OF RESULTS....................................5

1.4 APPROACH ....................................................................... 5

2. SAFETY-RELATED CONCRETE STRUCTURES DESCRIPTION

AND LONGEVITY CONSIDERATIONS ................................................ 13

2.1 DESIGN CRITERIA............................................................... 13

2.2 SAFETY-RELATED CONCRETE STRUCTURES .................................. 13

2.2. 1 Typical Plant Structures ..................................................... 14

2.2.1 .1 Boiling-water reactors .............................................. 14

2.2.1.2 Pressurized-water reactors .......................................... 15

2.2.2 Auxiliary Structures ......................................................... 16

2.2.2.1 Common structures................................................. 16

2.2.2.2 Plant-unique structures.............................................. 17

2.3 AGING AND LONG-TERM DURABILITY CONSIDERATIONS.................... 17

2.3. 1 Materials of Construction .................................................... 17

2.3. 1. 1 Concrete........................................................... 17

2.3.1.2 Conventional steel reinforcement .................................... 1is

2.3. 1.3 Prestressing steel................................................... 18

2.3.1.4 Liner plate ......................................................... 19

v NUREG/CR-6424
TABLE OF CONTENTS (cont'd)

Page

2.3.1.5 Embedment steel ............................................................ 19

2.3.2 Aging Factors and Environmental Stressors .......................................... 19

2.3.2.1 Environmental stressor considerations ..................................... 20

2.3.2.2 Service life models ............................................................ 29

2.3.3 Long-Term Performance of Concrete Materials and NPP Structures .............. 34

2.3.3.1 Structural materials information center ..................................... 35

2.3.3.2 Longevity of NPP reinforced concrete structures ......................... 42

2.3.3.3 Commentary on longevity ................................. 45

3. IN-SERVICE INSPECTION, CONDITION ASSESSMENT, AND

REMEDIAL MEASURES CONSIDERATIONS .................................................... 85

3.1 IN -SERVICE INSPECTION TECHNIQUES ................................................ 85

3.1.1 Current In-Service Inspection Techniques ............................................ 85

3.1.1.1 Concrete material systems .................................................... 85

3.1.1.2 Conventional mild steel reinforcing ......................................... 90

3.1.1.3 Prestressing system components ............................................ 91

3.1.1.4 Liner plate ...................................................................... 91

3.1.1.5 Embedment steel .............................................................. 92

3.1.2 New and Emerging Techniques ........................................................ 92

3.1.2.1 Magnetic methods ............................................................. 92

3.1.2.2 Electrical methods ............................................................. 93

3.1.2.3 Ultrasonic methods ........................................................... 94

3.1.2.4 Pulsed infrared imaging method ............................................. 95

3.1.2.5 Ground-probing radar method ............................................... 95

3.1.2.6 Utraviolet radiation method .................................................. 95

3.1.3 Commentary on Applicability of Methods to NPP Concrete Structures ........... 95

3.2 CONDITION ASSESSMENTS ................................................................. 96

3.2.1 Condition Surveys of Conventional Reinforced Concrete Structures .............. 97

3.2.1.1 Field survey .................................................................... 97

3.2.1.2 Laboratory tests, office studies, and design verification ................. 99

3.2.1.3 R eport .......................................................................... 100

3.2.2 Condition Assessments of NPP Reinforced Concrete Structures .................. 100

3.2.3 Commentary on Condition Assessments .............................................. 101

3.3 REMEDIAL MEASURES ....................................................................... 102

3.3.1 Initial Repair Considerations ............................................................ 102

NUREG/CR-6424 v i
TABLE OF CONTENTS (cont'd)

Page

3.3.2 Typical Remedial Measures Techniques for Reinforced Concrete Structures ..... 103

3.3.2.1 Techniques and materials for repair of damaged concrete ................ 103

3.3.2.2 Remedial measure techniques for corrosion-damaged concrete ......... 106

3.3.3 Commentary on Remedial Measures ................................................... 109

4. EVALUATION OF NPP REINFORCED CONCRETE STRUCTURES ........................ 135

4.1 SELECTION OF SAFETY-RELATED CONCRETE STRUCTURES .................... 136

4.1.1 Aging Assessment Methodology ....................................................... 136

4.1.1.1 Basic methodology ........................................................... 136

4.1.1.2 Ranking system to select reinforced concrete components for

evaluation ...................................................................... 138

4.1.2 Probabilistic Risk Assessment .......................................................... 140

4.1.3 Combined Methods ...................................................................... 141

4.2 CONSIDERATIONS FOR DEVELOPMENT OF ISI PROGRAMS ....................... 142

4.2.1 Inspection Methods ...................................................................... 142

4.2.1.1 Visual inspection .............................................................. 142

4.2.1.2 Nondestructive testing ........................................................ 143

4.2.1.3 Destructive testing ............................................................. 143

4.2.1.4 Analytical methods ............................................................ 143

4.2.2 Accessibility Limitations ................................................................. 144

4.2.3 Acceptance Criteria ....................................................................... 144

4.2.3.1 Visual approach ............................................................... 145

4.2.3.2 Degradation-based approach ................................................. 149

4.2.4 Inspection Scheduling and Personnel Qualifications ................................. 151

4.2.4.1 Inspection scheduling ......................................................... 151

4.2.4.2 Qualifications of inspection personnel ...................................... 152

4.3 EXAMPLE ILLUSTRATING DEVELOPMENT OF AN INSPECTION PROGRAM.. 153

4.3.1 Background on Example PWR Plant .................................................. 153

4.3.2 Selection of Components for Evaluation .............................................. 153

4.3.3 Initial Planning ............................................................................ 155

4.3.4 Baseline Inspection Program ............................................................ 155

4.3.5 Inspection Program Adjustments ....................................................... 157

4.4 COMMENTARY ON INSPECTION PROGRAMS .......................................... 157

vii NUREG/CR-6424
TABLE OF CONTENTS (cont'd)

Page

5. RELIABILITY-BASED METHODOLOGY FOR CURRENT AND FUTURE CONDITION

ASSESSMENTS ......................................................................................... 177

5.1 RELIABILITY-BASED METHODS FOR CONDITION ASSESSMENT ................ 177

5.1.1 Degradation Mechanisms ................................................................ 178

5.1.2 Statistical Data on Loads and Resistance .............................................. 178

5.1.2.1 Structural loads ................................................................ 178

5.1.2.2 Structural resistance ........................................................... 179

5.1.3 Time-Dependent Reliability Analysis .................................................. 180

5.1.4 Illustrations of Service Life Prediction ................................................. 181

5.1.4.1 Reinforced concrete slab ..................................................... 181

5.1.4.2 Reinforced concrete shear wall .............................................. 182

5.2 ROLE OF INSPECTION/REPAIR IN MAINTAINING RELIABILITY ................. 184

5.2.1 Degradation Function Based on Individual Damage Intensities ..................... 185

5.2.2 Degradation Function After Repair ..................................................... 187

5.2.2.1 Full inspection/repair ......................................................... 187

5.2.2.2 Partial inspection/repair ....................................................... 190

5.3 OPTIMUM INSPECTION/REPAIR STRATEGIES ......................................... 192

5.3.1 Full-Inspection Survey .................................................................. 194

5.3.2 Partial Inspection Strategy ............................................................... 195

5.3.3 Example Illustrating Development of Optimum Inspection/Repair Strategy ....... 196

5.3.3.1 Description of slab ............................................................ 196

5.3.3.2 Time-dependent strength in flexure ......................................... 197

5.3.3.3 Determination of optimum strategy ......................................... 198

5.4 COMMENTARY ON RELIABILITY-BASED CONDITION ASSESSMENTS ......... 200

6. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS ................................. 225

6.1 SUMMARY ........................................................................................ 225

6.2 CONCLUSIONS .................................................................................. 226

6.3 RECOMMENDATIONS ......................................................................... 227

7. REFERENCES ........................................................................................... 231

APPENDICES

A. U.S. COMMERCIAL NUCLEAR POWER REACTOR SUMMARY ........................... 255

B. LIST OF TECHNICAL REPORTS AND PAPERS PREPARED UNDER THE

SAG PROGRAM ........................................................................................ 259

NUREG/CR-6424 viii
LIST OF FIGURES

Figure Page

1.1 Listing of subcontracted activities for each primary task area .................................. 7

1.2 Level 3 work breakdown structure for SAG Task S. 1: Program Management ............. 8

1.3 Level 3 work breakdown structure for SAG Task S.2: Materials Property Data Base ..... 9

1.4 Level 3 work breakdown structure for SAG Task S.3: Structural Component

Assessment/Repair Technology .................................................................... 10

1.5 Level 3 work breakdown structure for SAG Task S.4: Quantitative Methodology

for Continued Service Determinations ............................................................ 11

1.6 Evaluation methodology for NPP reinforced concrete structures .............................. 12

2.1 BWR Mark I type reinforced concrete containment ............................................. 62

2.2 BWR Mark II type reinforced concrete containment ............................................ 63

2.3 BWR Mark Ill type reinforced concrete containment ........................................... 64

2.4 PWR subatmospheric type reinforced concrete containment ................................... 65

2.5 PWR reinforced concrete containment with ice condenser ..................................... 66

2.6 PWR large dry prestressed concrete containment ................................................ 67

2.7 PWR free-standing steel containment with elliptical bottom .................................... 68

2.8 Relationship between the concepts of concrete durability and performance .................. 69

2.9 Relationship between primary causes and types of cracks that can occur in concrete ....... 70

2.10 Examples of intrinsic cracks in a hypothetical concrete ......................................... 71

2.11 Time of appearance of cracks from placing of concrete ......................................... 71

2.12 Types of chemical reactions responsible for concrete deterioration ............................ 72

2.13 Schematic representations of forms of corrosion that may be found in metals ............... 73

2.14 Carbonation data for exterior sheltered concrete ................................................. 74

2.15 Proposed curves for surface chloride content versus cement content for different curing

conditions ............................................................................................. 74

2.16 Effective diffusion coefficient of chloride versus time for ordinary portland cement

concretes ............................................................................................. . 75

2.17 Example of page from Vol. 1 (Performance Values) of Structural Materials Handbook. 76

2.18 Example of page from Vol. 2 (Supporting Documentation) of Structural Materials

H andbook ............................................................................................ . 77

ix NUREG/CR-6424
LIST OF FIGURES (cont'd)

Figure Page

2.19 Example of page from Vol. 3 (Material Data Sheet) of Structural Materials Handbook.. 78

2.20 Material code arrangement .......................................................................... 79

2.21 Long-term compressive strength results for selected data from University of

Wisconsin and Portland Cement Association .................................................... 80

2.22 Relative compressive strength for concretes obtained from several U.S. and U.K.

nuclear pow er stations ..................................................................... 81

2.23 Recommended procedure for strength evaluation of existing concrete buildings ............ 82

2.24 Relationship between the concrete performance and service life ............................... 83

3.1 Techniques for assessing deterioration ............................................................ 128

3.2 General structural assessment and evaluation procedure for existing buildings .............. 129

3.3 Effect of multiple repairs over the life of a structure ............................................. 130

3.4 Steps to be taken in a repair process ............................................................. 131

3.5 Selection of repair techniques for dormant cracks ............................................... 132

3.6 Selection of repair techniques for active cracks .................................................. 133

3.7 Principles of repair to stop corrosion .............................................................. 134

4.1 Structural aging assessment methodology flow diagram ....................................... 174

4.2 Damage state chart relating environmental exposure, crack width, and necessity for

additional evaluation or repair ...................................................................... 175

4.3 Damage state chart relating environmental exposure, half-cell potential reading, and

necessity for additional evaluation or repair ..................................................... 176

5.1 Typical sample functions of structural loads ..................................................... 206

5.2 Mean degradation functions of one-way slab .................................................... 207

5.3 Failure probability of a one-way slab ............................................................. 208

5.4 Comparison of measured and calculated shear strength of low-rise reinforced

concrete w all ......................................................................................... 209

5.5 Mean degradation function of wall in shear without repair ..................................... 210

5.6 Failure probability of wall in shear without repair ............................................... 211

5.7 Hazard function of wall in shear without repair .................................................. 212

NUREG/CR-6424x X
LIST OF FIGURES (cont'd)

Figure Page

5.8 Mean degradation function of wall in flexure/compression ..................................... 213

5.9 Role of inspection/repair in controlling failure rate .............................................. 214

5.10 Probability of detection of a defect of size x ...................................................... 214

5.11 Effect of several detectability functions on mean degradation function of

inspection/repair ..................................................................................... 215

5.12 Effect of multiple inspection/repairs on mean degradation function ........................... 216

5.13 Partial inspection/repair strategies ................................................................. 217

5.14 Failure probability of a component with partial inspection ..................................... 218

5.15 Optimum solutions for full-inspection strategies for example 1 in Table 5.4 ................. 219

5.16 Optimum solutions for full-inspection strategies for example 2 in Table 5.4 ................. 220

5.17 Optimum solutions for full-inspection strategies for example 3 in Table 5A ................. 221

5.18 Mean degradation function of slab subject to rebar corrosion .................................. 222

5.19 Failure probability of slab with mean strength degradation in Fig. 5.18 ...................... 222

5.20 Optimum solutions for full-inspection strategies for slab ....................................... 223

5.21 System reliability approach to condition assessment ............................................ 224

xi xi NUREG/CR-6424

LIST OF TABLES

Table Page

2.1 Typical safety-related concrete structures in LWR plants and their accessibility

for visual exam ination ................................................................................ 46

2.2 Typical safety-related concrete structures at BWR plants ........................................ 47

2.3 Typical safety-related concrete structures at PWR plants ........................................ 48

2.4 Degradation factors that can impact the performance of safety-related concrete structures... 49

2.5 Locations in NPPs where concrete-related materials may exhibit degradation ................ 50

2.6 Reactivity of various chemicals with portland cement concrete and reinforcing steel/liner

p lates ................................................................................................... 5 1

2.7 Estimated irradiation levels at the outside boundary of a RPV for several operating

periods ................................................................................................. 52

2.8 Material code identification and description ........................................................ 53

2.9 Property code range descriptions ................................................................... 54

2.10 Q uality level definitions .............................................................................. 54

2.11 Background information and service history requirements for portland cement concretes

and embedded metallic materials .................................................................... 55

2.12 Mix proportions, baseline information, and plastic concrete properties for the portland

cement concrete used in the examples .............................................................. 56

2.13 Time-dependent compressive strength values for portland cement concretes obtained

from SM IC ............................................................................................ 57

2.14 Temperature-dependent compressive strength values for portland cement concretes

obtained from SM IC .................................................................................. 57

2.15 Ambient and temperature-dependent properties for ASTM A 615 (Gr 60) and A 36 steels.. 58

2.16 Requirements for a data base management system for the SMIC ............................... 59

2.17 Condition survey results for various NPP concrete structures .................................. 61

3.1 Commonly used evaluation procedures to assess concrete properties and physical

condition .............................................................................................. 111

3.2 Description of test methods for concrete ........................................................... 113

3.3 Recommended testing methods to assess concrete degradation ................................. 118

3.4 C auses of cracking .................................................................................... 120

... i NUREG/CR-6424
LIST OF TABLES (cont'd)

Table Page

3.5 Permissible crack widths to prevent corrosion of steel reinforcement .......................... 121

3.6 Properties of an epoxy and polyester chemical grout ............................................. 122

3.7 General guide to repair options for concrete cracking ............................................ 123

3.8 Typical properties of rapid set patching materials by generic family ............................ 124

3.9 General guide to repair options for concrete spalling ............................................. 125

3.10 Water absorption and vapor transmission of selected coating materials ........................ 126

3.11 Relative performance of several coating systems for concrete materials ....................... 127

4.1 Safety significance ranking criteria ................................................................. 159

4.2 Environmental exposure categories and ratings ................................................... 160

4.3 Relative weights assigned to criteria affecting degradation factor gradings .................... 161

4.4 Prioritization of consequent net effects by generic subelements ................................. 162

4.5 Range of values for degradation factor grading in terms of subelement type .................. 163

4.6 Standards for use in environmental assessments .................................................. 165

4.7 Classification and rating of cracks and surface damage developed by RILEM 104-DDC .... 166

4.8 Influence of moisture state on durability processes ............................................... 168

4.9 Relationship between environmental conditions and aggressivity to reinforced concrete .... 169

4.10 Exposure classes for steel reinforcement-related environmental conditions ................... 170

4.11 Assessment of the degree of chemical attack of concrete by waters and soils containing

aggressive agents ..................................................................................... 171

4.12 Results of application of structural aging assessment methodology: PWR prestressed

large-dry plant ......................................................................................... 172

5.1 Statistical properties of NPP loads .................................................................. 202

5.2 Statistical data for reinforced concrete .............................................................. 202

5.3 Probabilistic descriptions of resistance of concrete components ................................ 203

5.4 Models analyzed in examples ........................................................................ 203

NUREG/CR-6424 xivi
LIST OF TABLES (cont'd)

Table Page

5.5 Optimum solutions for full-inspection strategies for Example 1 in Table 5.4 ................. 203

5.6 Optimum solutions for full-inspection strategies for Example 2 in Table 5.4 ................. 204

5.7 Optimum solutions for full-inspection strategies for Example 3 in Table 5.4 ................. 204

5.8 Optimum solutions for partial inspection strategies for Example 1 in Table 5.4 .............. 204

5.9 Engineering properties of slab ....................................................................... 205

5.10 Optimum solutions for full-inspection strategies for slab ........................................ 205

XV xv NUREG/CR-6424

A CKNOWLEDGEMENTS

The authors would like to acknowledge the continuing support and guidance throughout the

program provided by the U.S. Nuclear Regulatory Commission Technical Monitors, Mr. Herman Graves,

III and Mr. Gunter Arndt. Appreciation is also extended to the many organizations that contributed to the

success of the program. Specific organizations that participated in the program through subcontracted

activities included Construction Technologies Laboratories; Corrpro Companies; Howard Universities;

Multiple Dynamics Corporation; National Institute of Standards and Technology; Sargent & Lundy

Engineers; Taywood Engineering; The Johns Hopkins University; and Wiss, Janney, Elstner, Associates.

Contributions were also made to the program by consultants that either served on a peer review board

(T. Brown, J. Clifton, W. Gamble, C. Hookham, R. Ravindra, and J. Reed) or researched special topics

(H. Hill and C. Hookham). Finally, special thanks are due Ms. Pam Abbott for typing the manuscript.

xvii xvii NUREG/CR-6424

EXECUTIVE SUMMARY

The Structural Aging (SAG) Program addressed safety-related concrete structures in nuclear power

plants and provided the United States Nuclear Regulatory Commission (USNRC) with the following:

(1) identification and evaluation of the structural degradation processes; (2) issues to be addressed under

nuclear power plant (NPP) continued service reviews, as well as criteria and their bases for resolution of

these issues; (3) identification and evaluation of relevant in-service inspection or structural assessment

programs in use, or needed; and (4) quantitative methodologies for assessing current or future structural

safety margins. In meeting these objectives activities were conducted under three technical task areas:

(1) materials property data base, (2) structural component assessment and repair technologies, and

(3) quantitative methodology for continued service determinations. Results obtained under these three

tasks are contained in this report that consists of six chapters.

Chapter one provides background information pertaining to the importance of continuing the

service of NPPs. License renewal activities sponsored by the U.S. utilities and the USNRC are

summarized. Also included is the overall objective of the SAG Program, objectives of the three technical

task areas, and information pertaining to subcontracted activities. A complete listing of reports and papers

prepared under this program is contained in the Appendix.

Safety-related concrete structures and longevity considerations are addressed in the second chapter.

Design criteria are summarized and typical safety-related concrete structures are identified as well as their

materials of construction. Aging factors and environmental stressors that can impact the performance of

these structures are described as well as service-life models for estimating future performance. An

application is presented in which service life models are used to estimate onset of corrosion in NPP

reinforced concrete structures. Details are provided on a structural materials information center consisting

of a handbook and an electronic data base that contain data and information on the time variation of

material properties under the influence of pertinent environmental stressors and aging factors. Information

is provided on the performance of U.S. and United Kingdom reinforced concrete structures in nuclear

power stations.

Chapter three addresses in-service inspection (ISI), condition assessment, and remedial measures.

Current and emerging ISI techniques are identified, described, and recommendations provided relative to

their use. Techniques for conducting condition assessments of general civil engineering structures and

existing requirements for NPP reinforced concrete structures are summarized. Repair considerations,

materials, and techniques are identified. Guidance is provided on repair options for various forms of

concrete degradation (e.g., cracking and spalling). Remedial measures specifically addressing corrosiondamaged concrete are described.

Background for use in development of a methodology for inspection of NPP reinforced concrete

structures is provided in the fourth chapter. Selection of structures for evaluation is described in terms of

an aging assessment methodology, based in part on probabilistic risk assessments. Applications of

commonly used inspection methods are noted (i.e., visual, nondestructive, destructive, and analytical).

Considerations are presented for situations where accessibility is an issue. A visual-based inspection

methodology and criteria for use in interpretation of results are developed. Inspection scheduling and

personnel qualification requirements are provided. Development of a NPP inspection program for

reinforced concrete structures is illustrated through an example.

A reliability-based methodology for current and future condition assessments is addressed in the

fifth chapter. Modeling of degradation mechanisms, statistical data on loads and resistance for NPP

concrete structures, and basic aspects of the time-dependent reliability analysis are described. Conceptual

examples are presented for a reinforced concrete slab and low-rise shear wall to illustrate the timedependent reliability analysis. The role of inspection and repair in maintaining reliability is discussed.

Both single full inspection/repair and multiple full inspection/repair strategies are considered as well as

partial inspection/repair strategies. Examples are used to illustrate the effect of an inspection/repair

xix NUREG/CR-6424
operation. Optimum inspection/repair strategies to minimize expected future costs of keeping the failure

probability of the structure below an established target failure probability are developed and demonstrated

through an example application.

Chapter 6 presents a summary of program results, general conclusions that can be derived from the

program, and recommendations for additional activities.

NTJREGICR-6424 x

XX
1. INTRODUCTION

1.1 BACKGROUND

There are 109 nuclear power reactors presently licensed for commercial operation in the

United States with 1 reactor still under construction and 5 reactors partially completed, but under a

deferred construction schedulel* (see Appendix A). The Atomic Energy Act (AEA) of 1954 limits

the duration of operating licenses for most of these reactors to a maximum of 40 years. Forty-nine

of these reactors have been in commercial operation for 20 or more years. Expiration of operating

licenses for these reactors will start to occur in the year 2000 when Big Rock Point's license

expires. Through 2010 an additional 13 plants will also reach the end of their initial operating

license period with a potential net loss of electrical generating capacity of approximately 9 GW. An

additional 30 GW loss of net electrical generating capacity will occur between 2011 and 2015

when the initial licenses for an additional 37 plants are scheduled to expire. Under current

economic, social, and political conditions in the United States, the prospects for early resumption

of building of new nuclear plants to replace lost generating capacity are very limited.2 In some

areas of the country it may be too late because of the 10 to 15 years required to plan and build

replacement power plants. Continuing the service of existing nuclear plants through a renewal of

their initial operating licenses provides a timely and cost-effective solution to the problem of

meeting future energy demand.

The 40 year term on the duration of an operating license that was provided in the AEA of

1954 apparently was based on various financial considerations (e.g., bond maturity) and was not

based on safety or technical concerns. No technical information was presented to suggest that the

nuclear power plants (NPPs) would become unsafe if they were to operate after 40 years. 3 In

fact, the AEA permits the renewal of operating licenses. Paragraph 50.51 of Part 10 of the Code

of Federal Regulations4 implements this authority; however, no prior standards or procedures

have been provided for preparing or evaluating license renewal applications.

1.2 U.S. UTILITY AND NUCLEAR REGULATORY COMMISSION (USNRC)

AGING AND LICENSE RENEWAL ACTIVITIES

As a consequence of the significant economic and energy supply implications due to

expiration of the operating licenses for existing NPPs, both the U.S. utilities and USNRC have

conducted extensive research programs addressing aging management and its relation to license

renewal.

1.2.1 U.S. Utilities

The nuclear utility industry has expressed considerable interest in operating nuclear power

plants beyond their initial term of operation and has undertaken several initiatives in support of

this. A Steering Committee on Nuclear Plant Life Extension (NUPLEX) was formed under the

direction of the Nuclear Management and Resources Council (NUMARC) [now part of Nuclear

Energy Institute (NEI)]. The Electric Power Research Institute (EPRI), in cooperation with the

U.S. Department of Energy (DOE) and two utilities, sponsored research on life extension,

including pilot studies on two nuclear plants, Surry-1 5 and Monticello. 6 This led to funding by

DOE of two lead plants (Yankee Rowe and Monticello) to develop formal requests for renewal of

their operating licenses. The technical aspect of the DOE program was to provide an initial

* References are collected and provided as Chap. 7 of the report

I NUREG/CR-6424
evaluation of the effects of aging on commercial nuclear power plants and establish the scope of the

effort required to extend the operating lifetime of these plants beyond the initial 40 years of licensed

operation. Since that time, Yankee Rowe has been permanently shut down, and although a license

renewal application has been prepared for Monticello as a Boiling-Water Reactor Owner's Group

document, submittal of the license renewal application has been delayed.

Ten industry reports sponsored by DOE and EPRI in support of the license renewal

process have been developed under the auspices of NUMARC and submitted for USNRC staff

review. The industry reports evaluate the age-related degradation effects on specific structures and

components (e.g., Class 1 structures, pressurized-water reactor containments, and pressurizedwater reactor internals), describe the bases for how existing programs required by various

regulations address the aging concerns, and provide specific recommendations for corrective

actions that should be implemented for specific components or structures not presently addressed

by effective age-related management programs. These reports are intended to provide a partial

demonstration of the viability and stability of the license renewal process by reaching agreement

with the USNRC staff on technical issues related to light-water reactor systems, structures, and

components for operation beyond the current licensing term. When approved by the USNRC,

renewal applicants could reference the industry reports in the same manner that they can reference a

topical report for other licensing actions. These reports were prepared prior to issuance of the

revised License Renewal Rule discussed in the next section.

A number of activities addressing license renewal are being conducted by the various

owners groups [e.g., Babcock & Wilcox (B&WOG), Boiling Water (BWROG), and

Westinghouse (WOG)], and individual utilities [e.g., Baltimore Gas and Electric (BG&E),

Virginia Power, and Duke Power]. 7,

8 The NEI recently has formed a license renewal working

group (LRWG) consisting of three owner's groups (B&WOG, WOG, BWROG), two utilities

(BG&E, Virginia Power), DOE, and EPRI to coordinate owner's group technical activities. The

LRWG is conducting activities under two primary tasks: (1) new license renewal technical issues

(e.g., generic technical issues, technical reports coordination, and definition of terms), and

(2) new license renewal implementation guide (e.g., input coordination, lead industry effort, and

technical direction). Also, a number of industry-sponsored workshops specifically addressing

license renewal have taken place (e.g., Baltimore Gas and Electric Life-Cycle and EPRI/DOE

License Renewal).

1.2.2 U.S. Nuclear Regulatory Commission (USNRC)

The USNRC program on the degradation of nuclear power plant systems, structures, and

components (SSCs) due to aging began in the 1980's. This program has addressed the

development of rules related to renewal of operating licenses and has conducted research in support

of the various rulemaking activities.

1.2.2.1 Rulemaking activities

The USNRC staff in 1982, recognizing the potential impact of plant aging phenomena on

the continued safe operation of NPPs, convened a workshop in Bethesda, Maryland. The

objective of the workshop was to focus attention on how best to proceed to identify and resolve

various technical issues relevant to plant aging. By 1986, age-related degradation had become a

higher priority with the recognition that utilities were interested in continuing the service of their

existing nuclear power plants past the initial licensing term. In May 1987, the Technical Review

Group for Aging and Life Extension (TIRGALEX) produced a document defining technical safety

and regulatory policy issues associated with plant aging. A primary conclusion of this group was

NUREG/CR-64242 2
that many aging phenomena are readily managed and do not pose major technical issues that would

preclude continuing the service of NPPs past 40 years, provided that aging was properly managed

through programs that maintained, surveyed, repaired, and replaced key SSCs.

In August 1988, the USNRC staff published an Advanced Notice of Rulemaking in the

Federal Register (53 FR 32919) announcing the intention to prepare a proposed rule on license

renewal. The draft proposed rule (10 CFR Part 54) was published for public review and comment

in the Federal Register (Vol. 55, No. 137, July 17, 1990). Its intent was to provide the

regulatory philosophy for license renewal through establishment of (1) the technical requirements

that a license renewal applicant must satisfy, (2) the nature of information to be provided in a

license renewal application, and (3) the application procedures. Two important principles

underlying license renewal are that (1) the regulatory process is adequate to ensure that the

licensing bases of all currently operating plants provide and maintain an acceptable level of safety

so that operation will not be inimical to public health and safety or common defense and security, and (2) the plant-specific licensing basis must be maintained during the renewal term in the same

manner and to the same extent as during the original licensing term. To satisfy these principles, the

rule requires that (1) all license renewal applications compile the current licensing basis (CLB) for

their plants, submit a summary list of documents that identify portions of the CLB relevant to the

Integrated Plant Assessment (IPA), and maintain all CLB documents in an auditable and retrievable

form; (2) age-related degradation of all SSCs important to license renewal be evaluated through

IPA; and (3) existing or newly established effective programs be used to manage age-related

degradation. The CLB is defined as all regulatory requirements and licensee commitments for the

plant that are in effect at the time of the license renewal application and thus evolves with time and

is plant specific. The IPA consists of a screening process to select SSCs important to license

renewal based on their intended safety functions or contribution to challenging safety systems; an

evaluation and demonstration of the effectiveness of the already ongoing licensee actions under

existing regulatory requirements and plant-specific programs to address aging concerns; and

implementation, as necessary, of supplemental programs to prevent or mitigate age-related

degradation during the renewed license term. To amplify and support the proposed rule, written

guidance was provided in the form of a draft regulatory guide Standard Format and Content of

Technical Information for Applications to Renew Nuclear Power Plant Operating Licenses

(DG 1009), and draft standard review plan Standard Review Plan for the Review of License

Renewal Applications for Nuclear Power Plants (NUREG 1299), both published for public

comment on December 4, 1990.9

In further action, the Commission amended its regulations on July 10, 1991, through addition of §50.65, "Requirements for Monitoring the Effectiveness of Maintenance at Nuclear

Power Plants (56 Federal Register 31324)," to be implemented by each licensee no later than

July 10, 1996. The maintenance rule requires that power reactor licensees monitor the

performance or condition of SSCs against licensee-established goals in a manner sufficient to

provide reasonable assurance that these SSCs are capable of fulfilling their intended functions. It

provides a basis for concluding that the effects of aging will be effectively managed during the

license renewal period.

Subsequently, as a result of interactions with industry and a senior management review at

the USNRC, the USNRC staff in 1993 provided its recommendations regarding key license

renewal issues in two Commission policy papers - Implementation of 10 CFR Part 54 (SECY93-049), and Requirements for Renewal of Operating Licenses for Nuclear Power Plants

(SECY-93-113). A staff requirements memorandum of June 28, 1993, noted that the Commission

desired a predictable and stable regulatory process that defined the Commission's expectations for

license renewal in a clear and unequivocal way. This would permit licensees to make decisions

about license renewal without these decisions being influenced by the regulatory process that is

perceived to be uncertain, unstable, or not clearly defined. The Commission directed the USNRC

3 NUREG/CR-6424
staff to convene a workshop to address alternative approaches for license renewal to take advantage

of existing licensee activities and programs in demonstrating that aging will be addressed in an

acceptable manner during the period of continued service. Also, it was directed that the USNRC

staff should examine the extent to which greater reliance can be placed on the maintenance rule

(10 CFR 50.65) as a basis for concluding that the effects of aging will be effectively managed

during the continued service term. As a result, the USNRC staff on September 30, 1993,

conducted a public workshop in Bethesda, Maryland, on license renewal. The results of this

workshop and recommendations for revision of 10 CFR Part 54 were provided to the Commission

in December 1993 in License Renewal Workshop Results and Staff Proposals for Revision to

10 CFR Part 54 (SECY-93-33 1). The USNRC staff recommended that the Commission direct it

to amend 10 CFR Part 54 to establish a more stable and predictable license renewal process. In

their staff requirements memorandum of February 3, 1994, the Commission agreed with the

USNRC staff s conceptual approach in SECY-93-331 and directed the staff to proceed with

rulemaking to amend 10 CFR Part 54. The Commission noted that the amendment should

establish a more stable and predictable license renewal process that identifies certain SSCs that

require review to provide the necessary assurance that these SSCs will continue to perform their

intended function for the period of continued service. The resulting proposed revisions developed

by the USNRC staff were published for public review and comment in the Federal Register

(Vol. 59; No. 174; September 9, 1994). The proposed rule, which has since been adopted,

revises certain requirements contained in 10 CFR Part 54 and establishes a regulatory process that

is simpler, more stable, and more predictable than the prior license renewal rule. Some of the

changes implemented in the rule are that (1) the review focuses on the adverse effects of aging

rather than identify all aging mechanisms; (2) the definitions of IPA and the [PA process are

clarified to be consistent with the rule; (3) SSCs within the scope of license renewal are defined

and important functions that must be maintained identified; (4) the IPA process is simplified

through delineation of a methodology for determination of the need for an aging management

review for structures and components, and only long-lived passive structures and components are

subject to an aging management review for license renewal; (5) requirements for the CLB and

conditions of renewed licenses would be changed to delete all references to age-related degradation

unique to license renewal; and (6) the requirement for additional record and record-keeping

requirements would be changed to be less prescriptive.

1.2.2.2 Research activities

In 1985, the Division of Engineering of the Office of Nuclear Regulatory Research issued

the first comprehensive program plan for NPP aging research. 10 The Nuclear Plant Aging

Research (NPAR) Program is aimed at developing the technical bases to ensure that the critical

SSCs will provide adequate reliability as reactors age (i.e., understand and manage aging). The

NPAR Program approach to understanding aging is to (1) define the component's boundary and all

interfaces of interest, identify materials used in the design and fabrication of component parts;

(2) identify applicable stressors and environments during the lifetime of the component, including

those expected during and post design-basis events; (3) identify aging mechanisms and where they

could be operative; and (4) determine age-related degradation effects and their significance on

operability or performance (performing, when necessary, in situ testing and testing under

controlled laboratory conditions; performing testing on naturally-aged components and samples of

materials for correlation and validation). The NPAR Program approach to manage aging is to

(1) identify detection and condition monitoring methods for evaluating age-related degradation

effects, (2) identify and review ongoing programs with respect to their effectiveness in detecting

and managing age-related degradation effects, and (3) develop appropriate recommendations to

overcome deficiencies. This hardware-oriented program has studied 22 electrical and mechanical

components, 13 safety-related systems, and 10 special topics (e.g., data needs and record keeping,

NUREG/CR-6424 4
risk evaluation of aging phenomena, and degradation modeling). Results obtained under this

program have been summarized in approximately 160 technical reports and papers. 11 More

detailed information on the overall program is provided elsewhere. 12

One of the reports prepared under the NPAR Program was used as the basis to formulate

the Structural Aging (SAG) Program. 13 The SAG Program was initiated in 1988 and had the

overall objective of developing technical bases for addressing aging of the safety-related concrete

structures and providing guidance for use in NPP evaluations for continued service. In meeting

this objective, over 90 technical reports have been prepared and over 70 technical presentations

have been made describing program results. These results can be used in the aging management of

safety-related concrete structures to identify and evaluate (1) potential structural degradation

processes; (2) issues to be addressed under NPP continued service reviews, as well as criteria,

and their bases for resolution of these issues; (3) relevant in-service inspection (ISI) or structural

assessment programs in use, or needed; and (4) quantitative methodologies for assessing current,

or estimating future, structural safety margins.

1.3 OBJECTIVE AND APPLICATION OF RESULTS

The objective of this report is to provide a summary of results developed over the seven

year duration of the SAG Program. These results provide background data and information that

can be used by a reviewer or licensee to determine if the intent of the license renewal (10 CFR Part

54) and maintenance (10 CFR Part 50.65) rules are being met with respect to the safety-related

concrete structures (i.e., necessary actions are being taken to ensure that these structures will

continue to meet an acceptable level of safety, and age-related degradation is being adequately

addressed to assure that there is not a loss of safety functions or an unacceptable reduction in safety

margins). The results developed under this program have application to the required assessments

related to selection of SCCs important to license renewal based on intended safety functions,

evaluation and demonstration of the effectiveness of ongoing programs to address aging concerns,

and implementation of programs to prevent or mitigate age-related degradation. Potential

regulatory applications of this program include (1) improved predictions of long-term material and

structural performance and available safety margins at future times, (2) establishment of limits on

exposure to environmental stressors, (3) reduction in total reliance by licensing on inspection and

surveillance through development of a methodology that will enable the integrity of structures to be

assessed (either pre- or post-accident), and (4) improvements in damage inspection methodology

through incorporation of results into national standards that could be referenced by Standard

Review Plans.

1.4 APPROACH

The SAG Program consisted of a management task and three technical task areas

(Fig. 1.1).

The objective of the program management task (Task S.1) was to effectively manage the

technical tasks undertaken to address priority structural safety issues related to continuing the

service of NPPs. A key function of the management task in addition to technology transfer was

integration of the technical objectives and efforts of various program participants Individuals and

organizations outside Oak Ridge National Laboratory (ORNL) that participated in the SAG

Program are also presented in Fig. 1.1. The level 3 work breakdown structure for this task

identifying subtasks and primary activities under each subtask is presented in Fig. 1.2.

5 5 NUREG/CR-6424
The materials property data base task (Task S.2) developed a reference source containing

data and information on the time variation of material properties under the influence of pertinent

environmental stressors and aging factors. The data base has use in the prediction of potential

long-term deterioration of critical structural components in NPPs and in establishing limits on

hostile environmental exposure for these structures. The level 3 work breakdown structure for this

task identifying subtasks and primary activities under each subtask is presented in Fig. 1.3.

The structural component assessment/repair technology task (Task S.3) identified a

systematic methodology that can be used to (1) make quantitative assessments of the presence,

magnitude, and significance of any environmental stressors or aging factors that could impact the

durability of safety-related concrete structures in NPPs; and (2) provide recommended ISI or

sampling procedures that can be utilized to develop the data required both for evaluating the current

structural condition as well as trending the performance of these components for use in continued

service assessments. Associated activities included identification and evaluation of techniques for

mitigation of any environmental stressors or aging factors that may act on critical concrete

components, and an assessment of techniques for repair, replacement, or retrofitting of concrete

components that have experienced an unacceptable degree of deterioration. The level 3 work

breakdown structure for this task identifying subtasks and primary activities under each subtask is

presented in Fig. 1.4.

The quantitative methodology for continued service determinations task (Task S.4)

developed a methodology to facilitate quantitative assessments of current and future structural

reliability and performance of concrete structures in NPPs, taking into account those effects that

might diminish the ability of the structures to withstand future operating, extreme environmental,

or accidental conditions. Associated activities in meeting this objective included identification of

models to evaluate changes in strength of concrete structures over time in terms of initial

conditions, service load history, and aggressive environmental factors; and formulation of a

methodology to predict structural reliability of existing concrete structures during future operating

periods from a knowledge of initial conditions of the structure, service history, aging,

nondestructive condition assessment techniques, and inspection/repair strategies. The level 3 work

breakdown structure for this task identifying subtasks and primary activities under each subtask is

presented in Fig. 1.5.

Appendix B presents a listing of reports and papers that have been developed under the

SAG Program. Contained in the balance of this report is a compendium of knowledge that is

intended to provide the necessary data and information required to either develop an effective aging

management program for the NPP reinforced concrete structures or to evaluate an existing

program. Basic components of one approach for use in setting up a program for these structures is

presented in Fig. 1.6. The relation of the various sections of the report to this approach is

identified in the figure.

NUREG/CR-64246 6
STRUCTURAL

AGING PROGRAM

I

PROGRAM

MANAGEMENT

I

MATERIALS

M DATA PROPERTY BASE

III

STRUCTURAL

COMPONENT

ASSESSMENT/REPAIR

TECHNOLOGY

,, I

QUANTITATIVE

METHODOLOGY

FOR CONTINUED

SERVICE

DETERMINATION

CONSTRUCTION

TECHNOLOGY I

LABORATORIES I"

LONG-TERM STUDY OF

CEMENT IN CONCRETE

TAYWOOD ENG.

LABORATORIES

BRITISH

NUCLEAR PLANT

STRUCTURES

NAT'L INST. OF|

ST'DS & TECH.

MATERIAL BEHAVIOR

MODELING & ACCEL.

AGING TESTING

I

CORRPRO CO.

REINFORCED

CONCRETE

CORROSION

ASSESSMENTS

HOWARD

UNIVERSITY

DAMAGE

CLASSIFICATION

SYSTEM

NAT'L INST. OF

ST'DS & TECH.

NDE STATISTICAL

DATA

MULTIPLE DYN.

CORPORATION

COMPONENT AND

DEGRADATION FACTOR

CLASSIFICATION

j• CONSTRUCTION

TECHNOLOGY

LABORATORIES

ISI AND SAMPLING

TECHNOLOGY L - - - - -- - - - - i

I JOHNS HOPKINS

CURRENT AND

RELIABILITY-BASED

FUTURE CONDITION

ASSESSMENTS

HOWARD HILL

POST-TENSIONING

SYSTEM AGING

MANAGEMENT

CHUCK HOOKHAM

CONDITION

ASSESSMENT

METHODOLOGY

PEER REVIEW

BOARD

T. BROWN

J. CLIFTON

W. GAMBLE

C. HOOKHAM

R. RAVINDRA

J. REED

II I

TAYWOOD ENG.

LABORATORIES

REMEDIAL MEASURES

TECHNOLOGY

EUROPE

zl

Pd1

WISS, JANNEY,

ELSTNER, ASSOC.

REMEDIAL MEASURES

TECHNOLOGY IN U. S.

Fig. 1.1 Listing of subcontracted activities for each primary task area.
Fig. 1.2 Level 3 work breakdown structure for SAG Task S. 1: Program Management.

NUREG/CR-6424 8
MATERIALS

PROPERTY

DATA BASE S.2

• ~I.

STRUCTURAL MATERIALS

INFORMATION CENTER (SMIC)

FORMULATION/

IMPLEMENTATION

S.2.1

DATA

COLLECTION

S.2.2

*1i

|

MATERIAL

BEHAVIOR

MODELING

S.2.3

REVIEW/ASSESS

EXISTING MATERIAL

PROPERTY DATA BASES

EXISTING DATA

ASSEMBLAGE

m

DEVELOP SMIC

FOR CONCRETE

MATERIAL

SYSTEMS

PROTOTYPICAL

MATERIAL

TESTS

AGING EFFECTS

ENVIRONMENTAL

STRESSOR

EFFECTS

0

0' -Is

r'3

-Is

EXPAND SMIC TO INCLUDE

OTHER STRUCTURAL

MATERIAL SYSTEMS

Fig. 1.3 Level 3 work breakdown structure for SAG Task S.2: Materials Property Data Base.
01

STRUCTURAL COMPONENT

ASSESSMENT/REPAIR

TECHNOLOGY S.3

I I

LWR CRITICAL

CONCRETE COMPONENT

CLASSIFICATION

S.3.1

I

NDE/SAMPLING

INSPECTION

TECHNOLOGY S.3.2

I

REMEDIAL/PREVENTATIVE

MEASURES

CONSIDERATIONS 

S.3.3

1 - *1 .1

CRITICAL

COMPONENT

IDENTIFICATION

I,.. 0

IN-SERVICE

INSPECTION

TECHNIQUES

STRUCTURAL

INTEGRITY

ASSESSMENTS

REMEDIAL

MEASURES AND

MITIGATION

PROCEDURES

DEGRADATION

FACTOR

DESCRIPTION

DAMAGE ASSESSMENT AND REPAIR

PRIORITIZATION

Fig. 1.4 Level 3 work breakdown structure for SAG Task S.3: Structural Component Assessment/Repair Technology.
QUANTITATIVE

METHODOLOGY

FOR CONTINUED SERVICE

DETERMINATIONS S.4

I

I

PREDICTIVE

MODEL

DEVELOPMENT

S.4.1 I

I

DATA

ASSEMBLAGE/

IMPLEMENTATION

S.4.2 I

I

METHODOLOGY

DEVELOPMENT

S.4.3 I

I

CONDITION

ASSESSMENT

MODELS

DESIGN/MATERIAL/

CONSTRUCTION

CONSIDERATIONS

CURRENT

CONDITION

ASSESSMENT I-.

I

RELIABILITYBASED LIFE

PREDICTION

TOOLS

SERVICE

HISTORY

EFFECTS

I

RELIABILITYBASED FUTURE

CONDITION

ASSESSMENT

a'•

4:.-

Fig. 1.5 Level 3 work breakdown structure for Task S.4: Quantitative

Methodology for Continued Service Determinations.
Applicable Section(s) of Report

Review Pertinent Background

Materials and Documentation

Component Selection 2.22

Select Components 4.1

for Evaluation

3.1.1

Visua'Field L tor Inspection 42.1

E[ a itions 2 tn lesing 4.2.4

Condition Assessment

3.2.1

3.2.2

4.2

Repair 3.3

Continued Service

Determination

2.3.1

2.3.2

3.1

3.3.2

5.1

5.2

5.3

Fig. 1.6 Evaluation methodology for NPP reinforced concrete structures.

NUREG/CR-6424 12
2. SAFETY-RELATED CONCRETE STRUCTURES DESCRIPTION

AND LONGEVITY CONSIDERATIONS

2.1 DESIGN CRITERIA

All commercial nuclear power plants (NPPs) in the U.S. contain concrete structures whose

performance and function are necessary for protection of the safety of plant operating personnel

and the general public, as well as the environment. The basic laws that regulate the design (and

construction) of NPPs are contained in Title 10 of the CFR4 that is clarified by Regulatory Guides

(e.g., R.G. 1.29),14 NUREG reports, Standard Review Plans (e.g., Concrete and Steel Internal

Structures of Steel or Concrete Containments), 15 etc. In addition, R.G. 1.29 and Part 100 to Title

10 of the CFR state that NPP structures important to safety must be designed to withstand the

effects of earthquakes without the loss of function or threat to public safety. These "safety-related"

structures are designated as seismic Category I. Seismic Category I structures typically include

those classified by the American Society of Mechanical Engineers (ASMiE) and the American

Nuclear Society (ANS) as Safety Classes 1, 2, and 3 (i.e., safety related).

Initially, existing building codes such as the American Concrete Institute (ACI) Standard

318, Building Code Requirements for Reinforced Concrete, 16 were used in the nuclear industry

as the basis for the design and construction of concrete structural members. However, because the

existing building codes did not cover the entire spectrum of design requirements and because they

were not always considered adequate, the U.S. Nuclear Regulatory Commission (USNRC)

developed its own criteria for the design of Category I structures (e.g., definitions of load

combinations for both operating and accident conditions). Current requirements for nuclear safetyrelated concrete structures, other than concrete reactor vessels and concrete containments, are also

based on ACI 318, but have incorporated modifications to accommodate the unique performance

requirements of NPPs. These requirements were developed by ACI Committee 349 and first

published in October 1976.17 This Code has been endorsed by the USNRC as providing an

adequate basis for complying with the general design criteria for structures other than reactor

vessels and containments.18 Reference 19 provides additional information on the design of

seismic Category I structures that are required to remain functional if the Safe Shutdown

Earthquake (SSE) occurs. Current requirements for concrete reactor vessels and concrete

containments were developed by ACI Committee 359 and first published in 1977.20 Supplemental

load combination criteria are presented in Sect. 3.8.1 of the USNRC Regulatory Standard

Review Plan.2 1 However, since all but one of the construction permits for existing NPPs have

been issued prior to 1978, it is unlikely that endorsed versions of either ACI 349 or ACI 359 were

used in the design of many of the concrete structures at these plants. Older plants that used early

ACI codes, however, have been reviewed by the USNRC through the Systematic Evaluation

Program to determine if there were any safety concerns. 22

2.2 SAFETY-RELATED CONCRETE STRUCTURES

A myriad of concrete-based structures are contained as a part of a light-water reactor

(LWR) plant to provide foundation, support, shielding, and containment functions. Table 2.1"

presents a listing of typical safety-related concrete structures that may be included as part of a LWR

plant. Only a general description of these structures is provided in the following sections because

detailed information of this type along with typical design parameters and operating conditions is

* Tables and figures appear at the end of each chapter.

13 NUREG/CR-6424
provided elsewhere. 13,

2 3- 26 Information pertaining to a particular structure at a plant of interest

can be obtained from sources such as the plant's safety analysis report or docket file. Concrete

structures that are considered to be "plant specific" or unique have not been addressed in the

discussion below, but some information provided for similar structures may be applicable.

Additionally, the names of certain structures may vary from plant-to-plant depending on the nuclear

steam supply system (NSSS) vendor, architect-engineering firm, and owner preference. The

safety-related concrete structures, for purposes of discussion, have been separated into two

categories - typical plant structures and auxiliary structures.

2.2.1 Typical Plant Structures

2.2.1.1 Boiling-water reactors

Typical safety-related concrete structures contained in boiling-water reactor (BWR) plants

can be grouped into four general categories: primary containments, containment internal

structures, secondary containments/reactor buildings, and other structures. Table 2.2 presents a

summary of BWR structures that typically are included in the categories.

Primary Containment Of the 37 BWR plants that have been licensed for commercial

operation in the U.S., 11 utilize either reinforced (2 Mark 1, 5 Mark II, 2 Mark III) or prestressed

(2 Mark II) concrete primary containments. Leak tightness of each of these containments is

provided by a steel liner attached to the containment inside surface by studs (e.g., Nelson studs) or

by structural steel members. Exposed surfaces of the carbon steel liner are typically painted to

protect against corrosion and to facilitate decontamination should it be required. A portion of the

liner toward the bottom of the containment and over the basemat is typically embedded in concrete

to protect it from damage, abrasion, etc. due to corrosive fluids and impact. A seal to prevent the

ingress of fluids is provided at the interface around the circumference of the containment where the

vertical portion of the liner becomes embedded in the concrete.

BWR containments, because of provisions for pressure suppression, typically have "normally dry" sections (drywell) and "flooded" sections (wetwell) that are interconnected via

piping or vents (see Figs. 2.1-2.3). Requirements for BWR containments include the following:

1. Provide an "essentially" leak-tight barrier against the uncontrolled release of

radioactivity to the environment for all postulated design basis accident conditions;

2. Accommodate the calculated pressure and temperature conditions resulting from a

loss-of-coolant accident;

3. Withstand periodic integrated leak-rate testing at the peak calculated accident

pressure that may be at levels up to and including the containment design pressure;

and

4. Permit appropriate periodic inspection of all important components and surfaces

and the periodic testing of the leak tightness of containment penetrations.

In addition, the containment vessel can provide structural support for the NSSS and other

internal equipment. The containment foundation, typically a basemat, provides the primary

support and transfer of load to the earth below.

Containment Internal Structures Each of the three BWR plant types (Mark I,

Mark II, and Mark IlI) incorporate a number of reinforced concrete containment internal

structures. These structures may perform singular or several functions including the following:

1. Radiation shielding;

2. Human accessibility provisions;

NUREG/CR-6424 14
3. NSSS and other equipment anchorage/support/protection;

4. Resistance to jet, pipe whip, and other loadings produced by emergency

conditions;

5. Boundary of wetwells and pool structures, allow communication between drywell

and wetwell (Mark II and III);

6. Lateral stability for containment;

7. Transfer of containment loads to underlying foundation; and

8. Transfer of fuel to reactor (Mark IlI).

As many of these functions are interrelated with the required containment functions, these

structures are considered safety-related.

Secondary Containments/Reactor Buildings Of the 26 BWR plants that utilize steel

primary containments (1 Pre-Mark, 22 Mark I, 1 Mark II, 2 Mark III), all but the pre-Mark plant

have reinforced concrete structures that serve as secondary containments or reactor buildings and

provide support and shielding functions for the primary containment. Although the design

parameters for the secondary containments of the Mark I and Mark II plants vary somewhat, the

secondary containments are typically composed of beam, floor, and wall structural elements.

These structures typically are safety-related because they provide additional radiation shielding;

provide resistance to environmental/operational loadings; and house safety-related mechanical

equipment, spent fuel, and the primary metal containment. Although these structures may be

massive in cross-section in order to meet shielding or load-bearing requirements, they generally

have smaller elemental thicknesses than primary containments because of reduced exposure under

postulated accident loadings. These structures may be maintained at a slight negative pressure for

collection and treatment of any airborne radioactive material that might escape during operating

conditions.

Other Structures Included in this category are such things as foundations, walls, slabs,

and fuel/equipment storage pools. The spent- and new-fuel storage pools, and the pools for

reactor internals storage, typically have a four wall-with-bottom slab configuration. The walls and

slab are composed of reinforced concrete members lined on the interior surface with stainless steel.

Cross-sections of these members are generally large because they must support a large pool of

water and heavy fuel/component loads, produced by high-density fuel storage considerations. The

fuel storage pool in Mark III plants is located within the primary containment.

2.2.1.2 Pressurized-water reactors

Typical safety-related concrete structures in pressurized-water reactor (PWR) plants also

can be grouped into the four general categories noted above for the BWR plants. Table 2.3

presents a summary listing of PWR structures that typically are included in these categories.

Primary Containment Of the 72 PWR plants that have been licensed for commercial

operation in the U.S., 59 utilize either reinforced (11 large dry, 8 subatmospheric, 2 ice condenser)

or prestressed (38 large dry) concrete primary containments. In meeting the same basic functional

and performance requirements as noted for BWR containments in Sect. 2.2.1.1, the concrete

containments in PWR plants are of three different functional designs (Fig. 2.4-2.6):

subatmospheric (reinforced concrete), ice condenser (reinforced concrete), and large/dry

(reinforced and prestressed concrete). The primary differences between these containment designs

relate to volume requirements, provisions for accident loadings/pressures, and containment internal

structures layout.

The PWR containment structure generally consists of a concrete basemat foundation,

vertical cylindrical walls, and dome. The basemat may consist of a simple mat foundation on fill,

natural cut or bedrock, or may be a pile/pile cap arrangement. Most of the plants have utilized the

15 NUREG/CR-6424
simple mat on fill or bedrock design. Interior containment surfaces are lined with a thin carbon

steel liner to prevent leakage. Two of the PWR plants (Bellefonte and Ginna) have rock anchor

systems to which the post-tensioning tendons are attached.

Containment Internal Structures The containment internal structures in PWR plants

are typically constructed of conventionally reinforced concrete and tend to be more massive in

nature than the internal structures in BWR plants because they typically support the reactor

pressure vessel, steam generators, and other large equipment and tanks. In addition, these

structures provide shielding of radiation emitted by the NSSS. Some of the specific functions that

these structures (typically floor slabs, walls, and columns) are required to perform include:

1. Provision of human accessibility;

2. Support and separation of various plant equipment;

3. Resistance to emergency loading conditions;

4. Transfer of containment loads to containment foundation;

5. Missile protection; and

6. Channeling/routing steam and air through ice condensers (PWR ice condenser

containments).

Secondary Containments/Reactor Buildings PWR plants that utilize a metallic

primary containment (large dry and ice condenser designs) are usually contained in reinforced

concrete "enclosure" or "shield" buildings. This secondary containment consists of a vertical

cylinder wall with shallow dome (Fig. 2.7) and is often supported by the containment basemat. In

addition to withstanding environmental effects, the secondary containment provides radiation

shielding and particulate collection and ensures that the free standing metallic primary containment

is protected from the natural environment.

Other Structures Except for differences in the spent- and new-fuel storage pools,

structures that fall into the other structures category are essentially the same at the PWR and BWR

plants. The spent- and new-fuel storage pools for PWR plants are typically located in an auxiliary

building proximate to the containment. These reinforced concrete wall and slab structures are

generally massive in cross-section to support a large pool of water and the fuel elements, and are

lined on the water side with stainless steel. The pools are connected to the reactor/refueling cavity

(inside containment) via a transfer channel that is also a safety-related structure since it must

provide radiation shielding and support for the fuel transport mechanism and fuel.

2.2.2 Auxiliary Structures

Auxiliary structures are considered to be those concrete structures in a NPP that may or

may not perform safety-related functions, depending on the plant-unique or site-specific design and

licensing or operating criteria. These structures typically house important plant equipment or

control-room facilities or provide additional radiation shielding/containment to meet 10 CFR

requirements. They may be located immediately adjacent to the secondary containment (e.g.,

auxiliary building, diesel generator building, etc.) or be separated on site (e.g., intake structures,

offgas stacks, etc.). Although these reinforced concrete structures may take many different

physical configurations in meeting their functional and performance requirements, they typically

fall into two broad categories: (1) common structures, and (2) plant-unique structures.

2.2.2.1 Common structures

Common building structures are typically configured in a rectangular box shape, and

consist of reinforced concrete floor slabs, walls, and mat foundation. These subelements are

typically of lighter construction (thinner sections with reduced conventional reinforcing) than the

NUREG/CR-6424 16
plant containment structures. They may also be composite with structural steel framing and contain

shear walls for vertical and horizontal load resistance. Primary functions of these structures are to

provide an enclosure for equipment important to plant safety and to provide secondary radiation

containment.

2.2.2.2 Plant-unique structures

Plant-unique concrete structures include components such as intake canal liners, offgas

stacks, and emergency cooling pathways. Although these structures are typically constructed of

conventional reinforced concrete, their configuration and methods of construction differ from that

of general building construction because the structures must meet specific design loading

conditions dictated by their function as well as that of potential extreme environmental conditions

(e.g., earthquake, flood, tornado, etc.). In addition, these structures may be required to resist the

effects of the natural environment, and may be exposed to cooling water (river, ocean, lake).

Typically, the plant-unique structures contribute to plant safety by serving to dissipate heat and

radiation, or to protect other safety-related components.

2.3 AGING AND LONG-TERM DURABILITY CONSIDERATIONS

As previously noted, the reinforced concrete structures at NPPs have been designed in

accordance with national consensus codes and standards (e.g., ACI 318).16 The rules in these

documents were developed over the years by experienced people and are based on the knowledge

that was acquired in testing laboratories and supplemented by field experience. Design procedures

have been dominated by analytical determinations based on strength principles. Durability

considerations require that the time element be factored into the design of reinforced concrete

structures. Associated with the design specifications developed for the concrete structures in

conformance with these calculations was a certain level of durability (e.g., minimum requirements

for concrete cover to protect embedded steel reinforcement under different anticipated

environmental conditions). Although the vast majority of reinforced concrete structures, and

particularly those associated with NPPs, have and continue to meet their functional and

performance requirements, examples can be cited where these structures have exhibited

degradation primarily due to the effects of environmental stressors. 13,24,27 Estimations of the

future performance of NPP concrete structures requires knowledge on their materials of

construction, potential sources of degradation, and performance history or longevity.

2.3.1 Materials of Construction

Nuclear safety-related concrete structures are composed of several constituents that, in

concert, perform multiple functions (e.g., load-carrying capacity, radiation shielding, and leak

tightness). Primarily, these constituents include the following material systems: concrete,

conventional steel reinforcement, prestressing steel, steel liner plate, and embedment steel. More

detailed information on materials of construction is available elsewhere. 13,

2 0,

24

2.3.1.1 Concrete

The concrete typically used in nuclear safety-related structures consists of Type II portland

cement, 28 fine aggregates (e.g., sand), water, various admixtures for improving properties or

performance of the concrete, and either normal-weight or heavyweight coarse aggregate. Type II

portland cement was used because of its improved sulfate resistance and reduced heat of hydration

relative to the general purpose Type I portland cement. Both the water and fine and coarse

aggregates are normally acquired from local sources and subjected to material characterization

testing prior to use. Various admixtures have been used to improve air entrainment (i.e., enhanced

17 NUREG/CR-6424
durability), improve workability (i.e., enhanced placement and compaction), modify hardening or

setting characteristics, aid in curing, reduce evolution of heat, or provide other property

improvements. 29 Coarse aggregate consists of gravel, crushed gravel, or crushed stone. For

those concrete structures in nuclear power plants that provide primary (biological) radiation

shielding, heavyweight, or dense aggregate materials, such as barites, limonites, magnetites,

ilmenites, etc., may have been used to reduce the section thickness requirements needed for

attenuation.

The constituents are then proportioned and mixed to develop portland cement concrete that

has specific properties. Depending on the characteristics of the specific structure, the concrete mix

may be adjusted to provide increased strength, higher durability, or better workability for

placement. The hardened concrete typically provides the compressive load-carrying capacity for

the structure. Reinforced concrete, in one form or another, has been used in the construction of all

LWR plants. Specified concrete unconfined compressive strengths typically have ranged from 13

to 55 MPa, with 28 MPa being most common.

2.3.1.2 Conventional steel reinforcement

Most of the mild, or conventional, reinforcing steels 30 used in nuclear power plants to

provide primary tensile and shear load resistance/transfer consists of plain carbon steel bar stock

with deformations (lugs or protrusions) on the surface. These bars typically conform with ASTM

A 61531 or A 70632 specifications (older vintage plants may contain bars conforming to ASTM

A 43233 or A 30534 specifications that have been superseded by the above). The minimum yield

strength of this material ranges from 270 MPa to 415 MPa, with the 415 MPa material being most

common. Conventional reinforcing steel also encompasses welded wire fabric (ASTM A 18535

and A 49736), deformed wire (ASTM A 49637), bar and rod mats (ASTM A 18438), and all

accessory steel components used in positioning/placing the reinforcement (e.g., seats, ties, etc.).

2.3.1.3 Prestressing steel

A post-tensioned prestressing system consists of tendons that are installed, tensioned, and

then anchored to the hardened concrete forming the structure. A number of concrete containment

structures utilize post-tensioned steel tendons to provide primary resistance to tensile loadings.

Three major categories of prestressing systems exist depending on the type of tendon utilized:

wire, strand, or bar. These systems typically conform to ASTM specifications A 421,39

A 416,40 and A 72241 and have minimum ultimate strengths ranging from 2000 kN to 10,000

kN. The tendons are installed within preplaced ducts in the containment structure and are posttensioned from one or both ends after the concrete has achieved sufficient strength. After

tensioning, the tendons are anchored by buttonheads, wedges, or nuts. Corrosion protection is

provided by filling the ducts with wax or corrosion-inhibiting grease (unbonded), or portland

cement grout (bonded). Supplemental conventional reinforcing is also used to minimize shrinkage

or temperature effects and to provide local load-carrying capacity or load transfer. With the

exception of Robinsion 2 (bar tendons) and Three Mile Island 2 (strand tendons), plants that have

post-tensioned containments utilize unbonded tendons. Bellefonte and Ginna each have grouted

tendons (rock anchors) to which tendons are attached.

NUREG/CR-6424 18
2.3.1.4 Liner plate

Leak tightness of reinforced and post-tensioned concrete containment vessels is provided

by steel liner plate. A typical liner is composed of steel plate stock less than 13 mm thick, joined

by welding, and anchored to the concrete by studs (Nelson studs or similar conforming to ASTM

A 10842), structural steel shapes, or other steel products. The drywell portions of BWR

containments and PWR containments are typically lined with carbon steel (ASTM A 3630 or

A 51643). The liners of LWR fuel pool structures typically consist of stainless steel (ASTM

A 276,44 Type 30445 is common). The liners of wetwells also have used carbon steel materials

such as ASTM A 285,46 A 516,43 and A 537.47 Certain LWR facilities also have used carbon

steel clad with stainless steel weld metal for liner members. Although the liner's primary function

is to provide a leaktight barrier for containment of radiation, it acts as part of the formwork during

concrete placement and may be used in the support of internal piping/equipment.

2.3.1.5 Embedment steel

Anchorage to concrete is required for heavy machinery, structural members, piping,

ductwork, cable trays, towers, and many other types of structures. Anchorage design had to meet

certain requirements such as ease of installation, load capacity, susceptibility to vibration, preload

retention, temperature range, corrosion resistance, postinstallation or preinstallation, and ease of

inspection or stiffness. In meeting its function, loads that the anchor must transfer to the concrete

vary over a wide combination of tension, bending, shear, and compression. Examples of types of

anchors available include embedded bolts (ASTM A 307,48 A 325,49 or A 49050), grouted bolts,

embedded studs, self-drilled expansion anchors, or wedge anchors. Embedded steel, typically

ASTM A 36,30 may also be constructed of structural plates or shapes installed during concrete

placement

2.3.2 Aging Factors and Environmental Stressors

As concrete ages, changes in its properties will occur as a result of continuing

microstructural changes (e.g., slow hydration, crystallization of amorphous constituents, and

reactions between cement paste and aggregates), as well as environmental influences. These

changes do not have to be detrimental to the point that reinforced concrete will not be able to meet

its functional and performance requirements. When specifications covering concrete's production

are correct and followed, reinforced concrete need not deteriorate.5 1 Concrete, however, can

suffer undesirable degrees of change with time because of improper specifications, a violation of

specifications, or environmental effects. Basic factors related to the production of durable concrete

include materials selection, design, execution, and curing, Fig. 2.8.52 Quality control/quality

assurance programs at NPPs generally have been very effective in ensuring that these factors are

adequately addressed. 53

Surveys of reported errors involving general civil engineering concrete structures in North

American and Europe54,

55 concluded that when errors occurred they were almost always the result

of faulty construction or design deficiencies.* Errors due to construction were generally

discovered during construction. A similar conclusion was derived from a survey questionnaire

* A limitation of the North American study was that the information presented was strongly biased toward errors that

escaped detection until revealed by the structure and thus did not present a true picture of the error-detection process of

the review check system. Also, the survey favored those structures and serviceability characteristics that revealed

themselves in a short period of time and thus does not represent the actual incidences of concrete degradation. 54

19 NUREG/CR-6424
sent to U.S. utilities to obtain information related to in-service inspection procedures, incidences of

degradation, and repair procedures that have been utilized with respect to the concrete structures. 56

Responses to the survey questionnaire, provided by slightly less than half the commercial NPPs in

the U.S., indicated that the majority of problems associated with the concrete structures initiated

during construction and have been corrected.

Results of the surveys noted above indicate that as these structures age, deterioration due to

the effects of environmental stressors is the primary threat to their durability. From an aging

management perspective, it is important not only to understand the potential degradation factors

and their manifestations, but to be able to estimate future performance through service life models.

2.3.2.1 Environmental stressor considerations

The longevity, or long-term performance of safety-related concrete structures is primarily a

function of the durability or propensity of these structures to withstand the potential effects of

degradation. Table 2.4 presents a summary of the degradation factors that potentially can impact

the performance of the basic constituents that compose safety-related concrete structures in NPPs

(i.e., concrete, mild steel reinforcement, post-tensioning system, and liner/structural steel

members). Also contained in the table is a listing of primary manifestations of each degradation

factor. A listing of several areas in NPPs that potentially may experience degradation is provided

in Table 2.5. More detailed information to that summarized below is available elsewhere. 13,23-27

Concrete Material Systems The durability of concrete materials can be limited as a

result of adverse performance of its cement-paste matrix or aggregate constituents under either

chemical or physical attack. In practice, these processes may occur concurrently to reinforce each

other. In nearly all chemical and physical processes influencing the durability of concrete

structures, dominant factors involved include transport mechanisms within the pores and cracks,

and the presence of water.

Cracking occurs in virtually all concrete structures and, because of concrete's inherently

low tensile strength and lack of ductility, can never be totally eliminated. Cracks are significant

from the standpoint that they can indicate major structural problems (active cracks); provide an

important avenue for the ingress of hostile environments (active or dormant cracks); and may

inhibit a component from meeting its performance requirements (active or dormant cracks) (e.g.,

diminished shielding capacity). As shown in Fig. 2.9,57 cracks in concrete can form either before

or after the concrete has hardened. Examples of cracks that form in concrete due to movements

generated within the concrete (e.g., shrinkage, and expansion and contraction due to temperature

change) or expansion of material embedded within the concrete (e.g., corrosion of steel

reinforcement) are presented in Fig. 2.10,57 and an indication of the potential time occurrence of

cracks in concrete due to several of these causes is noted in Fig. 2.11.52 Information on cracking

and its classification with respect to damage is provided in Ref. 58. From an aging management

perspective for existing structures, causes of concrete degradation due to either chemical or

physical causes are of primary interest.

Chemical Attack

Chemical attack involves the alteration of concrete through chemical reaction with either the

cement paste or coarse aggregate, or embedded steel reinforcement.** Generally, the attack occurs

on the exposed surface region of the concrete (cover concrete), but with the presence of cracks or

** Corrosion of embedded steel reinforcement due to carbonation of the concrete or the action of chloride ions is

covered under the section addressing mild steel reinforcement.

NUREG/CR-6424 20
prolonged exposure, chemical attack can affect entire structural cross sections. Chemical causes of

deterioration can be grouped into three categories: (1) hydrolysis of cement paste components by

soft water; (2) cation exchange reactions between aggressive fluids and the cement paste; and

(3) reactions leading to formation of expansion products. 59 The rate of chemical attack on

concrete is a function of the pH of the aggressive fluid and the concrete permeability, alkalinity,

and reactivity. Figure 2.12 presents a summary of the types of chemical reactions responsible for

concrete deterioration and the detrimental effects that can occur.60 Chemical attack of concrete may

occur in several different forms as highlighted in the following sections.

Leaching and Efflorescence Pure water that contains little or no calcium ions, or

acidic groundwater present in the form of dissolved carbon dioxide gas, carbonic acid, or

bicarbonate ion, tends to hydrolyze or dissolve the alkali oxides and calcium-containing products.

The rate of leaching is dependent on the amount of dissolved salts contained in the percolating

fluid, rate of permeation of the fluid through the cement paste matrix, and temperature. Extensive

leaching causes an increase in porosity and permeability thus lowering the strength of the concrete

and making it more vulnerable to hostile environments (e.g., water saturation and frost damage, or

chloride penetration and corrosion of embedded steel). The rate of leaching can be controlled by

minimizing the percolation of water through the concrete. Concretes produced using low watercement ratios, adequate cement content, and proper compaction and curing are most resistant to

leaching.

Efflorescence occurs on the surface of concrete following the percolation of a fluid (e.g.,

water) through the material, either intermittently or continuously, or when an exposed surface is

alternately wetted and dried. It forms due to crystallization of the dissolved salts as a result of

evaporation of the fluid or interaction with carbon dioxide in the atmosphere. As such,

efflorescence is primarily an aesthetic problem rather than a durability problem, but may indicate

that alterations to the cement paste are taking place in the concrete.

Sulfate Attack Magnesium and alkali sulfates present in soils, groundwater, and

seawater react with the calcium hydroxide and alumina-bearing phases of portland cement to form

gypsum and ettringite. These reactions, if enough water is present, result in expansion and

irregular cracking of the concrete that can lead to progressive loss of strength and mass. Structures

subjected to seawater are more resistant to sulfate attack because of the presence of chlorides that

form chloro-aluminates to moderate the reaction. Concretes that use cements low in tricalcium

aluminate (e.g., Type V sulfate resisting) and those that are dense and of low permeability are most

resistant to sulfate attack. Guidelines on concrete exposed to sulfate attack are provided in

Refs. 16 and 61.

Acids and Bases. Acids present in groundwater (e.g., sulfuric or carbonic) and certain

plant internal fluids (e.g., boric and sulfuric acids) can combine with the calcium compounds in the

hydrated cement paste (i.e., calcium hydroxide, calcium silicate hydrate, and calcium aluminate

hydrate) to form soluble materials that are readily leached from the concrete to increase its porosity

and permeability. The main factor determining the extent of attack is not so much the

aggressiveness of the attacking acid, but more the solubility of the resulting calcium salt. The rate

of deterioration is also accelerated if the aggressive chemical solution is flowing. Since under acid

attack there is a conversion of the hardened cement, the concrete permeability is not as important as

for other types of chemical attack (e.g., leaching and sulfate attack). Due to the large buffering

capacity of concrete and the relatively small amount of acid contained in rain, acid rain will convert

only an insignificant amount of the concrete. 52 Acid rain is even a smaller threat to NPP structures

than general civil engineering concrete structures because of their massive cross sections.

21 NUREG/CR-6424
As hydrated cement paste is an alkaline material, high quality concretes made with

chemically stable aggregates normally are resistant to bases. However, sodium and potassium

hydroxides in high concentrations (> 20%) can cause concrete to disintegrate.

Under mild chemical attack, a dense concrete with low water-cement ratio may provide

suitable resistance. As corrosive chemicals can attack concrete only in the presence of water,

designs to minimize attack by acids and bases generally involve the use of protective barrier

systems. Table 2.6 presents a listing of the reactivity with concrete of various chemicals that may

be found in NPPs or the surrounding environment. Reference 62 presents additional information

on the effect of chemicals on concrete.

Alkali-Aggregate Reactions Chemical reactions involving alkali ions (portland

cement), hydroxyl ions, and certain siliceous constituents that may be present in aggregate

materials can form a gel. As the alkali-silica gel comes in contact with water, swelling (i.e.,

hydraulic pressure) occurs that can cause cracking that eventually could lead to complete

destruction of the concrete.63 Visible concrete damage starts with small surface cracks exhibiting

an irregular pattern (or map cracking). The expansion will develop in the direction of least

constraint (i.e., parallel surface patterns developing inward from surface for slabs and cracking

parallel to compression forces in columns or prestressed members). Pop-outs and glassy

appearing seepage of varying composition can appear as a. result of alkali-silica reactions.

Expansion reactions also can occur as a result of alkali-carbonate reactions (i.e., dedolomitization).

Primary factors influencing alkali-aggregate reactions include the aggregate reactivity (i.e.,

amount and grain size of reactive aggregate), alkali and calcium concentrations in concrete pore

water, cement content (i.e., alkali content), and presence of water. Although alkali-aggregate

reactions typically occur within 10 years of construction, deterioration has not occurred in some

structures until 15 or even 25 or more years following construction. The delay in exhibiting

deterioration indicates that there may be less reactive forms of silica that can eventually cause

deterioration.64 Control of the alkali-aggregate reactions is generally through elimination of

deleteriously reactive aggregate materials from consideration through petrographic examinations,

laboratory evaluations, and use of materials with proven service histories. An improved rapid test

method for evaluating the alkali reactivity of siliceous aggregates has been proposed. 6 5 A method

for evaluating potential alkali reactivity of carbonate aggregate is also available.66 Known

deleterious reactive aggregate materials are provided in ACI 201.2R. 67 Additional mitigating

procedures include use of pozzolans, restricting the cement alkali contents to less than 0.6% by

weight Na2O equivalent, and application of barriers to restrict or eliminate moisture.

Physical Attack

Physical attack involves the degradation of concrete due to external influences and generally

involves cracking due to exceeding the tensile strength of the concrete, or loss of surface material.

Concrete attack due to overload conditions is not considered as an aging mechanism.

Salt Crystallization Salts can produce cracks in concrete through crystal growth

pressures that arise through physical causes (e.g., repeated salt crystallization due to evaporation in

the pores). Structures in contact with fluctuating water levels or in contact with groundwaters

containing large quantities of dissolved salts (e.g., NaCe, CaS0 4 , and NaS0 4) are susceptible to

this type of deterioration. The problem of salt crystallization is minimized for low permeability

concretes and where sealers or barriers have been effectively applied to prevent water ingress or

subsequent evaporation.

NUREG/CR-6424 22
Frost Attack Concrete, when in a saturated or near saturated condition, can be

susceptible to damage during freezing and thawing cycles caused by hydraulic pressure generated

in the capillary cavities of the cement paste as water freezes. Damage to concrete resulting from

frost attack can take several forms: scaling, spalling, and pattern cracking (e.g., D-cracking).64

The damage is incurred after an extended number of cycles and is observed on exposed surfaces of

affected structures. Factors controlling the resistance of concrete to frost action include air

entrainment (i.e., size and spacing of air bubbles as opposed to entrapped air), water-cement ratio,

curing, strength, and degree of saturation. Selection of durable aggregate materials is also

important. An estimate of the susceptibility of concrete aggregates for known or assumed field

environmental conditions can be provided from ASTM C 682.68 Guidelines for production of

frost resistance concretes are provided in ACI 201.2R67 and ACI 31816 in terms of total air content

as a function of maximum aggregate size and exposure condition. Areas in the U.S. subjected, on

average, to the largest number of freeze-thaw cycles per year are provided in a weathering index

chart contained in Ref. 69.

Abrasion/Erosion/Cavitation Progressive loss of material at the concrete surface can

occur due to abrasion, erosion, or cavitation. Abrasion generally refers to dry attrition, while

erosion is normally used to describe wear by the abrasive action of fluids containing solid particles

in suspension. Cavitation relates to the loss of surface material by formation of vapor bubbles and

their subsequent collapse, due to sudden change of direction or pressure in rapidly flowing water,

on the surface of the structure. Resistance of concrete to abrasion and erosion is dependent on the

quality of the concrete (low porosity, high strength) and in particular the aggregate particles used in

the mix. While good quality concrete may show good resistance to abrasion and erosion, it may

still suffer severe loss of surface material due to cavitation. The best way to guard against the

effects of cavitation is to eliminate the cause(s) of cavitation. Reference 70 provides additional

information on the effects of erosion on concrete structures.

Thermal Exposure/Thermal Cycling Elevated-temperature exposure and thermal

gradients are important to concrete structures in that theyf affect the concrete's strength (i.e., ability

to carry loads) and stiffness (i.e., structural deformations and loads that develop at constraints).

The mechanical property variations result largely because of changes in the moisture content of the

concrete constituents and progressive deterioration of the cement paste and aggregate (especially

significant where thermal expansion values for cement paste and aggregate are markedly different).

Significant deterioration of the concrete strength does not generally occur until the exposure

temperature reaches -40 0 'C at which dehydration of calcium hydroxide occurs. 7 1 Reference 72

suggests that concrete exposed to temperatures of 90'C may lose only up to 10% of its roomtemperature strength and modulus of elasticity values.* The response of concrete to elevatedtemperature exposure depends on a number of factors (e.g., type and porosity of aggregate, rate of

heating, permeability, moisture state, etc.). In addition to potential reductions in strength and

modulus of elasticity, thermal exposure of concrete can result in cracking, or when the rate of

heating is high and concrete permeability low, surface spalling can occur. Elevated temperatures

also are important in that they affect the volume change and creep of concrete. 73 References 74-77

provide additional information on the effects of elevated temperature on concrete materials and

structures.

Thermal cycling, even at relatively low temperatures (<65°C), can have some deleterious

effects on concrete's mechanical properties (i.e., compressive, tensile and bond strengths, and

modulus of elasticity are reduced). Most reinforced concrete structures are subjected to thermal

cycling due to daily temperature fluctuations and are designed accordingly (i.e., inclusion of steel

* Except near certain penetrations carrying elevated temperature process fluids (e.g., steam lines), concrete

temperatures are not expected to exceed 65'C.

23 NUREG/CR-6424
reinforcement). At higher temperatures (200 to 300'C), the first thermal cycle causes the largest

percentage of damage, with the extent of damage markedly dependent on aggregate type and is

associated with loss of bond between the aggregate and matrix. Temperature variations, or thermal

cycles, also can become important if the deformation of the structure resulting from the temperature

variations is constrained.

A design-oriented approach for considering thermal loads on reinforced concrete structures

is provided in ACI 349.1 R.78 Limited information on the design of temperature-resistant concrete

structures is available through guides. 79,

80 Currently available codes pertaining to NPP

structures1 7,20 generally handle elevated temperature applications by requiring special provisions

(e.g., cooling) to limit the concrete temperature to <65°C, except for local areas where temperatures

can increase to 93°C. These codes, however, do allow higher temperatures if tests have been

performed to evaluate the reduction in strength and the reduction is applied to the design allowable.

Irradiation Irradiation in the form of either fast and thermal neutrons emitted by the

reactor core or gamma rays produced as a result of capture of neutrons by members (particularly

steel) in contact with concrete can affect the concrete. The fast neutrons are mainly responsible for

the considerable growth, caused by atomic displacements, that has been measured in certain

aggregate (e.g., flint). Nuclear heating occurs as a result of energy introduced into the concrete as

the neutrons or gamma radiation interact with the molecules within the concrete material.

Reference 81 indicates that nuclear heating is negligible for incident energy fluxes less than 1010

MeV/cm2 per s. Gamma rays produce radiolysis of water in cement paste that can affect concrete's

creep and shrinkage behavior to a limited extent and also result in evolution of gas. Prolonged

exposure of concrete to irradiation can result in decreases in tensile and compressive strengths and

modulus of elasticity. Irradiation has little effect on shielding properties of concrete beyond the

effect of moisture loss due to temperature increase. Approximate threshold levels necessary to

create measurable damage in concrete have been reported in limited research studies. 82 These

levels are 1 x 1019 neutrons/square centimeter for neutron fluence and 1010 rads of dose for

gamma radiation. Table 2.7 from Ref. 83 provides data for estimated radiation environments at the

outside surface of LWR pressure vessels for a 1000 MW(e) plant operating at a capacity factor of

80%. Results indicate that radiation levels may approach the limits provided above in a concrete

primary shield wall after 40 years of operation (32 equivalent full-power years). However, these

values are upper limits and are probably higher than would be experienced because of the

attenuating effects that would occur due to the presence of air gaps, insulation, etc., that could be

positioned between the pressure vessel and concrete structures. Additional information on the

interaction of radiation and concrete is available in Ref. 84.

Fatigue/Vibration Concrete structures subjected to fluctuations in loading, temperature,

or moisture content (that are not large enough to cause failure in a single application) can be

damaged by fatigue. Fatigue damage initiates as microcracks in the cement paste, proximate to the

large aggregate particles, reinforcing steel, or stress risers (e.g., defects). Upon continued or

reversed load application, these microcracks may propagate to form structurally significant cracks

that can expose the concrete and reinforcing steel to hostile environments or produce increased

deflections. Ultimate failure of a concrete structure in fatigue will occur as a result of excessive

cracking, excessive deflections, or brittle fracture. Fatigue failure of concrete is unusual because

of its good resistance to fatigue, 85,

86 and concrete structures are designed using codes that limit

design stress levels to values below concrete's endurance limit. However, as structures age, there

may be instances of local fatigue damage at locations where reciprocating equipment is attached, or

at supports for pipes that exhibit flow-induced vibrations.

NUREG/CR-6424 24
Mild Steel Reinforcing Systems Mild steel reinforcing systems are provided in

concrete structures to control the extent of cracking and the width of cracks at operating

temperatures, resist tensile stresses and compressive stresses for elastic design, and provide

structural reinforcement where required by limit condition design procedures. 20 Potential causes

of degradation of the mild reinforcing steel are corrosion, elevated temperature, irradiation, and

fatigue. Of these, corrosion is the factor of most concern for aging management of NPP

structures. Information on the other potential degradation factors is provided for completeness and

special situations that might occur.

Corrosion

Corrosion of steel in concrete is an electrochemical process that can assume the form of

either general or pitting corrosion. Both water and oxygen must be present for corrosion to occur.

The electrochemical potentials that form the corrosion cells may be generated in two ways:

(1) composition cells formed when two dissimilar metals are embedded in concrete, such as steel

rebars and aluminum conduit, or when significant variations exist in surface characteristics of the

steel; and (2) concentration cells formed due to differences in concentration of dissolved ions in

the vicinity of steel, such as alkalies, chlorides, and oxygen.59 As a result, one of two metals (or

different parts of the same metal when only one metal is present) becomes anodic and the other

cathodic. Other potential causes of corrosion include the effects of stray electrical currents or

galvanic action with an embedded steel of different metallurgy. The transformation of metallic iron

to ferric oxide (rust) is accompanied by an increase in volume that can cause cracking and spalling

of the concrete. In addition, corrosion will result in a reduction in effective steel cross-section and

capacity. Depending on the source, local embrittlement may also be produced.

In good-quality, well-compacted concretes, reinforcing steel with adequate cover should

not be susceptible to corrosion because the highly alkaline conditions present within the concrete

(pH > 12) causes a passive iron oxide film to form on the iron surface (i.e., metallic iron will not

be available for-anodic activity). However, when the concrete pH falls below 11, a porous oxide

layer (rust) can form on the reinforcing steel due to corrosion. Carbonation and the presence of

chloride ions can destroy the passive iron oxide film.

Reduction of the concrete pH can occur as a result of leaching of alkaline substances by

water or carbonation [i.e., calcium hydroxide is converted to calcium carbonate (calcite)]. The

penetration of carbon dioxide from the environment is generally a slow process dependent on the

concrete permeability, the concrete moisture content, and the carbon dioxide content and relative

humidity of the ambient medium. Carbonation may be accelerated due to the concrete being porous

(i.e., poor quality) or the presence of microcracks.

The passive iron oxide film on the steel reinforcement can also be destroyed by the

penetration of chloride ions, even at high alkalinites (pH > 11.5). Maximum permissible chloride

contents, as well as minimum recommended cover requirements, are provided in codes and

guides. 16,67 For typical concrete mixtures, the threshold chloride content to initiate steel corrosion

is about 0.6 to 0.9 kg Ci-/m3 (Ref. 59). Chlorides may be present in concrete due to external

sources (seawater effects, deicing salts, etc.) or may be introduced naturally into the concrete via

aggregate or mix water transport. Furthermore, when large amounts of chloride are present,

concrete tends to hold more moisture, which also increases the risk of steel corrosion by lowering

concrete's electrical resistivity. Once the passivity of the steel is destroyed, the electrical resistivity

of concrete and availability of oxygen control the rate of corrosion. Methods of excluding external

sources of chloride ions from concrete are provided in Refs. 62 and 87. Detailed information on

corrosion of steel embedded in concrete, its detection, and its repair, as well as the potential for

stray electrical current corrosion to occur is available elsewhere. 88

25 NUREG/CR-6424
Elevated Temperature

The property of mild steel reinforcement of most importance to design is the yield strength.

The reinforcing steel yield strength can be affected by elevated temperature exposure, but the

temperatures to cause a strength decrease are significantly higher (except under unusual conditions)

than would be experienced by a NPP concrete structure. Data for German reinforcing steels 89

indicate that for temperatures up to -200'C, the yield strength is reduced by 10% or less, and at

5001C it falls to about 50% its reference room temperature value. Hot-rolled steels tend to resist

the effects of temperature better than cold drawn or twisted steel. The steel modulus of elasticity

exhibits similar reductions with increasing temperature. Other data 90 ,

9 1 confi-rn the effects of

temperatures above 200'C on the mild steel reinforcing as well as providing a threshold

temperature of about 300'C for loss of bond properties with the concrete.

Irradiation

Neutron irradiation produces changes in the mechanical properties of carbon steels (e.g.,

increased yield strength and rise in the ductile-to-brittle transition temperature). The changes result

from the displacement of atoms from their normal sites by high-energy neutrons, causing the

formation of interstitials and vacancies. A threshold level of neutron fluence of 1 x 1018 neutrons

per square centimeter has been cited for alteration of reinforcing steel mechanical properties. 92 As

noted previously, 83 fluence levels of this magnitude are not likely to be experienced by the safetyrelated concrete structures in NPPs, except possibly in the concrete primary biological shield wall

over an extended operating period.

Fatigue

Fatigue of the mild reinforcing system would be coupled with that of the surrounding

concrete. The result of applied repeated loadings, or vibrations, is generally a loss of bond

between the steel reinforcement and concrete. For extreme conditions, the strength of the mild

steel reinforcing system may be reduced or failures may occur at applied stress levels less than

yield. However, there have been few documented cases of fatigue failures of reinforcing steel in

concrete structures and those published occurred at relatively high stress/cycle combinations. 85

Because of the typically low normal stress levels in reinforcing steel elements in NPP safety-related

concrete structures, fatigue failure is not likely to occur.

Post-Tensioning Systems The post-tensioning systems used in NPPs are designed to

have (1) consistently high strength and strain at failure, (2) serviceability throughout their

lifetime, (3) reliable and safe prestressing procedures, and (4) ability to be retensioned and

replaced (nongrouted systems). Potential causes of degradation of the post-tensioning systems

include corrosion, elevated temperature, irradiation, fatigue, and loss of prestressing force. Of

these, corrosion and loss of prestressing force are most pertinent from a NPP aging management

perspective.

Corrosion

Corrosion of prestressing systems can be highly localized or uniform. Most prestressing

corrosion-related failures involving general civil engineering structures have been the result of

localized attack produced by pitting, stress corrosion, hydrogen embrittlement, or a combination of

these. Pitting is the electrochemical process that results in locally intensified material loss at the

tendon surface, potentially reducing the cross section to the point where it is incapable of

supporting load. Stress corrosion cracking results in the fracture of a normally ductile metal or

alloy under stress (tensile or residual) while in specific corrosive environments. Hydrogen

NUREG/CR-6424 26
embrittlement, frequently associated with hydrogen sulfide exposure, occurs when hydrogen

atoms enter the metal lattice and significantly reduce its ductility. Hydrogen embrittlement also

may occur as a result of improper application of cathodic protection to post-tensioning

systems. 9 3-

95 Failure of post-tensioning systems can also occur as a result of microbiologicallyinduced corrosion. Due to the stress state in the post-tensioning systems, the tolerance for

corrosion attack is much less than for the mild steel reinforcement.

Elevated Temperature

The effect of elevated temperature on all heat-treated and drawn wires can be significant,

and on cooling the wires may not regain their initial strength because the heating destroys the

crystal transformations achieved by the heat-treating process. Short-term heating, on the order of

3 to 5 min., even to temperatures as high as 400'C, however, may not harm the prestressing

wire's mechanical properties. 96 Results of a Belgian study 89 involving 30 types of prestressing

steels indicate that thermal exposures up to -200'C do not significantly reduce (< 10%) the tensile

strength of prestressing wires or strands. References 91 and 97 support results of the Belgian

study.

Elevated-temperature exposures also affect the relaxation and creep properties of

prestressing tendons. Reference 98 indicates that losses in a 15.2-mm-diameter strand initially

stressed to 75% of its guaranteed ultimate tensile strength at 40'C will be 5 to 6.4% after 30 years.

Relaxation losses of tendons composed of stress-relieved wires are of about the same magnitude as

stress-relieved strand, but relaxation of a strand is greater than that of its straight constituent wire

because of the combined stress relaxation in the helical wires. 99 Creep (length change under

constant stress) of stress-relieved wire is negligible up to 50% its tensile strength. Also, the creep

effect in steel varies with its chemical composition as well as with mechanical and thermal treatment

applied during the manufacturing process. As temperature levels experienced by the prestressing

tendons in LWR facilities are below 200'C, the possibility for thermal damage to the prestressing

steels under normal operating conditions is low.

Irradiation

Irradiation of post-tensioning system steel affects its mechanical properties because atoms

are displaced from their normal sites by high-energy neutrons to form interstitials and vacancies.

These defects can propagate or combine and effectively both strengthen the steel and reduce its

ductility; or, at higher temperatures, they can recombine and annihilate each other and, for a given

neutron dose, reduce the irradiation damage. 92 Results obtained from studies 92 in which 2.5-mmdiam prestressing wires were stressed to 70% of their tensile strength and irradiated to a total dose

of 4 x 1016 neutrons per square centimeter (flux of 2 x 1010 neutrons-cm2 .per s) showed that for

exposures up to this level, the relaxation behavior of irradiated and unirradiated materials was

similar. These flux levels are higher than the level likely to be experienced in a LWR containment

vessel.

Fatigue

Repeated reversals of stress, or variations in stress, applied to concrete structural elements

(beams in particular) can result in fatigue failure in any of the following modes: (1) failure of the

concrete due to flexural compression; (2) failure of the concrete due to diagonal tension or shear;

(3) failure of the prestressing steel due to flexural, tensile-stress variations; (4) failure of pretensioned beams (grouted tendons) due to loss of bond stress; and (5) failure of the end

anchorages of post-tensioned structures. 100 The majority of fatigue failures that occurred while

testing prestressed concrete beams have resulted from fatigue of the tendons due to stress

27 NUREG/CR-6424
concentrations that occur in the tendon at a location where a crack occurs. In unbonded posttensioned construction, the end anchorages could be subjected to some variation in stress under the

action of changing external load, but unbonded tendons are not generally used in members

subjected to frequent variations in stress. Reference 20 presents high-cycle and low-cycle dynamic

tensile test requirements for prestressing tendon systems used in concrete containments.

Loss of Prestressing Force

Maintaining an adequate level of prestressing force in post-tensioned concrete containments

is important to the overall safety of the NPP, especially during postulated accident conditions.

Primary contributors to the loss of initial force level that was applied by the prestressing tendons

include (1) friction, (2) end anchorage deflection (take up end slip), (3) elastic shortening,

(4) tendon relaxation, and (5) concrete creep and shrinkage. 101-104 Of these factors, tendon

relaxation and concrete creep and shrinkage are time-dependent factors and thus aging related.

Stress relaxation, defined as loss of stress (force) in the steel when the strain (elongation)

does not vary, is related to tendon material properties, initial stress level, exposure temperature,

and time. Creep and shrinkage of concrete represent volume changes of the concrete that occur

over the life of the structure that can significantly affect the force levels in the tendons. Guidelines

for developing surveillance programs acceptable to the USNRC and for providing reasonable

assurance (when properly implemented) that the structural integrity of the containment is being

maintained are provided in Regulatory Guides. 105,

10 6 Reference 106 is a companion to Ref. 105

and provides clarification with respect to determination of prestressing forces and prediction of

prestressing force losses over the service life of the structure.

Liner and Structural Steel Liner and structural steel members are subject to the same

general degradation mechanisms as the steel reinforcement. Of these, corrosion and fatigue are of

most importance to aging management. Except for structural steel members that assist in providing

support for the reactor pressure vessels in certain plants (e.g., Trojan and Turkey Point), these

members are generally not subjected to the effects of elevated temperature or irradiation.

Corrosion

The primary degradation factor for the liner plate and structural steel (both embedded

sections and those within containment) is corrosion. Typically the liner plate and any installed steel

are coated, either with a primer or a primer-finish coat system to prevent corrosion (e.g., zinc-rich

primer with polyamide epoxy or modified phenolic coatings). Depending on the component, a

corrosion allowance may also have been provided during the design stage. However, little

allowance will have been provided for the relatively thin liner plate (i.e., -6.3-mm thick).

The corrosion process that affects these components is similar to that for conventional

reinforcing steel. Figure 2.13 presents a schematic representation of forms of corrosion that may

be found on metals.88 For liner plates, the influence of local attack that can lead to loss of leak

tightness is of most concern. Local attack may result due to accumulation of moisture in areas

experiencing loss of coating integrity, or failure of adjoining floor-liner sealant. The rate of attack

may be rapid, depending on the aggressiveness of the environment. Reference 107 contains

corrosion data for structural steel in numerous environments. For an industrial environment, the

atmospheric (general) corrosion rate was found to be 0.02 to 0.04 mm/yr. This same reference

reported pitting rates of 0.056 mrn/yr for low carbon steels placed in polluted seawater. In general,

depending on the environmental parameters, surface corrosion rates were noted to range from

0.001 mm/yr to 0.03 mm/yr.

NUREG/CR-6424 28
Corrosion of structural steel piles, used in certain containment configurations for

transferring foundation loadings to greater depths below grade, is also a possible degradation

mechanism. Similar to other containment steel, the concern for piles is from localized corrosion

resulting in significant loss of cross-sectional area. One study' 08 examined corrosion data from

43 piling installations of varying depths (up to 41.5 m) with times of exposure ranging from 7 to

50 years in a wide variety of conditions. The conclusion of this study was that the type and

amount of corrosion observed in steel pilings driven in undisturbed soil, regardless of soil

characteristics and properties, was not sufficient to significantly affect the piling's performance as

load-bearing structures. However, pilings placed in oxygen-enhanced fills, those exposed above

grade, or those exposed to seawater or salt spray may be somewhat affected. 109

Fatigue

The effects of repeated loads such as from polar crane operations or flow-induced

vibrations may possibly detract from the function and performance of liner plate and structural steel

members. The influence of repeated loads generally has been addressed at the design stage per

national design codes. However, the effects of conditions outside of design predictions and local

stress intensification points (material flaws, etc.) may result in fatigue-related problems. With

respect to the liner plate, possible fatigue sites include base metal delaminations, weld defects, arc

strikes, shape changes near penetrations, structural attachments, and concrete floor interfaces. For

structural steel members (liner attachments and anchorages), the locations most susceptible to

fatigue include large containment penetration framing (hatches, etc.) and liner anchorages near

vibrating load conditions (such as those generated in structural attachments).

Anchorage Embedments Anchorage to concrete is required for heavy machinery,

structural members, piping, ductwork, cable trays, towers, and many other types of structures.

An anchorage might have to meet certain requirements for ease of installation, load capacity, susceptibility to vibration, preload retention, temperature range, corrosion resistance, postinstallation or pre-installation, and ease of inspection and stiffness. 110 In meeting its function,

loads that the anchor must transfer to the concrete vary over a wide combination of tension,

bending, shear, and compression. Several potential factors related to failure or degradation of the

anchorage systems include design detail errors, installation errors (improper embedment depth or

insufficient lateral cover, improper torque), material defects (low anchor or concrete strengths),

shear or shear-tension interaction, slip, and preload relaxation. 110 Aging effects that could impair

the ability of an anchorage to meet its performance requirements would be primarily those that

result in deterioration of concrete properties, because if a failure did occur, it would most likely

initiate in the concrete.

2.3.2.2 Service life models

The actual safety and functional response of a structure in service depends on parameters

chosen prior to construction (e.g., structural dimensioning and detailing, and choice of materials),

and presumed parameters (e.g., environmental and loading conditions) that depend on subsequent

service conditions that may be somewhat unpredictable. Prediction of the remaining service life* of

concrete requires information on the condition of the concrete, major environmental stressors and

aging factors, processes causing the deterioration, and rates of deterioration. 112 An integral part of

structural safety and serviceability assessments is inspection that provides a link between the

* Service life is the period of time after construction during which all properties exceed the minimum acceptable

values when routinely maintained; whereas, durability is the capability of maintaining the capacity of the structure to

perform the function(s) for which it was designed and constructed. 11 1 Durability incorporates the concept of design

requirements being met for a specific time and service life incorporates the concept of predicting the time that the

design requirements will be met.

29 NUREG/CR-6424
environmental conditions to which the structure is subjected and the manner in which it performs

with time. Maintenance and repair in conjunction with a systematic inspection program can

effectively extend the service life of structures. In-service inspection, repair, and the role of ISI

and repair in extending the usable life of a structure are addressed in Chapter 5 of this report.

Contained in the balance of this section is information primarily related to estimating the service life

of in-service reinforced concrete structures.

Approaches for Estimating Service Life Approaches that have been used for

predicting the service lives of construction materials include (1) estimates based on experience,

(2) deductions from performance of similar materials, (3) accelerated testing, (4) application of

stochastic and reliability concepts, and (5) mathematical modeling based on the chemistry and

physics of degradation processes. Often two or more of these processes may be used in

combination. A description and critical review of these approaches has been completed. 113 The

most promising approaches were considered to be accelerated testing, applications of stochastic

and reliability concepts, and use of mathematical models.

Accelerated testing programs for determining concrete durability involve the use of elevated

stressors (e.g., concentration of reactants, temperature, humidity, etc.) to expedite degradation. If

the testing programs are properly designed, performed, and interpreted, they should provide a

sound basis for estimating the concrete performance and service life. An important requirement for

accelerated testing is that the degradation mechanism in the accelerated test be the same as

responsible for in-service deterioration. Mathematical modeling is used to relate the accelerated

degradation rate to the in-service rate. The performance of concrete exposed to frost (i.e., freezethaw cycles) or sulfate attack is frequently estimated using accelerated testing. 114,115

Service life models using stochastic methods are based on the premise that service lives can

not be precisely predicted because of the large number of factors involved, as well as their

interaction. 116 Stochastic approaches include the reliability method, and the combination of

statistical and deterministic models. The reliability method combines the principles of accelerated

degradation testing and probabilistic concepts. The reliability method takes into account the broad

distribution in times to failure exhibited by supposedly identical specimens tested under identical

conditions. By changing a parameter such as applied stress, families of curves relating probability

of failure to failure time can be developed. These curves can then be. used to develop stressor

(e.g., degradation factor) vs time-to-failure diagrams for different probabilities of failure. 117 Often

the statistical models are combined with deterministic models. Carbonation of concrete and

corrosion of embedded steel reinforcement have been addressed using the stochastic

method. 116,118

Most of the degradation processes for concrete are associated with concrete intrusion by

water, ions, or gases. Mathematical models for estimating service life under the influence of these

processes can be developed by considering (1) the rate of penetration of aggressive media into the

concrete, and (2) the rate of chemical reactions and physical processes. References 119-122

describe service life models for degradation processes that are considered most likely to be

applicable to NPP safety-related concrete structures (e.g., corrosion, sulfate attack, frost attack,

and leaching). Service life models for degradation caused by radiation, salt crystallization,

microbiological attack, and alkali-aggregate reactions do not appear to have been developed.

Service Life Methods Most Applicable to NPP Concrete Structures

Estimation of the remaining service life of reinforced concrete structures in nuclear power facilities

essentially involves determining the present condition of the concrete and then estimating the time

NUREG/CR-6424 30
required for concrete to degrade to the failure state. In estimating the time-to-failure, the specific

process(es) responsible for the degradation needs to be identified and the degradation rates for the

process(es) determined.

The amount of degradation of reinforced concrete is dependent on the environment,

geometry of the structure, properties of the concrete, the specific degradation processes, and the

concentration of the aggressive specie(s). Assume that these factors are invariant and thus can be

represented by a constant, kd, that does not change from year-to-year. Also assume that climatic

variations from season-to-season smooth out over several decades so that only the number of years

is represented by a time function, ty. The cumulative amount of deterioration, Ad (e.g., loss of

steel reinforcement section or depth of chloride ion penetration), at ty can be represented by

Ad = kdtyn (2.1)

where n is the time order. 122 The overall rate of deterioration, Rd, is given by

Rd = nkdtyn-. (2.2)

Equation (2.2) indicates that when n < 1, the rate of degradation decreases with time; when n = 1,

the rate is constant; and when n > 1, the rate increases with time. Values of n that have been

established for models of importance to potential degradation of NPP concrete structures are

provided below:

Degradation Factor Time Order,

n

Sulfate Attack

Wetting and Drying 1

Immersed 0.5

Corrosion

Chloride Ion Diffusion 0.5

Active Corrosion 1

Acid Attack (Siliceous Aggregate) 0.5

Frost Attack 1

In general, n = 0.5 for diffusion-controlled processes, n = 1 for a reaction-controlled process, and

n is between 0.5 and 1 if both convection and diffusion are involved in the transport of a reactive

substance. Parameters for input into the particular degradation model ideally would come from

inspections and testing of samples removed from the structure of interest. If the structure of

interest is located in an area that can not be inspected or where samples cannot be removed for

evaluation (e.g., environmental or accessibility constraints), values for material properties could be

obtained from structures located elsewhere in the facility that were cast using the same materials

and mix design. Other less desirable approaches would be to obtain material properties from

specimens fabricated using the same materials, mix proportions, and curing conditions, or to

obtain samples from facilities at other plants that have been shut down or decommissioned so that

the areas of interest are accessible for inspection and sample removal.

Defining Adf as the amount of damage-at-failure, and rearranging Eq. (2.1) yields

tyf = (Adf/kd)1/n (2.3)

31 NUREG/CR-6424
where tyf is the time-to-failure. The estimated remaining service life is obtained by subtracting the

age of the concrete when the inspection was made from tyf.

Example Application of Service Life Models Survey data were obtained from

reinforced concrete structures at several nuclear power stations in the United Kingdom ranging in

age from 17 to 24 years after first commissioning - Windscale Advanced Gas-Cooled Reactor,

Hinkley Point "A," Bradwell, and Wylfa. 123 One of the primary applications of this data was to

provide input for the development of models to predict the onset of corrosion of the steel

reinforcement at various locations in these structures. Current corrosion activity was assessed

through half-cell potential and resistivity measurements. Core samples were removed from each of

the, structures and tested to determine the depth of carbonation and chloride ion content profiles -

primary potential causes of corrosion of embedded steel at these facilities.

Corrosion of steel is recognized to be a two sage process (i.e., activation and propagation).

The activation period is the time required for the depth of carbonation or the threshold level of

chloride ions to reach the level of the steel and initiate the corrosion process. Once the corrosion

process is activated, corrosion will propagate at a rate defined by the nature of the concrete and the

environment, eventually resulting in cracking, staining, and spalling of the cover concrete. Most

research has concentrated on the initiation stage of the corrosion process.

Carbonation results from the reaction of atmospheric carbon dioxide with the calcium

hydroxide produced by cement hydration. It occurs most rapidly in relatively dry conditions (i.e.,

about 65% relative humidity) and results in the formation of calcium carbonate with a pH = 9 at

which steel will corrode. Traditionally, the depth of carbonation, x, after an exposure time, t, is

described by

x = kt 1/

2. (2.4)

The constant k is determined by the concentration of carbon dioxide in the atmosphere, its effective

diffusivity through the concrete, and the amount of carbon dioxide consumed in the reaction with

hydrated cement. As a result of a related research activity, 124 Eq. (2.4) was modified to

accommodate the rapid early rate of carbonation in the near surface concrete layer as follows

x = A + kt 1/

2 (2.5)

where A is primarily a function of the concrete mix proportions (i.e., compressive strength), but

also includes exposure conditions [i.e., interior (0 < A < 3), sheltered exterior (0 < A < 2), and

exposed exterior (0 < A 5 1). Nomograms based on an extensive review of literature have been

developed that correlate the k factor with concrete compressive strength and exposure condition

(e.g., see Fig. 2.14).

Chlorides are generally assumed to migrate into concrete by a diffusion process and Fick's

Second Law is assumed to apply as follows

ac a2c

-=D - , (2.6)

where C = chloride content by weight of cement,

t = time of exposure to chloride,

x = distance from concrete surface, and

Dc = diffusion coefficient of chloride.

NUREG/CR-6424 32
If the surface chloride level is Cs and the level of chloride'at depth x is Cx, then Eq. (2.6) can be

solved to yield

Cx=Cs I-erf x (2.7)

where Dce is the effective diffusion coefficient of chloride in cover concrete and erf is the error

function. Research noted previously 124 in conjunction with a study of old structures indicates that

Dce tends to decrease with age and varies through the depth of the cover concrete. In order to

predict the time to activation of steel corrosion using Eq. (2.7), several factors must be defined

[i.e., surface chloride content (Cs), effective diffusion coefficient of the cover zone (Dce), and

threshold level of chloride at which steel corrosion activates (Ca)]. Research12 4 indicates that Cs

reaches its ultimate value very quickly (i.e., within weeks), remains relatively constant thereafter if

the exposure does not change drastically, and is determined in part by the particular concrete, Fig.

2.15. The effective diffusion coefficient, Dce, at any point in time is determined by analysis of the

chloride ingress profile. Figure 2.16 presents effective diffusion coefficients vs age for various

concrete mixes based on results presented in Ref. 125. These results can be used to predict future

values of Dce based on a value determined at a known concrete age. The level of chloride at which

steel corrosion activates, Ca, is generally assumed to be 0.4% by weight of cement. A lower value

of Ca (e.g., 0.2%) can be used if more conservative estimates of service life are desired.

Results of the condition surveys at the United Kingdom nuclear power stations indicated

that (1) depths of carbonation of the internal concrete were 50 mm or less, with coated concrete

being much less susceptible to carbonation; and (2) levels of internal and external chloride ingress

were generally low, being less than 0.05% by weight of cement in the surface 10 mm of concrete.

Except for Wylfa, corrosion potential measurements generally indicated that there was no active

corrosion. At Wylfa, the cooling water plant exhibited severe cracking, rust staining, spalling, and

substantial loss of steel reinforcement section due to the harsh seawater environment. The

carbonation depth and chloride profile data were input into Eqs (2.5) and (2.7), respectively, to

estimate onset of steel corrosion at various locations in the stations. With the exception of Wylfa,

estimates of minimum number of years to corrosion activation due to carbonation ranged from

about 31 for an internal area near a heat exchanger at Windscale to over 140 in various areas of all

other stations. Estimates of minimum number of years to corrosion activation due to chlorides

were in excess of 100 for all locations examined in the stations except for Wylfa.

Needed Developments Quantitative design of reinforced concrete structures for

durability requires an improved understanding of the degradation mechanisms, improved

characterization of service environments, the development of advanced models, and the

development of standards and guidelines of acceptance for durability estimations. 126,127 It has

been suggested that a new set of codes and standards be required to include the interaction between

the environment and applied loads in the estimation of service life. 128

Extensive research and studies have been carried out to determine the durability of concrete

under various service conditions (e.g., Refs. 129-132) and, thus, information on the progressive

changes in the physical and chemical nature of concrete under such conditions is available.

However, using this information to develop criteria and models for service life prediction is far

from complete. Each concrete structure is somewhat unique due to variability in materials,

geometry, construction practices, and environments experienced. Also, properties of the concrete

have changed over the years (cement characteristics in particular). Furthermore, applications of the

service life models to estimating the remaining service life of in-service concrete structures are few,

33 NUREG/CR-6424
especially when compared to applications for new concretes. If the value of n is known, the time

order method [e.g., Eqs. (2.1) and (2.2)] is the most straight forward approach for estimating

service life of in-service concrete when one process is rate controlling. Application of the time

order method to specific degradation processes should be evaluated through additional modeling

using field and experimental data. Also, the method requires additional study where more than one

degradation process may be occurring simultaneously, with different time orders, but with no

predominant rate-controlling process. Severe climatic changes, either transient or permanent, have

not been addressed. Also, a technical basis does not exist for quantitatively estimating the effects

of some degradation mechanisms (e.g., alkali-aggregate reactions) on the service life of concrete.

2.3.3 Long-Term Performance of Concrete Materials and NPP Structures

Data on the long-term performance of reinforced concrete materials and structures is of

importance for demonstrating the durability of reinforced concrete structures in NPPs, and in

predicting their performance under the influence of pertinent aging factors and environmental

stressors. This information also has application to establishing limits on hostile environmental

exposure for these structures and to development of inspection and maintenance programs that will

prolong component service life and improve the probability of the component surviving an extreme

event such as a loss-of-coolant accident.

Concrete, originally based on lime that hardened by atmospheric carbonation, has been

utilized as a construction material for several thousand years. The oldest known concrete is from

Yugoslavia and is about 7600 years old. 133 Gypsum mortars were used by the Egyptians to

fabricate the Great Pyramid at Giza about 2500 BC. The Romans were the first to use hydraulic

limes and discovered the benefits of pozzolans. The survival of several ancient concrete structures

(e.g., Colosseum in Rome and Pont du Guard at Nimes) attests to the durability that concrete can

attain. A detailed study involving an examination of samples obtained from several ancient

concrete structures utilizing physical and chemical techniques concluded that these structures

survived primarily because of careful materials selection and construction, mild climatic conditions,

and the lack of steel reinforcement. 133 These structures, however, were not fabricated using

current hydraulic portland "cement" that was not in existence until about 1824. However, in

Ref. 133 it was noted that samples were obtained for testing from several structures fabricated in

the mid- to late 1800's. It was concluded that the durability of these structures was primarily due

to high cement contents, but also the relatively slow cement-setting times and high construction

quality. These portland cements differ somewhat from the portland cements used to fabricate NPP

concrete structures in that the formulations have changed significantly as well as the fineness of the

cement. Also, modem concretes have incorporated admixtures to improve workability, modify

hardening or setting characteristics, aid in curing, and enhance the performance or durability.

Results from the ancient and old portland cement-based concretes, however, do point out the

importance to durability of material selection, good quality construction, and having adequate

cementitious materials to produce dense concretes resistant to penetration by deleterious agents.

Prior reviews13 of research conducted on concrete materials and structures indicated that

only limited data are available on the long-term (40 to 80 years) properties of portland cement

concretes, or on the examination of reinforced concrete structures that have an extended service

time. Where concrete properties have been reported for conditions that have been welldocumented, the results were generally for concretes having ages < 5 years, or for specimens that

had been subjected to extreme, nonrepresentative environmental conditions such as seawater

exposure or accelerated aging. Few investigations were reported providing results on

examinations of structures that had been in service for the time period of interest, 20 to 100 years,

and they did not generally provide the "high quality" information (e.g., baseline material

characteristics and changes in material properties with time) that is desired for meaningful

assessments to indicate how the structures have changed under the influence of aging factors and

environmental stressors.

NUREG/CR-6424 34
Over the last few years, however, data of use in continued service assessments and welldocumented results from concrete specimens fabricated about 50 years ago and maintained under

controlled conditions have become available (e.g., Ref. 134). Also, the testing of prototypical

concrete specimens associated with the NPP structures (e.g., Ref. 123) and surveys to provide

information on the condition of these structures has increased significantly as utilities have become

interested in continuing the service of these facilities (e.g., Refs. 135 and 136). As a result, the

Structural Aging (SAG) Program developed a Structural Materials Information Center containing

information on the time variation of material properties under the influence of pertinent

environmental stressors and aging factors. Also, information was assembled on the performance

and condition assessments of the NPP safety-related concrete structures.

2.3.3.1 Structural materials information center (SMIC)

Results of a comprehensive review and assessment of existing material properties data

bases indicated that a system of the type desired for use by the SAG Program did not exist (e.g.,

personal computer-based system that can easily access and retrieve material property data and

information). 137 As a result, a plan was prepared for development of a data and information

management system. 138 The plan was then implemented through development of the Structural

Materials Information Center (SMIC). 139

Description The SMIC has been developed in two formats - a Structural Materials

Handbook and a Structural Materials Electronic Data Base. Since initial development, two updates have been prepared.14 0,

14 1 Provided below are general descriptions of the handbook and

electronic data base.

Structural Materials Handbook

The Structural Materials Handbook has been developed as an expandable, hard-copy

reference document that contains complete sets of data and information for each material in the

SMIC. The handbook consists of four volumes that are provided in loose-leaf binders for ease of

revision and updating. Volume 1 contains design and analysis information useful for structural

assessments and safety margins evaluations (e.g., performance curves for mechanical, thermal,

physical, and other properties presented as tables, graphs, and mathematical equations). Test

results and data used to develop the performance curves in Vol. 1 are provided in Vol. 2.

Volume 3 contains material data sheets that provide general information, as well as material

composition and constituent material properties, for each material system contained in the

handbook. Volume 4 contains appendices describing the handbook organization, as well as

updating and revision procedures. Examples of pages that are contained in Vols. 1-3 that have

been prepared for a long-term study on concrete properties (Ref. 134) are presented in Figs. 2.17-

2.19, respectively.

Volumes 1, 2, and 3 of the handbook each contain four chapters of materials property data

and information, with the chapters consistent between the volumes. Each material in the data base

is assigned a unique seven-character material code that is used in the handbook and the electronic

data base to organize materials with common characteristics. This code consists of a chapter index,

a group index, a class index, and an identifier. The chapter index is used to represent the various

material systems in the data base. The group index is used to arrange materials in each chapter into

subsets of materials having distinguishing qualities such as common compositional traits. The

class index is used to organize groups of materials with common compositional traits into subsets

having a similar compositional makeup or chemistry. The identifier is used to differentiate

structural materials having the same chapter, group, and class indices according to a specific

35 NUREG/CR-6424
concrete mix, ASTM standard specification for metallic reinforcement, etc. The arrangement of the

material code parameters is shown in Fig. 2.20. Table 2.8 lists material code parameters that have

been developed for the concrete and concrete-related materials presently in the data base.

A wide variety of information and materials property data has been collected and assembled

for each material system included in the data base (e.g., general description, composition,

mechanical property data, etc.). In setting up the data base, each material property has been

identified by a unique four-digit property code selected from an established set of material

properties categories. Table 2.9 lists the property code ranges and corresponding material property

categories. Reference 139 presents a breakdown of the individual material property code values in

each of these ranges.

Associated with each entry of data (numerical results of tests) or values (results of

evaluation of data) into the data base is an assessment of the quality of the entries presented in the

form of a letter grade. Although the criteria for assessing the quality of data and values are

somewhat subjective, five quality levels have been developed. These levels are represented, in

order of descending quality, by the letters A through E, Table 2.10. The 11 requirements utilized

in the evaluation of the quality of data and values are listed in Ref. 139 along with specific criteria

for each of the quality levels.

Each reference document that is used as an information source is assigned a unique integer

identifier. In Vols. 1 and 2, reference numbers are listed to identify each information source, and

all references that are used to develop a reported property for a particular material are provided in

Vol. 3. Since each reference may be used for more than one property or structural material, a

complete listing of references appears in Appendix E of Vol. 4. The integer identifier assigned to

each reference source is consistent in both the handbook and the electronic data base.

Structural Materials Electronic Data Base

The Structural Materials Electronic Data Base is an electronically accessible version of

the Structural Materials Handbook. It has been developed on an IBM-compatible personal

computer using a commercially-available data base management system designed specifically for

maintaining and displaying properties of engineering materials. To ensure that the handbook and

electronic data base are compatible, each material included in the electronic data base is identified

by the same common name and material code that has been used to represent the material in the

handbook. Also, each electronic data base material record contains data and information taken

directly from the handbook. Due to software limitations, the electronic data base is not as

comprehensive as the handbook, but it does provide an efficient means for searching the various

data base files to locate materials with similar characteristics or properties.

The electronic data base management system includes two software programs: Mat.DB

(Ref. 142) and EnPlot (Ref. 143). Mat.DB is a menu-driven software program that employs

window overlays to access data searching and editing features. This software is capable of

maintaining, searching, and displaying textual, tabular, and graphical information and data

contained in electronic data base files. Although Mat.DB has been developed for metallic

materials, its formatting can be modified to accommodate nonmetallic and composite materials such

as portland cement concretes. EnPlot is a software program that incorporates pop-up menus for

creating and editing engineering graphs. This software includes curve-fitting and scale-conversion

features for preparing engineering graphs and utility features for generating output files. The

engineering graphs generated with EnPlot can be entered directly into the Mat.DB data base files.

These graphs are compatible with Microsoft Word, the work processing software used to prepare

NUREG/CR-6424 36
the handbook. The software was developed to run under Microsoft Windows Version 3.0 on IBM

personal computers, or compatibles, using an Intel 80286 Processor, or higher, and DOS 3.1, or

higher.

Material Property Data Bases Contained in the SMIC Two primary approaches

have been utilized to obtain data and information for input into SMIC - open-literature references

and testing of prototypical samples. A total of 144 data bases for concrete and concrete-related

materials have been developed (i.e., 129 concrete, 12 metallic reinforcement, 1 prestressing steel,

and 2 structural steel). Reference 141 provides a complete listing of the material property data

bases contained in SMIC.

Examples of concrete material property data and information files currently available

include compressive strength, modulus of elasticity and flexural strength vs time for several

concrete materials cured under a variety of conditions (i.e., air drying, moist, or outdoor exposure)

for periods up to 50 years; ultimate compressive strength and modulus of elasticity vs temperature

at exposures up to 600'C for durations up to four months; weight loss vs time for specimens

subjected to sulfuric acid concentrations (by weight) of 0.0016 to 0.02%; length change vs time for

specimens subjected to wet (2.1% Na 2SO 4 solution) - dry cycling; and bond stress vs slip for

reinforced concrete bond test specimens exposed for 14 days to either direct or alternating current

(potential up to 20 volts). Metallic reinforcement (ASTM A 615 and A 15) performance curves are

available for fatigue, and ambient and temperature-dependent (A 615 material only) engineering

stress vs strain. Temperature-dependent engineering stress vs strain, and tensile yield strength,

ultimate tensile strength, and ultimate elongation vs temperature performance curves are available

for both prestressing tendon (ASTM A 421, Type BA) and structural steel (ASTM A 36) materials.

Figure 2.21 presents information on the relative compressive strength of concrete specimens vs age

for selected specimens tested under the University of Wisconsin 134 and Portland Cement

Association144, 145 long-term concrete properties programs. The results presented have been

normalized in terms of the 28-day reference concrete compressive strength and indicate the effect of

mix design (i.e., water-cement ratio) and curing conditions (i.e., continuous moist, continuous air,

inside laboratory environment, and outside environment). In general, the compressive strengths of

the concretes tended to increase with age (e.g., the University of Wisconsin specimens having a

water-cement ratio of 0.67 and continuously stored outside exhibited about a 180% increase in

compressive strength at an age of about 25 years). The results presented in this figure indicate that

two factors influenced the magnitude of increase in concrete compressive strength with age -

relative value of 28-day reference strength and curing conditions. Specimens in each series that

had the highest water-cement ratio generally exhibited the largest increases in compressive strength

with age because of their lower 28-day reference strength values. Also, specimens for which

moisture was available for continued cement hydration tended to exhibit greater increases in

compressive strength with age. These results indicate that in estimating the increase in

compressive strength with age of NPP concretes, the selection of a factor to apply to the reference

28- or 60-day reference strength should not be done "a priori." Although the strength increases

experienced by the concretes in different locations in a NPP will vary somewhat due to different

environments, the magnitude of increase in concrete compressive strength would not be expected

to be nearly as great as that exhibited by some of the data in Fig. 2.21. The primary reason for this

is that the reference strengths of NPP concretes would be relatively high (e.g., 27 to 40 MPa vs

12.3 MPa for the Wisconsin specimens exhibiting the 180% increase noted above). Also, the

same factor should not be applied to all concrete structures in a NPP because of different

environmental exposure conditions that would affect the strength gain (e.g., elevated temperatures

and relative humidities).

Over the course of the SAG Program, a number of concrete samples were obtained from

U.S. and U.K. nuclear power stations and tested. In the U.S., compressive strength tests were

conducted on concretes from the Shippingport, EBR-II, Vallecitos, Palisades, Midland,

37 NUREG/CR-6424
Braidwood, Byron, Dresden, La Salle, Quad Cities, and Zion stations. In the U.K., results were

obtained from specimens cast in conjunction with fabrication of the Wylfa, Heysham I, Heysham

II, Hartlepool, Tormess, and Sizewell "B" stations. 146 With the exception of the Wylfa station

specimens, all the U.K. specimens had been initially cured under heat-cycled conditions to

simulate the early in situ temperature rise due to cement hydration. The specimens were then

continuously stored in a sealed, stable moisture state at temperatures from 100 to 95°C, with some

under sustained loading (13.8 MPa) until tested. Figure 2.22 presents relative concrete

compressive strength data obtained from the U.S. and U.K. data for which baseline data could be

obtained. Results obtained from these tests indicate that although the concrete compressive

strengths generally increased with age, the magnitude of increase for specimens tested at an age of

about 25 years was 60% or less. Specimens obtained and tested from U.S. stations for which

results were not presented in Fig 2.22 all exhibited concrete compressive strength values in excess

of design requirements. These results could not be presented in the figure, however, because

records providing the reference 28-day concrete compressive strength could not be located. This is

likely to be a recurring problem at several of the older NPPs. Samples removed from these plants

and tested will provide valuable information on the current concrete strength, but are of somewhat

limited use for estimating future performance. Under ideal conditions it is desirable to have

concrete compressive strength results obtained at several ages so that trending can be used to

estimate future performance. One solution might be to periodically conduct tests on concrete

materials that are removed as a result of plant modifications.

Potential Applications of the SMIC The structural integrity of a concrete structure

can be evaluated using either a static load test or an analytical approach as outlined in Fig. 2.23.147

For NPP concrete structures, the analytical approach is preferred due to such things as their

massive size and accessibility restrictions.* Application of the analytical approach in the form of a

theoretical stress analysis must take into account the physical characteristics of the structural

members and their connection details, material properties, quality of construction, and the

structure's current condition. Typically, two methods are available to obtain properties of the

structural materials - nondestructive or destructive testing methods. When test methods are

calibrated and certified and standardized procedures used, nondestructive testing can provide the

required information. Destructive testing requires that samples be removed from the structure.

Results from these tests in conjunction with visual examinations can be used to characterize the

overall condition of a structure that is accessible. However, inspections, sampling, and

nondestructive testing of all reinforced concrete structures in a NPP may not always be possible

(e.g., primary biological shield or basemat). One potential approach for this situation is to use a

comparative approach.

The comparative approach is based on the concept that the performance characteristics for

materials in one structure can be estimated based on the results that are available on the

performance of similar materials that have been exposed to similar service or environmental

conditions (i.e., removal of samples for testing or application of nondestructive testing techniques

is not required). Establishing material properties using the comparative approach requires a large

knowledge base on material behavior under the influence of aging factors or environmental

stressors such as provided by SMIC.

The comparative approach in conjunction with SMIC can be used to estimate current as

well as future material properties.

* A structural integrity test is performed on the containment vessel prior to placing the plant in service and integrated

leak-rate tests are periodically conducted that require pressurization of the containment. Ultimate strength evaluations

would use the analytical approach.

NUREG/CR-6424 38
Estimating Current Material Property Values A step-by-step procedure to estimate

current material property values for use in a theoretical stress analysis of an existing concrete

structure is provided below. This procedure should only be used as a guide, however, because

each concrete structure has unique characteristics and features, and each theoretical stress analysis

should be performed on a case-by-case basis.

1. Collect background information so that the structural materials used to construct the

structure can be uniquely identified (Table 2.11).

2. Establish the service history (Table 2.11).

3. Identify material properties needed for the analysis (e.g., concrete strength and

modulus of elasticity, and yield strength of metallic reinforcement).

4. Search SMIC to identify materials having similar compositions, characteristics, and

exposure conditions (i.e., the Electronic Data Base can be searched based on either

the seven-character material code or the four-digit property code).

5. From candidate materials provided, select properties for similar materials that reflect the

service history of the structure being evaluated.

6. Estimate numerical values for each needed property by using the performance curves

provided in Vol. 1 and the supporting documentation contained in Volume 2 of the

Structural Materials Handbook.

Application of the above procedure can be illustrated through a hypothetical example. In

this example, a utility is considering increasing the capacity of an overhead crane that is supported

by reinforced concrete beams and columns that are part of a NPP safety-related concrete structure.

In order to assess the feasibility of increasing the crane capacity, the current structural capacity of

the reinforced concrete beams and columns is required. This entails an evaluation of the

compressive strength of the concrete and the tensile yield strength of the reinforcing steel. One

method would be to obtain material samples for destructive testing, but this is not feasible because

the beams and columns are heavily reinforced and sample removal would degrade the structure.

An alternate approach is to apply the six-step procedure noted above.

1. Collection and review of background information shows that the structures were

fabricated 30-years ago using a normal-weight concrete (Table 2.12) and ASTM A15

No. 10 deformed (flexure and axial reinforcement) bars having a minimum yield

strength of 276 MPa.

2. Review of plant records and operating logs indicates that the reinforced concrete has not

been exposed to harsh environmental conditions. A visual inspection indicated no

problems.

3. Compressive strength of the in situ concrete and modulus of elasticity and tensile yield

strength of the steel reinforcement are required for the stress analyses.

4. Using information provided in Table 2.12, a search of the Electronic Data Base

revealed three concretes having similar compositions and service histories to the

concrete in question. Information on plain and deformed ASTM A 15 steel

reinforcement was also located through a search of the Electronic Data Base.

5. Table 2.13 presents pertinent compressive strength information for the three concrete

materials contained in SMIC.

6. The 30-year compressive strength value for the in situ concrete (41.7 MPa) is estimated

by multiplying the 28-day value (24.4 MPa) by the ratio (1.71) of the average 30-year

concrete strength to the average 28-day concrete strength for the similar concretes

contained in SMIC. Since the steel properties do not vary significantly (if at all) with

age, the original design properties were used.

39 NUREG/CR-6424
Estimating Future Material Property Value A step-by-step procedure for

estimating future material property values for use in theoretical stress analyses of an existing

structure is provided below. Again, this information is provided only as a guide. This procedure

has primary application to existing concrete structures that are required to remain in service for a

specified time.

1. Establish current properties for structural materials using procedures described above,

or destructive and nondestructive testing techniques can be used if feasible.

2. Establish the required continued service period, and estimate the service conditions for

pertinent structural materials during this period.

3. Search SMIC to identify materials having similar compositions that were subjected to

service conditions representative of that anticipated during the desired continued service

period.

4. Establish numerical values for properties of interest for the pertinent structural materials

using performance curves and supporting documentation obtained from the Structural

Materials Handbook.

Application of the above procedure can be illustrated through another hypothetical example.

In this example, a utility is investigating the possibility of annealing the reactor pressure vessel

(RPV) at its NPP. In order to anneal the RPV steel, the temperature of affected parts of the RPV

must be maintained at 445°C for approximately 168 hours. Results of a thermal analysis indicate

that during the annealing operation, some concrete components located adjacent to the RPV could

be exposed to temperatures up to 1770C. Prior to the annealing operation, an evaluation of the

effects of the elevated temperature exposure on the performance of the reinforced concrete

structures is required. The required analyses must reflect the current properties of the concrete

materials as well as any changes in properties resulting from the elevated temperature exposure. A

comparative approach is required to provide the required properties because the concrete is not

accessible for visual examination, nondestructive examination, or removal of samples for testing.

Application of the procedure described above can be used to obtain the required information for the

concrete, metallic reinforcement, and structural steel materials that were used to fabricate the walls

and floors adjacent to the RPV.

1. Collection and review of background information shows that the structures were

fabricated 20-years ago using a normal-weight concrete (Table 2.12), various sizes of

plain and deformed ASTM A 615 (Gr 60) steel reinforcement, and ASTM A 36

structural steel.

2. Review of plant records and operating logs indicates that the structural materials have

not been exposed to harsh environmental conditions.

3. Using information provided in Table 2.12, a search of the Electronic Data Base,

revealed three concretes having similar compositions and service histories (Table 2.13),

and two similar concretes that had been exposed to elevated temperatures covering the

magnitude and length of exposure of interest (Table 2.14). Ambient and elevated

temperature results for the ASTM A 615 steel reinforcement and ASTM A 36 structural

steel were also obtained from a search of the Electronic Data Base and are presented

in Table 2.15.

4. Material property values just prior to the start of the annealing process are estimated.

Procedures described above are used to estimate the current concrete compressive

strength (i.e., 40.7 MPa). In this case, however, the multiplication factor (1.67) is

determined using the 20-year old compressive strength data in Table 2.13. Steel

material properties at this time are assumed to be time invariant because of the relatively

benign service history of the structures.

5. Compressive strength of the concrete after the annealing operation is estimated by

multiplying the estimated strength just prior to annealing (i.e., 40.7 MPa) by the ratio

NUREG/CR-6424 40
(i.e., 0.59) of concrete strength after 177°C exposure for 7-days to the strength before

exposure. Values for computing this ratio are provided in Table 2.14. The resulting

estimated concrete compressive strength (i.e., 24 MPa) still exceeds the design

compressive strength (i.e., 20.7 MPa) provided in Table 2.12. Pertinent material

property values for the steel materials after the annealing process are provided in

Table 2.15.

Recommendations for Continued Development of SMIC As noted previously, the SMIC contains 144 material property data bases. Reviews and assessments of data bases have

indicated that SMIC is the only comprehensive information and data management system that has

been developed specifically for the storage and retrieval of concrete and concrete-related materials,

and in particular those materials that are representative of NPP concretes. As more and more

information and data become available as a result of activities associated with continued service

assessments of NPPs (e.g., testing of concrete core samples removed from the plants), it is

recommended that this data and information continue to be incorporated into SMIC. Also, as

SMIC has the capability to add other structural materials through additional files in the Electronic

Data Base and chapters in the Structural Materials Handbook, it is recommended that SMIC be

expanded to include information and data on other materials of importance to continuing the service

of NPPs (e.g., reactor pressure vessel steels, structural steels, and sealants and coatings).

Also, as noted previously, Mat.DB, the data base management system for SMIC, was

developed primarily for metallic materials. This has required the adaption of some of the

information fields so that they could be used to present compositional information and timedependent properties of composite materials such as portland cement concretes. Restrictions in the

type of data that could be entered into a specific field were also encountered (e.g., concrete mixture

proportions are reported in a field that was setup for percentage values ranging from 99.999 to

0.001 rather than units of mass per unit volume). This "force fitting" of information and data has

resulted in a data base that new or occasional users may find objectionable or confusing. Due to

software constraints and data-field limitations, the current version of Mat.DB (Version 1.22) is

considered adequate only for examination of individual Structural Materials Electronic Data

Base files and is not well suited for engineering evaluations in which properties for similar

materials are combined and compared (e.g., data for only one material can be displayed on the

computer screen at any given time). Table 2.16 presents a list of data base management system

requirements considered necessary for storing and accessing materials property data and

information at the SMIC. 14 8 Also identified in the table are some of the perceived weaknesses and

limitations of Mat.DB (Version 1.22). A new version of Mat.DB (Version 2.0) based on

Microsoft Windows (Ref. 149) is being developed that contains features making the software

easier to use, but it is not anticipated to address all the requirements provided in Table 2.16.

Based on experience gained during development of the SMIC, advances in personal

computer hardware capabilities, and corresponding developments in software tools for building

customized data bases, a reassessment of candidate systems was conducted. 148 Data base

management system software, computer hardware, and networking vs local operation were

considered in the overall evaluation. Three classifications of data base management system

software were considered: (1) standard - currently available software that can be used as-is to

store and access properties of materials, (2) adaptable - currently available software that can be

modified to accommodate the specific needs of the user, and (3) custom - software developed

from "scratch" to user specifications using either commercially available data base management

system development tools or a lower level computer language. Computer hardware requirements

depend to some degree on the data base management system software selected and whether the

software is operated locally or accessible using a wide-area network. Communication with a widearea network requires a personal computer with a terminal emulator that is compatible with the

operating system of the server that supports the data base management system software. Terminal

41 NUREG/CR-6424
emulators operate on almost any type of computer platform. When data base management system

software is operated from individual systems that may or may not be connected to a local-area

network, hardware such as IBM-compatible personal computers can be used. The greatest

advantage of accessing a data base using a wide-area network is the ease with which updates and

revisions can be distributed. Generally, only one copy of the data is produced and transmitted thus

eliminating the need for mailing electronic media to individual users. Disadvantages of a wide-area

network include relatively slow transmission rates over telephone lines and relatively high

development and maintenance costs. The advantages of accessing a data base using a personal

computer include faster data processing, that is especially valuable for graphical representations,

and relatively low development and maintenance costs. The main disadvantage of using a personal

computer is the increased distribution effort needed to mail updated electronic media to individual

users. A feasible alternative to these two approaches is a combined approach in which distribution

of updates is handled electronically and actual use of the system is performed using a personal

computer. It was concluded from the assessment that custom software provides enough flexibility

to satisfy the requirements presented in Table 2.16 and also permits entry of existing data and

information files. Object-oriented relational data base software, such as Microsoft Access

(Ref. 150), would provide the foundation for a new data base management system, and the data

base could be completely designed and built locally. It is, therefore, recommended that in

conjunction with the expansion and continuation of SMIC as a living document, a new data base

management system be developed that incorporates the requirements in Table 2.16.

2.3.3.2 Longevity of NPP reinforced concrete structures

In general, the performance of NPP safety-related concrete structures has been very good.

However, there have been several isolated incidences that if not remedied could challenge the

capacity of the containment and other safety-related structures to meet future functional and

performance requirements. Table 2.17 presents a summary of local degradation mechanisms that

have been observed by one organization during condition surveys of various concrete structures at

both U.S. and foreign NPPs located in areas having several different climatic conditions. 27 Some

general observations derived from these results were that virtually all NPPs have experienced

cracking of the concrete structures that exceeds typical acceptance criteria for width and length,

numerous NPPs had groundwater intrusion occurring through the power block or other subsurface

structures, few NPPs currently have a program for conducting periodic inspections of the concrete

structures, and aging concerns exist for subsurface concrete structures as their physical condition

cannot easily be verified. Collectively, it was concluded in this study that the general performance

of the NPP concrete structures has been quite favorable, and proper evaluation and treatment of

observed degradation at an early stage is both a cost effective and necessary approach to long-term

plant operations. More specific results on the performance of U.S. and U.K. NPP concrete

structures is provided below.

U.S. Experience Most of the instances related to degradation of NPP concrete

structures in the U.S. occurred early in their life and have been corrected. 13,

56 Causes were

primarily related either to improper material selection and construction/design deficiencies, or

environmental effects. Examples of some of the problems attributed to these deficiencies include

low 28-d compressive strengths, voids under the post-tensioning tendon bearing plates resulting

from improper concrete placement (Calvert Cliffs); cracking of post-tensioning tendon anchor

heads due to stress corrosion or embrittlement (Bellefonte, Byron, and Farley); and containment

dome delaminations due to low quality aggregate materials and absence of radial steel

reinforcement (Crystal River) or unbalanced prestressing forces (Turkey Point 3).151-153 Other

construction-related problems have included occurrence of excessive voids or honeycomb in the

concrete, contaminated concrete, cold joints, cadweld (steel reinforcement connector) deficiencies,

NUREG/CR-6424 42
materials out of specification, higher than code-allowable concrete temperatures, misplaced steel

reinforcement, post-tensioning system buttonhead deficiencies, and water-contaminated corrosion

inhibitors. 13

Although continuing the service of a NPP past the initial operating license period is not

expected to be limited by the concrete structures, several incidences of age-related degradation have

been reported. 15 1- 154 Examples of some of these problems include corrosion of steel

reinforcement in water intake structures (San Onofre), corrosion of post-tensioning tendon wires

(Fort St. Vrain), leaching of tendon gallery concrete (Three Mile Island), low prestressing forces

(Ginna, Turkey Point 3, Zion, and Summer), and leakage of corrosion inhibitors from tendon

sheaths (Palisades, Trojan, and Fort Calhoun). Other related problems include cracking and

spalling of containment dome concrete due to freeze-thaw damage, low strengths of tendon wires,

contamination of corrosion inhibitors by chlorides, and corrosion of concrete containment liners.

As the containment provides the final barrier between the NSSS and the outside

environment, it contains radioactive material at elevated temperature in the unlikely event of an

accident. Continuing the integrity of a containment that potentially can deteriorate under the action

of various aging factors and environmental stressors, therefore is an essential aspect of NPP

safety. Since post-tensioned concrete containments constitute the single largest type of

containment structure and an in-depth evaluation of potential aging concerns of a post-tensioning

system had not been performed, a study was conducted. 155 Primary aging mechanisms

considered were corrosion, loss of prestressing force, and (potentially) loss of strength and

ductility of the post-tensioning elements. Results provided by the study indicate that deterioration

of the system hardware (including bearing zone concrete and sheathing filler) has not been

significant. Water has occasionally been found in end caps that cover the anchor heads, but

generally has been of no consequence or in a few instances has produced only minor surface

staining of load-bearing components. Leakage of tendon sheathing filler (i.e., petroleum

petrolatum wax type base material used to inhibit corrosion) has been observed at the end cap

regions and on exterior concrete surfaces of several containments. The cause in the end cap region

primarily has been defective gaskets/fittings, whereas filler on the concrete surface is believed to

result from its migration under high head from the tendon duct seams through concrete cracks.

This leakage generally has been accepted by the utilities as messy, but minor. Corrective actions

have involved cleaning up the excess filler, replacement of the end cap gasket/fitting seals, and

injection of additional filler into the ducts, if necessary. According to Ref. 156, this occurrence

has been more prevalent in older plants that used an earlier version of sheathing filler (i.e., pre1974) that had a lower melting point than the current product. Wire (or strand) strength and

ductility do not appear to decrease with age under load. Tendon end anchorage forces* determined

as part of examinations of the post-tensioning tendons conducted at regular intervals generally were

above the time-dependent confidence limits; however, a few of the older containments have been

found at or below these limits. Limited results obtained under this study tend to indicate that the

minimum tendon forces obtained from end anchorage lift-off force measurements may exceed the

actual prestressing force. Where end anchorage force measurements have been sufficiently low

that tendon retensioning was required, it is recommended that the force in these tendons be

monitored on a regular basis to determine how relaxation of retensioned tendons proceeds. The

study concluded that there was essentially no evidence of physical deterioration of system

hardware and that current examination programs 10 5,

10 6,15 7 appear adequate to ensure the

continuing physical integrity of post-tensioning systems. The one aging issue noted was the

apparent discrepancy between the actual prestressing force and the force indicated by tendon lift-off

load measurements. Potential contributors to this apparent discrepancy could be friction and

* Containment design criteria specify that the minimum force along the length of a tendon be sufficient to provide

the required compressive force in the concrete. Since it is not feasible to measure this force, end anchorage force is

used as a substitute for this value.

43 NUREG/CR-6424
nonuniform prestressing forces along the tendon length. As the containments age, the significance

of overestimation of the prestressing force takes on added importance because the losses may

already be approaching the lower limit of acceptability (i.e., structural margins may be lower than

indicated and unexpected cracking of the concrete can occur). Also, the impact of leakage of

sheathing filler that has been observed at several plants should be evaluated to determine if there is

an effect on mechanical properties of the concrete or structural performance (i.e., degradation of

the bond that develops between the steel reinforcement and concrete).

U.K. Experience Condition surveys have been periodically conducted of U.K. nuclear

power station concrete structures. 146,158,159 These surveys were part of a program to develop

information on the likely durability of reinforced concrete components of buildings and structures

at nuclear power stations over the currently-envisaged period of up to 100 years that includes

completion of decommissioning. The studies included one Advanced Gas-Cooled Reactor and six

Magnox plants that ranged in age from 13 to 26 years. Results from French nuclear stations at

Chinon and Marcoule were also factored into the study. The condition surveys consisted of three

primary elements: (1) visual inspection, (2) nondestructive testing (NDT) in situ, and

(3) laboratory testing of retrieved samples. Pertinent results obtained from NDT in situ and

laboratory testing of retrieved samples were described previously in this chapter. Visual

inspections indicated that in general the components were in good condition, with the internal steel

reinforcement in good condition and visible degradation of external concrete limited to a few

localized areas of cracking and spalling due to corrosion of steel reinforcement or to exposed

bolting. Little significant deterioration was expected throughout the 100 year period of interest for

these structures provided adequate heating and ventilation was maintained in the buildings (e.g.,

T > 15'C and relative humidity < 50%). The most significant environmental stressor was

determined to be corrosion of steel reinforcement from chlorides due to the close proximity of

many of the U.K. stations to the coastline. Corrosion of the water intake structures due to

chlorides was observed at one station (i.e. Wylfa). Carbonation depths and chloride ion profiles

were obtained at selected locations and the results input into models to estimate the onset of

corrosion (see previous discussion). A system for regular planned inspection and maintenance

was proposed to monitor and protect against the processes causing corrosion. Inspections of the

French stations indicated that the concrete structures, some dating from 1959, were generally

considered to be in good condition. Some cracking and spalling was apparent in the externallyexposed concrete, however. Recently, through-wall corrosion of the liner of several of the French

pressurized-water reactor plants has been observed due to failure of a seal at the lower floor-liner

interface where the liner becomes embedded in the concrete. 160

An investigation was conducted to develop additional data on longevity of NPP concrete

structures.16 1 Surveillance results for two prestressed concrete pressure vessels (PCPVs) each at

the Wylfa, Hartlepool, and Heysham I stations were examined. Records reviewed included

prestressing tendon anchorage lift-off load measurements, results of corrosion examinations, and

visual examination results. Surveillance data for the Wylfa, Hartlepool, and Heysham I stations

covered time periods since prestressing of 23, 14.5, and 13.4 years, respectively. It was

concluded that performance of the PCPVs generally had been good. As expected, the tendon

anchorage lift-off load measurements showed a trend for the loads to decrease with time due to a

combination of prestressing steel relaxation and concrete creep. Examination of prestressing

strands removed for inspection and testing revealed only a few minor, structurally insignificant pits

indicating that the combination of waxes and greases used to inhibit corrosion of the ungrouted

prestressing systems has been effective. Tensile test results for the prestressing strands exceeded

design requirements. Visual examination of the concrete surfaces revealed a few surface cracks,

with the cracks <0.30-mm-wide, and when active their growth rate was <20 microns per year. A

comparison of concrete crack widths measured while the PCPVs were pressurized and

unpressurized indicated that the changes in crack widths with pressure were insignificant. The

cracks were associated with drying shrinkage.

NUREG/CR-6424 44
2.3.3.3 Commentary on longevity

As concrete ages, changes in its properties will occur as a result of continuing

microstructural changes (i.e., slow hydration, crystallization of amorphous constituents, and

reactions between cement paste and aggregates), as well as environmental influences. These

changes do not have to be detrimental to the point that concrete will not be able to meet its

performance requirements. When specifications covering concretes production are correct and are

followed, concrete will not deteriorate. 5 1 Concrete, however, can suffer undesirable changes with

time because of improper specifications, a violation of specifications, or adverse performance of its

cement paste matrix or aggregate constituents under either physical or chemical attack. Guidelines

for production of durable concrete are available in national consensus codes and standards such as

ACI 31816 that have been developed over the years through knowledge acquired in testing

laboratories and supplemented by field experience. Serviceability of concrete has been

incorporated into the codes through strength requirements and limitations on service load

conditions in the structure (e.g., allowable crack widths, limitations on midspan deflections of

beams, and maximum service level stresses in prestressed members). Durability generally has

been included through specifications for maximum water-cement ratios, requirements for entrained

air, minimum concrete cover over reinforcement, etc.

Water is the single most important factor controlling the degradation processes of concrete,

apart from mechanical deterioration (i.e., the process of deterioration of concrete with time is

generally dependent on the transport of a fluid through concrete). The relationship between the

concepts of concrete durability and performance is illustrated in Fig. 2.8 that was obtained from

Ref. 52. The rate, extent, and effect of fluid transport are largely dependent on the concrete pore

structure (i.e., size and distribution), presence of cracks, and micro climate at the concrete surface.

The primary mode of transport in uncracked concrete is through the cement paste pore structure

(i.e., its permeability). Although the coefficient of permeability for concrete depends primarily on

the water-cement ratio and maximum aggregate size, it is influenced by the curing temperature,

drying, and addition of chemical or mineral admixtures as well as the tortuosity of the path of flow.

Concrete strength, although a reasonable indicator of potential durability under most scenarios,

may not be sufficient. It is important also that adequate cementitious materials be included in the

concrete mix to reduce its permeability.

In general, the performance of reinforced concrete structures in NPPs has been very good.

Incidents of degradation reported generally occurred early in the life of the structures and primarily

have been attributed to construction/design deficiencies, improper material selection, or

environmental effects. Although the vast majority of these structures will continue to meet their

functional and performance requirements during the current licensing period (i.e., nominally 40

years) as well as the continued service period being considered (i.e., 20 years), it is reasonable to

assume that there will be isolated examples where the structures may not exhibit the desired

durability (e.g., water intake structures at San Onofre) without some form of intervention. Aging

concerns of most interest are primarily related to corrosion of steel reinforcement and liner

materials, loss of prestressing force, and possibly the effects of tendon sheathing filler leakage.

Since these structures have already been designed and constructed, outside of possibly the addition

of barrier coatings and sealants to accessible structures to prevent ingress of hostile environments,

the most prudent approach for maintaining adequate structural margins as well as extending usable

life is through an aging management program that involves application of ISI and maintenance

strategies. Figure 2.24 illustrates the relationship between structural performance, service life, and

time, and the impact of ISI/repair activity.

45 NUREG/CR-6424
Table 2.1. Typical safety-related concrete structures in LWR plants and their

accessibility for visual examination.

Concrete Structure Accessibility

Primary containment

Containment dome/roof

Containment foundation/basemat

Slabs and walls

Containment internal structures

Slabs and walls

Reactor vessel support structure (or

pedestal)

Crane support structures

Reactor shield wall (biological)

Ice condenser dividing wall (ice

condenser plants)

NSSS equipment supports/vault

structures

Weir and vent walls (Mark III)

Pool structures (Mark III)

Diaphragm floor (Mark II)

Drywell/wetwell slabs and walls

(Mark II)

Secondary Containment/Reactor Buildings

Slabs, columns, and walls

Foundation

Sacrificial shield wall (metallic

containments)

Fuel/Equipment Storage Pools

Walls, slabs, and canals

Auxiliary building

Fuel storage building

Control room (or building)

Diesel generator building

Piping or electrical cable ducts or tunnels

Radioactive waste storage building

Stacks

Intake structures (inc. concrete water it

piping and canal embankments)

Pumping stations

Cooling towers

Plant discharge structures

Emergency cooling water structures

Dams

Water wells

Turbine building

Internal liner/complete external

Internal liner (not embedded) or top surface

Internal liner/external above grade

Generally accessible

Typically lined or hard to access

Generally accessible

Typically lined

Lined or hard to access

Generally accessible

Lined with limited access

Lined

Lined with limited access

Internal liner/partial external access

Accessible on multiple surfaces

Top surface

Internal lined/external accessible

Internal lined/partial external

Generally accessible

Generally accessible

Generally accessible

Generally accessible

Limited accessibility

Generally accessible

Partial internal/external above grade

ake Internal accessible/external above grade and

waterline

Partially accessible

Accessible above grade

Internal accessible/external above grade and

waterline

Limited accessibility

External surfaces above waterline

Limited accessibility

Generally accessible

NUREG/CR-6424 46
Table 2.2. Typical safety-related concrete structures at BWR plants.

Importance Factor*

A. Primary Containment

Concrete Containment

I. Basemat Foundation

2. Drywell Pedestal

3. Vertical Walls (Mark I)

4. Steel Liner

5. Suppression Chamber (Mark I)

6. Chamber Steel Liner (Mark I)

7. Vertical Walls (Mark II)

8. Vertical Walls (Truncated Cone - Mark 1I)

9. Concrete Dome (Mark III)

10. Polar Crane Support (Mark III)

Steel Containment

1. Basemat Foundation

B. Containment Internal Structures

1. Bottom Slab (Steel Mark I and Pre-Mark Containments)

2. Reactor Pedestal/Support Structure

3. Biological (Reactor) Shield Wall

4. Floor Slabs

5. Walls

6. Columns

7. Diaphragm Floor (Mark II)

8. NSSS Equipment Pedestals/Supports

9. Upper and Fuel Pool Slabs (Mark III)

10. Drywell Wall (Mark III)

11. Weir/Vent Wall (Mark III)

12. Crane Support Structure (Mark III)

C. Secondary Containments/Reactor Buildings

1. Basemat Foundation (if isolated from containment basemat)

2. Walls

3. Slabs

4. Columns

5. Equipment Supports/Pedestals

6. Sacrificial Shield Wall (Metal Containments)

7. Spent/New Fuel Pool Walls/Slabs

8. Drywell Foundation (Mark I)

D. Other Structures (Category I)

1. Foundationsa

2. Wallsa

3. Slabsa

4. Cable Ducts

5. Pipe Tunnels

6. Stacks

7. Concrete Intake Piping

8. Cooling Tower Basins

9. Dams

10. Intake Crib Structures

11. Embankments

12. Tanks

13. Water Wells

10

10

8

4

8

4

8

8

8

10

10

10

9

6

6

7

7

7

5

5

6

7

5

10

8

6

7

4

6

6

9

10

8

6

5

5

5

6

5

6

6

4

5

4

* See Chapt. 4 for description and use of importance factors.

47 NUREG/CR-6424
Table 2.3. Typical safety-related concrete structures at PWR plants.

Importance Factor*

A. Primary Containment

Concrete Containment

1. Basemat Foundation 10

2. Tendon Access Galleries 3

3. Vertical Walls (and Buttresses) 8

4. Ring Girder (Prestressed Concrete Containment Vessel) 9

5. Dome 8

Steel Containment

1. Basemat Foundation 10

B. Containment Internal Structures

1. Bottom Floor (Metal Containments) 10

2. Floor Slabs 6

3. Walls 7

4. Columns 6

5. NSSS Equipment Pedestals/Supports 5

6. Primary Shield Wall (Reactor Cavity) 8

7. Reactor Coolant Vault Walls 7

8. Beams 5

9. Crane Support Structures 4

10. Ice Condenser Divider Wall and Slab 5

11. Refueling Pool and Canal Walls 6

C. Secondary Containment Buildings (metal containments)

1. Foundation 10

2. Walls 8

3. Slabs 6

D. Other Structures (Category I)

1. Foundationsa 10

2. Wallsa 8

3. Slabsa 6

4. Cable Ducts 5

5. Pipe Tunnels 5

6. Stacks 5

7. Concrete Intake Piping 6

8. Hyperbolic Cooling Towers 5

9. Dams 6

10. Intake Crib Structures 6

11. Embankments 4

12. Tanks 5

13. Water Wells 4

* See Chapter 4 for description and use of importance factors.

a Components of other site buildings such as Auxiliary, Turbine, Control, and Diesel Generator Buildings.

NUREG/CR-6424 48
Table 2.4. Degradation factors that can impact the performance

of safety-related concrete structures.

Material Degradation Factor Primary Manifestation

System

Concrete Chemical Attack

Leaching and efflorescence Increase porosity

Sulfate attack Volume change/cracking

Acids and bases Increased porosity/erosion

Alkali-aggregate reactions Volume change/cracking

Physical Attack

Salt crystallization Cracking

Frost attack Cracking/spalling

Abrasion/erosion/cavitation Section loss

Thermal exposure/thermal cycling Cracking/spalling/strength loss

Irradiation Volume change/cracking

Fatigue/vibration Cracking

Mild Steel Reinforcement Corrosion Concrete cracking/spalling

Elevated temperature Decreased strength

Irradiation Reduced ductility

Fatigue Bond loss

Post-Tensioning Corrosion Section loss/capacity loss

Elevated temperature Reduced strength

Irradiation Reduced ductility

Fatigue Concrete cracking

Stress relaxation/end effects Prestress force loss

Liner/Structural Steel Corrosion Section loss

Fatigue Cracking

Elevated temperature Reduced strength

Irradiation Reduced durability

49 NUREG/CR-6424
Table 2.5. Locations in NPPs where concrete-related materials

may exhibit degradation.

Material Degradation Potential Areas of Deterioration

System Factor

Concrete Chemical Attack

Freeze/Thaw Cycling

Thermal Exposure/Thermal

Cycling

Irradiation

Abrasion/Erosion/

Cavitation

FatigueNibration

Mild Steel Reinforcement Corrosion

Irradiation

• Subterranean areas

• Surfaces exposed to cooling water or

decontamination fluids

* In-containment floors and slabs subject

to chemical spills

• Containment/shield, auxiliary building

structures (ocean atmosphere, alkaliaggregate reaction)

• External structures where water may

collect

• Intake/discharge structures, particularly

at water line of cooling water source

• Containment/shield structures (diurnal

and seasonal effects)

• Biological shield

* Areas located near reactor coolant

pressure boundary or hot penetrations

" In-containment structures proximate to

reactor coolant pressure boundary (e.g.,

biological

• Localized areas of specific containment

designs

* Floor and slab elements

* Cooling water intake or discharge

structures

• Local areas in containment (e.g., near

liner anchors)

• Local areas at equipment supports or

piping vibrations

* Outer layer of conventional steel

reinforcing in all structures

SIn-containment structures proximate to

reactor coolant pressure boundary (e.g.,

biological shield)

• Containment buildings

* Containment buildings

• Areas of moisture storage or

accumulation

• Areas exposed to chemical or borated

water spills * Hot penetrations

• Discontinuities and equipment supports

* Penetrations subject to cyclic thermal and

mechanical loads.

Post-tensioning Systems

Liner/Structural Steel

Corrosion

Relaxation/concrete creep

and shrinkage

Corrosion

Thermal exposure/

thermal cycling

Fatigue

NUREG/CR-6424 50
Table 2.6. Reactivity of various chemicals with portland cement concrete

and reinforcing steel/liner plates.

Material Reactivity Effect on Concrete Reactivity Effect on

Reinforcing Steel/Liner Plate

Acetone

Acidic Water (less

than 6.5 pH)

Boric Acid

Borated Water

(and boron)

Chlorine Gas

Deicing Salts

Diesel Exhaust

Gases

Formaldehyde

(formic acid)

Hydrochloric Acid

Hydroxides

Lubricating Oil

Seawater

Sodium

Hydroxide

Sodium

Pentaborate

Sulfates

Sulphuric Acid

(sulphurous)

Liquid loss by penetration (may cause

slow disintegration)

Disintegrates concrete slowly

Negligible effect unless immersed

Negligible effect unless immersed

Concrete (moist) slowly disintegrates

Scaling of non-air entrained concrete

May disintegrate moist concrete by

action of carbonic, nitric, or

sulphurous acid; minimal effect on

hardened dry concrete

Disintegrates slowly

Disintegrates concrete rapidly

At low concentrations, slow

disintegration; at high concentrations,

greater disintegration

Fatty oils, if present, slowly

disintegrate concrete

Disintegrates concrete with inadequate

sulfate resistance

Not harmful below 20%

concentration, disintegrates at

concentrations above 20%

Disintegrates at varying rates

depending on concentration

Disintegrates at varying rates with

concentration (concretes with low

sulfate resistance such as Type I

Portland cement concrete)

Disintegrates rapidly in concentration

between 10 and 80%

May attack rebar and embedments

Severely corrosive to liner and

reinforcing steel

Very corrosive at high concentration

Highly corrosive

Highly corrosive

Minimal

Minimal

Highly corrosive

Unknown

Minimal

Highly corrosive

Minimal

Dependent on concentration

Harmful at certain concentrations

Harmful

51 NUREG/CR-6424
Table 2.7. Estimated irradiation levels at the outside boundary of a

RPV for several operating periods.

BWR* PWR*

40 Year 60 Year 80 Year 40 Year 60 Year 80 Year

(32 EFPY) (48 EFPY) (64 EFPY) (32 EFPY) (48 EFPY) (64 EFPY)

Neutron Fluence (n/cm2)

Slow (E < 1.0MeV) 3.7 x 1018 5.6 x 1018 7.5 x 1018 2.0 x 1019 3.0 x 1019 4.0 x 1019

Fast (E> 1.0 MeV) 5.1 x 1017 7.7 x 1017 1.0 x 1018 1.0 x 1018 1.5 x 1018 2.0 x 1018

Gamma Total Integrated 1.6 x 1010 2.4 x 1010 3.2 x 1010 4.7 x 109 7.0 x 109 9.3 x 109

Dose (rads)

* 1000 MW(e) plant with 80% capacity factor.

EFPY = effective full-power years.

Source: Copyright © 1977. Electric Power Research Institute. EPRI NP-152. PWR and BWR Radiation

Environment for Radiation Damage Studies. Reprinted with Permission.

NUREG/CR-6424 52
Table 2.8. Material code identification and description.

Portland Cement Concretes

Handbook Chapter Index 01 - Electronic Data Base File CONCRETE.DB

Group Group Index Description Class Class Index Description

Index Type of Concrete Index Type of Aggregate

A Insulating A Stone

B Structural Lightweight B Gravel

C Normal-Weight C Manufactured or

By-produce

D Heavyweight

Metallic Reinforcements

Handbook Chapter Index 02 - Electronic Data Base File REBAR.DB

Group Group Index Description Class Class Index Description

Index Type of Reinforcement Index Characteristic Feature

A Carbon Steel Bars A Uncoated without Deformations

B Stainless Steel Bars B Coated without Deformations

C Steel Wires C Uncoated with Deformations

D Bar Mats/Wire Fabric D Coated with Deformations

Prestressing Tendons

Handbook Chapter Index 03 - Electronic Data Base File TENDON.DB

Group Group Index Description Class Class Index Description

Index Type of Tendon Index Characteristic Feature

A Carbon Steel Bars A Materials without

Deformations

B Carbon Steel Wires

B Materials with

Deformations

C Strand

D Nonmetallic Materials

Structural Steels

Handbook Chapter Index 04 - Electronic Data Base File STEEL.DB

Group Group Index Description Class Class Index Description

Index Type of Reinforcement Index Characteristic Failure

A Carbon Steels A Hot- or Cold-Rolled Steels

B Stainless Steels B Bolting Materials

C Special Materials

53 NUREG/CR-6424
Table 2.9. Property code range descriptions.

Property Code Property Code Range Description

Ranges

1000-1999 General Information

2000-2999 Constituent Material and Plastic Concrete Properties

3000-3999 Mechanical Properties

4000-4999 Thermal, Physical, and Other Properties

5000-9999 Available for Data Base Expansion

Table 2.10. Quality level definitions.

Quality Levels and Corresponding Term Definitions

Quality Quality Level Relative

Level Term Description Quality Level Rating

A Recommended Property Highest

B Selected Property

C Typical Property

D Provisional Property

E Interim Property Lowest

NUREG/CR-6424 54
Table 2.11. Background information and service history requirements for

portland cement concretes and embedded metallic materials.

Background Information

Portland Cement Embedded Metallic

Concrete Materials

Material Lightweight Metallic Reinforcement

Description Structural Lightweight Prestressing Tendon

Normal-Weight Structural Steel

Heavy-Weight

Material Common Name or Designation Material Specification

Identification (including Type, Class, Grade, etc.)

Baseline 28-Day Compressive Strength Yield Strength or

Properties (or other reference properties) Ultimate Tensile Strength

Specified Design Strength

Material Constituent Materials Mill Certificate

Composition Mixture Proportions Chemical Analysis

Petrographic Reports Inspection Reports

Plastic Water-Cementitious Materials Ratio Not Applicable

Concrete Cement Content

Properties Air Content

Unit Weight

Service History

Portland Cement Embedded Metallic

Concrete Materials

Physical Inside/Outside Inside/Outside

Location Above/Below Grade Above/Below Grade

Submerged Submerged

Soil and Groundwater Conditions Soil and Groundwater Conditions

Natural Temperature Extremes Temperature Extremes

Environment Freeze-Thaw Cycles Weather Conditions

Weather Conditions

Normal Temperature Range Temperature Range

Operating Humidity Range Radiation Exposure

Environment Radiation Exposure Chemical Exposure

Chemical Exposure Electrical Currents

Electrical Currents

Abnormal Fire Fire

Exposure Chemical Spills Chemical Spills

Conditions Abnormal Temperature Exposure Abnormal Temperature Exposure

55 NUREG/CR-6424
Table 2.12. Mix proportions, baseline information, and plastic concrete

properties for the portland cement concrete used in the examples.

Mix Proportions

Property Code 1210 Material Composition

Mix Proportions per Unit Volume Property

Constituent Material kg/m 3 lb/yd 3 Code

Portland Cement 279 470 2001

ASTM C 150, Type I

Fine Aggregate 653 1100 2211

Coarse Aggregate

Maximum Size

38 mm (1-1/2 in.) 1246 2100 2221

Water 139 235 2421

Total 2317 3905

Baseline Information

Property Code 1310 Baseline Information

Data or Value

Property Description Property

MPa psi Code

Design Compressive Strength 20.7 3000 3021

Average Compressive Strength 24.4 3540 3022

at 28 Days

Plastic Concrete Properties

Property Code 2600 Plastic Concrete Properties

Property Description Property Property

or Designation Value Code

Cement Content 279 kg/m 3 470 lb/yd 3 2601

Water-Cement Ratio 0.50 0.50 2603

Air Content 1.0% 1.0% 2613

Unit Weight 2317 kg/m 3 144.6 lb/ft3 2621

Slump 76 mm 3.0 in. 2631

NUREG/CR-6424 56
Table 2.13. Time-dependent compressive strength values for

portland cement concretes obtained from SMIC.

Compressive Strength, MPa

Material Code 28-day 20-year 30-year

01CB002 19.2 35.9 37.8

O1CB003 12.3 25.5 26.1

O1CB007 33.6 47.3 47.4

Average= 21.7 36.2 37.1

Table 2.14. Temperature-dependent compressive strength values for

portland cement concretes obtained from SMIC.

Compressive Strength, MPa

Material Code Before Exposure After Exposure to

177°C for 7 days

O1CA004 42.8 26.6

01CA005 63.3 35.7

Average= 53.1 31.2

57 NUREG/CR-6424
Table 2.15. Ambient and temperature-dependent properties for

ASTM A 615 (Gr 60) and A 36 steels.

Property Value at

Material Property

Modulus of Elasticity, GPa

Tensile Yield Strength, MPa

Ultimate Tensile Strength, MPa

Elongation, %

Plates and Bars

Shapes

#3, #4, #5 Bars

#7, #8 Bars

#9, #10, #11, #14, #18 Bars

Thermal Coefficient of

Expansion, mm/mm/°C

ASTM A 615 Steel

Ambient 177 0C

200 191

414 414

621 621

ASTM

Ambient

200

248

621

20

23

A 36 Steel

1770C

191

220

420

15

17

9

8

7

11.7 x 10-6

9

8

7

11.3 x 10-6 11.7 x 10-6 11.3 x 10-6

NUREG/CR-6424 58
Table 2.16. Requirements for a data base management system for the SMIC.

Data Base Management System

Requirement

a. Each material and its associated properties should be represented in the

handbook and the electronic data base using the same format. In addition,

handbook pages should be printed automatically using information and data

stored in the electronic data base. These two capabilities would eliminate the

need for double entry of data and information and for maintaining the data base

in two separate formats. This would ensure that both presentation formats

were compatible and would greatly reduce the effort required to reproduce and

distribute hard copies of the data base.

b. The data base management system needs to accommodate variable-length field

names so that complete material names, property designations, and other

types of descriptive information can be recorded. "Material Codes" and

"Property Codes" are used in both the handbook and electronic data base to

identify the various materials and to distinguish one property from another

(Refs. 139 and 140). These codes are used as an indexing system for

organizing the handbook and provide a convenient way to subdivide materials

and properties into common groups. However, there is no inherent reason for

displaying code abbreviations on the computer screen when corresponding text

could be inserted as appropriate. The ability for the end user to edit these

names could potentially be desirable.

c. The ability to simultaneously display multiple windows of tabular or

graphical data is a desirable data base management system characteristic. This

feature would greatly enhance the usefulness of the data base because it would

provide a way for the end user to compare the same types of properties and

information for materials with similar compositions or characteristics. This

capability would also allow the end user to superimpose curves from different

materials onto the same plot making comparison of time- and environmentdependent performance relatively easy and accurate.

d. Material composition is an important data base parameter that needs to be

represented accurately and precisely. While dimensionless units such as

percent may be suitable for reporting the composition of metallic materials,

units of mass per unit volume may be required for composite materials such

as concrete. The data base management system must be able to accommodate

a broad range of units for reporting material composition. Representing these

units in both the International System of Units (SI) and customary units is

also desirable.

Mat.DB, Version 1.22

Perceived Weaknesses and Limitations

Does not exist in Mat.DB

Codes and other cryptic abbreviations are used in the Structural Materials

Electronic Data Base because Mat.DB has limited space for representing

this type of information. This limitation makes the data base confusing

to use and somewhat difficult to learn because the codes are only identified

and defined in the Structural Materials Handbook. The field names used

in Mat.DB are not always appropriate for all types of materials and in

particular concrete. Adapting to this limitation occasionally introduces

interpretation problems for the end user.

A new version of Mat.DB is currently being developed using Microsoft

Windows (Ref. 149). This version is expected to include the ability to

overlay multiple windows of data, but superimposing graphs may not be

possible.

Units of percent are the only ones available in Mat.DB for reporting the

composition of materials. This limitation is particularly inconvenient for

concrete in which mixture proportions are typically reported as mass per

unit volume.

4-'
Table 2.16. (Cont'd)

0

as

Data Base Management System

Requirement

e. The data base management system needs to be capable of storing and

displaying mathematical equations in such a way that they can be used to

construct tables and generate graphical representations of performance curves.

The performance curves reported in the Structural Materials Handbook were

developed from test results or synthesized from minimum property values.

These curves are one of the most important features in the data base because

they provide the basis for comparing time- and environment-dependent

properties for different materials. In order for this feature to be interactive, the

equations must be solved in real time and the results displayed upon demand.

f. The data stored in a data base management system need to be used to construct

tables and create graphs. This feature is essential for limiting files to a

manageable size and economizing data input efforts

g. Customized help features are needed to assist the end user by answering

fundamental questions associated with terminology such as differences

between various data categories and material designations. Suggested

guidelines for using material properties data, and information are also

considered necessary so that the end user can take full advantage of the data

base and its features. Help files are often necessary to clarify terminology,

prevent misuse, and enhance the significance of the reported data and

information.

h. Notes are a very important part of any materials property data base. The

ability to search the notes for keywords and phrases is considered necessary.

Mat.DB, Version 1.22

Perceived Weaknesses and Limitations

This capability does not currently exist within Mat.DB. The graphs

included in the Structural Materials Electronic Data Base are simply

pictorial representations that were developed using EnPlot, and the

property values presented in the spreadsheets were entered as numerical

values. Mat.DB was not designed to accommodate mathematical

equations.

The same data and values that are used by EnPlot to prepare engineering

graphs must also be entered into Mat.DB. These two programs do not

share a common data file.

Customized prompt text files can be displayed using Mat.DB utility

features, but these files only provide information that enhances the

identify of reported data and values. These fields do not contain

information that provides guidance to the end user.

Recent advances in data base technology have made this feature possible,

but the current version of Mat.DB does not have this capability.

O
Table 2.17. Condition survey results for various NPP concrete structures.

Local Plant

Degradation

Mechanisms I I I I I F I I I

Concrete

Chemical Attack

Efflorescence

and Leaching

Alkali-Aggregate

Reaction

Freeze/Thaw

Cycling

Thermal Exposure

Abrasion/Erosion

Fatigue/Vibration

Cracking

Conventional

Reinforcing

Corrosion

Prestressin2

System

Corrosion

Block Walls

Excessive Cracking

Structural Steel

and Liners

Corrosion

Soil/Structure

Issues

Differential

Settlement

Soil Erosion (Scour)

b,c

d

c,d,

fg

b,d

n/a

d

c

d

c

b,c,d

c

a,b,

c,d

b,d

el

c

e

b

b,c

c

c

c,d,g

b

n/a

c,d

c

a,d

c

c,d

n/a

c

b,d

c

a,b,

c,d,g

b,d

n/a

d

c

b,d

b,e,

d,f,g

b

n/a

c

d

d

c,d

b,f

n/a

c

c,e

c

b,d,f

f

c

b,c,

d,f

b,d

n/a

a

a,b,

c,d

b,c,

d,f

b

n/a

e

b,f

a

b,f,g

b,f

n/a

g

Key:

a - External Structure (Power Block)

b - Subgrade Structure (Power Block)

c - Internal Structure (Power Block)

d - Water Control Structure (Intake, Discharge, Etc.)

e - Containment Vessel

f - Other Site Structure

g - Equipment Supports

Notes:

1. Corrosion limited to exposed grease can and

bearing plate surfaces (no tendon corrosion noted).

Source: F. E. Gregor and C. J. Hookham, "Remnant Life Preservation of LWR Plant Structures," Transactions of

the 12th International Conference on Structural Mechanics in Reactor Technology held August 15-20,

1993, in Stuttgart, Germany, Elsevier Science Publishers, Amsterdam, The Netherlands, 1993; reprinted

with permission from author.

61 NUREG/CR-6424
Reactor building

Fig. 2.1 BWR Mark I type reinforced concrete containment. Source: Copyright © 1994.

Electric Power Research Institute. EPRI TR-103840-R 1. BWR Containments License Renewal

Industry Report; Revision 1. Reprinted with Permission.

NUREG/CR-6424 62
25130 ton polar crane

Expansion Joint rwl -

(continuous at

ExpasionJoit [• ' :•,: Personell

some plants)

"• "'"--'--- • .- RPV :'•Z::i .

pedestal ':..

Acces YI . J 

lock I I I I

Downcomef

Suppression twel

•.......:•..,• .•..•.....,,. .:...,,...:.., .,,...

Isolation Joint

(at some plants)

Fig. 2.2 BWR Mark II type reinforced concrete containment. Source: Copyright © 1994.

Electric Power Research Institute. EPRI TR-103840-RI. BWR Containments License Renewal

Industry Report: Revision 1. Reprinted with- Permission.

63 NUREG/CR-6424
Pressure wlvveTweii suppression V

pool Pedestal -..:Horizontal

vents

.. .. -Containment

basemat

Fig. 2.3 BWR Mark III type reinforced concrete containment. Source: Copyright © 1994.

Electric Power Research Institute. EPRI TR-1 03840-R 1. BWR Containments License Renewal

Industry Report: Revision 1. Reprinted with Permission.

NUREGICR-6424 64
Fig. 2.4 PWR subatmospheric type reinforced concrete containment.

Source: Insights for Aging Management of Major Light Water Reactor Components, Volume 2 - Reinforced and Prestressed Concrete Containments, NUREG/CR-5314 (EGG-2562), Idaho National

Engineering Laboratory, Idaho Falls, July 1994 (Draft).

65 NUREG/CR-6424
Fig. 2.5 PWR reinforced concrete containment with ice condenser.

Source: Insights for Aging Management of Major Light Water Reactor Components, Volume 2 - Reinforced and Prestressed Concrete Containments, NUREG/CR-5314 (EGG-2562), Idaho National

Engineering Laboratory, Idaho Falls, July 1994 (Draft).

NUREG/CR-6424 66
8m

2.5m

Reactor vessel" Reactor/\\>>

Fig. 2.6 PWR large dry prestressed concrete containment.

Source: Insights for Aging Management of Major Light Water Reactor Components, Volume 2 - Reinforced and Prestressed Concrete Containments, NUREG/CR-5314 (EGG-2562), Idaho National

Engineering Laboratory, Idaho Falls, July 1994 (Draft).

67 NUREG/CR-6424
Fig. 2.7 PWR free-standing steel containment with elliptical bottom. Source: Copyright © 1994.

Electric Power Research Institute. EPRI TR-103835. PWR Containment Structures License Renewal

Industry Report. Revision 1. Reprinted with Permission.

NUREG/CR-6424 68
a'

Physical Chemical and Corrosion I Biological

PERFORMANCE

Surface - - Resistance Rigidity Condition

Safety Serviceability Appearance

Fig. 2.8 Relationship between the concepts of concrete durability and performance. Source: Comit6 Euro-International

du Bdton (CEB), Durable Concrete Structures-Design Guide, published by Thomas Telford Services Ltd., London, England,

1989; reprinted with permission from CEB and Thomas Telford Services Ltd.
_ After

hardening

-Physical-

- Chemical

- Thermal -

-Shrinkable aggregates

.Drying shrinkage

-Crazing

-Corrosion of reinforcement

Alkali-aggregate reactions

-Cement carbonation

-Freeze/thaw cycles

External seasonal temperature variations

External

-Earlv thermal contraction--[ restraint

Types of crack- ----- j .........

Accidental overload

temperature

Creep gradients

-Design loads

Early frost damage

-Before .s F Plastic shrinkage

hardening Plastic L ---Plastic settlement

Construction Formwork movement

movement Sub-grade movement

Fig. 2.9 Relationship between primary causes and types of cracks that can occur in concrete.

Source: Comit6 Euro-International du B6ton (CEB), Durable Concrete Structures-Design Guide,

published by Thomas Telford Services Ltd., London, England, 1989; reprinted with permission from

CEB and Thomas Telford Services Ltd.

NUREG/CR-6424 70
rust

slains

A = plastic settlement, over reinforcement

B = plastic settlement, arching

C = plastic settlement, change of depth

D = plastic shrinkage, diagonal

E = plastic shrinkage, random

F -- plastic shrinkage, over reinforcement

G = early thermal contraction, external restraint

H = early thermal contraction, internal restraint

I = long-term drying shrinkage

J = crazing, against formwork

K = crazing, floated concrete

L = corrosion of steel reinforcement

Fig. 2.10 Examples of intrinsic cracks in hypothetical concrete. Source: Comit6

Euro-International du BMton (CEB), Durable Concrete Structures-Design Guide, published

by Thomas Telford Services Ltd., London, England, 1989; reprinted with permission from

CEB and Thomas Telford Services Ltd.

Loading.

service

conditions

Alkali-silica

reaction

*,:' rw mar. ///////////

Corrosion

DrOying

Z shrinkage

Early thermal o contraction

Plastic

shrinkage

Plastic

settlement

.,./7,,7/7ZW7-2Z

nr--rr 7 , t7

z7wr

I hour Ap I day I week month

rime from placing of concrete

year

-j 50 years

Fig. 2.11 Time of appearance of cracks from placing of concrete. Source: Comit:

Euro-International du BMton (CEB), Durable Concrete Structures-Design Guide, published

by Thomas Telford Services Ltd., London, England, 1989; reprinted with permission from

CEB and Thomas Telford Services Ltd.

71 NUREG/CR-6424
DETERIORATION OF CONCRETE BY CHEMICAL REACTIONS

Exchange Reactions Between 11![ReactionslInvolving Hydrolysis ] ] ReactionslInvolving ]

-- Aggressive Fluid and Components and Leaching of the Components Formation of

of Hardened Cement Paste of Hardened Cement Paste Expansive Reactions

Removal of Removal of Ca++ Substitution Reactions

Ca++ Ions as Ions as Replacing Ca++

Soluble Products Non-Expansive in C-S-H

Insoluble Products

I Increase in Porosity Increase in

and Permeability Internal Stress

3-

+~

Loss of Increase in Lossof Cracking,

Alkalinity Mass Deterioration Strength Spalling, Deformation

Processes and t Popouts

A: Softwater attack on calcium hydroxide and C-S-H present in hydrated portland cements;

B(I): acidic solution forming soluble calcium compounds such as calcium sulfate, calcium acetate, or calcium bicarbonate;

B(II): solutions of oxalic acid and its salts, forming calcium oxalate;

B(III): long-term seawater attack weakening the C-S-H by substitution of Mg++ for Ca++;

C(l): sulfate attack forming ettringite and gypsum;

C(2): alkali-aggregate attack;

C(3): corrosion of steel in concrete; and

C(4): hydration of crystalline MgO and CaO.

Fig. 2.12 Types of chemical reactions responsible-for concrete deterioration. Source: P. K. Mehta and B. C. Gerwick, Jr.,

"Cracking-Corrosion Interaction in Concrete Exposed to Marine Environment," Concrete International 4(10), pp. 45-51, Detroit,

Michigan, October 1982; reprinted with permission of author.
Cyclic Load More noble

metal

...........................

...........................

...........................

...........................

W,ýM

........................... ............................ ........................... ............................

Metal or

Non -metal

REFERENCE

No Corrosion

a. Uniform b. Galvanic c. Fretting d. Crevice

•.,.o..o...., ..O°oo... ..... ,.,.'.°......•

•. .O.0.o.. o.... .... o..°,..., .',. .. ',. ..

e. Pitting ,...

f. Selective Leaching g. Intergranular h. Stress corrosion

cracking

i. Corrosion fatigue

• ...o ......... o ° .° °. •.,,. • •.o... . ... ...

........ 0................... O..o °o......o,.,.°. , o.°.o.... o,-...'..,.."

j. Hydrogen

z embrittlement

0'3 Fig. 2.13 Schematic representations of forms of corrosion that may be found in metals.
20

E

0

0

6

Cu

15

10

5

0

0 10 20 30 40 50 60 70

Concrete Strength, MPa

Fig. 2.14 Carbonation data for exterior sheltered concrete. 12 3

w,

1-

LV

1.2

1

0.8

0.6

0.4

0.2

0

0 100 200 300 400 500 600 700

Cement content, kg/m3

Fig. 2.15 Proposed curves for surface chloride content versus cement content

for different curing conditions (Refs. refer to Ref. 123).

NUREG/CR-6424 74
cJ

0

2

C

U

'3'

I~6

C

U

C

U

C

U

C

Cao

~p.

U

C'-

10.9

10"10

10-11

10-12

10-13

28-d. Strength, MPa

30

40

50

60

1 10 100

Age, years

Fig. 2.16 Effective diffusion coefficient of chloride versus time for

ordinary portland cement concretes. 12 3

75 NUREG/CR-6424
STRUCTURAL MATERIALS HANDBOOK Volume 1 - Performance Values

Material Code OICB004 Property Code 3621 Page 1. 1

Portland Cement Concrete Ultimate Update Package Number 2

Normal-Weight

Gravel Aggregate Compressive Strength Revision Control Code 1.0

Series B, Janesville, 0.41 versus Time Quality Level A

University of Wisconsin

Series B, Janesville, Water-Cement Ratio = 0.41

Time, years

0 10 20 30 40 50

60 I I I

Performance Curve, Outside

500

-6

40Ž

4) 4)ý

rE0 0

0 5000 10000 15000 20000

Time, days

Preparer: C. B. Oland Sources: References 24, 25, 26, 27

Organization: Oak Ridge National and 28

Laboratory

See Page 1.2 for a list of computed compressive strength values and the

equations used to generate the ultimate compressive strength versus time

performance curve.

Fig. 2.17 Example of page from Vol. 1 (Performance Values) of Structural Materials Handbook.

NUREG/CR-6424 76
STRUCTURAL MATERIALS HANDBOOK Volume 2 - Supporting Documentation

Material Code 01CB004 Property Code 3621 Page 1.3

N ora C-Weight CUltite Update Package Number 0

| Gravel Aggregate Compressive Strength Revision Control Code 0.0

Series B, Janesville, 0.41 versus Time Quality Level A

Compressive Strength Test Results

for Specimens Stored Outside, MPa (psi) at:

Cement 7 28 1 5 10 25 50

Vendor Days Days Year Years Years Years Years

Medusa I= 22.9 31.6 46.7 50.1 51.0 4970

(3M) (2325) (3315) (4580) (6780) (7260) (7400) (7110)

Lehigh 18.1 23.8 33.9 47.8 49.6 52.1 59.7

(4M) (2620) (3455) (4910) (6930) (7195) (7555) (8660)

Universal 17.4 27.8 34.8 48.0 47.6 55.1 48.7

(5M) (2530) (4035) (5050) (6960) (6900) (7990) (7070)

Marquette 19.3 28.2 35.8 49.3 52.5 54.1 60.1

(7M) (2805) (4095) (5190) (7145) (7615) (7850) (8715)

Average 17.7 25.7 34.0 48.0 49.9 53.1 54.4

(2570) (3725) (4930) (6955) (7240) (7700) (7890)

Test specimens were cast with each of these four cements, moist

cured for 28 days, and then placed outside in Madison, Wisconsin

for long-term storage. Each value listed above is the average

compressive strength (Property Code 3023) from five test specimens (Reference 134).

Fig. 2.18 Example of page from Vol. 2 (Supporting Documentation)

of Structural Materials Handbook.

77 NUREG/CR-6424
STRUCTURAL MATERIALS HANDBOOK

Material Code 01CB004 Property Code 1000

Volume 3 - Matedal Data Sheet

Page 2

IPortland Cement Concrete . =r

Normal-Weight General RUpdate Package Code 0

Gravel Aggregate Information Revision Contr

Series 5. Janesville, 0.41 Quality Level A

Property Code 1210 Material Composition

Mix Proportions per Unit Volume Constituent Material Property

kg/M3 lb/yd' Code

Portland Cement

ASTM C 150, Type I 369 622 2001

Fine Aggregate 724 1220 2211

Coarse Aggregate 1236 2084 2222

Water 151 255 2421

Total 2421 4081

The mix proportions were 1:1.5:3 by volume or 1:1.8:3.35 by weight

(derived from Reference 134).

Property Code 1220 1 Processing Information

Each concrete specimen was moist cured for 28 days and then placed

outside in Madison, Wisconsin for long-term storage. Outside

storage consisted of placing each specimen on level ground in an uncovered cage having a northeast exposure until 1950, and then

each specimen was moved to an uncaged location in an open area for

the remaining time. The relative humidity in Madison normally

varies from 65 to 100 percent and averages about 75 percent. The

annual precipitation including snowfall is about 810 mm (32 in.).

Annual air temperatures usually range between -32 and 35"C (-25 and

95"F).

Fig. 2.19 Example of page from Vol. 3 (Material Data Sheet)

of Structural Materials Handbook.

NUREG/CR-6424 78
Typical Material Code (01CBOO1) for a Normal-Weight

Portland Cement Concrete made with Gravel Aggregate

Chapter Group Class Identifier

Index Index Index Index

(01-99) (A-Z) (A-Z) (001-999)

Fig. 2.20 Material code arrangement.

79 NUREG/CR-6424
z

3 - 1 1

r'3

................................................................. ............ ........................... | ..... .......... • ....... ]. 2 . .; .......................

2D A c

Lo 2- LPC

.• 15 .... .............. .......... ..... i ................................. . ................. • ............ .. •

.. . . . .. . . . . . . . . . . . ....... .. . .. . . . . . . l P

KPC U)U 1 .5 ........... ,

................... .........0 ...................... ............. ........................ .........................

.

o 1 • +Wi,,

CD0 A AWh

> ~~ ~Reference au

01

100 101 102 10 3 10 4 10 5

Age, Days

Fig 2.21 Long-term compressive strength results for selected data from

University of Wisconsin and Portland Cement Association.
3

2

0

2.5

2

1.5

1

0.5

0

100 101 102 104 105

Age, Days

a. Concretes obtained from several U.S. nuclear power facilities

2.5

Q

W,

2• 0

2

1.5

1

0.5

0

100 101 102 103 104 10 s

Age, Days

b. Concretes obtained from several U.K. nuclear power facilities

Fig. 2.22 Relative compressive strength for concretes obtained from several U.S. and U. K. nuclear power stations.

81 NUREG/CR-6424
PROCEDURE APPLIES

TO ALL OR PART OF

EXISTING BUILDINGS

EVALUATION BY

STATIC THI

LOAD TEST

h

EVALUATION BY

EORETICAL STRESS

ANALYSIS

* CONDUCT A PRELIMINARY

STRUCTURAL ANALYSIS

" PERFORM REPAIRS NECESSARY

FOR THE STATIC LOAD TEST

" ESTABLISH THE MAGNITUDE OF

THE TEST LOADS

* INSTALL DISPLACEMENT AND

STRAIN INSTRUMENTATION

" PROVIDE SCAFFOLDING TO

SUPPORT THE STRUCTURE IN

CASE OF FAILURE

* APPLY THE TEST LOADS WITHOUT SHOCK OR VIBRATION AND

RECORD THE DISPLACEMENT

AND STRAIN DATA

* AFTER 24 HOURS, RECORD THE

DISPLACEMENT AND STRAIN

DATA

" REMOVE THE TEST LOADS AND

AFTER 24 HOURS, RECORD THE

DISPLACEMENT AND STRAIN

DATA

* IF THE PORTIONS OF THE

STRUCTURE THAT WERE

TESTED SHOW NO SIGNS OF

FAILURE, THE STRUCTURE

PASSES THE TEST

" OBTAIN SECTIONAL CHARACTERISTICS AND DETAILS OF THE

STRUCTURAL MEMBERS AND

CONNECTIONS

* IDENTIFY PROPERTIES OF THE

STRUCTURAL MATERIALS

USING DESTRUCTIVE OR A

COMBINATION OF DESTRUCTIVE

AND NONDESTRUCTIVE

TESTING METHODS

* ASSESS THE QUALITY OF THE

CONSTRUCTION AND THE

PRESENT CONDITION OF THE

STRUCTURE

" CONDUCT A THEORETICAL

STRESS ANALYSIS TO

DETERMINE THE SAFE SERVICE

LOAD CARRYING CAPACITY OF

THE STRUCTURE FOLLOWING

RECOGNIZED PRINCIPLES FOR

STRENGTH DESIGN IN ACI 318

(Ref. 16).

Fig. 2.23 Recommended procedure for strength evaluation of existing concrete buildings.

NUREG/CR-6424 82
Initial[.• Repair

Implemen

Im~I

C I

E• IMinimum Required

Performance

Service

I Life

to tmi tr ts T

mm r An Ti

Fig. 2.24 Relationship between concrete performance and service life.

me

83 NUREG/CR-6424

3. IN-SERVICE INSPECTION, CONDITION ASSESSMENT, AND REMEDIAL

MEASURES CONSIDERATIONS

As noted in the previous chapter, concrete structures in nuclear power plants (NPPs) are

subjected in use to many types of environmental influence that can impact their ability to continue

to meet functional and performance requirements. Due to the significant safety as well as economic

consequences that could result if these structures were to deteriorate to unacceptable performance

levels, it is important that these structures be inspected at regular intervals and maintained. Data

and information developed from periodic condition assessments can be used to indicate the current

structural condition as well as to estimate future performance (i.e., continued service assessments).

Maintenance and repair activities also are essential elements in an overall program to effectively

manage the aging of these structures.

3.1 IN-SERVICE INSPECTION TECHNIQUES

Detection of age- or environmental stressor-related degradation, as well as its magnitude

and rate of occurrence, are key factors in maintaining the readiness of safety-related concrete

structures in NPPs to continue their functions in the unlikely event that a condition such as a lossof-coolant accident (LOCA) would occur.

3.1.1 Current In-Service Inspection Techniques

Continuing satisfactory service of a reinforced concrete NPP structure over an extended

period of time is dependent in large measure on the durability of its constituents. Techniques for

detection of degradation of reinforced concrete structures should, therefore, address evaluation of

the concrete, steel reinforcement, liner plate, and anchorage embedments. Table 3.1 presents a

summary listing of evaluation procedures routinely used to determine concrete properties, the

physical condition of concrete, and the occurrence or potential for occurrence of corrosion of steel

either embedded in or in contact with concrete. 162 Pertinent information relating to several of these

procedures is provided in Table 3.2.163 A summary description of commonly used techniques is

provided below. Additional background information on inspection of concrete materials and

structures is available elsewhere. 164-1 67

3.1.1.1 Concrete material systems

Primary manifestations of distress that are present or can occur in reinforced concrete

structures include (1) cracking, voids, and delaminations; and (2) strength losses. Although not an

aging-related phenomenon, whether the concrete was fabricated using the proper constituents and

mix design, as well as properly placed, compacted and cured, could also become a service life

consideration and should be considered as part of the overall evaluation process. For reinforced

concrete structures in NPPs that may not be accessible for inspection without the removal of soil,

portions of neighboring structures, etc. (e.g., basemat), inspection can be addressed indirectly

through an assessment of exposure conditions.

Detection of Cracking, Voids, and Delaminations Methods used to detect

discontinuities in concrete structures generally fall into two categories: direct and indirect. Direct

techniques involve a visual inspection of the structure, removal/testing/analysis of material(s), or a

combination of the two. The indirect techniques generally measure some parameter from which an

estimate of the extent of degradation can be made through existing correlations. Quite often,

however, evaluation of concrete materials and structures requires the use of a combination of test

methods since no single testing technique is presently available that will detect all potential

85 NUREG/CR-6424
degradation factors. Detection of concrete cracking, voids, and discontinuities generally involves

either visual inspection, nondestructive testing, destructive testing (e.g., removal and examination

of cores), or a combination of these methods.

Visual Inspection

Periodic visual examination of exposed concrete provides a rapid and effective method for

identifying and defining areas of distress (e.g., cracking, spalling, volume change, or

cement/aggregate interaction). By locating, marking, identifying by type and orientation, and

measuring and recording conditions associated with cracks (e.g., seepage, edge spalling,

differential settlement, etc.), a history can be established that will be of assistance in identifying the

cause and establishing whether a crack is active or dormant. A crack comparator capable of width

determinations to an accuracy of 0.025 mm can be used to establish cracks that are above a critical

size that permit the entry of hostile environments to attack either the concrete or its steel

reinforcement. 168,

169 Subsurface cracking, delarninations and voids, and extent of cracking,

however, cannot be established through visual examination.

Nondestructive Testing

Nondestructive testing techniques are available that can be utilized to determine the presence

of internal cracks, voids or delaminations, and the depth of penetration of cracks visible at the

surface. Ultrasonic pulse velocity techniques are generally used, but acoustic impact, radiography

and radar, and thermal techniques have also been used.

Detection of cracks or voids in concrete by using ultrasonic through-transmission

measurements is based on the principle that the amplitude and direction of travel of ultrasonic

compressional waves propagating through concrete will be changed when they encounter a

discontinuity. 170 The ultrasonic pulses are emitted by a transmitter, and the transit time to a

receiver is measured by electronic means in terms of either transit time (microseconds) or path

length. Large internal flaws in concrete can be detected by an abnormally long transmission time

and/or a large decrease in the magnitude of the ultrasonic pulses as they pass around a

discontinuity. The technique can also be used to indicate concrete changes due to freezing and

thawing or other aggressive environments. Primary advantages of this technique are that it is an

excellent method for rapidly estimating the quality and uniformity of concrete and a low level of

user expertise is required to make measurements. Disadvantages are that sound transmission

through concrete is influenced by a number of conditions (e.g., aggregate characteristics, mix

designs, presence of reinforcement, etc.) and quantitative interpretation of results can be difficult.

Stress wave reflection/refraction methods are based on monitoring the interaction of

acoustic (or stress) waves with the internal structure of an object. 17 1 Striking an object by

mechanical impact or an electromechanical transducer and listening to or recording and analyzing

the stress waves is a common method for detecting the presence of voids, cracks, or

delaminations. Stress wave propagation methods include pulse-echo, impact-echo, and stress

wave refraction. In the pulse-echo method, a stress pulse is introduced into an accessible surface

by impact and as the stress wave propagates into the test article with flaws or other interfaces

causing reflections. The surface response in the form of the reflected wave is monitored by either

the transmitter acting as a receiver (true pulse-echo) or by a second transducer located near the

pulse source (pitch-catch). Receiver output is displayed on an oscilloscope and, by using a time

base display, the round-trip travel time of the pulse to the reflecting interface can be determined.

Pulse-echo techniques have been used to measure the thickness of thin concrete sections and to

identify and locate defects and discontinuities. Interpretation of results can be difficult with

optimum performance requiring calibration and referencing. The impact-echo system consists of

an impact source (e.g., an instrumented hammer having a characteristic transfer function), a

NUREG/CR-6424 86
receiving transducer, and a digital processing oscilloscope or wave form analyzer that is used to

capture the transient output of the transducer, store the digitized wave forms, and perform signal

analysis. 172 Reflections of P- and S-waves by internal interfaces or external boundaries are sensed

at the receiving transducer as displacements and recorded by the oscilloscope. Examination of the

amplitude spectra from scans taken at several positions on the test article surface are used to

indicate the approximate size of defects. Impact-echo can detect various types of interfaces and

defects (e.g., cracks, voids, and honeycombed concrete), the depth of surface-opening cracks,

voids in prestressing ducts, and thickness of wall and slab sections. 17 3 Interpretation of the forcetime history requires relatively complex signal processing. Stress wave refraction uses a hammer

to transmit signals into a material. Receivers are located in four directions at equal distances from

the impactor. The signal travels through the material as spherical and surface waves and the time

required to travel from the point of impact to the closest transducer is measured. 174 Deteriorated

concrete will cause an increase in transit time making the method capable of indicating the relative

quality and detecting cracking at different locations in a structure.

Audio or acoustic impact methods utilize the frequency characteristics or "ringing" resulting

from striking a concrete surface with a hammer, chain, rod, etc., to detect cracks, voids, and

delaminations. The theory underlying this technique is that when a metallic object strikes a solid

(nondelaminated) area, a metallic ringing sound will be heard. If a defect is present, a "hollow"

sound would be emitted. Advantages of the technique include relative ease of use and simplicity of

the equipment. However, the technique requires experience to interpret results, it can miss small

delaminations, and results can be affected by geometry and mass of the test object.

Radioactive (X- and gamma-ray) techniques consist of a beam from a radiation source, the

object examined, and an image collector. These systems employ a photographic film placed on the

opposite side of the specimen from the source as a detector. The intensity of radiation passing

through the test article varies according to the article's thickness, density, and absorptive

characteristics. Applications of X-ray radiography in the field, because of its relatively high initial

cost and limited mobility of testing equipment, have been limited to establishing steel reinforcement

location, investigating bond stress in prestressed concrete, and showing concrete density

variations. Gamma-ray radiography, because of its use of less costly portable equipment and its

ability to make measurements up to concrete thicknesses of 450 mm, has been widely used to

determine the condition and position of steel reinforcement, voids in post-tensioning tendon grout,

and variable compaction of concrete. 175 Depending on the source, structures up to 1--m thick can

be examined and voids as small as 5 mm detected. Radiography, however, is fairly expensive,

skilled operators and a license to operate are required as well as appropriate radiation safety

protection, and thin cracks or planar defects parallel to the radiation beam may not be detected.

Penetrating- or impulse-radar is based on the principle of electromagnetic wave reflection

and consists of a main radar unit that radiates short-time-duration electromagnetic pulses (0.5 to

1.5 nanoseconds) into the material examined, a graphic printer or computer-aided analyzer, and an

antenna. The main radar unit is the primary processing section of the radar equipment. The

antenna houses the transmitter as well as the receiver. When an electromagnetic wave travels

through different mediums (different dielectric constants), a portion of the wave is reflected back

to the antenna and the remainder of the wave is refracted into the next medium. Loss of strength of

the reflected wave form, measured in volts, is used in the evaluation of data. Voids in concrete are

indicated by differences in dielectric constants between concrete and air. Advantages of the

technique are that large areas of concrete can be rapidly surveyed and internal construction details

and defects identified. Also, depth of penetration can be changed by changing the antenna.

Disadvantages are that where material differences are small - such as a crack in sound material or a

87 87 NUREG/CR-6424
contact delamination - transmission differences are hard to detect and evaluate, material

permittivity must be known to determine the interface depth, and results are influenced by the

presence of moisture and steel reinforcement.

Infrared thermography is based on heat-transfer principles with subsurface anomalies

detected as irregular surface temperature distributions (i.e., discontinuities do not transmit heat as

fast). A thermographic scanning and analysis system consists of (1) an infrared scanner head and

detector, capable of detecting temperature variations as small as 0.1 'C; (2) real-time microprocessor

coupled to a monitor that displays different temperature levels as different colors or as gray-tone

images; (3) data acquisition and analysis equipment consisting of an analog-to-digital converter,

computer with high resolution color monitor, and data storage and analysis software; and (4) image

recording and retrieving device. 176 Advantages of infrared thermography include the ability to

scan a large area quickly using portable equipment to determine the location and horizontal

dimensions of a defect, and testing can be done without direct access to the surface. Disadvantages

are that the equipment is expensive, extensive operator experience is required, and it can not

provide information on the cause and extent of deterioration or the depth or thickness of a void.

Destructive Testing

Visual and nondestructive testing methods are effective in identifying areas of concrete

exhibiting distress, but often cannot quantify the extent or nature of the distress. This is generally

accomplished through removal and visual examination of core samples. 177

In Situ Strength Determinations In general, limited attention is given to the in situ

compressive strength of existing concrete structures because 28-day (or older) moist-cured control

specimens were used to demonstrate that the required strength was achieved,* and very few

concrete structures actually fail. 179,180 However, for a structure that either has had a change in

functional or performance requirements, been subjected to a less than ideal operating environment,

or is being evaluated to provide an indication of its service life, the in situ strength takes on new

meaning. Available methods used to indicate concrete strength include both direct (testing of core

specimens) and indirect (ultrasonic pulse velocity, surface hardness, penetration, pullout, and

breakoff resistance). The indirect methods measure some property of concrete from which an

estimate of the strength is made through correlations that have been previously developed.

When core samples are removed from areas exhibiting distress, 177,18 1 a great deal can be

learned about the cause and extent of deterioration through strength and petrographic studies. 182

Additional applications of concrete core samples include calibration of nondestructive testing

devices, chemical analyses, visual examinations, determination of steel reinforcement corrosion,

and detection of the presence of voids or cracks. Additional information on the use of core

samples is presented elsewhere. 183,

184

The ultrasonic pulse velocity methods are based on the fact that the velocity of sound in a

material is related to the elastic modulus and material density. 1

85 The test equipment consists of a

means of producing and introducing a pulse into the concrete (pulse generator and transmitter) and

a means of accurately measuring the time taken for the pulse to travel through the concrete to the

receiver. Introduction of the pulse can be either direct, semi-direct, or indirect, with direct being

* In situ compressive strength of 28-day concrete is normally less (20 to 25%) than the 28-day standard control

specimen strength of the same concrete because of different compaction and curing conditions. 17 8 Also, systematic

variation of concrete occurs in a structure because of segregation that can reduce concrete strength at the top of a lift

by 15 to 30%.179

NUREG/CR-6424 88
the preferred method since the maximum energy is transmitted. Because the pulse velocity

depends only on the elastic properties of the material and not on the geometry, it is very convenient

for evaluating concrete quality. Advantages of the technique are that it is a rapid and cost-effective

method for measuring in situ concrete uniformity, the method is totally nondestructive, and it can

be utilized to "estimate" concrete strength within about 15 to 20% if a good correlation curve has

been developed. Disadvantages are that results are affected by factors such as contact surface

smoothness (i.e., coupling), aggregate and concrete mix characteristics, curing conditions,

concrete age, and presence of steel reinforcement.

Surface hardness methods impact the concrete surface using a given mass activated by a

given energy. The size of the indentation is used to indicate the concrete compressive strength. A

special form of surface hardness method is the Schmidt rebound hammer that impacts a springloaded weight against the concrete surface and a rebound number is obtained. Concrete strength is

determined either from a manufacturer-supplied chart or from a laboratory-generated calibration

chart. The primary usefulness of this device is in assessing concrete uniformity in situ, delineating

zones (or areas) of poor quality or deteriorated concrete in structures, and indicating changes with

time of concrete characteristics. Advantages include minimal user expertise requirements and a

large amount of data can be obtained in a short period of time at minimal expense. However, for

results obtained in the field the probable variation in compressive strength is ± 25%.186 Test

results are affected by factors such as smoothness of the test surface, specimen geometry and

rigidity, moisture condition, and mix characteristics.

Probe penetration techniques involve measurement of the resistance of concrete to

penetration of a steel probe driven by a given amount of energy. 187 The most common device of

this type is the Windsor Probe, consisting of a powder-activated driving unit that propels a

hardened alloy probe into concrete and a gage for measuring depth of penetration. Compressive

strength is derived from calibration curves based on depth of penetration. Advantages of the

technique are that it is simple to operate and correlates fairly well with core compression strength

results. Limitations of the method are related to size of specimen that can be tested (minimum

distance between test locations and edge of specimen is 150 to 200 mm; minimum specimen

thickness of about three times depth of penetration), and results are influenced by aggregate

characteristics.

The pullout test measures the force required to pull an embedded metal insert with an

enlarged head from a concrete specimen or structure.188 The method provides a measure of the

shear strength of concrete that is converted to tensile or compressive strength through correlations.

The method was developed primarily for determining the in-place strength of concrete during

construction because of the requirement to embed or cast the insert into the concrete. Recently,

new techniques have been developed in which holes are drilled into the hardened concrete and

either pullout inserts or split-sleeve assemblies/wedge anchors are installed and pulled.

Advantages of the method are that it is the only nondestructive (actually, semi-destructive) method

that provides a direct measure of in-place strength, and it is rapid and economical. It does not

measure the interior strength of mass concrete, minor surface repairs are required, and results are

affected by aggregate characteristics.

Mix Composition Analysis Questions concerning whether the concrete structure was

cast using the specified mix composition can be answered through examination of core samples. 189

By using a "point count" method, 190 the nature of the air void system (volume and spacing) can be

determined by examining under a microscope a polished section of the concrete. An indication of

the type and relative amounts of fine and coarse aggregate, as well as the amount of cementitious

matrix and cement content, can also be determined. 182,

19 1 Determination of the original watercement ratio is not covered by a standard test procedure, but the original water (volume of capillary

89 NUREG/CR-6424
pores originally filled with capillary and combined water) can be estimated.192 A standard method

also does not exist for determination of either the type or amount of chemical admixtures used in

the original mix. Determination of mix composition becomes increasingly difficult as a structure

ages, particularly if it has been subjected to leaching, chemical attack, or carbonation.

3.1.1.2 Conventional mild steel reinforcing

Assessments of mild steel reinforcing materials are primarily related to determining its

characteristics (e.g., location and size) and evaluating the occurrence of corrosion.

Steel Reinforcement Characteristics Equipment for determining the location, size,

and depth of mild steel reinforcement in concrete are based on the induction principle. 192 Magnetic

induction is only applicable to ferromagnetic materials. The equipment consists of a primary coil

connected to a power supply that delivers a low frequency (10 to 50 Hz) alternating current and a

second coil that feeds into an amplifier circuit. When a ferromagnetic test object, such as a rebar,

is introduced near the coils, a much higher secondary voltage is introduced with the amplitude of

the induced signal being a function of the magnetization characteristics, location, and geometry of

the object. Steel reinforcement between 9.5 and 57 mm (No. 3 to No. 18 bars) and as deep as

30.5 cm can be detected in concrete. 193 Although a variety of probes can be used with a meter,

each probe needs to be calibrated. Also, several factors can affect the magnetic field within the

range of the instrument (e.g., presence of multiple rebars, bar supports, and temperature).

Corrosion Occurrence The primary distress to which mild steel reinforcement would

be subjected is corrosion. Implications of safety and serviceability of structures undergoing

appraisal as a result of rebar corrosion should consider effects on three levels: (1) effect on rebars

themselves (cross section reduction), (2) development of fine hairline cracks in concrete cover

parallel to rebars (indication of deterioration), and (3) structural cracking or voids (preferential

corrosion sites). Safety aspects of corrosion depend primarily on the structural form or system of

construction; second, on the way in which the geometry of the structural components may be

affected; and, third, to a lesser degree, on the total amount of corrosion of the rebars. 194 As

shown in Fig. 3.1, a variety of techniques are available establishing both the depth of penetration

of carbon dioxide or chloride ions, and the extent of steel corrosion. 195 Methods available for

corrosion monitoring and inspection of steel in concrete include (1) visual inspection,

(2) mechanical and ultrasonic tests, (3) core sampling with chemical and physical testing,

(4) electrical methods, and (5) rate of corrosion probes. The mechanical and ultrasonic methods

were discussed previously.

Visual inspection generally provides the first indication of a corrosion problem. Buildup of

corrosion products around reinforcement will eventually reach a point where the internal tensile

forces generated form hairline cracks in the concrete following the line of reinforcement. Rust

staining and spalling also occur as the corrosion progresses. Chipping of concrete cover to expose

rebar will indicate the degree of corrosion and may provide clues to its cause.

Chemical analysis of materials obtained during a coring process can be used to obtain

information on chloride distribution. Depth of carbonation can be easily determined by treating a

freshly broken surface with phenolphthalein - the carbonated portion will be uncolored. Periodic

measurements can be used to establish rate of penetration information that is useful in predicting

the onset of degradation (e.g., time required for C- to reach level of steel reinforcement).

Electrical methods use resistance and potential difference measurements of a structure to

determine the moisture content, estimate thicknesses, estimate probability of significant corrosion,

and indicate the current rate of corrosion. The four-electrode method measures the resistivity of

NUREG/CR-6424 90
concrete and relates it to the probability of presence of steel corrosion. 19 6 Four contact points,

linearly arranged and equally spaced in the concrete at a depth of approximately 6.4 mm, are used.

Alternating current is passed through the outer electrodes and the potential difference determined

using the inner electrodes. Depending on the resistivity measured, corrosion is likely, corrosion

will probably occur, or corrosion is certain to occur. Potential measurements using a reference

copper-copper sulfate half cell can be used to estimate the likelihood of corrosion, but can not

indicate the corrosion rate or amount of corrosion present. 197 The half cell consists of a copper

rod submerged in a capsule filled with copper sulfate. An electrical connection is made to the

reinforcement at a convenient position enabling electrode potentials to be measured at any desired

location by moving the half cell over the concrete surface in an orderly manner. Temperature and

presence of moisture can influence the results.

Two types of probes are available that can be embedded into concrete to provide an

indication of the rate of corrosion. 19 8 The first type uses two to three electrically isolated short

sections of steel wire or reinforcing steel and linear polarization techniques. The second device

uses a steel wire or hollow cylinder embedded into concrete to provide cumulative rate of corrosion

data from periodic measurements. One of the primary applications of these devices has been to

evaluate the effect of rehabilitation procedures on the corrosion rate.

3.1.1.3 Prestressing system components

Prestressing system components in NPPs are routinely inspected to identify the occurrence

of corrosion or excessive loss of prestressing force. In the U.S., the condition and functional

capability of the unbonded post-tensioning systems must be periodically assessed. This is

accomplished, in part, systematically through an ISI program that must be developed and

implemented for each post-tensioned concrete containment. Guidelines for development of tendon

surveillance programs are provided in Refs. 105, 106, and 157.* Basic components of these

documents include sample selection, visual inspection, prestress force monitoring, tendon material

tests and inspections, and evaluation of the tendon sheathing fillers (i.e., corrosion inhibitors).

Reference 155, presents results of a study conducted specifically to identify aging concerns, if any,

for the NPP post-tensioning systems (see Sect. 2.3.2.2), and describes the tendon surveillance

program in detail (i.e., examination schedule; tendon sample size requirements; determination of

prestressing force; tendon material visual examination and property determinations; and sheathing

filler analysis and limits for water content, reserve alkalinity, and concentrations of chloride,

nitrate, and sulfide ions). Additional information pertaining to inspection of existing prestressed

concrete structures is provided in Ref. 200.

3.1.1.4 Liner plate

Inspections of metallic liners and penetration liner plates of NPP concrete components are

performed in compliance with requirements contained in Subsection IWE of Section XI of the

American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code. 20 1

Contained in this reference are examination and inspection requirements, acceptance standards,

repair requirements, and system pressure test requirements for these components. Inspection

generally involves a visual examination with supplemental surface (e.g., liquid penetrant or

magnetic particle), volumetric examinations (e.g., ultrasonic or radiographic), or an engineering

evaluation required if abnormal conditions are noted. Although abrasion or impact damage and

fatigue are possible degradation mechanisms for metallic liner materials, corrosion is the largest

threat to the ability of the liner to provide a leaktight pressure boundary. Techniques for inspection

* The USNRC has issued a draft regulation 19 9 that will, when finalized, endorse the requirements for containment

examination contained in this reference.

91 NUREG/CR-6424
of inaccessible regions of the pressure boundary (i.e., portions embedded in concrete) require

development. The integrity of coatings applied to liner materials to inhibit corrosion can be

evaluated visually with supplemental testing in suspect areas to measure dry-film coating thickness

and determine if holidays are present.

3.1.1.5 Embedment steel

Failure of anchorage embedments will generally occur as a result of either improper

installation or deterioration of the concrete within which it is embedded. Visual examinations can

be used to evaluate the general condition of the concrete near an embedment and to provide a

cursory examination of the anchor or anchor plate to check for improper anchor embedment, weld

or plate tearing, plate rotation, or plate buckling. Mechanical testing can be used to verify that

pullout and torque levels of embedments equal or exceed minimums required by design. Welds or

other metallic components can be inspected using magnetic particle or liquid penetrant techniques

for surface examinations, or if a volumetric examination is required, radiographic, ultrasonic, and

eddy current techniques are available. Additional information on anchorage to concrete is available

elsewhere. 202

3.1.2 New and Emerging Techniques

Several nondestructive testing techniques in the developmental stage possess potential for

application to evaluation of NPP reinforced concrete structures.

3.1.2.1 Magnetic methods

Leakage Flux When ferromagnetic materials are magnetized, magnetic lines of force

(i.e., flux) flow through the material and complete a magnetic path between the poles. When the

magnetic lines of flux are contained within a ferromagnetic object, it is difficult to detect them in the

air space surrounding the member. If the surface of the member contains a crack or defect,

however, its magnetic permeability is changed and leakage flux will emanate from the

discontinuity. 193 Measurement of the intensity of the leakage flux provides a basis for

nondestructive identification of such discontinuities.

A magnetic field method based on leakage flux has been investigated for use in detecting

loss of area (> 10%) due to corrosion and fracture of reinforcing bars and prestressing strands. 203

A steady-state magnetic field is applied to the member under inspection, and a scanning magnetic

field sensor (Hall-Effect device) is used to detect protuberances in the applied field caused by

anomalies such as deterioration or cracks. The technique has been demonstrated in the laboratory

relative to its ability to detect varying degrees of deterioration, influence of adjacent unflawed steel

elements, type of tendon duct, type of reinforcing steel, transverse rebar configuration, etc. The

laboratory results indicated good overall sensitivity to loss-of-section and excellent sensitivity to

fracture. Field tests have produced similar results. Additional work has confirmed the ability of

the method to detect fractures in reinforcing strands developed due to fatigue loadings. 19 3 The

capability of this technique in testing concrete components with congested reinforcement has not

been reported.

Nuclear Magnetic Resonance Nuclear magnetic resonance (NMR) is an

electromagnetic method capable of determining the amount of moisture present in a material by

detection of a signal from the hydrogen nuclei in water molecules. The method exposes the

material examined to a static magnetic field and to a pulsed radio frequency (RF) magnetic field

corresponding to the NMR frequency of the nuclei of interest.204 A prototype NMR moisture

measurement system has been developed that utilizes a two.-pulse sequence to provide the

NUREG/CR-6424 92
capability of distinguishing between NMR signals from free moisture and signals from bound

hydrogen. The system utilizes a sensor assembly in which the test article is external to both the RF

coil and magnet structure. The system specifications indicate that it can detect a moisture content

range from 1 to 6% by weight of material with an error of ± 0.2% for depths up to 70 mm and

± 0.4% for depths from 70 to 95 mm. No information regarding the accuracy of this method in

massive concrete structures is available.

3.1.2.2 Electrical methods

Capacitance Instruments Experimental investigations at frequencies up to 100 M-Hz

have shown that components (real and imaginary) of the dielectric constants of building materials

increase significantly with increasing moisture contents.205- 207 The dielectric constant provides a

measure of the ability of a material to store a charge. Several capacitance instruments are available,

having various electrode configurations, to measure moisture content of building materials. The

electrodes are attached to a constant frequency alternating current source and establish an electric

field in the material to be tested. The current flow power loss is used to indicate moisture content.

Reference 193 indicates that moisture content of laboratory concrete specimens could be

determined to ± 0.25% for values less than 6% using a 10 MHz frequency. Reference 208

suggests that further study is required to establish the reliability of the method. Also, for best

results, the instruments should be calibrated to the material.

Polarization Resistance A system has been developed for measuring the corrosion rate

of steel in concrete. 20 9, 2 10 The system is computer controlled and uses the polarization resistance

to compute the corrosion current which indicates the rate at which steel is corroding. The

polarization technique employs a three electrode system using the specimen as one electrode, a

voltage reference as the second electrode, and a counter electrode from which polarizing current is

applied to the specimen. The reliability and accuracy of the system are being assessed.

An electrochemical impedance system, somewhat similar to the system described above,

has been developed that can be used for locating deteriorated areas and for corrosion rate

measurements of rebar embedded in concrete. 2 11 Corroded and noncorroded rebars are

distinguished based on different impedances in a low-frequency region. Corrosion reaction of

rebars is indicated by differences in impedances between the low-frequency region and the highfrequency region. The method was investigated using a 20-year-old precast concrete slab

reinforced by welded wire fabric. Results showed that the electrical potential best serves as an

indicator for a threshold value for corrosion to occur, and that the reciprocal value of polarization

resistance of the corrosion reaction correlates well with actual corrosion rate (e.g., at high

corrosion rates the potential is low and reciprocal value of polarization resistance is high). The

system is undergoing evaluation and is not presently commercially available.

Two instruments, also somewhat similar to the system above and based on the linear

polarization method, have been developed to measure the instantaneous corrosion rate of steel in

reinforced concrete. 2 12 The first device uses a counter electrode and reference electrode in

conjunction with the working electrode (i.e., corroding rebar). The second device incorporates a

guard ring with the counter and reference electrodes to confine the polarizing current being applied

to the rebar from the counter electrode. As a result, the area of rebar being measured is known.

After polarization of the rebar, the change in potential and input current is obtained, recorded, and

corrosion rate determined.

Exploratory work has been completed on use of ultralow frequency alternating current

(AC) impedance spectroscopy to characterize corrosion of steel reinforcement in concrete. 2 13 The

principle of operation is that corrosion may be located by imposing a sinusoidal current at a

93 NUREG/CR-6424
monitoring point on the surface to measure the concrete/rebar impedance as a function of frequency

(typically from 104 to 10-3 Hz). Location of corrosion is estimated on the basis that the distance

traveled by the AC wave down the rebar increases as the frequency is lowered. At some

characteristic frequency, the AC wave intersects the corroding region resulting in a sudden,

perceptible change in measured impedance. Impedance is used to estimate the true polarization

resistance of the rebar from which the corrosion rate is determined. It has been demonstrated

experimentally that the electrical impedance of the rebar/concrete system is sensitive to the presence

of corrosion and that by scanning the reference electrode used to detect the alternating voltage

across the surface, areas of corrosion in a reinforced concrete structure can be resolved spatially

(i.e., survey structure for corrosion damage). Although the feasibility of this approach has been

demonstrated, further development and field testing are required.

Half-Cell Potential Test Using Impulse Radar When performing half-cell

potential measurements using the copper-copper sulfate method to detect the potential for corrosion

of steel reinforcement, one end of the cell is connected to the reinforcement and the other end is

placed in direct contact with the test area. The most recent addition to this technique is the

capability to measure the resistance to passing a charge through the concrete. Because of the "electric potential" of the concrete, it is of interest to correlate the return data from the impulse radar

to that of the copper-copper sulfate half cell. The theory behind this concept is that as the

resistance of the concrete increases, the dielectric constant of the concrete decreases. The opposite

occurs when the resistance of concrete decreases. The change in dielectric constants can be

detected by the impulse radar. This method is still in the experimental stage, and at present, no

literature is available to assess its application to massive concrete structures. 2 14

3.1.2.3 Ultrasonic methods

Laser Ultrasonics A pulsed Nd-YAG laser system has been developed to detect voids,

delaminations, and defects in asphalt and reinforced concrete materials. 2 15 The laser is used to

introduce ultrasonic waves in the test sample and a He-Ne output laser interferometer is used to

measure the minute movements in the surface caused by the ultrasonic waves. The system can be

operated in three modes: direct transmission, near-coincident, and same surface-separated.

Moderate success was obtained in the laboratory in detecting steel reinforcement and simulated

voids in sections up to 200-mm thick. Development of a portable more powerful laser system and

improved signal enhancement and recognition capabilities (i.e. overcoming the effects of surface

texture and poor reflectiveness of asphalt and concrete) are required before the system will be

suitable for field use.

Acoustic Tomography Imaging A preliminary study has been conducted to determine

the feasibility of using acoustic tomographic imaging for defect location and identification of the

interior of concrete sections. 216 The method uses a large number of ultrasonic pulse readings

obtained on the exterior of a concrete object as input to a tomographic reconstruction computer

program to create a map of the velocities of the interior object. From the velocity profile, the

location, shape, and type of internal feature can be identified. Preliminary results obtained from

medium-sized concrete specimens in which various types of objects simulating flaws were

embedded are encouraging (i.e., objects such as steel bars, voids, and zones of low density were

identified). Work is presently underway to modify and optimize the computer software to meet the

specific needs for examination of mass concrete, and to improve the data acquisition equipment and

transducer characteristics.

NUREG/CR-6424 94
3.1.2.4 Pulsed infrared imaging method

A preliminary study has been conducted in which thermal pluses and infrared images were

used to inspect subsurface features of concrete. 2 17 The equipment consisted of a controlled heat

supply (i.e., bank of tungsten-filament lamps) and a thermal camera to capture the changing

surface temperature of the concrete. The depth of concrete observed was controlled through the

length of time that the heaters were activated. The method was able to identify the presence and

location of steel reinforcement and it was theorized that corrosion could be detected because of the

relatively poor conduction characteristics of corrosion products. A commercial version of a pulsed

infrared imaging method has been developed for nondestructive examination of metallic and

composite materials. 2 18

3.1.2.5 Ground-probing radar method

Ground-probing radar 164 is the electromagnetic analog of sonic and ultrasonic pulse-echo

methods. It is governed by the process involved in the propagation of electromagnetic energy

through materials of different dielectric constants. The systems operate by transmitting a short

pulse that is followed by a "dead time" in which the reflected signals are returned to the antenna.

As the radiated pulses travel through a material, different reflections will occur at interfaces that

represent changing dielectric properties. Each reflected electromagnetic pulse arrives back at the

receiving antenna at a different time that is governed by the depth of the corresponding reflecting

interface and the dielectric constant of the intervening material. A receiver circuit reconstructs the

reflected pulses at an expanded time scale using the time-domain sampling technique. The

resulting replicas of the received radar signals are amplified and further conditioned in the control

unit before they are fed to an output. Applications include detection of voids under concrete,

delamination of concrete, scour, thickness measurements, and subsurface soil exploration. Skill is

required in interpreting the various radar signatures. With refinements such as increased resolution

of the antenna, ground-probing radar may have potential application to inspection of massive

concrete structures such as NPP basemats.

3.1.2.6 Ultraviolet radiation method

A simple and rapid test has been developed that provides conclusive evidence of the

presence of alkali-silica reactions (ASRs) in in situ concrete structures. 2 19 The method consists of

applying a dilute solution of uranyl acetate on the suspected concrete surface, and viewing the

treated surface under ultraviolet light in a dark background. The presence of ASR is revealed by a

yellowish-green fluorescent glow caused by incorporation of the solution in the ASR gel product.

Some user experience is required and there are precautions associated with handling the uranyl

acetate solution.

3.1.3 Commentary on Applicability of Methods to NPP Concrete Structures

Nondestructive testing techniques that have been used on concrete structures include visual,

acoustic, ultrasonic, electrical resistivity, electromagnetic, vibration, strain gage, dye penetrant,

radar, magnetic particle, infrared thermography, and radiography. The use of any of these

methods depends largely on the sensitivity desired, adaptability to field use, cost of

instrumentation, and survey requirements. In general, most methods detect defects by the

observation of changes in the response of the interrogation medium. The most common forms of

defects that can occur in concrete components include cracking, surface scaling, corrosion of steel

reinforcement, honeycombing or air pockets, popouts, surface deposits, chemical attack, wear,

and erosion. For steel structures, the most common forms of deterioration include corrosion

(uniform, pitting, galvanic, or dissimilar metal) and cracking (fatigue or embrittlement). Table 3.3

95 NUREG/CR-6424
provides a summary of recommended testing methods to identify and assess the extent of damage

resulting from factors that can degrade reinforced concrete. 2 14

The ideal nondestructive testing technique for reinforced concrete structures would provide

information about the existence and location of deterioration, corrosion, and structural damage.220

Rates of corrosion are important in estimating service life. Given the variables of exposure,

geometry, and materials that are present in NPP concrete structures, there is no single test method

that can provide all the desired information. Best results are obtained through use of a combination

of testing methods. Many of the nondestructive testing methods require the development of

correlation curves that relate the material property of interest to a measured parameter (e.g.,

compressive strength and rebound number). Best correlation results are obtained when destructive

and nondestructive testing are done in tandem using representative materials obtained from the

structure of interest.* Detectability functions that relate flaw characteristics to probability of

detection require development. Many of these techniques operate on the surface of a structure

where the properties can be significantly different relative to internal concrete due to effects such as

carbonation. This is particularly true for NPP reinforced concrete structures that tend to be more

massive than conventional civil engineering reinforced concrete structures. No "field-ready"

nondestructive testing method was identified that can be used to assess the in situ condition of the

post-tensioning tendons with respect to occurrence of corrosion. Some encouraging results have

been provided by the use of magnetic field disturbance to sense discontinuities (i.e., wire breaks)

in prestressing tendons, but considerable development is still required. 203 Also, due to the

massive size, steel reinforcement congestion, and accessibility problems associated with the

basemats of NPPs, no nondestructive testing techniques are currently available that can reliably

inspect these structures. Nondestructive testing methods suitable for inspection of inaccessible

portions of the metal pressure boundary where it is embedded in concrete have not been identified.

Ultrasonic-based methods have been successfully applied to the interface region between the

pressure boundary and concrete to detect corrosion effects, but they are only capable of detecting

sharp pits located within a distance equal to about four times the pressure boundary thickness (i.e.,

about 25 mm for the liner of reinforced concrete containments). 222

Despite the limitations associated with many of the nondestructive testing methods, their

proper use and application provides vital input for accessing the structural condition of reinforced

concrete structures. Nondestructive testing methods provide a vital element in an aging

management program to maintain required safety margins, (i.e., they aid in the diagnosis of

problems, specifying repair activities, and quantifying the extent of adverse conditions and

deterioration).

3.2 CONDITION ASSESSMENTS

Determining the existing performance characteristics and extent and causes of any observed

distress is accomplished through a condition assessment. Several documents have been prepared

to aid in conduction of condition surveys: Guide for Making a Condition Survey of Concrete InService;2 23 Causes, Evaluation, and Repair of Cracks in Concrete;224 Practices for Evaluation

of Concrete in Existing Massive Structures for Service Conditions;2 25 Strength Evaluation of

Existing Concrete Structures;226 Guide for Concrete Inspection;227 Guidelines for Structural

Condition Assessment of Existing Buildings;163 Strength Evaluations of Existing

* In many of the NPP concrete structures (e.g., containment and biological shield) it may not be possible to obtain

the samples required to develop the correlation curves. For this situation statistical data have been developed for use

with techniques commonly used to indicate concrete compressive strength (i.e., ultrasonic pulse velocity, rebound

hammer, breakoff, pullout, and Windsor probe methods). 22 1

NUREG/CR-6424 96
Buildings;147 and Guide for Evaluation of Structures Prior to Rehabilitation.16 2 General

information pertaining to condition surveys of reinforced concrete structures, current inspection

requirements for NPP reinforced concrete structures, and needed developments follows.

3.2.1 Condition Surveys of Conventional Reinforced Concrete Structures

Common in the condition assessment approaches proposed by several of the references

noted above is the conduct of a field survey involving visual examination and application of

nondestructive and destructive testing techniques, followed by laboratory and office studies.

Results of the field survey, and laboratory and office studies are analyzed to either verify the

adequacy of the structure to successfully meet its functional and performance requirements, or to

provide data and information necessary for implementation of a maintenance or repair activity.

Figure 3.2 presents a flow diagram of one procedure proposed for use in a general structural

assessment and evaluation of an existing building. 163

3.2.1.1 Field survey

Visual Survey General direction on conducting surveys of concrete structures is

provided in Ref. 228 from which information presented in the balance of this section has been

abstracted. The condition survey usually begins with a review of the "as-built" drawings and other

information pertaining to the original design and construction so that information, such as

accessibility and the position and orientation of embedded steel reinforcing and plates in the

concrete, is known prior to the site visit. Next is a detailed visual examination of the structure.

This initial visual examination is used to document easily obtained information on instances that

can result from or lead to structural distress such as cracking, spalling, leakage, and construction

defects such as honeycombing and cold joints in the concrete. The instances of cracking, spalling,

leakage, delamination, efflorescence, chemical attack, or structural distress are observed and

documented.

Crack Survey Cracking can be due to effects such as shrinkage of the concrete, thermal,

effects, corrosion of embedded steel, alkali-aggregate reaction, or structural loads. A crack survey

is usually done by drawing the locations and widths of cracks on copies of project plans. Cracking

patterns may appear that suggest weaknesses in the original design, construction deficiencies,

unanticipated thermal movements, chemical reactivity, detrimental environmental exposure,

restrained drying shrinkage, or overloading. Distress associated with cracks such as efflorescence,

rust stains, or spalling are noted. Photographs or video recordings are made to provide a

permanent record of this information, and notes are made on the survey sheets to indicate the area

photographed.

Delamination Plane Survey After the visual survey has been completed, the need for

additional surveys may be indicated. One such survey is for internal delaminations that are not

visible. These internally cracked regions are usually caused by corrosion of embedded metals or

internal vapor pressure. For horizontal surfaces, chains can be dragged over the concrete surface

to quickly locate hollow sounding delamination planes. Hammers or lengths of steel bar impacted

against the surface sometimes result in more distinctive differences in tone. On vertical or

overhead surfaces, a hammer or bar must be used to locate the hollow sounding areas.

If a more detailed study is then warranted, the results of the visual and delamination

surveys are used to select portions of the structure that will be studied in greater detail. The

detailed study on the suspect areas can include measuring half-cell potentials, removing and testing

of concrete and steel samples, removing concrete powder samples for chloride analyses, locating

embedded steel, additional nondestructive testing evaluations and, in extreme cases, load testing.

97 NUREG/CR-6424
Corrosion Survey Concrete normally provides a high degree of corrosion protection to

embedded metals because of the high pH environment within the concrete. Steel in high pH

concrete is polarized anodically by a thin protective film of iron oxide that forms on its surface.

However, the passivity of this film can be disrupted by a reduction in the pH value of the moisture

within the concrete due to carbonation or by the ingress of sufficient quantities of chloride ions, or

other ions, to the steel surface.

This disruption of the protective film causes some steel areas to become more anodic,

creating an electrochemical cell with the more cathodic unaffected steel, causing corrosion to occur.

The electromotive force in these cells depends on the pH value, the concentration of chloride, and

the moisture content adjacent to the anodic steel and on the transfer of oxygen through the concrete

to the cathodic areas. To locate areas of corrosion activity within reinforced concrete, coppercopper sulfate half-cell studies can be performed. By taking readings at multiple locations on the

concrete surface, an evaluation of the corrosion activity probability of embedded reinforcing steel

(or other metals) can be made. If sufficient readings are taken on a grid pattern, a diagram can be

prepared that resembles a contour map. On such a diagram, points of equal electrical potential are

connected by isopotential lines, permitting areas of high potential or high corrosion probability to

be readily identified.

Reinforcing Bar Location Survey A pachometer survey may be performed as part of

the detailed study. The pachometer consists of a search coil that emits a magnetic field and is

connected to electronic circuitry that senses any disruption in this field. A display dial is graduated

to indicate the depth of the steel reinforcing bars, if the size of the bar is known. The equipment

can be calibrated for depth using a reinforcing bar and various thicknesses of an inert material, or

on the job by drilling or coring to the depth of the steel.

Where there is evidence of severe corrosion, the steel bar should be uncovered to allow

visual inspection and measurement of cross-sectional area loss. In many cases, old delaminations

or spalls will have already accomplished most of the concrete removal, aiding in the bar

examination. A concrete coring rig may also be used to retrieve steel samples.

Concrete Chloride Survey Concrete power samples or cores should be removed from

the structure where significant chloride penetration is suspected. The samples are usually removed

from several depths, extending to and beyond the embedded outer layer of reinforcing steel. If the

concrete contains more acid soluble chloride than about 300 ppm by weight (0.7 kg/m3), as

measured by ASTM Cl 152,229 it is considered to contain sufficient chloride to support

electrochemical corrosion of embedded steel when in a moist environment that has oxygen

availability. Particularly susceptible to damage by chloride ions are dissimilar metals. Where

aluminum or galvanized electrical conduit has been embedded in concrete and is in contact with the

reinforcing steel, the conduit will corrode rapidly, acting as an anode to the steel.

In elements exposed to a marine environment, most of the concrete elements will eventually

contain significant amounts of chloride. Water intake and discharge structures present unique and

severe environmental conditions where water with dissolved chlorides, sulfates, and other minerals

and salts is routinely in contact with the concrete. Some regions are always wet while other

regions may experience wet and dry periods. Experience has shown that regions experiencing wet

and dry cycles exhibit the greatest distress.

Concrete columns and walls contain capillary channels that can cause saltwater to wick

upward for several feet. The columns and walls in such structures can exhibit delaminations and

spalls caused by corrosion due to this upward moisture movement. Chloride contents should even

NUREG/CR-6424 98
be determined in indoor structures if cracking patterns suggest reinforcing steel corrosion.

Numerous high-rise office buildings in the U.S. have suffered severe corrosion due to the

inclusion of concrete admixtures containing chloride ions.

3.2.1.2 Laboratory tests, office studies, and design verification

Laboratory Tests During the site survey, samples of concrete and steel are collected for

testing within the laboratory. Concrete cores are removed for examination of the steel,

compressive strength determination, petrographic examination, or laboratory sampling for chloride

contents or other chemical ingredients known to create distress. The samples taken from a

structure may be investigated using many different techniques, such as

Petrographic methods: thickness, distribution of cement, aggregate studies,

estimation of water-cementitious materials ratios, air-void distribution, types of

distress, recognition of unstable aggregates, deterioration mechanisms, and age at

which cracking occurred.

Chemical techniques: chemical constituents of the cementitious materials,

characteristics of the paste and aggregates, presence and quantity of chemical

admixtures, quantification of chemical compounds within the cement paste,

efflorescence, and carbonation effects.

Concrete strength testing: compressive strength, modulus of elasticity, tensile

strength, flexural strength, and bond strength of patches or overlays.

Steel materials: yield and ultimate strengths of reinforcing bars per ASTM A615,31

or prestressing tendons per ASTM A421. 39

Petrographic examination 182 can determine the quality of the cement paste, whether or not

the aggregates are unstable, if the concrete contains entrained air, or if it has been damaged by

freezing and thawing, chemical attack, or other deterioration mechanisms. Petrographic

examination of core samples is recommended for any concrete that may be showing distress or

when the concrete components or durability of the concrete is not known. A trained petrographer

experienced in concrete deterioration is needed for this evaluation. The petrographic examination is

done on a section of concrete that is usually sawed vertically through the center of the core, then

polished to a high luster. Examination is done both with the naked eye and through a low power

microscope.

Office Studies A crack survey map is prepared and studied for meaningful patterns.

Half-cell data are studied and isopotential lines are drawn to assist in determining active corrosion

sites. Chloride ion results are plotted vs depth to determine the profile and the chloride content at

the level of the steel. Any elements that appear to be structurally marginal, due either to

unconservative design or to the effects of deterioration, are identified and appropriate calculation

checks made. If the calculations are inconclusive, suitable load testing may be indicated.

Design Verification Based on physical test results, chemical analysis of materials, and

present condition of the structure, a redesign of various questionable elements should be

accomplished to verify compliance with current codes and design requirements. Based upon the

initial survey results, additional testing such as impact-echo, linear polarization, infrared

thermography, X-ray, or numerous other destructive or nondestructive tests may be appropriate.

99 99 NUREG/CR-6424
From design documentation and from measurements made in the field, structural analyses

may then be accomplished. Compressive and tensile strengths and elastic properties of materials

may be determined from laboratory measurements and used in the structural analyses. These

analyses may identify distress in the structure that has been caused by structural overload and

indicate safety factors.

3.2.1.3 Report

After all of the field and laboratory results have been collected and studied, and all

calculations have been completed, the report is prepared. It should start with an introduction

stating how and by whom the work was instigated, who did the investigation, why the

investigation was performed, and when. A brief description of the structure should be included.

A photograph of the entire structure is helpful.

A description of the field investigation follows, with a short explanation of the various

testing techniques used and a short summary of findings. Photographs of significant features and

exploratory excavations should be included, along with maps showing crack, delamination, and

spall locations, and where core and powder samples were removed.

The testing techniques used and the results determined in the laboratory should then be

described, and the results interpreted. Any structural analyses performed should be presented and

discussed. A general discussion that summarizes all of the findings and characterizes the condition

of the structure should follow. Any unsafe conditions should be identified, and temporary

corrective actions suggested.

The final section of the report should discuss the possible repair techniques, and which

appear to be appropriate in view of the results of the investigation and the environment of the

structure. Appendices may be added if a complete compilation of the data is desired. A condition

survey done in this manner will provide information in which a sound, economical repair

specification can be based.

3.2.2 Condition Assessments of NPP Reinforced Concrete Structures

ISI programs for NPPs traditionally have focused on the periodic inspection of safetyrelated components at various intervals or frequencies depending primarily on component

importance; however, passive components such as safety-related concrete structures have generally

received minor attention in the overall ISI strategy.

The concrete containment vessel is generally the only structure included in a defined ISI

program. This has resulted from the requirement contained in General Design Criteria 53,

"Provisions for Containment Testing and Inspecting," to 10 CFR Part 50 that the reactor

containment be designed to permit periodic inspection of all important areas and an appropriate

surveillance program.4 Condition assessments of the containment are conducted in conjunction

with the performance of periodic Type "A" leak-rate tests required by Appendix J to 10 CFR Part

50. This only includes a general visual inspection of the accessible interior and exterior surfaces of

the containment structures and components. For conventionally-reinforced (non-post-tensioned)

concrete containments, a general visual inspection currently is the only in-service inspection that is

required for the concrete structures.

Examinations of the unbonded post-tensioning systems are conducted at regular intervals to

assess the state of the hardware as well as the level of prestressing force in the containment. These

examinations are mandated by the Plant Technical Specifications. Detailed examination procedures

and acceptance criteria are provided in Regulatory Guide 1.35 (Ref. 105), and the ASME Boiler

NUREG/CR-6424 100
and Pressure Vessel Code Section XI, Subsection IWL.15 7 These examinations typically consist

of a visual examination of accessible hardware; measurement of tendon end anchorage force;

measurement of prestressing wire (or strand) strength and ductility; and analysis of sheathing filler

for water content, concentration of corrosive ions (i.e., chlorides, sulfides, and nitrates), and

reserve alkalinity. The visual assessment considers cracks in the bearing zone concrete; cracking,

corrosion, and deformation of load bearing metallic hardware; the presence of water within the end

caps or ducts; and the degree of sheathing filler coverage as well as sheathing filler leakage.

Currently under development by the American Concrete Institute (ACI) is a guideline for

use in evaluation of existing nuclear safety-related concrete structures, other than the

containment. 2 30 Contained in the guideline is information related to determiination of critical

structures for evaluation, potential degradation mechanisms, inspection techniques and

frequencies, and evaluation of results. Two evaluation methods are proposed: selective and

periodic. The selective evaluation method is used to investigate a specific structure or subelement

to provide existing materials or condition data, input for structural reanalysis or design

modification, or some other specific purpose. Generally the desired end result is predefined for

this method. The periodic evaluation method is used to demonstrate satisfactory performance,

identify the presence and activity of age-related degradation, or provide data and information to

support continued service (i.e., aging management program). Desired end results from this

method are generally not predefined. Evaluation criteria are provided in terms of acceptance

without further evaluation, acceptance after review (i.e., additional inspection and testing to

identify cause, activity, and effect of deterioration), and further evaluations required (i.e., more

extensive application of testing and analytical methods to assess current capabilities and develop a

remedial measure program, when required). Also included in the guideline are suggested periodic

evaluation intervals (i.e., generally at 5- or 10-year intervals, but shortened if deterioration has

occurred), qualification requirements of the evaluation team, and considerations for damage

mitigation (i.e., repair, monitor at increased frequency, or replacement).

3.2.3 Commentary on Condition Assessments

Condition assessment methodologies have been used effectively to determine the

performance characteristics and the cause and extent of observed distress in reinforced concrete

structures. The common approach is to start with a general visual examination of accessible

concrete surfaces. These examinations provide a rapid and effective means for identifying areas of

distress because most of the degradation mechanisms of concern for reinforced concrete structures

manifest themselves in the cover concrete in the form of cracks, spalls, or stains. For structures

that are essentially inaccessible for visual examinations, either the surfaces must be exposed for

examination, or an indirect approach must be used (e.g., evaluation of the environment adjacent to

the structure for its potential to cause degradation). Identified suspect areas are then investigated in

more detail through nondestructive, destructive, or a combination of these test methods. Best

results are generally provided through application of a combination of testing methods. Although

much has been published on the basic methodologies and mechanics of conducting condition

surveys on reinforced concrete structures, only limited information is available on classifying or

rating the damage state. Where results have been presented, they generally were "application

specific" because of the uniqueness of each concrete structure and the environment to which it is

subjected. Some general information has been provided in which observed deterioration in the

form of cracks, joint deficiencies, surface damage, changes in member shape, and texture features

have been ranked through an appearance parameter (e.g., longitudinal cracks are rated through

width measurements). 58 Condition assessment programs that address the unbonded tendon

systems in NPP post-tensioned concrete containments generally have been shown to be

effective. 155 With the exception of the cursory visual examination conducted in conjunction with

101 NUREG/CR-6424
periodic leak-rate tests, no specific condition assessment requirements exist for the NPP reinforced

concrete containments or other safety-related concrete structures. Due to the importance of

condition assessments in effectively managing aging of structures and the likelihood for incidences

of degradation to increase as the structures age, it seems prudent to periodically assess the

condition of these structures. One of the conclusions of a report in which assessments of the inservice condition of structures at several NPPs were performed was that these structures need to be

periodically inspected and maintained. 154 The importance of periodic inspections, evaluations, and

maintenance in managing the aging of NPP concrete structures was also noted in Ref. 27 where

results of observations made at several U.S. and foreign NPPs were presented. ACI 349 (Ref.

230) has drafted guidelines for evaluation of safety-related concrete structures relating damage state

(or current condition) to a recommended action defined through acceptance limits. However,

further development of damage assessment criteria is required for concrete structures to relate

parameters such as damage state to the environment (e.g., extent of remedial action based on a

relationship between crack width and chloride ion content of its environment).

3.3 REMEDIAL MEASURES

Reinforced concrete structures almost from the time of construction will start to deteriorate

in one form or another due to exposure to the environment (e.g., temperature, moisture, cyclic

loadings, etc.). 195 The rate of deterioration is dependent on the component's structural design,

materials selection, quality of construction, curing, and aggressiveness of its environmental

exposure. Figure 3.3 presents a relationship between concrete performance, time, and repair.

Termination of a component's service life occurs when it no longer can meet its functional and

performance requirements. As noted by the deterioration of many roadways and bridges in the

U.S.,220 this often occurs prior to achieving the desired service life.

It was noted in the previous section that ISI techniques are available that can indicate the

occurrence and extent of age- or environmental-stressor-related deterioration. Periodic application

of these techniques as part of a condition assessment program can monitor the progress of

deterioration. Results obtained from these programs can be used to develop and implement a

remedial action prior to the structure achieving an unacceptable level of performance. Depending

on the degree of deterioration and the residual strength of the structure, the function of a remedial

measures activity may be structural, protective, cosmetic, or any combination of these three

functions. Basic components of a remedial measures program include diagnosis (damage

evaluation), prognosis (can repair be made and is it economical), scheduling (priority

assignments), method selection (depends on nature of distress, adaptability of proposed method,

environment, and costs), preparation (function of extent and type of distress), and application.23 1

Figure 3.4 indicates the basic steps of a typical repair strategy.232

3.3.1 Initial Repair Considerations

The first step in any repair activity is a thorough assessment of the damaged structure or

component including evaluation of the (1) cause of deterioration, (2) extent of deterioration, and

(3) effect of deterioration on the functional and performance requirements of the structure or

component. Basic elements of the assessment include (1) preplanning and accumulation of

background data (e.g., age, previous condition surveys, design documents, as-built drawings,

materials data sheets, etc.), (2) visual examination, (3) in situ and laboratory testing, and

(4) evaluation of collated survey data and determination of cause(s) of deterioration. From this

information a remedial measures strategy is developed based on consequence of damage (e.g.,

affect of degradation on structural safety), time requirements for implementation (e.g., immediate

or future safety concern), economic aspects (e.g., partial or complete repair), and residual service

NUREG/CR-6424 102
life requirements (e.g., desired residual service life will influence action taken). 233 Basic remedial

measures options include (1) no active intervention; (2) carry out repairs to restore deteriorated or

damaged parts of structure to a satisfactory condition; (3) if safety margins are presently

acceptable, take action to prevent deterioration from getting worse; and (4) demolish and rebuild all

or part of structure. Quite often options (2) and (3) are considered jointly.

3.3.2 Typical Remedial Measures Techniques for Reinforced Concrete Structures

Application of the basic remedial measures strategy includes the repair of damaged concrete

and mitigation of the cause of deterioration.

3.3.2.1 Techniques and materials for repair of damaged concrete

Deterioration of reinforced concrete structures generally will result in cracking, spalling, or

delamination of the cover concrete. Of these, surveys of general civil engineering construction234

and NPP structures 27,

56 ,235 indicated that cracking was by far the most frequently occurring

problem. Seepage of water through construction joints or cracking was also reported in the NPP

surveys as well as the presence of honeycombs and voids.

Crack Repairs Cracking in concrete structures can be expected to occur for a number of

reasons, including plastic and drying shrinkage,, thermal effects, fatigue, reactive aggregates, and

excessive loads, Table 3.4.56 All concrete structures will have cracks that can be classified into

two categories: microcracks and macrocracks. Microcracks form within the cement paste adjacent

to the aggregate particles and are discontinuous, very narrow, and require no repair action. The

microcracks are important from the standpoint that under increased loadings they become wider

and propagate, and can eventually reach a size (i.e., macrocracks) sufficient to deteriorate the

concrete, accelerate corrosion of embedded steel, or produce leakage. Macrocracks are important

to service life.

Crack Width and Durability

Two theories have been proposed regarding how cracks reduce the service life of

reinforced concrete structures. 236 The first is that they permit access of chloride ions, moisture,

and oxygen to the steel, not only accelerating the onset of corrosion, but providing space for the

deposition of corrosion products. The second theory agrees that cracks may accelerate the onset of

corrosion at localized regions on the steel, but after a few years of service there is little difference

between cracked and uncracked concrete. Factors that may influence corrosion occurrence include

crack arrangement, width, depth, shape orientation with respect to steel, intensity, origin; type of

structure; and service environment.237

Both the crack geometry (i.e., width and depth) and environmental exposure condition are

important to the durability of reinforced concrete structures. A wide surface crack that quickly

narrows with depth may not be as detrimental as a narrower surface crack that penetrates to the

steel reinforcement. For structures that are located in controlled environments, cracks are primarily

an aesthetic concern. Researchers have provided a number of relationships between maximum

permissible crack width and exposure condition. Table 3.5 presents a summary listing of

permissible crack widths (and environmental factors) that have been proposed to prevent

corrosion.56 Larger cracks widths increase the probability of corrosion,2 38 however, as noted in

Ref. 224, the values of crack width are not always reliable indicators of corrosion and the

deterioration to be expected. In general, Ref. 56 provides the following guidance: for severe

exposure to deicing chemicals or for water tightness, widths < 0.1 mm; for normal exterior

103 NUREG/CR-6424
exposures or interior exposures subjected to high humidities, widths < 0.2 mm; for internal

protected structures, widths • 0.3 mm; and for structures containing chemicals or fluids that must

remain leaktight, widths • 0.05 mm. These limits are also consistent with those recommended by

the Commission of European Communities.52

Selection of Crack Repair Technique

After identifying that the crack is of sufficient size to require repair, it is important to

determine if the crack is active or dormant. Active cracks are those for which the mechanism

causing cracking is still at work, whereas dormant cracks are those caused by a condition that is

not expected to recur. A basic procedure for use in identifying the cause of cracks has been

developed:239

Step 1 Examine the appearance and the depth of the cracking to establish the basic

nature of the occurrence, such as pattern or individual cracks, depth of cracks,

open or closed cracks, and extent of cracking.

Step 2 Determine when the cracking occurred. This step will require talking with the

individuals who operate the structure and possibly those involved in the

construction.

Step 3 Determine if the cracks are active or dormant. This step may require

monitoring the cracks for a period of time to determine if crack movement is

taking place. Also, attempt to determine if the crack movement detected is

growth or simply cyclical opening and closing such as caused by thermal

expansion. Cracks that are moving but not growing should be treated as active

cracks.

Step 4 Determine the degree of restraint. This step will require a thorough

examination of the structure and the construction drawings, if available. Both

internal restraint (caused by reinforcing steel, embedded items, etc.) and

external restraint caused by other elements may be present. A checklist for

determining the cause of cracking is presented below. Using the checklist,

eliminate as many potential causes as possible. If more than one potential

cause remains, the final determination may require a laboratory analysis of

concrete samples or a detailed stress analysis.

0 Check for major errors in design;

* Check easily identifiable causes, such as

corrosion of reinforcement,

accidental or impact loading,

poor design detailing, and

foundation movement; and

* Check other possible causes, such as

incidents during construction,

shrinkage induced stresses,

volume changes,

chemical reactions,

moisture changes, and

freezing and thawing.

Having established the cause of cracking, several questions should be addressed:

NUREG/CR-6424 104
* Is repair necessary? Repair of cracking caused by expansion products of internal

chemical reactions may not be necessary. " Should repair be treated as spalling rather than cracking? If the damage is such that

loss of concrete mass is probable, treatment of the cracks may not be adequate. For

example, cracking due to corrosion of embedded metal or freezing and thawing

would be better treated by removal and replacement of concrete than by one of the

crack repair methods.

* Is it necessary that the condition causing the crack be corrected? Is doing so

economically feasible?

* What will be the future movement of the crack?

* Is strengthening across the crack required? * What is the moisture environment of the crack?

With these questions answered, a repair technique can be selected. Figures 3.5 and 3.6 present

repair methodologies for dormant and active cracks, respectively.2 39 Typical properties of epoxy

and polyester chemical grouts used to repair concrete are provided in Table 3.6.240 General

guidance on crack repair options including perceived durability is presented in Table 3.7.56

Detailed descriptions of techniques available for repair of dormant or active cracks in reinforced

concrete are available elsewhere. 13,56,223,239,241 Final selection of a repair technique should take

into account durability, life-cycle costs, and labor skill and equipment requirements.

Spall Repairs Spalls can occur due to impacts, corrosion of embedded metals, erosion,

or problems such as alkali-aggregate reactions, freeze-thaw, and fire exposure. Surface

preparation is critical to a successful spall repair. The concrete substrate must be sound, and the

exposed surface dry and free of grease, oil, and loose particles. Suitable techniques for surface

preparation include use of small chipping hammers (followed by abrasive blasting, and removal of

dust and chips by compressed air) and high-pressure water blasting. If steel reinforcement is

exposed during the removal of degraded material, the excavation should be extended so that the

steel will be enclosed in the patch material. If the steel reinforcement is corroded, the corrosion

products should be removed and the steel coated with a barrier material such as epoxy resin, or a

high electrically-resistant patch material utilized.56 Corrosion-inhibiting admixtures (e.g., calcium

nitrate and organic-based) can also be included in the concrete patch material* Shallow spalls

(• 20 mm) are generally repaired using portland cement-based mortar materials. Polymer

concretes containing epoxies or methyl methacrylates have also been successfully utilized. Deep

spalls are treated in a similar manner to shallow spalls except coarse aggregate is added to the repair

material. To ensure good durability of the repair it is important that the repair material have

mechanical and physical properties similar to the in-place concrete and that it is properly

consolidated and cured. For mass concrete requiring extensive replacement of material, the repair

patch may be built up in two or more layers to prevent excessive heat build-up due to cement

hydration. Fly ash can be used as a partial replacement for the cement to reduce the maximum

temperature build-up of the repair mass. Also, if the spall to be repaired is located in a vertical or

overhead surface, special precautions are required because of the increased difficulty in application

of material due to gravity effects. Generally, dry packing is used in which a very harsh mix (i.e.,

dry) is applied and compacted using a blunt instrument.242 Typical properties of materials

commonly used for spall repairs are presented in Table 3.8.243 General guidance on spall repair

options including perceived durability is provided in Table 3.9.56 Additional information on spall

repair techniques is available elsewhere. 13,56,239,242

* Additional information specifically addressing remedial measures for corrosion damaged concrete is presented in the

next section.

105 NUREG/CR-6424
Delamination Repairs Delaminations are horizontal voids in concrete domes or slabs

that commonly occur due to corrosion of steel reinforcement or separation of concrete layers that

do not develop adequate bond. Spalls will occur if the delaminations are not repaired.

Delaminations can be repaired by removal and replacement of the delaminated concrete using

procedures similar to those for repair of spalls. In areas where removal of concrete is not required,

the delaminated area can be repaired by injection of an epoxy resin. Several holes are made into the

delamination using either a drill with a vacuum attachment to remove the fines developed or by

coring. If water is used in either process, the concrete should be permitted to dry prior to epoxy

injection under low pressure or by gravity feed. Dowell pins can be used to enhance shear

transfer. Additional information on delamination repair techniques is available

elsewhere. 13,

56,

2 39,

24 1

Water Seepage Repair Water seepage through construction joints and cracks may

result in leaching of the concrete, entry of aggressive environments into the concrete matrix, or

unacceptable flow of fluids either into or out of a facility. Long-term reactions that require the

presence of moisture such as efflorescence, sulfate attack, or alkali-aggregate reactions also may

initiate as seepage into the concrete occurs. Implementation of a repair activity, therefore, can

prevent possible future deterioration and the unacceptable migration of fluids either into or out of

the facility.

A properly implemented repair procedure first will identify the source and then repair the

path. Chemical grouting using silicate, acrylamide, lignin, or resin (e.g., epoxy, polyester, and

urethane) systems is the most effective repair technique when moisture is present. The chemical

grouts consist of solutions of two or more chemicals that react to form a gel or solid precipitate, as

opposed to cement or clay grouts that consist of suspensions of solid particles in a fluid. The

reaction of the chemical grout, that may be purely chemical or physiochemical, produces a decrease

in fluidity and a tendency to solidify and form occlusions in channels or fill voids in the

material. 24 1 Reaction of the chemical grout can be in the form of either soft flexible, semirigid, or

rigid gels. When the seepage is intermittent and the path through the concrete periodically dries, it

can be injected with epoxy resins, or water can be incorporated into a urethane injection system to

promote expansion and curing to form a flexible foam material.

Honeycomb and Void Repair Nonvisible voids such as rock pockets, honeycomb, or

excessive porosity can be repaired by drilling small diameter holes to intercept the voids;

determining the extent and configuration of the void system by injection of compressed air or water

into the void system, or by visual inspection using a borescope; and, depending on the magnitude

of the delamination, injecting either epoxy resin, expansive cement grout or mortar, or epoxyceramic foam. Proper injection of the cement grouts requires prewetting of the substratum with

excess water removed prior to injection.

3.3.2.2 Remedial measure techniques for corrosion-damaged concrete

Corrosion of steel reinforcement is by far the greatest threat to the durability of reinforced

concrete structures. A high percentage of the corrosion damage that has occurred to general civil

engineering concrete structures has been the result of insufficient planning, incorrect estimation of

environmental actions, and bad workmanship. 195 As a consequence, many of these structures

have had to be repaired, or under extreme conditions, removed from service because of corrosion

of steel reinforcement. Fortunately, incidences of corrosion of NPP concrete structures have been

limited,56 probably due to more detailed considerations associated with material selection,

construction workmanship, and effective use of quality assurance/quality control procedures.

However, the history of these structures is somewhat limited and as they age, incidences of

corrosion can be expected to increase.

NUREG/CR-6424 106
Preventative Measures for In-Service Concrete

Corrosion of steel reinforcement can be expected if the concrete is sufficiently moist and

either carbonation or chlorides have reached the surface of the steel. The most cost effective

approach to treating corrosion of reinforced concrete structures obviously is to provide preventative

protection, or if necessary, intervention early in the process. The application of barriers in the

form of sealers, coatings, or membranes to exposed surfaces provides one commonly used

measure of intervention.

Sealers are liquids applied to the surface of hardened concrete to either prevent or decrease

the penetration of liquid or gaseous media (e.g., water, carbon dioxide, or aggressive

chemicals). 244 A number of materials have been applied to concrete (e.g., boiled linseed oil,

sodium or potassium silicates, stearates, silicones, asphaltic emulsion, and cementitious

formulations). Five categories of sealers have been found to be effective in bridge deck

applications - polyurethanes, methyl mathacrylates, certain epoxy formulations, relatively low

molecular weight siloxane oligomers, and silanes. 245 Of these, the silanes and oligomers are most

commonly used.* Newer formulations of these materials penetrate the concrete surface to some

degree, but still permit the transmission of air or water vapor. Therefore, coatings also have to be

applied to protect against carbonation.

Coatings and membranes differ from sealers in that they are applied in some thickness,

generally measured in hundredths of a millimeter, and generally do not penetrate the concrete.

Coating types include epoxy resins, polyester resins, acrylics, vinyls, polyurethanes, and

cementitious materials. Membrane types include liquid applied acrylics, urethanes, neoprenes,

vinyls, rubberized asphalts, silicones, and preformed membranes such as rubberized asphalts,

neoprenes, and butyl rubbers, hypalons, vinyls, and ethylene propylene diene. Characteristics,

advantages, and disadvantages of these materials are provided in Ref. 56.

Selection of sealer, membrane, or coating materials involves a number of factors (e.g.,

compatibility with new or old concrete, compatibility with joint sealant materials, crack-bridging

ability, effective service life, weatherabiliy, etc.). Water absorption and vapor transmission

properties for selected coating materials are presented in Table 3.10.56 An indication of the relative

performance of several coating systems is provided in Table 3.11.233 Guidelines for selection of

barrier systems for concrete are provided in Ref. 62. Surface preparation is extremely important in

the use of any sealer or coating material (i.e., cleanliness and moisture condition). Adhesion of

film forming coatings can be evaluated by ASTM D 3359,246 direct tension (elcometer), or by

direct peel. For membrane coatings, 25.4-mm wide strips can be cut in the membrane, clamped to

a force gage, and pulled at 180' to the surface.56

Remedial Measures for Active Corrosion

In situations where application of preventative measures may not be possible, or the

corrosion process has initiated, remedial measures are required. Basic remedial measures to

strengthen or repair reinforced concrete structures damaged by corrosion include (1) taking no

action, (2) replacement of damaged components, (3) stopping the corrosion process, and

(4) reducing the corrosion rate. 232 An example where no action might be taken would be in

situations where the structure may be nearing the end of its desired service life and an assessment

indicates that it can continue to meet its functional and performance requirements. Some local

* Polyurethanes may degrade under ultraviolet exposure and the methacrylate monomer is highly volatile.

107 NUREG/CR-6424
repairs and monitoring may be part of this strategy. When corrosion damage is localized on

exposed surfaces, replacement or partial reconstruction may provide the most feasible solution.

Often, however, some form of intervention is required to inhibit the corrosion process (i.e., reduce

the corrosion rate to negligible values or repassivate corroding areas).

Basically three processes are necessary for corrosion occurrence - anodic, cathodic, and

electrolytic. Repair activities are directed at halting one or more of these processes.* Basic

principles for halting the anodic and electrolytic processes are presented in Fig. 3.7.232 Brief

descriptions of each of these basic principles are provided below.** More detailed information on

each of these principles, including proper application, effectiveness, advantages and

disadvantages, and any limitations is provided elsewhere. 87,

232,

233,

247

Repassivation - Three basic methods for repassivation of steel reinforcement are

available: (1) use of alkaline cement or mortar, (2) electrochemical realkalization,

and (3) chloride extraction.

(1) Use of alkaline cementitious materials involves the placement of a cementbased mortar or concrete in the form of a patch (local) or layer over the entire

concrete surface (general) and relies on the migration of alkalis into the old

concrete. If cracks or spalls are present, loose material should be removed

and loose rust cleaned from steel. The depth of placement of new material

should be greater than the estimated depth of carbonation during the remaining

desired service life. This method is not effective if carbonation has penetrated

greater than 20 mm below the depth of reinforcement or if steel depassivation

has been caused by chlorides. 232

(2) Electrochemical realkalization restores a high pH to the concrete by generation

of hydroxide ions at the steel and transport of alkaline material from the

electrolyte (e.g., one molar sodium carbonate) into the concrete by capillary

absorption, diffusion, and possibly by electro-osmosis. This method has

been shown in the field to arrest corrosion, but its long-term effectiveness is

questionable. Also, the method has been shown to be suitable for preventing

corrosion in structures where cover concrete has been carbonated, but it is

unknown if it can stop ongoing corrosion due to carbonation. Due to

evolution of hydrogen during the process, this method is not recommended

where prestressing steel is present or where steel-to-concrete bond is

important. Also, the potential for alkali-aggregate reactivity is increased by

the high concentration of sodium ions.233

(3) Chloride extraction removes chloride ions from the concrete in order to

achieve a residual chloride ion concentration low enough to stop the corrosion

process. A direct current field is applied through the concrete by means of an

external anode on the concrete surface. The chloride ions migrate to the

surface where they are captured in the electrolyte and removed when the

process is completed. This method has often been combined with

electrochemical realkalization and has the same basic precautions. 232,

233

Halting the cathodic process requires the total blockage of oxygen access to the steel reinforcement. Since this can not

generally be accomplished, this process will not be addressed.

** Reduction of the moisture content will not be discussed as it is essentially the same as described in the previous

section covering sealers, coatings, and membranes as preventative measures.

NUREG/CR-6424 108
" Steel Reinforcement Coating - Some repair systems require or include the

application of a physical barrier (e.g., epoxy) to protect the steel and provide

electrical resistance. The procedure requires the removal of concrete to a depth

below the steel, a clean steel surface free of rust, and sometimes, if the steel is

badly corroded, replacement of portions of the steel reinforcement. If the remainder

of the steel in the structure is uncoated, and the coating in the repaired area is

damaged and becomes corrosively active, a high rate of corrosion can occur. Also,

in chloride environments, the chlorides may penetrate the ends of the coating to

cause crevice corrosion where the steel has not been repassivated. Use of low

water-cement ratio cementitious coatings may be preferred in chloride-contaminated

concrete. 232

* Cathodic Protection - The corrosion process can be effectively halted, or its rate

decreased, through application of a small direct current to the steel reinforcement to

make it slightly cathodic relative to an externally applied anode at or near the

concrete surface. Two systems of cathodic protection systems are available:

(1) impressed current, and (2) sacrificial anode. Impressed current systems use a

direct current power supply (rectifier) to force current flow from a relatively inert

electrode (anode) through the concrete to the steel to be protected. In the sacrificial

systems, a metal that is more anodic (higher tendency to corrode) than the

embedded steel is used as the source of energy. Cathodic protection has been

successfully used for several decades and is best suited for applications where the

concrete is extensively contaminated and high chloride levels exist at the level of the

steel. Removal of contaminated concrete is not required. Special precautions are

required when the following are encountered: alkali-aggregate reactivity, lack of

steel reinforcement continuity, highly electrically-resistant concrete, epoxy coated

rebars, or galvanized steel. Also, generation of hydrogen at the cathode may

embrittle prestressing wires or strands and, since cathodic protection systems are

direct current, there is the potential for stray current corrosion in other

structures. 87,

2 32

3.3.3 Commentary on Remedial Measures

Reviews of repair procedures for reinforced concrete structures have been conducted under

the Structural Aging Program from both the European and North American perspectives. 56 ,

233

Although a number of codes and standards have been developed for new construction, none are

presently available that specifically address repair of degraded structures. Activities are presently

underway, but this is expected to be a rather lengthy process. However, several documents are

available in the form of guidelines or recommended practices.

In the U.S., ACI Committee Reports 201.2223 and 546242 discuss concrete repair. The

ACI also has produced several documents used in educational seminars that are of use -

Troubleshooting Concrete Problems - And How to Prevent them in the Future248 and

Concrete Repair Basics.2 49 Both the U.S. Army Corps of Engineers and U.S. Bureau of

Reclamation have produced concrete repair manuals. 239,

24 1 The manual by the Corps of

Engineers provides a standard format for repair techniques and includes chapters on evaluation of

concrete structures, causes of distress and deterioration, selection of materials and methods for

repair or rehabilitation, maintenance of concrete, specialized repairs, and case histories.

Information on material applications and limitations is somewhat brief, however, and service life of

repairs is not discussed. The Corps of Engineers has recently developed a notebook250 in the form

of a computer data base that provides material data sheets on specific products.

109 NUREG/CR-6424
In Europe, the most widely developed regulations for repair of concrete have been prepared

by the German Committee on Reinforced Concrete. 251 The German guidelines address four major

areas: general regulations and basic design rules, design and performance, quality assurance and

execution, and technical delivery conditions and test regulations. Outside Germany, the Austrians

have revised their standards to define an "orderly" basis for the future repair of concrete structures

and a basis for the evaluation of existing structures. 252 Guidelines or recommended practices have

been produced by the Concrete Society,2 53 The Construction Industry Research and

Information Association,2 47 The United Kingdom Department of Transport,254 ,

25 5 Comite

Euro-International du Beton,2 56 and Fiddration Internationale de la Prdcontrainte.200 Most

of the European regulations address repair of corrosion-damaged concrete, indicating the size of

the problem.

Although codes and standards have not been developed, sufficient documentation, such as

noted above, is available to develop an effective repair strategy for safety-related NPP concrete

structures. As structures in the general civil engineering community have aged and incidences of

degradation have increased, there is increasing awareness of potential problems and research being

conducted to address these problems can be transferred to NPP concrete structures. The basic

mechanisms leading to concrete degradation are generally understood. The importance of

recognizing the critical role played by the environment, at both the macro and micro level, is being

recognized, as well as the importance of workmanship during installation. Repair strategies are

becoming more global in that they are looking at the entire repair process as opposed to merely the

selection and application of a repair material based on information provided by the vendors. Longterm data on the effectiveness or durability of various remedial measures is required. Knowledge

of the durability of various repair materials is required to formulate the most effective repair.

NUREG/CR-6424 110
Table 3.1. Commonly used evaluation procedures to assess concrete

properties and physical condition.

a. Evaluation of properties of concrete.

EVALUATION

ALKALICAR O ATE URACTION

A PROCTEDURE I I U

0U U

CEEN CONTENTX U•C4

CHEMICALAND OMOS N •0IN 

PHYSICAL 0" U H

NPROPERTIES U U W 2-~ W rU _1 _

ACIDITY 10 1 - --- 1 

.AIRICONTENT -A

ALKALI-CARBONATE REACTION 0

ALKALI -SILICA REACTION

CEMENT CONTENTVO E3

CHEMICAL COMPOSITIONN

CHLORIDE CONTENT @0•

COMPRESSIVE STRENGTH 0

CONTAMINATED AGGREGATE 0

CONTAMINATED MIXING WATER

CORROS ION ENVIRONMENT0

CREEP T0

DENSITY 0 •

.ELONGATION

FROZEN COM PONEN17S _

MODULUS OF ELASTICITYII

MODULUS OF RUPTURE S0_

,MOISTURE CONTENT

'PERMEABILITY @

'PULL OUT STRENGTH0

nIJATITY OF AQUGREGATE - ---- -

RESISTANCE TO FREEZING

AND THAWING

SOUNDNESS000

SPL~ITTING TENSILE STRENGTH ___ 0

SULFPATE RESISTANCE

TENSILE STRENGTH--

WATER-CEMENT RATIO i

From ASCE 11-90, ASCE Guideline for Structural Condition Assessment of Existing Buildings,

American Society of Civil Engineers, New York, New York, August 1, 1991; adapted with

permission of ASCE, 1996.

III NUREG/CR-6424
Table 3.1. (Cont'd)

b. Evaluation of physical conditions of concrete.

EVALUATION O

CONDTIO AC = ON ••

CROSSSECT.PROP EDTE

PROCEDURE MFLRSEC

z 0

E onH•

PYSICALN

CON~TIO _~_ C4 

SPLLEINGCHANNBLS 9

CHEMTIICADERORTION - • --- 9- - - - •

DiSOrATion o9 CE

CODISNERTION 0.13- ý >

DHETORALDTERORTONNe

EOROSION OFSTELs

SCRALKING AAA 9-A 

SLALLINGTO 0 00 0A 0 0

DSTORATIFCTON 999

STFLRUSCTA ERFRMNE low 0.

EROIORiYOCON CEE0- @ _ 01

wCAIth pemsso ofAC196

NUREG/CR-6424 112
Table 3.2. Description of test methods for concrete.

Method Applications Principle of Operation User Expertise Advantages Limitations Standards

Acoustic Continuous Monitoring During crack growth or plastic Extensive knowledge Monitors structural Expensive test to run;

Emission of structure during deformation, rapid release of strain required to plan test response to applied can be used only when

service life to detect energy produces acoustic (sound) and to interpret load; capable of structure is loaded and

impending failure; waves that can be detected by results. detecting onset of when flaws are

monitoring performance sensors attached to surface of test failure; capable of growing; interpretation

of structure during proof object. locating source of of results requires an

testing. possible failure; expert; currently largely

equipment is portable confined to laboratory;

and easy to operate. further work required.

Acoustic Used to detect debonds, Surface of object is struck, with the Low level of expertise Portable equipment; Geometry and mass of

Impact delaminations, voids, frequency, through transmission required to use, but easy to perform; test object influence

and hairline cracks. time, and damping characteristics of experience needed for electronic device not results; poor

resulting sound giving indication of interpreting results, needed for qualitative discrimination; reference

presence of defects; equipment may results. standards required for

vary from simple hammer or drag electronic testing.

chain to sophisticated trailermounted electronic equipment.

Core Testing Direct determination of Drilled cylindrical core is removed Obtaining drilled core Most widely accepted Process of drilling and ASTM

concrete strength; from structure; tests may be is routine; moderate method to reliably analyzing cores is C 42

concrete evaluation of performed on core to determine level of expertise determine strength and expensive; coring

condition of aggregate, compressive and tensile strength, required to test and quality of in place damages structures and

cement, and other torsional properties, static modulus evaluate results. concrete. many may be required;

components. of elasticity, etc. analysis of cores is

time-consuming.

Cover Meters/ Cover meters measure Presence of steel in concrete affects Moderate; easy to Portable equipment, Difficult to interpret

Pachometers depth of reinforcement magnetic field of probe; closer probe operate; training good results if concrete results if concrete is

cover in concrete; is to steel, the greater the effect. needed to interpret is lightly reinforced, heavily reinforced or if

pachometers measure results. wire mesh is present.

cover and size of

reinforcement, and

locations of

delaminations.

From ASCE 11-90, ASCE Guideline for Structural Condition Assessment of Existing Buildings, American Society of Civil Engineers, New York,

New York, August 1, 1991; adapted with permission of ASCE, 1996.

Z-
Table 3.2. (cont'd)

0

0~'

-'S

I-.

-IS

Method Applications Principle of Operation User Expertise Advantages Limitations Standards

Electrical Determining condition Electrical potential of steel Moderate. User must Portable equipment; Information on rate of ASTM

Potential of steel rebars in reinforcement indicates probability be able to recognize field measurements corrosion not provided;

Measurements concrete or masonry. of corrosion. problems. readily made; appears to access to rebars required. C 876

give reliable

information.

Electrical Determination of Determination of moisture content High level of Equipment is automated Equipment very

Resistance moisture content of of concrete is based on principle that expertise required to and easy to use. expensive and requires

Measurements concrete. the dielectric properties of concrete interpret results; high frequency

change with changes in moisture equipment is easy to specialized applications,

content. use. dielectric properties also

depend on salt content

and temperature of

specimen that poses

problems in

interpretation of results.

Fiber Optics To view portions of a Fiber optics probe consisting of Equipment is easy to Gives clear high- Equipment expensive;

structure that are flexible optical fibers, lens, and handle and operate. resolution images of limited application to

inaccessible to the eye. illuminating system is inserted into remote objects. concrete members;

crack or drilled hole in concrete; many bore holes

eyepiece is used to view interior to required to give adequate

look for flaws such as cracks, voids, access.

or aggregate debonds; commonly

used to look into areas where cores

have been removed or bore holes

have been drilled.

Infrared Test Detection of internal Flaws detected by using selective High level of Has potential for Method still in

flaws, crack growth, infrared frequencies to detect various expertise required to becoming a relatively developmental phase,

delamination, and passive heat patterns that can be interpret results. inexpensive and accurate requires dry surface and

internal voids; currently identified as belonging to certain method for detecting bright sunshine that

used primarily in defects. concrete defects, limits application.

laboratory.

From ASCE 11-90, ASCE Guideline for Structural Condition Assessment of Existing Buildings, American Society of Civil Engineers, New York,

New York, August 1, 1991; adapted with permission of ASCE, 1996.
Table 3.2. (cont'd)

L-n

Z"

Method Applications Principle of Operation User Expertise Advantages Limitations Standards

Load Testing Determine performance Test load is applied to structure in a High level of Provides highly reliable Very expensive and ACI 437R

of structure under manner that will simulate the load expertise required to prediction of structure's time-consuming;

simulation of actual pattern under design conditions, formulate the test ability to perform testing may cause

loading conditions. programs and to satisfactorily under limited or even

evaluate the results. expected loading permanent damage to

conditions. the structure or some of

its elements.

Nuclear Estimation of moisture Moisture content in concrete Must be operated by Portable moisture Equipment very

Moisture Meter content of hardened determined based on the principle trained and licensed estimates can be made sophisticated and

concrete. that materials (such as water) personnel. of in-place concrete. expensive; NRC license

decrease the speed of fast neutrons in required to operate;

accordance with the amount of moisture gradients in

hydrogen produced in test specimen. specimen may give

erroneous results.

Petrographic Used to determine a Used in conjunction with coring; Considerable skill Provides very detailed Qualified petrographer ASTM

Analysis variety of properties of chemical and physical analysis of required to perform and reliable required; relatively C 856

concrete cores removed core is performed by qualified and analyze test information. expensive.

from structure; some of petrographer. results.

these include (1) denseness of cement,

(2) homogeneity of

concrete, (3) location of

cracks, (4) air content,

and (5) proportions of

aggregate, cement, and

air voids.

Pullout Testing Estimation of Measure the force required to pull Low level of expertise Only NDE method that Pullout devices must be ASTM

Cast in-place compressive and tensile out the steel rod with enlarged head required; an be used by directly measures in- inserted during C 900

strengths of concrete. cast in concrete, pullout forces field concrete testers place strength of construction; cone of

produce tensile and shear stresses in and inspectors, concrete; appears to concrete may be pulled ACI 503R

concrete. give good prediction of out, necessitating minor

concrete strength. repairs.

From ASCE 11-90, ASCE Guideline for Structural Condition Assessment of Existing Buildings, American Society of Civil Engineers, New York,

New York, August 1, 1991; adapted with permission of ASCE, 1996.
Table 3.2. (cont'd)

01%

Method Applications Principle of Operation User Expertise Advantages Limitations Standards

Pull-off Estimation of Circular steel probe is bonded to Highly skilled Simple and Standard test procedure

Testing compressive strength of concrete. Tensile force is applied operator is not inexpensive; can be not yet available.

existing concrete. using portable mechanical system required. conducted on horizontal Concrete must be

until concrete fails. Compressive and vertical surfaces. repaired at test

strength can be estimated using locations.

calibration charts.

Radar Detection of substratum Uses transmitted electromagnetic High level of More time- and cost- Equipment is

voids, impulse signals for void detection, expertise required to efficient and less expensive; reliability of

operate equipment and destructive than "guess void detection greatly

interpret results. and drill" techniques, reduced if reinforcement

present; procedure still

under development.

Radiographics, X-ray-- density and Based on principle that the rate of Use of gamma- Internal defects can be X-ray has limited field

X-ray and internal structure of absorption of X-rays or gamma-rays producing isotopes is detected; applicable to application because

Gamma-ray concrete; location of is affected by density and thickness closely controlled by variety of materials; equipment is heavy and

reinforcement. Gamma- of test specimen; X-rays or gamma- NRC; gamma permanent record on costly; X-ray and

ray - location, size, and rays are emitted from source, equipment must be film; gamma-ray gamma-ray sources

condition of rebars; penetrate the specimen, exit on operated by licensed equipment easily harmful to organic

voids in concrete; opposite side, and are recorded on inspectors. portable. tissue; requires access to

density and thickness. film. both sides of specimen.

Schmidt Compares quality of Spring-driven mass strikes surface of Simple to operate; can Equipment is light- Results affected by ASTM

Rebound concrete from different concrete and rebound distance is be readily operated by weight, simple to condition of concrete C 805

Hammer areas of specimen; given in R values; surface hardness field personnel. operate, and surface; does not give

estimates of concrete is measured and strength estimated inexpensive; large precise prediction of

strength based on from calibration curves provided by amount of data can be strength; estimates of

calibration curves with hammer manufacturer. quickly obtained; good strength should be used

limited accuracy. for determining with great care; frequent

uniformity of concrete calibration of equipment

and areas of potentially required.

low strength.

From ASCE 11-90, ASCE Guideline for Structural Condition Assessment of Existing Buildings, American Society of Civil Engineers, New York,

New York, August 1, 1991; adapted with permission of ASCE, 1996.
Table 3.2. (cont'd)

Method Applications Principle of Operation User Expertise Advantages Limitations Standards

Ultrasonic Gives estimates of Operates on principal that High level of Equipment relatively Good coupling between ASTM

Pulse Velocity compressive strength, vibrational wave propagation is expertise required to inexpensive and easy to transducer and test C 597

uniformity, and quality affected by quality of concrete; pulse interpret results. operate; accurate substrate critical;

of concrete; internal waves are induced in materials and assessment of strength interpretation of results

discontinuities can be those reflected back are detected. and quality; estimates of can be difficult; density

located and their size concrete strength to and amount of aggregate

estimated; most widely + 20%. may affect results;

used vibrational method calibration standards

for field use. required.

Visual Evaluation of the Visual examination with or without Experience required in Generally low-cost; Trained evaluation ACI

Examination surface condition of optical aids, measurement tools, order to determine rapid results except for required; primary 201.1R

concrete (finish, rough- photographic records, or other low- what to look for, what surveying method; evaluation confined to

ness, scratches, cracks, cost tools, differential movement measurements to take, evaluation of both surface of structure. ASTM

color); determining determined over long periods of time and what follow-up surface and interior of C 823

deficiencies in joints; with surveying methods. testing to specify. concrete member

and determining differen- possible.

tial movements of

structures.

Windsor Probe Estimates of Probes are gun driven into concrete Simple to operate; can Equipment is simple May not yield accurate ASTM

compressive strength, specimen; depth of penetration be readily operated in durable, and requires estimates of concrete C 803

uniformity, and quality converted to estimates of concrete the field with little little maintenance, strength; interpretation

of concrete. strength by using calibration curves training, useful in assessing the of results depends on

provided by manufacturer. quality and relative correlation curves.

strength of concrete;

does relatively little

damage to specimen.

From ASCE 11-90, ASCE Guideline for Structural Condition Assessment of Existing Buildings, American Society of Civil Engineers, New York,

New York, August 1, 1991; adapted with permission of ASCE, 1996.

C-1

N0

T0
Table 3.3. Recommended testing methods to assess concrete degradation.

DEGRADATION TESTING METHODS

FACTOR SYMPTOM TO IDENTIFY TO ASSESS EXTENT OF

OCCURRENCE DAMAGE*

Alkali-aggregate Cracking 1. Core/petrography 1. Visual and petrography

reactivity (concrete) Expansion 2. SHRP Rapid Test 2. Pulse velocity

3. Impact echo

4. Pulse echo

5. Modal analysis

Sulfate attack Cracking 1. Core/petrography 1. Visual and petrography

(concrete) Expansion 2. Core/chemical 2. Pulse velocity

3. Impact echo

4. Pulse echo

5. Modal analysis

Efflorescence and Surface deposits 1. Visual 1. Visual and petrography

leaching (concrete) of efflorescence 2. Core/petrography

3. Sample/X-ray diffraction

Bases/acids/salt Disintegration 1. Core/petrography 1. Visual and petrography

crystalization and loss of paste 2. Chemical analysis

(concrete)

Moisture changes Cracking 1. Visual I. Visual and petrography

(concrete) 2. Core/petrography 2. Infrared thermography

3. Pulse velocity

4. Impact echo

5. Pulse echo

6. Modal analysis

Freeze/thaw Scaling 1. Visual 1. Visual and petrography

(concrete) Spalling 2. Core/petrography 2. Pulse velocity

Cracking 3. Impact echo

4. Pulse echo

5. Modal analysis

Thermal exposure/ Spalling 1. Visual 1. Visual and petrography

cycling (concrete) Cracking 2. Core/petrography 2. Pulse velocity

Loss of strength 3. Impact echo

4. Pulse echo

5. Modal analysis

Irradiation (concrete) Spalling 1. Visual 1. Visual and petrography

Cracking 2. Core/petrography 2. Pulse velocity

Loss of strength 3. Impact echo

4. Pulse echo

* Methods are rated in order of choice based on simplicity of the method and practical experience in applying the method.

NUREG/CR-6424 118
Table 3.3. (cont'd)

DEGRADATION TESTING METHODS

FACTOR SYMPTON TO IDENTIFY TO ASSESS EXTENT OF

OCCURRENCE DAMAGE*

Abrasion/erosion/ Surface wear 1. Visual 1. Visual and petrography

cavitation (concrete)

Fatigue/vibration Microcracking 1. Visual 1. Visual and petrography

(concrete) Cracking 2. Core/petrography 2. Pulse velocity

Excessive deflection 3. Impact echo

4. Pulse echo

5. Modal analysis

Creep Cracking 1. Visual 1. Visual and petrography

(conventionally Excessive deflection 2. Modal analysis

reinforced concrete)

Corrosion Corrosion 1. Visual 1. Visual and petrography

(conventionally 2. Core/visual 2. Impact echo

reinforced concrete) 3. Electrical method 3. Pulse echo

4. Chemical method 4. Radiography

5. Air permeability

6. Nuclear

Cracking 1. Visual 1. Visual and petrography

Delaminaion 2. Core/petrography 2. Infrared thermography

3. Audio method 3. Audio method

4. Impact echo 4. Pulse velocity

5. Pulse echo 5. Impact echo

6. Pulse echo

Corosion/temperature Corrosion 1. Visual 1. Visual

/irradiation 2. Mechanical testing

(prestressing system) 3. Chemical analysis

Loss of force Lift-off test Lift-off test

* Methods are rated in order of choice based on simplicity of the method and practical experience in applying the method.

119 NUREG/CR-6424
Table 3.4. Causes of cracking.

Type of Crack

Cause Active I Dormant Comment

Accidental loading

Design error

(inadequate reinforcement)

Temperature stresses

(excessive expansion due

to elevated temperature

and inadequate expansion

joints)

Corrosion of reinforcing steel

Foundation settlement

Alkali-aggregate reaction

Poor construction procedures

(inadequate curing,

formwork, etc.)

Design faults

• use of exposed rigidly

connected material to

concrete that has a much

different modulus of

expansion * stress concentrations

* faulty joint systems

Limit loading according to

current capacity and repair,

or redesign and repair as

indicated by the redesign.

It may be desirable to redesign

to include adequate expansion

joints.

Simple crack repair methods

should not be used as the steel

will continue to corrode and

crack the concrete.

Measurements must be made to

determine if the foundation is

still settling.

Concrete will continue to

deteriorate as long as moisture

is present. Crack repair

methods will be ineffective.

NOTE: This listing is intended to serve as a general guide only. It should be recognized

that there will be exceptions to all of the items listed.

Source: Corps of Engineers, Washington, D.C.

NUREG/CR-6424 120
Table 3.5. Permissible crack widths to prevent corrosion

of steel reinforcement.

Autho'Environment- factors, Permissible'iwidth, ~ini.

Dangerous crack width 1.0 to 2.0

Rengers Crack width allowing corrosion within 1/2 year 0.3

saline environment

Abeled Structures not exposed to chemical influences 0.3 to 0.4

Tremper Found no direct relation between crack width and corrosion

Boscard Structures exposed to a marine environment 0.4

de Bruyn Found no direct relation between crack width and corrosion

Engel and Unprotected structures (external) 0.2

Leeuwen

_______ Protected structures (internal) 0.3

Safe crack width up to 0.2

Voellmy Crack allowing slight corrosion 0.2 to 0.5

Dangerous crack width over 0.5

Indoor structures 0.25 to 0.35

Bertero Normal outdoor exposure 0.15 to 0.25

Exposure to seawater 0.025 to 0.15

Protected structures (interior) 0.3

Haas Exposed structures (exterior) 0.2

Fairly harmless crack width 0.1

Brice Harmful crack width 0.2

Very harmful crack width 0.3

For all structures under normal conditions 0.2

Salinger Structures exposed to humidity or to harmful chemical influences 0.1

Structures subjected to dead load plus half the live load for which 0.4

they are designed Wastlund Structures subject to dead load only 0.3

Exterior (outdoor) structures exposed to attack by seawater and 0.05 to 0.25

fumes

Efsen Exterior (outdoor) structures under normal conditions 0.15 to 0.25

Interior (indoor) structures 0.25 to 0.35

Ordinary structures 0.3

RUsch Structures subjected to the action of fumes and sea environment Structures subjected to the action of fumes and sea environment 0.2

121 NUREG/CR-6424
Table 3.6. Properties of an epoxy and polyester chemical grout.

~Pfoyekrty .Epoxy resin. sy~stelA. Plser esin system '

Tensile strength, MPa 35.2 55.8

Tensile modulus, MPa 1.17 x 103

Elongation, % 15.8 2.6

Flexural strength, MPa 60.0 84.8

Flexural modulus, MPa 1.45 x 103 4.2 x 103

Deflection, mm 13.5 _

Compressive strength, MPa 52 -

Compressive modulus, MPa 1.3 x 103

Deflection at yield, mm 4.6 -

Izod impact strength, Jim 55 -

Hardness at 25°C (Shore D) 79 38--40**

Water absorption, % 0.21 0.15

Shrinkage, % (volume) 0.001"** 6.0

* Typical values

* * BARCOL

Effective (after gel information)

Source: Chemical Grouting, Engineer Manual EM 1110-2-3504, U.S. Army Corps of Engineers,

Office of the Chief of Engineers, Washington, D.C., May 1973.

NUREG/CR-6424 122
Table 3.7. General guide to repair options for concrete cracking.

Description Repair Options Perceived Commentary durability

rating (1-5")

Dormant pattern or fine Judicious neglect 4 Only for fine cracks

cracking Autogenous healing 3 Only on new concrete

Penetrating sealers 2 Use penetrating sealer for

H20, Cl resistance

Coatings 3 Use coating for abrasion &

chemical resistance

HMWM or epoxy treatment 2 Topical application, bonds

cracks

Overlay or membrane 2 For severely cracked areas

Dormant isolated large Epoxy injection 1 Needs experienced

cracking applicator

Rout and seal 3 Requires maintenance

Flexible sealing 4 Requires maintenance

Drilling and Plugging 3

Grout injection or dry packing 4

Stitching

Additional reinforcing 5

Strengthening 4

3

Active cracks Penetrating sealer 3 Cracks less than 0.5 mm

Flexible sealing 3 Requires maintenance

Route and seal 3 Use for wide cracks

Install expansion joint 2 Expensive

Drilling and plugging 4 May cause new cracks

Stitching 4 May cause new cracks

Additional reinforcing 3 May cause new cracks

Seepage Eliminate moisture source I Usually not possible

Chemical grouting 2 Several applications may

be necessary

Coatings 4 May have continued

seepage

Hydraulic cement dry packing 4 May have continued

seepage

*Scale from 1 to 5, with 1 being most durable.

123 NUREG/CR-6424
C.'

41.

Table 3.8. Typical properties of rapid set patching materials by generic family.5 6

Approx. Approx. Compressive Abrasion Flexural Bond E Linear working time to o hrnkg time traffic strength loss strength strength (103 (10-6kage

(ime.) (mi.) (MPa) (g) (MPa) (MPa) MPa) per C)

@ 24 hr

MATERIAL @ 22°C @ 22°C @ 3hr @ 24 hr @ 24 hr @ 24 hr Dry Wet

PCC PCC

Inorganic I

PCC w/accelerator 120+ 300+ - 20 22 3.0 2.0 2.5 15-40 7-20 0.02-0.08

Magnesium phosphate 15 60 28 42 25 5.6 3.3 1.1 25-35 11-14 0.o10-0.15"

High alumina cement 15 60 35 46 20 4.2 2.8 2.6 25-35 7-20 0.02-0.08

Gypsum based 20 60 25 42 18 2.8 2.1 2.6 15-20 7-20 0.03-0.05

Organic

Failed Failed

Epoxy 30-60 90-200 15 55-80 0-1 16-21 in led in PCC 0.7-40 27-54 0.02-0.2

Methacrylate 20-40 60-120 50 55-65 10 14-21 Failed Failed 7-25 13-23 1.5-5.0 ____ ____ ____ __ ____ _ ____ __ _ _ ____ ____ ___in PCC in PCC_ _ _ _

Polyester-styrene 15-40 60-120 15 20-35 3 10-14 Failed Failed 7-35 32-54 0.3-3.0 _______ __________ ______ _______ _______in PCC in PCC ~~ 25 .-.

Failed

Urethane 5-45 30-90 3 - 15 3-35 3 10-27 in PCC 3.4 0.7-40 54-126 0.02-0.2

E = Modulus of elasticity in compression

a = Thermal coefficient of expansion *High exotherm

Source: H. Jerzak, "Unpublished Test Data," Transportation Laboratory, California Department of Transportation, Sacramento, California, 1988.
Table 3.9. General guide to repair options for concrete spalling.

Perceived Description Repair Options durability Commentary

rating (1-5*)

Shallow Spalling Portland cement grouts 3 Not good for acid attack

Polymer-modified grout 2 Different thermal coefficient

Coatings 4 Limited to shallow areas

Membranes 3 Acids-epoxy, methacrylate, butyl,

neoprene

Polymer grouts 2 Acids-use polyester grout

Deep Spalling Portland cement concrete 2 Inexpensive

Expansive cements 3 Unreliable expansion

Gypsum-based concrete 5 Do not use in moist environments

High alumina (modified) 3 Bonds best to dry concrete

Magnesium phosphate 2 Base concrete must be dry

Polymer-modified 2 Thermal stress can be high

Polymer patching materials 3 Less than 40 mm thickness

Polymer overlays 2 25 to 50 mm thickness

Latex-modified concrete 2 Greater than 30 mm thickness

overlays

Portland cement concrete 3 Use low water/cement ratio and

overlays high-range water reducer

Silica fume overlays 3 High strength

Pre-placed aggregate 2 Low shrinkage

Shortcrete 3 Good for large areas

*Scale from 1 to 5, with 1 being most durable.

125 NUREG/CR-6424
(7

Table 3.10. Water absorption and vapor transmission of selected coating materials.

7-day water absorption, % 7-day water vapor transmission, %

Application rate

Generic N o. Average Mean Range Average Mean Range range

type I sq ft/gal

Control 1 4.79 - - 3.53 - -

Acrylics 9 0.82 1.84 0.50-3.08 1.56 1.47 0.83-2.81 75-110

Cementitious 5 2.75 2.94 0.49-4.56 1.80 2.18 1.02-2.28 -

Epoxy 7 0.31 0.16 0.06-0.37 0.20 0.21 0.06-0.46 60-125

Hypalon 2 0.36 - 0.29-0.42 0.23 - 0.18-0.37 75-95

Neoprene 1 1.06 - - 0.37 - - 70

Polyurethane 16 0.55 0.20 0.04-3.79 0.60 0.54 0.15-1.54 70-175

Polyester 1 0.12 - 0.27 - 90

Silicone 1 1.76 - - 2.76 - - 80

I-O
Table 3.11. Relative performance of several coating systems for concrete materials.

Application* Protection*

Coating Damp Alkali Ease to Low Resist Resist Vapor Resist

Conditions Resistance Apply Hazard Ci- CO2 Transmission Rain

Film Forming

Epoxy resin 4 1 5 5 1 1 5 1

Coal tar epoxy 4 1 5 5 1 1 5 1

Polyurethane 4 1 5 5 1 1 5 1

Chlorinated rubber 2 1 1 5 1 1 5 1

Bituminous 2 3 1 3 3 3 3 1

Acrylic resin 2 3 1 1 3 1 1 1

Densifying

Silicate/flurosilicate 1 1 1 1 5 5 1 3

Cementitious 1 1 1 1 3 3 1 3

Non-film Forming

Silane/siloxane 2 2 1 5 1 5 1 1

Oil impregnation 4 5 1 1 3 5 1 4

Silicone 4 5 1 2 3 5 1 3

*I = very good, 5 = very poor.

9-.

C)

0~'

p.
tl

0'

Electrochemical Potentials

Carbona on Deptb1 Cover Depth Chloride sMoisture Content Conten Resistivity Rebar C°nditi°" lhndtamn

00

C Core Samples

I i

Permeability Porosity Absorption] Oxygen Diffusion Chloride Diffusion

Fig. 3.1 Techniques for assessing deterioration. Source: R. D. Browne, "Durability of Reinforced Concrete

Structures," Pacific Concrete Conference, Proceedings Vol. 3, Auckland, New Zealand, New Zealand Concrete

Society, November 1988; reprinted with permission from author.
Fig. 3.2 General structural assessment and evaluation procedure for existing buildings.

From ASCE 11-90, ASCE Guideline for Structural Condition Assessment of Existing Buildings,

American Society of Civil Engineers, New York, New York, August 1, 1991; adapted with

permission from ASCE, 1996.

129 NUREG/CR-6424
0 30 60 90 Life, years

a

0

5-

0u

0

7@

I.j _ J -- L - - Unacceptable Level

repair 1 repair 2 repair 3

Fig. 3.3 Effect of multiple repairs overthe life of a structure.

NUREG/CR-6424 130
I Condition Initial Assessment of the Structure of the

No

Decision on Strategy to be

Based on:

- Cause of damage - Degree of damage - Aggressivity of environment

- Consequences of damage - Structural aspects - Timing

- Economy

- Remaining period of use - Local availability of the

technique and materials

considered

I I Action j

DetIle einpu

Detailed design plus choice of materials

Execution including

quality control

I Assessment of

completed repair work

Definition of I maintenance requirements

Fig. 3.4 Steps to be taken in a repair process. Source: Technical Committee 124- SRC,

"Draft Recommendation for Repair Strategies for Concrete Structures Damaged by Reinforcement

Corrosion," pp. 415-436 in Materials and Structures 27(171), International Union of Testing and

Research Laboratories for Materials and Structures (RILEM), Cachan, France; adapted with

permission from RILEM.

131 NUREG/CR-6424
O'•i 

r~o •Cracks

Dormant

S Pattern Is'olated Cracks Cracks

SutgtennuoYes 4. Healngt.enint No

occurrence

Noa~nr water SvrNoe Water SereNnWar

Codto Conditiondiio

1. Routing and Sealing 2.PlmrImpregnation S

2. Judicious Neglectie. be n 3. Autogenous Healing • ]iMinor

4. Overlay

5. Polymer Impregnation 1. Additional Rein trcement + Epoxy Injection I 2.Autogenoue HeelIng

1. Epoxy Injection I 2. Routing and Sealing 1. Epoxy Injection 3. Grouting 2. Routing and Sealing I FI

4. Autogenous Healing 3. Grouting 1. poy njcton5. Judicious Neglect 4. Flexible Sealing

3. Autogenous Healing + Epoxy Injection 7. Drilling and Plugging Auigenou Healuing 4. Additional Reinforcemnent 8. Dry Pecking.DrligndPuig

Fig. 3.5 Selection of repair technique for dormant cracks. Source: Evaluation and Repair of Concrete Structures,

EM 1110-2-2003, U.S. Army Corps of Engineers, Washington, D.C., 1986.
Yes No Yes No

Occurrence ob Unbonded Overlay 1. Stitching 1 xible Sealing 2. Additional 2. Drilling and Plugging Reinforcement_

Fig. 3.6 Selection of repair technique for active cracks. Source: Evaluation and Repair of Concrete Structures, EM 1110-2-2003, U.S.

Army Corps of Engineers, Washington, D.C., 1986.
Aims Basic Principles Examples of Techniques

0'

-is

N.)

.15

I

I

I

I

I

I

I

Fig. 3.7 Principles of repair to stop corrosion.

Source: Technical Committee 124- SRC, "Draft Recommendation for Repair Strategies for Concrete Structures

Damaged by Reinforcement Corrosion," pp. 415-436 in Materials and Structures 27(171), International

Union of Testing and Research Laboratories for Materials and Structures (RILEM), Cachan, France;

adapted with permission from RILEM.
4. EVALUATION OF NPP REINFORCED CONCRETE STRUCTURES

The importance of reinforced concrete structures to the overall safety of nuclear power

plants (NPPs) has been established. In contrast to mechanical and electrical components, civil

structures are intended to have service lives on the order of 50 to 100 years or more, and current

codes of practice (e.g., Refs. 16 and 20) do not explicitly address aging. Structural systems in a

NPP are designed to be safe, serviceable, and durable. However, most structural systems are

designed for safety directly, but serviceability and durability issues are dealt with only indirectly, if

at all. The inherently conservative nature of design is intended to ensure the structure's

performance in service. Structural deterioration is not considered explicitly, and there is no

presumption in code development that periodic inspection, maintenance, and repair are carried out.

Current periodic inspections -only involve assessments of the post-tensioning systems and a general

visual inspection of accessible concrete surfaces in conjunction with a leak-rate test. Some utilities,

however, do much more.

Properly designed and maintained structures normally perform well over extended periods

of time. However, these structures are subject to a phenomenon known as aging, which refers to

time-dependent changes that may impact the ability of these structures to withstand various

demands from operation, the environment, and accident conditions. The time-dependent behavior

of these changes should be considered in the overall condition assessment of a structural system.

Failures can occur when excessive degradation takes place, frequently due to design or

construction errors or an unanticipated aggressive service environment. Such failures often are

related to serviceability rather than to safety. Structural systems and components are considered to

be passive (i.e., there is no change of state) in mitigating design-basis conditions. Recent

probabilistic risk assessments of NPPs confirm that structural systems are important to the overall

safe operation of a NPP.2 57 Structural components are more likely to be involved in commoncause failures, since structural failures may affect plant safety systems. Thus, aging in structural

components may also be significant in common-cause failures. Moreover, in comparison with

mechanical and electrical equipment, structural components are less readily inspectable.

Furthermore, it may be very difficult to access and repair structural components and systems

without major impact on the operation of the NPP. In fact, replacement of some structural

components (e.g., containment and basemat) may not be feasible.

Condition assessment and management of aging in NPP concrete structures requires a more

systematic approach than simple reliance on existing code margins of safety. 258 What is required

is the integration of structural component function, potential degradation mechanisms, and

appropriate control programs into a quantitative evaluation procedure. A methodology for

demonstrating the continued reliable and safe performance of these structures should include

(1) identification of structures important to public health and safety; (2) identification of

environmental stressors, aging mechanisms and their significance, and likely sites for occurrence;

and (3) a monitoring or in-service inspection (ISI) based methodology that includes criteria for

resolution of existing conditions. Basic background information for formulation of such a

methodology has been provided previously in this report. As each NPP is unique, only guidance

for development of a plant-specific evaluation program is provided in the balance of this chapter.

Utilization of results from the inspection methodology to estimate future performance and evaluate

the impact of in-service inspection/repair programs on structural reliability is provided in the next

chapter.

135 NUREG/CR-6424
4.1 SELECTION OF SAFETY-RELATED CONCRETE STRUCTURES

Category I (safety-related) NPP reinforced concrete structures were described in Chapter 2

and Table 2.1. Tables 2.2 and 2.3 listed primary reinforced concrete structures and their

subelement division for boiling-water reactor (BWR) and pressurized-water reactor (PWR) plants,

respectively. Because of the complexity of many of the Category I structures, their subelement

division was based on geometric differences, structural behavior/performance characteristics, and

environmental exposure. Selection of subelements of importance to structural aging for evaluation

can be based on an aging assessment methodology, use of probabilistic risk assessments, or a

combination of these two approaches.

4.1.1 Aging Assessment Methodology

An aging assessment methodology has been developed that can be used to rank structural

components with respect to aging considerations. 259 Basic components of the methodology,

determination of structural components important to aging assessments, and examples illustrating

application of the methodology are presented below.

4.1.1.1 Basic methodology

Basic components of the methodology are described below.

Subelement Importance (I) The performance, or importance, of the subelements of a

Category I structure is difficult to assess quantitatively. Factors related to structural behavior,

response to environmental effects, site-specific requirements, etc., must be considered.

Additionally, the physical boundaries of each subelement must be defined. A qualitative, or "relative," importance among subelements may be established, however, and associated

importance factors assigned. The primary purpose for these importance factors is to provide a

measure of the structural contribution of a particular subelement. For simplicity, assignment of

importance factors is on a 1 to 10 scale, with 10 being most important. Importance factors that

have been assigned to the various subelements of the reinforced concrete structures at typical BWR

and PWR plants are also provided in Tables 2.2 and 2.3, respectively.

Safety Significance (SS) The SS of a particular subelement is also of importance in

the selection of components for evaluation. Based on criteria provided in sources such as Parts 50

and 100 of 10 CFR,4 and Refs. 14 and 260, nine specific safety-significance functions have been

identified and include

* prevention of uncontrolled liquid or airborne radiation release,

* radiation attenuation and shielding,

* structural support for nuclear steam supply system and containment internal equipment,

" structural support for redundant safety-related equipment and components,

* structural support for ultimate heat sink equipment and components, " support for new/spent fuel pools and other pool loads,

• protection of safety-related equipment/components, * separation or "communication" function, and

* failure could damage safety-related component.

A system has been developed to assign a ranking to a subelement in terms of the

significance of the safety function(s) that the subelement performs. The SS ranking system is

based on a 0 to 10 scale, with 10 being most significant. Table 4.1 lists general criteria for use in

NUREG/CR-6424 136
assigning a safety significance ranking to a subelement. Definitions of the safety-significance

functions and more detailed description of the ranking system is provided in Ref. 259.

Environmental Exposure (EE) Since the environment within which the subelement

operates is of great importance to its durability, the severity of the EE must also be included in the

selection process. Table 4.2 lists seven EE categories that have been developed. An EE rating has

been assigned to each category based on several considerations: (1) historical performance data,

(2) degree of exposure of the subelement, (3) accessibility of subelement's surfaces for

inspection, and (4) severity of exposure. EE ratings have been assigned to each of the seven

categories presented in Table 4.2. The numerical ratings are based on a 0 to 10 scale, with 10

being most severe.

Degradation Factor Considerations The effect of an environmental stressor on a

reinforced concrete subelement is initially related to alteration of the component's constituent

materials. As the severity of the degradation advances, the integrity, as well as the functional

characteristics of the subelement may be compromised. Four criteria are considered to be of

importance in assessing the significance of various degradation factors to which NPP reinforced

concrete structures can be subjected: (1) rate of deterioration, (2) capability for inspection and

early detection of degradation, (3) repairability of the subelement affected, and (4) ultimate impact

of the degradation factor(s). The relative significance of these four criteria in terms of degradation

factors of most significance to each of the primary materials utilized in the construction of

reinforced concrete NPP structures is presented in Table 4.3. The relative effects with respect to

the four criteria for each material system degradation factor were assigned on the basis of the

potential impact on the subelement's serviceability or integrity [i.e., high (creates a major limiting

condition or has a major adverse effect), moderate (creates a minor effect), or low (creates very

little or no effect)].

Assignment of numerical values (i.e., degradation factor grading values) to degradation

significance has been done through consideration of historical performance of reinforced concrete

NPP structures and the potential impact of the specific degradation factor. In developing

composite degradation factor grading values, a range of values was developed to accommodate

differing conditions at light-water reactor facilities associated with environmental or material

characteristics, presence of protective coatings, etc. The limits for the range of possible

degradation factor grading values are based on a maximum of 10, indicating potential major

significance, and a minimum of 1, indicating minor significance. The ranges of degradation factor

grading values presented in Table 4.3 were developed based on information such as provided in

Refs. 13 and 23-25.

The net effect of degradation in a reinforced concrete structure may be loss of monolithic

behavior, damage to the steel reinforcing system, loss of concrete section, or decreased structural

material performance. In order to prioritize the consequent net effects of degradation, relative

importance multiplier values were assigned to each of the net effects of degradation (i.e., numerical

value from 0 to 1, with 1 most critical). These values are provided in Table 4.4 and are based on a

comparative assessment of the potential impact of each of the net effects listed on the performance

of the particular subelement. Importance values were then used to prioritize the overall importance

of the subelement to the performance of its parent structure. Subelement importance values

assigned to pertinent structural subelements (e.g., shell, foundation, etc.) are presented in

Table 4.4. The assignment of values is based on a hierarchy of structural subelements. 16

Prioritization of consequent net effects is then developed by multiplying the particular subelement

relative importance value by the relative importance multiplier value. The resulting matrix is

provided in Table 4.4. The resulting prioritization of consequent net effects also is based on a 1 to

10 scale, with 10 being most important.

137 NUREG/CR-6424
These results were used to develop a generic range of degradation factor grading values for

specific degradation factor/subelement combinations. The procedure included

(1) identifying specific degradation factors of importance to each subelement and using

Table 4.3 to assign appropriate ranges of degradation factor grading values (e.g.,

corrosion of steel reinforcement indicates a range of 6 to 10),

(2) classifying each subelement into one of six classifications provided in Table 4.4

(e.g., beam or column) and determining "worst case" consequent net effect for

each subelement (e.g., corrosion of steel reinforcement of beam results in a value

of 7.0),

(3) calculating ranges of degradation factor grading values in terms of degradation

factors and structural subelements by multiplying the upper and lower degradation

factor grading values obtained from Table 4.3 [e.g., 10 and 6 from step (1)] by the

pertinent subelement relative importance value provided in Table 4.4 [e.g., 7 from

step (2)] and dividing by 10 [e.g., range is 7 to 4.2 for this example], and

(4) making appropriate adjustments to resulting range of degradation values such as

rounding to closest integer and accounting for specific subelement applications and

age-related degradation experience data base.

Table 4.5 provides a generic range of degradation factor grading values that was developed for

concrete and metallic materials using the above procedure. Selection of a particular degradation

factor grading value would be based on an assessment of conditions at the specific plant being

addressed.

4.1.1.2 Ranking system to select reinforced concrete components for evaluation

A standard ranking system has been developed to characterize reinforced concrete

components in terms of aging importance. The aging assessment methodology takes into account

the structural and safety aspects as well as the likelihood for the structure to degrade over time.

The quantitative ranking system is based on four criteria: (1) structural importance of

subelements, (2) safety significance, (3) environmental exposure, and (4) degradation factor

significance. The approach for the methodology is outlined in Fig. 4.1 and involves five primary

activities: (1) identification of Category I structures, (2) subelement division of each structure,

(3) determination of degradation factor grading values, (4) calculation of degradation significance,

and (5) ranking of subelements and primary structures.

Identification of Category I Structures Safety-related reinforced concrete structures

in the plant of interest are obtained from documentation such as the plant safety analysis report

(SAR), Q-listings, and structural drawings/specifications. Other pertinent information such as

design and construction documents, maintenance and in-service inspection records, and operating

records should also be collected for later use, as required.

Subelement Division Using structural drawings, original design calculations, and plant

specifications, each Category I concrete structure is divided into pertinent subelements. A

subelement is defined as a component such as a floor, column, beam, etc., that performs a specific

or unique function (structural or otherwise), or that is exposed to a different operating

environment. The boundaries for the subelements are established at this stage in the overall

procedure. The intent of subelemental division is to enable the most critical components or

structures to be identified with respect to structural aging importance.

NUREG/CR-6424 138
Constituent materials, as well as their properties and characteristics, should be identified

and listed for each subelement. These materials are typically identified in the safety analysis report,

structural drawings, specifications, and construction-stage material test reports.

Determination of Degradation Factor Grading Values As noted previously, four

criteria are used to develop the quantitative ranking system for use in assessing aging importance of

the structures/subelements.

(1) Importance factors for the subelements are available from Tables 2.2 and 2.3 for

BWR and PWR plants, respectively.

(2) SS ranking values are obtained from Table 4.1 for each primary structure and the

appropriate value is applied to each structure's subelements.

(3) EE ratings are developed for each subelement based on Table 4.2.

(4) Degradation factor grading values are determined. Subelements are evaluated in

terms of materials of construction, existing conditions, and environmental

exposure. Potential degradation factors are listed and degradation factor grading

values assigned using the ranges presented in Table 4.5.

Calculation of Degradation Factor Significance After listing and assigning a

degradation factor grading value to each subelement as described in the previous section, a

conditional degradation factor significance value is calculated. The degradation factor significance

value for a subelement is a simple average of up to three of the most significant degradation factor

grading values and is calculated as follows:

DFS = {DFGi /n, (4.1)

where

DFS = degradation factor significance value,

DFG = degradation factor grading value, and

n = number of degradation factors, up to a maximum of three.

The resulting DFS value is to be rounded to the nearest integer, with a maximum possible value of

10.

Ranking Of Subelements and Structures Determination of the relative ranks of the

Category I concrete structures and subelements is based on the following procedure. A subelement

rank within each Category I concrete structure is determined as follows:

SR = wl (I) + w2 (SS) + w 3 (DEG), (4.2)

where

SR = subelement rank,

I = subelement importance,

SS = safety significance,

DEG = (EE + DFS)/2, rounded to nearest integer,

EE = environmental exposure,

DFS = degradation factor significance,

w 1 = weight factor applied to subelement importance,

139 NUREG/CR-6424
w2 = weight factor applied to safety significance, and

W3 = weight factor applied to degradation effects.

Use of weighting factors (1 to 10, with 10 highest) permits certain components of Eq. (4.2) to be

emphasized. The degradation factor significance was considered to be heavily influenced by the

surrounding environmental exposure. Therefore, these two criteria were combined, averaged, and

rounded to the nearest integer. Recommended values for w I, w2, and w3 are 4, 9, and 7,

respectively. These values are the result of a sensitivity analysis.259 Therefore, Eq. (4.2)

becomes

SR = 4 (I) + 9 (SS) + 7 (DEG). (4.2)'

Utilization of this equation results in a possible range of subelement ranks between 20 and 200.

The cumulative rank for each Category I concrete structure is determined as follows:

N

CR = I SRi/N, (4.3)

i=1

where

CR = cumulative rank,

SR = subelement rank, and

N = number of subelements.

Application of this equation ensures that the cumulative rank of a Category I concrete structure is

based on aging importance rather than total number of subelements. Since the cumulative rank that

results for a structure may not adequately reflect the importance of a specific subelement or

degradation factor due to the balance of subelements or degradation factors being ranked low,

results of both Eqs. (4.2)' and (4.3) should be considered when identifying critical structures and

degradation factors of concern.

A computer-based matrix format was developed to provide a simple method for

implementing and documenting results of the structural aging assessment methodology. "R:BASE

for DOS" was used as the software because of its flexibility of input format (integers, text, etc.),

compatibility with other IBM software, and user friendliness. Also, the software allows the

programming of important equations [Eqs. (4.1-4.3)] such that the ranks for each

subelement/structure are automatically computed after data entry. The end product of the

computer-based matrix presentation is a concise listing of Category I concrete structures and their

subelements. The ranking values that result may be used to identify structures/subelements and

degradation factors that are of most significance with respect to aging management. The resulting

list may also be used to assist in determining appropriate methods and schedules for

inspection/testing/maintenance.

4.1.2 Probabilistic Risk Assessment

An evaluation of the impact on plant risk due to structural aging can also be used in the

selection of structural components for evaluation. Probabilistic risk assessments conducted to date

indicate that the structural systems generally play a passive role in mitigating design basis (or

larger) internal initiating events; a notable exception being the pressure-retaining function of the

containment following a degraded core incident involving failure of the reactor pressure vessel. On

the other hand, the structural components play an essential role in mitigating extreme events

NUREG/CR-6424 140
initiated by earthquake, wind, and other extreme influences, and their failure probabilities due to

external events can be higher. Moreover, failure of major structural components may impact the

operation of a number of mechanical and electrical systems and lead to so-called "common cause

failures." Thus, deterioration of structural components and systems due to aging and other

aggressive environmental influences may be more serious in terms of overall plant risk than might

be evident from a cursory examination of their role in accident mitigation. The significance of

structural aging and deterioration to plant risk can be evaluated by considering the impact that they

have on risk associated with external initiating events, especially earthquakes. It is in mitigating

the effects of strong ground motion due to earthquakes that structural systems play a particularly

significant role.

The increase in risk due to structural aging and deterioration was examined within the

framework of a seismic probabilistic risk assessment. Zion was selected as the paradigm NPP

because it has been widely reviewed and studied previously, its probabilistic risk assessment

(particularly the seismic hazard analysis, fragility analysis, and plant logic) is scrutable, and the

core damage and dominant plant damage state depend on a mix of structural, mechanical, and

electrical components. The approach was typical of that for a seismic probabilistic risk

assessment 26 1 and involved four primary steps: (1) identify seismic hazard from potential

seismogenic sources, historical seismicity in the vicinity of the plant, and attenuation of ground

motion to the site; (2) develop plant logic to explain the interaction of various plant components

and systems in mitigating the effects of initiating events; (3) develop fragility models to determine

capacity of plant components and systems probabilistically; and (4) measure risk by calculating

core damage probability and plant damage state. With respect to measurement of risk, probability

of seismically-induced core damage on plant damage state leading to release was used as a

surrogate. Plant logic models identified the shear wall between the auxiliary and turbine buildings,

the roof of the crib house enclosure, and the pressurizer enclosure roof as structural components of

most importance. The impact of aging on the fragility parameters was determined. The impact of

structural component aging on plant risk and seismic margins was evaluated through changes in the

cumulative distribution function of the probability of core damage; the high-confidence, lowprobability-of-failure; and a point estimate of risk. Results of this study, presented in Ref. 262

indicate that substantial damage to structural components due to aging leads to less than an orderof-magnitude increase in core damage probability. The apparent impact of structural aging

becomes more important if a margins analysis is used to assess suitability for continued service.

Sensitivity analysis can help to identify the structures of importance that should warrant particular

attention.

4.1.3 Combined Methods

The recommended strategy for use in the selection of structural subelements for evaluation

is to use a combination of the aging assessment methodology and probabilistic risk assessment

approach. In this manner, in-service condition assessments (or periodic maintenance actions) that

may be required for continued service can focus on a selected subset of structural components that

have the potential to impact plant safety. Other structural components should also receive at least a

visual inspection during routine operation or maintenance, but would require no detailed or

invasive inspections/evaluations unless demonstrable problems are apparent. Critical structural

components could be identified through a three step process:

(1) apply aging assessment methodology described in Sect. 4.1.1 to develop a

ranked list of structures and their subelements;

(2) the plant logic models developed under the Individual Plant Examinations for

External Events (IPEEE) program for the particular NPP should be overlaid

141 NUREG/CR-6424
with results of step (1) to develop a master list of structural components

important to aging, and

(3) sensitivity analyses, as described in Ref. 262, should be performed to identify

those components that may be critical in terms of aging.

One of the goals of the sensitivity analysis in step (3) is to establish whether aging of any

inaccessible structural component or system is likely to have a significant impact on plant risk.

These components will require special consideration for inspection as noted previously.

4.2 CONSIDERATIONS FOR DEVELOPMENT OF ISI PROGRAMS

ISI programs for safety-related NPP reinforced concrete structures have the primary goal of

ensuring that these structures have sufficient structural margins to continue to perform in a reliable

and safe manner. A secondary goal of these programs is to provide a means to identify any

environmental stressor or aging factor effects before they reach sufficient intensity to potentially

degrade structural margins.

Results of a survey questionnaire 56 sent to U.S. utilities indicated that only a few utilities

are conducting inspections of the safety-related concrete structures beyond the minimum to comply

with requirements of the Code of Federal Regulations.4 An approach for use in selecting

reinforced concrete structures and identifying potential degradation factors was described in the

previous section. General information on an approach for conducting a condition survey was

provided in Sect. 3.2.1. Considerations for use in the establishment of an effective in-service

inspection program to meet the above goals and to manage the aging of these structures are

provided in the form of information on inspection methods, accessibility limitations, and

acceptance criteria. Also presented is material pertaining to scheduling and qualification

requirements for inspection personnel.

4.2.1 Inspection Methods

Inspection and testing methods generally fall into four categories: (1) visual inspection,

(2) nondestructive testing, (3) destructive testing, and (4) analytical assessments. Detailed

information on the various testing methods and condition assessments was provided in the

previous chapter. General discussions addressing applications of these methods is provided

below.

4.2.1.1 Visual inspection

Although relatively simple in principle, visual inspections are one of the most valuable of

the condition survey methods because many of the manifestations of concrete deterioration appear

as visible indications or discontinuities on exposed concrete surfaces. Visual inspections

encompass a variety of techniques (e.g., direct and indirect inspection of exposed surfaces, crack

and discontinuity mapping, physical dimensioning, environmental surveying, and protective

coatings review). To be most effective, the scope of the visual inspection should include all

exposed surfaces of the structure; joints and joint materials; interfacing structures and materials

(e.g., abutting soil); embedments; and attached components (e.g., base plates and anchor bolts).

Comprehensive direct viewing may require the temporary installation of ladders, platforms, or

scaffolding. Use of binoculars, fiberscopes, and other optical aids (e.g., indirect inspection) may

be required under certain conditions. Resolution capabilities should be comparable to those

contained in Ref. 157. Degraded areas of significance are measured. For crack investigations, a

feeler gage, optical crack comparator, or crack width meter can be used to quantify width and depth

NUREG/CR-6424 142
(if possible).2 24 The condition of the surrounding structures should also be examined to detect

occurrence of differential settlement or note aggressiveness of the local operating environment.

Results obtained should be documented and photographs or video images taken of any

discontinuities and pertinent findings. The physical condition and alignment results can be

documented through close-range photograrnmetry that provides a computer file of the mapped

surface or geometry for reference to results obtained from future inspections.2 63

4.2.1.2 Nondestructive testing

Nondestructive testing techniques employ specialized equipment to obtain specific data

about the structure in question, and in certain instances (e.g., inaccessible surfaces) its surrounding, environment (i.e., structure-specific or environment-specific). The structure-specific methods are

used to inspect internal portions of the structure for discontinuities (e.g., presence of voids,

cracks, and steel reinforcement) or to provide an indication of constituent material characteristics

(e.g., compressive strength, modulus of elasticity, and size of steel reinforcement). Available

techniques, their applications, and advantages and disadvantages were described in Sect. 3. 1.

Generally, the most comprehensive means of assessing structural condition and increasing the

probability of defect detection is to use two or more of these techniques in tandem (e.g., ultrasonic

pulse velocity and rebound hammer). Environment-specific methods are used where surfaces of

structures are not accessible for direct inspection due to the presence of soils, protective coatings,

or portions of adjacent structures. These methods are used to provide an indirect assessment of the

physical condition of the structure (i.e., potential for degradation) by qualifying the aggressiveness

of the environment adjacent to the structure (e.g., air, soil, and groundwater). Methods employed

are primarily based on chemical evaluations that provide results such as chloride or sulfide contents

of groundwater adjacent to the structure. Table 4.6 provides a listing of several candidate test

methods. If results of these tests indicate that the environment adjacent to the structure is not

aggressive, there is some justification that the structure is not deteriorating. However, when

conditions indicate that the environment is potentially conducive to degradation, additional

assessments are required that may include exposure of the structure for visual or limited destructive

testing.

4.2.1.3 Destructive testing

Destructive testing involves the removal of samples of material from the structure for the

purpose of determining physical, chemical, or mechanical characteristics. These methods were

described in Sect. 3.1. Since destructive testing involves a direct examination of the material

sample removed, it provides information of significant value for use in aging management

programs. Both the presence and impact of deterioration can be determined quantitatively. Also,

supplemental testing can be done using these samples to indicate future performance (e.g.,

durability evaluations through accelerated testing techniques and demonstrating that alkaliaggregate reactions are or are not a future concern). Where material sampling is permitted,

generally only a limited number of samples can be removed to minimize the impact on the ability of

the structure to meet future functional and performance requirements. However, many structures

in NPPs may not be suitable for removal of test samples because of accessibility limitations or

structural considerations. One relatively easy and cost-effective approach for providing material

samples for testing is to retain materials that are removed during a plant modification. These

samples can be used either to evaluate current material characteristics or for potential future

requirements.

4.2.1.4 Analytical methods

Analytical methods involve the use of supplemental calculations or analytical procedures to

reevaluate the behavior and resistance of the structure (e.g., structural margins). This reevaluation

143 143 NUREG/CR-6424
may be required due to either a change in performance requirements (e.g., plant modification) or

the identification of deterioration. Finite-element and ultimate strength design methods1 6,17

provide two techniques for reanalysis. Reference 264 provides additional information on use of

analytical methods.

4.2.2 Accessibility Limitations

Access to the reinforced concrete structures in NPPs may be limited due to a number of

conditions: radiation and radioactive contamination (both during normal operations and outages);

thermal gradients and gaseous environments; massive size of structures; and presence of

surrounding liners, protective coatings, or soils. Few of these structures are accessible on all

surfaces for conduct of inspections. Depending on the extent of inaccessibility, this can result in

requirements for use of indirect methods such as environmental assessments to supplement results

obtained from accessible surfaces. This approach has been used successfully at NPPs to

extrapolate limited results to indicate the general condition of the entire structure. 2 7,

123,

2 65,

26 6 If

deterioration is identified or suspected, additional testing and evaluation is required. Information

pertaining to accessibility of subelements of NPP reinforced concrete structures was presented in

Table 2.1.

When the entire structure is inaccessible, or only a very small section is available for

inspection, the inspection method shifts from visual-based towards environmental qualification. In

this case, the environmental conditions potentially affecting inaccessible portions of the structure

are quantified using methods such as listed in Table 4.6. If environmental evaluations indicate that

the ambient exposure is non-aggressive, no further action is probably required. However, if the

ambient exposure is found to be potentially aggressive, additional testing and evaluation is

required. This will involve exposing the structure for visual or nondestructive inspections,

removal of material samples for testing, or a combination of these two approaches. The extent of

the material removed to expose the structure and the inspection methods utilized will depend on the

objective of the inspection (e.g., compressive strength, type of deterioration, detectability

requirements, and overall importance of the structure to safety).

4.2.3 Acceptance Criteria

The influence of degradation on the performance and function of reinforced concrete

structures is difficult to assess. Material discontinuities, such as steel impurities or local regions of

improper concrete consolidation, unless excessive, are generally of minor structural significance.

However, errors during construction and the initiation and propagation of various degradation

mechanisms may result in loss of function and inability to provide resistance to applied loadings.

Often the degradation mechanisms occur at time-varying rates (e.g., chemical attack or migration of

chloride ions). In-service inspections of structures at risk are conducted to identify and mitigate the

potential degradation factor effects before a repair is required or structural margins have eroded to

unacceptable levels. The mechanisms of primary concern for NPP reinforced concrete structures

and their manifestations have been identified,5 6,

2 14,259 and are summarized below in terms of

material affected and potential effect.

NUREG/CR-6424 144
Constituent Mechanism Structural Effect

Concrete Chemical Attack Cracking, erosion, leaching of paste,

increased permeability

Thermal Exposure Cracking, loss of mechanical properties

Irradiation Cracking, loss of mechanical properties

Vibration/Fatigue Cracking, loss of strength and damping

Cement-Aggregate Reaction Cracking, loss of certain mechanical

properties

Conventional Corrosion Loss of monolithic behavior; loss of

Reinforcing Steel strength; cracking and spalling of

concrete

Other Mechanisms Loss of bond, change in mechanical

properties

Prestressed Corrosion Loss of section and capacity

Reinforcing Steel Fracture Loss of capacity, loss of ductility,

increased concrete cracking under

load

Stress Relaxation Loss of prestress force, increased

concrete cracking under load

dCorrosion Reduction in leak-tightness for

Liner andC postulated loads, loss of

E m 1 section/capacity

Several common characteristics may be observed from this categorization. First, concrete

cracking is a very common damage by-product from a large number of degradation mechanisms.

Active concrete cracking is difficult to assess in terms of impact on structural behavior and is

difficult to repair. Thus, inspection methods that support the early identification, sizing, and

determination of cracking cause and means for propagation in concrete structures are of primary

interest for future inspections. Second, the primary concern for all metallic constituents of concrete

structures is corrosion and corrosion-related damage. Inspections that identify early signs of

corrosion cell initiation and indicate the rate of propagation are similarly valuable.

Two approaches have been developed for assistance in the classification and treatment of

conditions or findings that might emanate from in-service inspections of NPP reinforced concrete

structures. These approaches are based primarily on the results of visual inspections since these

inspections provide the cornerstone of any condition assessment program for concrete structures.

Also, with the exception of some guidance on half-cell potential 197 and ultrasonic pulse velocity

measurements, 239 few standards have been published presenting acceptance criteria for results

obtained from nondestructive evaluation tests. Background information is presented below for use

in formulating acceptance criteria based on results obtained from visual inspections.

4.2.3.1 Visual approach

The visual-based approach uses a "three-tiered" hierarchy similar to that under development

by American Concrete Institute (ACI) Committee 349.230 Through use of different levels of

acceptance, minor discontinuities can be accepted and more significant degradation in the form of

defects can be evaluated in more detail. The three acceptance levels include acceptance without

further evaluation, acceptance after review, and additional evaluation required.

145 NUREG/CR-6424
Acceptance Without Further Evaluation Conditions presented below are considered

to be acceptable and require no further evaluation at present. Reference 223 provides definitions

and pictorial representations of typical forms of concrete degradation. In the event that the

conditions provided below are exceeded, or observed conditions are determined to be deserving

further evaluation, a more detailed review is required. Structures that are partially or totally

inaccessible for visual inspections may require supplemental evaluations as environments may be

present that are conducive to degradation.

1. Concrete Surfaces

Concrete surfaces that are exposed for inspection and meet the following surface

condition attributes are generally acceptable without further evaluation if the

following criteria are met:

a. Absence of leaching and chemical attack;

b. Absence of abrasion, erosion, and cavitation;

c. Absence of drummy areas (poorly consolidated, with paste deficiencies);

d. Popouts and voids less than 20 mm in diameter or equivalent surface area;*

e. Scaling less than 5 mm in depth;

f. Spalling less than 10 mm in depth and 100 mm in any dimension;

g. Absence of any signs of corrosion in reinforcing steel system or anchorage

components (including concrete staining or spalling);

h. Passive cracks less than 0.4 mm in maximum width ("passive cracks" are

defined as those having an absence of recent growth and absence of other

degradation mechanisms such as leaching at the crack);

i. Absence of excessive deflections, settlements, or other physical movements that

may affect structural performance; and

j. Absence of cement-aggregate reactions, chemical attack, or other active

degradation mechanism.

2. Concrete Surfaces Lined by Metal or Plastic

Concrete structures with surfaces protectively lined with either a metallic or plastic

(non-metallic) system are judged to be acceptable without further evaluation if the

following criteria are met:

a. Without Active Leak Detection System

1. Absence of bulges or depressions in liner plate (those that appear age-related

as opposed to being created during construction);

2. Absence of corrosion or other liner damage; and

3. Absence of cracking in liner weld or base metal.

b. With Active Leak Detection System

1. No detectable leakage observed in leak detection system;

2. Absence of any liner damage, such as noted in 2(a) above; and

3. Absence of fluid penetration indications by leak chases or other detection

system components.

* Dimensions of degradation presented in this section are meant only as guidelines.

NUREG/CR-6424 146
3. Areas Around Embedments in Concrete

The condition of the concrete around embedments is acceptable without further

evaluation if the following criteria are met:

a. Concrete surface condition attributes of Criteria 1 above are met;

b. Absence of corrosion of the exposed portions of embedded metal surfaces and

corrosion staining around the embedded metal;

c. Absence of detached embedments or loose anchorages; and

d. Absence of degradation due to vibratory loads from piping and other attached

equipment.

4. Joints, Coatings, and Non-Structural Components

The condition of joints, protective coatings, waterproofing membranes, and other

non-structural elements is acceptable without further evaluation if the following

criteria are met:*

a. No signs of separation, environmental degradation, or water in-leakage are

present in coatings, joints, or joint sealant material;

b. Loss or degraded areas of coatings for structures that do not serve as a barrier to

aggressive chemical flows are limited in surface area to 4000 square millimeters

or less at one area, and 0.01 square meters over the gross surfaces of the

structure;

c. Absence of degradation in any waterproofing membrane protecting below-grade

concrete surfaces (within the inspected area); and

d. Non-structural components such as dewatering systems are serving their

intended function.

5. Post-Tensioning Systems

Components of post-tensioning systems are acceptable if requirements such as

provided in Refs. 105 and 157 are met. These requirements are well defined.

Acceptance After Review Findings listed below require review and interpretation in

order to evaluate acceptability. Such a review involves determining the likely source of

degradation, its activity level, and its net effect on the component. Based on results of the review

and evaluation, possible approaches include acceptance as-is, further evaluation using enhanced

visual inspection (e.g., magnification), scheduling follow-up inspections at a later date, or use of

nondestructive or destructive testing techniques. An analytical assessment of the necessity for

repair may also be required. The analytical assessment should examine the impact of existing

degradation on the performance characteristics of the structure. Accessibility of the components in

question will also enter into the decision process relative to the action to be taken.

1. Concrete Surfaces

The following surface conditions shall be reviewed to determine if they are either

acceptable, require further evaluation, or require repair. Measurable discontinuities

exceeding the quantitative limits below require additional evaluation.

* Information on protective coatings for NPP applications is provided in Refs. 267 and 268.

147 NUREG/CR-6424
a. Appearance of leaching or chemical attack;

b. Areas of abrasion, erosion, and cavitation degradation;

c. Drummy areas that may exceed the cover concrete thickness in depth;

d. Popouts and voids greater than 20 mm but less than 50 mm in diameter or

equivalent surface area;

e. Scaling greater than 5 mm but less than 20 mm in depth;

f. Spalling greater than 10 mm but less than 20 mm in depth, and less than 200

mm in any planar dimension;

g. Corrosion staining on concrete surfaces;

h. Passive cracks greater than 0.4 mm but less than 1 mm in maximum width; and

i. Passive settlements or deflections exceeding the original design limits or

expected values.

2. Concrete Surfaces Lined by Metal or Plastic

a. Without Active Leak Detection System

Presence of any condition listed in Criteria 2(a) of previous section shall be

further evaluated to determine acceptability; and

b. With Active Leak Detection System

Presence of leakage in excess of amounts and flow rates committed to in the

original design or Plant Technical Specification will necessitate a root cause

investigation and assessment of the need for follow-up action. Leakage within

the prescribed limits may be acceptable if the source is known and found to be

inconsequential.

3. Areas Around Embedments in Concrete

Presence of any condition listed in Criteria 1 for concrete surfaces shall be further

evaluated to determine acceptability.

4. Joints, Coatings, and Non-Structural Components

Presence of any condition exceeding the descriptions and limits of Criteria 4 in

previous section shall be further evaluated to determine acceptability. Any

observation of widespread adhesion/cohesion problems, environmental attack, or

poor performance indicators are considered unacceptable.

5. Post-Tensioning System

Presence of conditions exceeding limits provided in references noted under

Criteria 5 of previous section shall be reviewed according to criteria provided in

these references.

Additional Evaluation Required Conditions outside the criteria provided in the

previous two sections must be evaluated to determine the appropriate coarse of action. This will

generally involve extensive application of both nondestructive and destructive testing methods.

Detailed analytical evaluations frequently will be required to better characterize the current condition

of the structure and provide the basis for formulation of a repair strategy (if needed). Even if the

analysis results indicate that the component is acceptable at present, additional assessments should

be conducted to demonstrate that the component will continue to meet its functional and

performance requirements during the desired service life (i.e., take into account the current

structural condition and use service life models to estimate the future impact of pertinent

degradation factors on performance). If the structure's desired service life is short, and its loss of

NUREG/CR-6424 148
function due to degradation is occurring at a rate such that sufficient structural margins will be

maintained during this period, no action may be required. However, when the opposite is true and

loss of function due to degradation is occurring at a rate such that structural margins will not be

adequately maintained during the desired service life period, the analytical and test results should

be utilized to develop an in-service inspection/repair strategy that will maintain structural margins

during the desired service life.

4.2.3.2 Degradation-based approach

The effects of degradation mechanisms on the performance of a structure can range from

cosmetic to structurally degrading. Provided below is information that is intended to be of

assistance in quantifying the significance of degradation that is detected through visual inspections,

nondestructive testing, or a combination of these methods. Forms of degradation considered

include concrete cracking and cement-aggregate reactions. Guidance for detection and resolution

of degradation in the form of corrosion of post-tensioning system components and loss of

prestressing force is not addressed as detailed information on these topics is available in

Refs. 105, 106, and 157. Degradation of metal liners or coating materials is also not addressed as

these materials were not generally addressed under the SAG Program.

Concrete Cracking Cracking in concrete can result from a number of factors as shown

previously in Fig. 2.9. Designs of reinforced concrete structures generally consider that the

concrete is incapable of supporting tensile forces. Steel reinforcement is included in the structural

members to both carry the tensile loadings and to provide control of cracking (i.e., limit width and

spacing of concrete cracks). From an aging perspective, both the width of concrete cracks and the

environmental exposure are important.

Limited information on cracking and its classification with respect to damage is available in

Ref. 58. In this reference, damage resulting from different crack types (e.g., diagonal and

longitudinal) is rated on a one to five scale, with five being the most significant. Table 4.7

presents the classifications and ratings that were developed for concrete cracks and surface defects.

Environmental influences have not been included in this classification scheme. The more severely

rated (i.e., greater crack widths) cracks generally would be related to an overload condition that

would require a structural evaluation. Active cracking, settlements, or deflections that are observed

must be carefully classified and evaluated as the cause may continue to act and its effect intensify.

From an aging management perspective, the presence of concrete cracks is of importance

because they provide possible avenues of access for environmental stressors (e.g., chloride ions

and sulfate solutions). Some work has been done in classifying environmental exposure

conditions in terms of degree of aggressivity. Table 4.8 provides an indication of the influence of

moisture state on several durability processes.52 The Comite European de Normalisation has

prepared a chart classifying environmental conditions in terms of severities of exposure classes

(Table 4.9).269 Exposure classes have also been developed specifically relating environmental

conditions to steel reinforcement corrosion (Table 4.10), 52 and degree of chemical attack of

concrete by water and soils containing aggressive agents (Table 4.11 ).52,270

As noted previously, the corrosion of steel reinforcement is one of the more significant (if

not the most) forms of degradation that could potentially impact NPP reinforced concrete

structures. There have been a number of studies over the years that have related maximum

permissible concrete crack widths to environmental factors, and these results were summarized in

Table 3.5. Limits in this table were provided to reduce the potential for enhanced degradation

through ingress of contaminants, primarily leading to corrosion of steel reinforcement.

149 NUREG/CR-6424
Two damage-state charts have been prepared to assist in the resolution of results obtained

from in-service inspections or testing.! Figure 4.2 provides a relationship between environmental

exposure in terms of extent of carbonation or chloride ion content of the environment, the width of

cracks present, and the necessity for additional evaluation or repair. As noted in the figure, the

extent of action required increases as the severity of environmental exposure increases or the width

of cracks present increases. Figure 4.3 provides a relationship between environmental exposure,

half-cell potential readings, and necessity for further evaluation or repair. Superimposed on the

half-cell potential axis are visual inspection results that might be anticipated for different degrees of

severity of corrosion of steel reinforcement. Crack width information presented in Table 3.5 and

limited industry-published acceptance criteria1 97,

247 were used to develop the relationships

between threshold levels and recommended actions presented in Figs. 4.2 and 4.3. Results are

presented in terms of a "three tier" hierarchy: (1) condition acceptable as-is without additional

assessment; (2) structure requires further evaluation, supplemental tests, or review after next

operating period to classify as acceptable or in need of repair or maintenance; and (3) structure

requires repair, maintenance, or replacement. Further evaluation would consider the use of other

inspection, testing, or analytical tools to obtain additional information on the current condition of

the structure and the potential for further degradation of its functional and performance

requirements with time.

Alkali-Aggregate Reactions"* Limited research results are available presenting the

impact of alkali-aggregate reactions on structural integrity (i.e., alkali-silica, alkali-carbonate, and

alkali-silicate). As a result of these reactions, expansion and cracking occurs that can lead to loss

of strength, reduced stiffness, or decreased durability of concrete. A quantitative ranking

methodology has been developed for beam and plate elements that potentially can be used as

guidance for NPP reinforced concrete structures should the presence of alkali-aggregate reactions

be confirmed.27 1 The criteria below were developed on the basis of visual inspections and

petrographic analyses of core specimens removed from a large number of structures exhibiting

various intensities of alkali-aggregate reactions.

CATEGORY VISUAL INSPECTION PETROGRAPHY RESULTS

1 Crack Width: 0-0.2 mm Internal and external circumferential cracks

Crack Depth: Superficial developed. Some gel exuded. Internal cracks in

Pop-outs, no.: 0-5 per m2 reactive aggregate.

Pop-outs, dia.: 0-5 mm

2 Crack Width: 0.2-1.0 mm Gel in air voids and small external cracks.

Crack Depth: Superficial to Deep Short cracks open to environment along major

Pop-outs, no.: 5-20 per m2 axis of structure.

Pop-outs, dia.: 5-15 mm

3 Crack Width: 1.0-2.0 m Gel in many air voids and cracks.

Crack Depth: Deep to Penetrating Marked increase in crack width (0.05 mm internal).

Pop-outs, no.: 15-30 per m2 Longer cracks along major axis of structure.

Pop-outs, dia.: 5-15 mm

4 Crack Width: > 0.2 mm Most reactive aggregate shows signs of reactivity.

Crack Depth: Penetrating Larger cracks along major axis intersect with

Pop-outs, no.: > 25 per m2 transverse cracks showing pattern. Gel exudation

Pop-outs, dia.: 5-20 mm easily seen in cracks and voids.

"The damage-state charts are intended only to represent the type of information that can be developed to aid in the

structural condition assessment. Every reinforced concrete structure is unique with respect to its constituent

materials and physical characteristics, functional and performance requirements, and environmental exposure

conditions.

** The ultraviolet radiation method2 19 described in Sect. 3.1.2.6 can be used to detect the presence of alkali-silica

reactions in in situ concrete structures.

NUREG/CR-6424 150
For structures having sustained Category 1 or 2 damage, the reactions have likely not caused

significant structural damage. Structures observed as having damage in Category 1 should be

considered for more frequent inspection and possibly for rehabilitative measures similar to that for

Category 2 damage. For structures in Category 2, maintenance measures aimed at preventing

exposure to moisture, such as adding a protective coating or sealer, should be considered.

Additional core samples may be needed to assess the degree of reaction. More frequent inspection

is also warranted. Structures in Category 3 and 4 require evaluation for structural repair. Because

a single aggregate source was generally used in the construction of a NPP, the balance of plant

structures should be inspected if Category 3 or 4 conditions are observed in one structure.

4.2.4 Inspection Scheduling and Personnel Qualifications

4.2.4.1 Inspection scheduling

ISI programs for NPPs have traditionally focused on the concrete containment vessel. A

general visual inspection of accessible surfaces is conducted in conjunction with performance of

periodic Type "A" leak-rate tests required by Appendix J to 10 CFR Part 50.4 Examination of the

unbonded post-tensioning tendon system is conducted at regular intervals to assess the state of the

hardware as well as level of prestressing force in the containment. 155 Schedules for the

containment-related inspections are considered to be "outage based" in that most or all of these

inspections are performed during planned plant outages (e.g., refueling) to provide improved plant

access. The plant owner does have the option of electing to perform certain inspections (e.g.,

tendon surveillances) at other times as long as the code-mandated frequencies and schedules are

met. Only containments are covered by current regulatory requirements.

The AC1230 is developing recommended frequencies and schedules for conducting

inspections of safety-related concrete structures other than containments. These schedules take into

account the relative aggressiveness of environmental conditions and physical exposures of these

structures, and will help assure that any age-related degradation is detected at an early stage of

development so appropriate mitigative actions can be taken. In general, the ACI document

proposes that all safety-related reinforced concrete structures be visually inspected at intervals not

to exceed 10 years. The frequency of ISI proposed is dependent on the exposure category into

which the structure of interest falls. The exposure categories are essentially the same as presented

in Table 4.2. Recommended ISI frequencies are provided below.

H Structure Exposure Category I Frequency of Visual Inspection

* Below-Grade 10 Years (each ISI Interval)

* Natural Environment 5 Years (two per ISI interval)

(Direct/Indirect)

" Inside Primary Containment 5 Years (two per ISI interval)

" Continuous Fluid (without liner) 5 Years (two per ISI interval)

* Fluid/Pressure Retaining (with liner) 5 Years (two per ISI interval)

" Controlled Interior (i.e., 10 Years (each ISI interval)

secondary containment, auxiliary

building, etc.)

151 NUREG/CR-6424
The above frequencies may be modified to smaller intervals if plant environments are particularly

severe or degradation has been observed to occur. When the observed degradation exceeds criteria

provided previously, increased visual inspections should be supplemented by nondestructive, and

possibly destructive testing.

Reliability-based methods can also be used to schedule inspections of safety-related

concrete structures. These concepts are discussed in detail in the next chapter and can be used to

assess the reliability of the NPP reinforced concrete structures in terms of damage state and rate of

degradation, inspection methods, detectability functions, remedial actions, and frequency of

inspections. Optimized strategies for inspection and repair can be developed that minimize future

costs associated with inspection, repair, and loss of service, while maintaining the component

probability of failure at or below a target value over the service life of the structures.

4.2.4.2 Qualifications of inspection personnel

The quality and usefulness of results obtained from inspections of existing NPP reinforced

concrete structures are dependent to a great deal on the qualifications and capabilities of the

personnel involved. To ensure that these inspections are properly performed, minimum

qualifications and skills should be defined. Although guidelines are available for inspection of new

concrete construction, few standards exist for qualification and certification of inspectors for

existing reinforced concrete structures. Some recommendations have been proposed by the ACI

Committee 349230 and they are summarized below.

As a minimum, the complete inspection team should include both civil/structural engineers

and concrete inspectors and technicians familiar with concrete aging and degradation mechanisms

and long-term performance issues. The qualifications for the person(s) responsible for the inservice inspection of safety-related concrete structures should meet the minimum recommendations

of Ref. 230. The individual responsible for the administration of an ACI-defined evaluation is the

responsible engineer, who will possess one of the following sets of qualifications:

A. Registered Professional or Structural Engineer, knowledgeable in the design,

evaluation, and in-service inspection of concrete structures and performance

requirements of nuclear safety-related structures; and

B. Civil/structural engineering graduate of an accredited college or university who has

successfully completed the experience, training, and testing requirements of the

ACI Level III Concrete Inspector Program and is knowledgeable of the

performance requirements of safety-related structures.

Personnel performing the balance of inspection or testing at the plant, under the direction of

the responsible engineer, should meet one of the following qualification sets, or equivalent:

A. Civil/structural engineering graduate (4-year) of an accredited college or university

who has over 1 year experience in the evaluation of in-service concrete ,structures

or quality assurance related to concrete structures;

B. Personnel possessing Level I or II Concrete Inspector certification from the plant

owner; and

C. Personnel meeting the requirements for Level I or II Concrete Inspector, as defined

in Section III, Division 2, Appendix VII.20

NUREG/CR-6424 152
Personnel inspecting metallic components such as liner plates should be certified in

ultrasonic thickness, liquid penetrant, or magnetic particle testing if these methods are to be used.

ISI results may need to be examined in terms of net effect on the structure. Structural

calculations addressing the as-designed structure and projected behavior under observed

degradation should be performed under the direction of the responsible engineer.

4.3 EXAMPLE ILLUSTRATING DEVELOPMENT OF AN INSPECTION

PROGRAM

An approach for establishing an inspection program for safety-related NPP concrete

structures is illustrated in the example presented below. The approach presented is applicable to

concrete structures in both PWR and BWR plants.

4.3.1 Background on Example PWR Plant

The example plant is a PWR with a large-dry post-tensioned concrete containment located

in the midwestern U.S. The containment vessel consists of a conventionally reinforced basemat

foundation located on compacted fill material, post-tensioned (vertical and 1200 hoop tendons)

cylinder wall, and post-tensioned (three groups with each group at 1200 with respect to other two

groups) dome. The post-tensioning system consists of unbonded wire tendons encased in ducts

filled with petrolatum-type corrosion inhibitor. The internal surfaces of the containment are lined

with 6.25-mm thick carbon steel plate, while the spent-fuel pool is lined with 6.25-mm thick

stainless steel plate. The dome external surface is coated by a high-solids urethane paint system.

All concrete structures were constructed of concrete having a minimum compressive strength of 28

MPa and conventional steel reinforcement having a minimum yield strength of 414 MPa.

Subterranean structures are protected by waterproofing materials and water stops. External

structures are subjected to several annual freeze-thaw and wet-dry cycles. Also, seasonal

groundwater fluctuates from one to six meters below grade.

Presently the utility is performing regular in-service inspections that comply with

requirements in Refs. 4 (Appendix J leak-rate testing) and 105 (post-tensioning system

evaluations). A review of the results of these inspections indicates that no significant degradation

has been observed to date. Formal inspections of the other safety-related reinforced concrete

structures are not done on a routine basis. Spent fuel pool liner leakage is monitored on a regular

basis to maintain cooling water inventory. A settlement monitoring program was in-place early in

the life of the plant, but was abandoned after uniform settlements stabilized at about 6 to 13 mm.

4.3.2 Selection of Components for Evaluation

The aging assessment methodology described in Sect. 4.1.1 is used to select structural

components for evaluation.* The initial step is to identify all Category I concrete structures. The

most recent edition of the plant's safety analysis report and other pertinent plant documentation

(e.g., Q-listing) are reviewed as well as structural drawings. Primary structures identified for this

plant include (1) containment vessel, (2) containment internal structures, (3) auxiliary building,

(4) turbine building (portions of slabs/walls), (5) cribhouse (portions), and (6) intake crib.

* For completeness, results of a probabilistic risk assessment identifying structures that would increase plant risk due

to aging and deterioration also should be included in the ranking.

153 NUREG/CR-6424
These structures were subdivided into subelements and materials of construction. This information

is entered into the matrix format as shown in Table 4.12.* Importance factors are assigned to each

subelement using information presented in Sect. 4.1.1.1.

Using conservative judgment, review of plant conditions, and guidelines presented in

Sect. 4.1.1.1, safety significance and environmental exposure values are established and entered

into the matrix format of Table 4.12 for each subelement. The primary containment structure, as

well as many of the other safety-related structures at this plant, are exposed to a natural

environment. However, a number of these structures have been protected with coatings to prevent

external degradation. As noted, the plant is founded on fill material. Also, cooling is provided by

fresh water. Certain groundwater parameters are periodically measured, and the post-tensioning

system is partially accessible for inspection. Each of these conditions was considered as being

favorable with respect to assignment of criteria values.

As a result of variability in existing protective media and exposure conditions at this plant,

evaluation of the degradation factors and their grading values requires careful consideration. Such

degradation mechanisms as corrosion of the containment post-tensioning system and irradiation of

the reactor pressure vessel pedestal were considered to be relatively important. Key potential

degradation factors for other structural subelements are then identified and entered into the matrix

format of Table 4.12. Degradation factor grading values for each of the potential degradation

factors for each of the subelements are assigned based on information provided in Table 4.5.

Based on known existing conditions at the plant, these values may be adjusted appropriately.

Equation (4.1) is then utilized to determine a degradation factor significance value for each

subelement using up to a maximum of three degradation factor grading values for each subelement.

A ranking of each subelement is determined using Eq. (4.2)' in conjunction with the

subelement importance, safety significance, environmental exposure, and degradation factor

significance values determined above. Finally, the cumulative rank for each structure is determined

by summing the subelement ranks and dividing by the number of subelements. The results of

these calculations indicate that the highest ranking primary reinforced concrete structure is the

containment vessel, and the highest ranking subelement is the mat foundation of the containment

vessel. A complete listing of the ranking of primary structures is provided below.**

1. Containment vessel (dome, ring girder, cylinder walls, and mat);

2. Reactor cavity walls and support;

3. Auxiliary building foundation;

4. Reactor coolant compartment walls;

5. Containment-internal walls and short columns;

6. Fuel pool walls and slab;

7. Polar crane support wall;

8. Containment-internal lower slab;

9. Intake crib foundation;

10. Containment-internal slabs (others);

11. Diesel generator vault walls;

12. Control room walls;

13. Diesel generator vault slabs;

14. Control room slab;

15. Auxiliary building walls;

* Detailed results are presented in Table 4.12 only for the post-tensioned concrete containment. Results for other

safety-related concrete structures are available in Ref. 259.

** Structures ranked higher than 20 have been omitted from the listing.

NUREG/CR-6424 154
16. Crib house walls;

17. Auxiliary building floor slabs (internal);

18. Turbine building safety-related walls;

19. Crib house slabs;

20. Turbine building safety-related floors;

4.3.3 Initial Planning

Using results presented above, planning for development of an inspection program is

initiated. The intent of this initial planning is to define the scope and type of inspections to be

performed on the structures identified above, establish initial inspection frequencies, set up

program documentation requirements, and identify personnel to oversee and conduct inspections.

The plan should consider all procedures currently in place to meet Plant Technical Specification

requirements. A brief visual inspection and review of pertinent documentation, such as related

plant modifications or repair activities, is of assistance in developing the inspection plan. This

information is then used to develop a detailed plan for conduct of an initial baseline survey of the

structures. Guidance on condition assessments including a visual baseline survey is available in

Sect. 3.2. In addition, there are several documents that have been prepared by the Corps of

Engineers for use in conducting condition assessments and quantifying degradation in civil works

structures that provide information of use in planning inspections of NPP concrete structures. 272-

274

4.3.4 Baseline Inspection Program

Initial planning results are implemented through a baseline inspection program. The

emphasis of the baseline inspection program is on the primary structures and subelements that were

ranked highest by the aging assessment methodology. The approach developed for the baseline

inspections includes the following.

" Post-tensioned concrete containment has been inspected at regular intervals under a

well-defined in-service inspection program. Since no degradation has been

experienced, these inspections will be continued unchanged.

1. Leakage-rate testing at current specified intervals according to Plant Technical

Specification requirements,

2. General visual inspections of liner and external concrete surfaces, and

3. Post-tensioning system inspections according to Plant Technical Specifications.

* High-ranking safety-related concrete structures (nos. 2-10 in Sect. 4.3.2) will be

visually inspected.

1. General visual inspection using visual-based approach acceptance criteria in

Sect. 4.2.3.1,

2. Detailed documentation of findings for future reference, and

3. Reinspect structures using inspection scheduling criteria presented in

Sect. 4.2.4.1, or at more frequent intervals for suspect areas.

" Remaining safety-related concrete structures (nos. 11-20 in Sect. 4.3.2) will also

be visually inspected, but not in the detail that the previous structures received.

1. General visual inspection to provide initial baseline information, and

2. Reinspect structures at five-year intervals with inspection interval increased to

ten years if first three inspections indicate no potential deterioration.

155 NUREG/CR-6424
The baseline inspection program approach was implemented to examine the 20 highestranking concrete structures noted previously. In addition, portions of the intake structure below

the water line were inspected, surface hardness measurements (rebound hammer) were made on

exposed concrete surfaces to provide an indication of concrete quality, and dry-film measurements

were made of coating thicknesses. Environmental exposure information for the structures was

estimated from existing plant data. Conclusions from the baseline inspection program were that

1. All safety-related structures were found to be in acceptable condition except for the

lower tendon gallery vaults, which were found to have been damaged by leaching,

cracking, and steel reinforcement corrosion. Several wall cracks were over 4-mm

wide, and had propagated through-thickness over the full height of the wall.

Intruding groundwater had caused significant leaching of the concrete, and staining

from steel reinforcement corrosion was severe. This cracking was not a current

safety issue, but should be addressed in the near future.

2. Significant concrete cracking was discovered in the intake structure walls, Turbine

Building walls and basemat, and lower Auxiliary Building walls and basemat.

Signs of groundwater intrusion, efflorescence, and reinforcing corrosion were

noted. Measured concrete crack widths were generally under 1.5 mm, except for

one crack with a maximum width of 15 mm that appeared to be through-wall.

Leakage of borated spent fuel pool water was found on many Auxiliary Building

slabs. Cracks were also identified in the nuclear steam supply system (NSSS) vault

walls and structures supporting NSSS equipment inside the containment vessel,

although they appeared to be passive in nature.

3. The environmental exposures of all structures, including below-grade structures,

was judged to be "mild" with limited chlorides present in the soil and groundwater.

The atmosphere was found conducive to carbonation, given the surrounding heavy

industry and environmental exposure witnessed at the plant. The groundwater

elevation fluctuates from 1 to 6 meters below grade during the year and was found

to have a fairly high dissolved oxygen content.

4. Review of tendon lift-off test results indicated that losses to date were well-within

expectations. Lift-off tests and tendon inspections have been performed per

Regulatory Guide requirements, although no trending was performed. Some water

was found in tendon grease samples at several anchorages. Significant grease

leakage was noted from grease cans and through anchorage-area concrete cracks,

with measured crack widths to 0.5 mm.

5. The containment liner was found to be locally corroded, with pitting and exfoliation

to depths of about 50% of the nominal liner thickness at several locations. Liner

coating systems were also degraded beyond acceptance criteria and in need of

repair. The liner is not a structural member, but a remedial action should be

implemented so that continued corrosion will not penetrate the liner thickness to

cause a loss of its pressure-retaining function.

6. One badly cracked wall (non-safety related) was identified in the Turbine Building.

7. Only limited local cracking and impact-related spalling was observed in the

containment vessel wall. This damage was primarily within a 3-m high band above

grade. One spall from vehicle impact measured 200 mm by 370 mm by 38 mm

deep.

NUREG/CR-6424 156
8. Containment basemat and lower cylinder walls, where accessible and near grade,

were found to be in good condition.

4.3.5 Inspection Program Adjustments

Several modifications were made to the initial inspection program as a result of conditions

observed during the baseline inspections. Acceptance criteria presented in Sect. 4.2.3 were used to

develop the modified inspection program. Modifications recommended to help ensure continued

integrity of structures that showed signs of or exhibited increased potential to degrade included the

following:

1. Establishing aggressiveness of soil and groundwater through annual sampling and

testing of groundwater and soil around containment basemat perimeter.

2. Conducting inspections at intervals not to exceed two years of structures that

exhibited water intrusion (i.e., lower portions of containment vessel walls,

Auxiliary Building, Turbine Building, and intake structure).

3. Repairing tendon gallery walls and bottom slab with post-repair inspections

conducted annually until a performance data base is developed.

4. Checking integrity of Auxiliary and Turbine Building basemats with impact-echo to

examine for subsurface damage.

5. Conducting visual examinations of Turbine Building wall to determine significance

of cracks that were observed during inspections. Results will be used to formulate

a repair strategy using guidance provided in Sect. 3.3.

6. Developing a program within one year to identify source of leakage of spent-fuel

pool water and assess potential impact of borated water on concrete structures at

risk (i.e., corrosion potential).

7. Evaluating significance to structural performance of tendon corrosion inhibitor

leakage at anchorage zone areas.

8. Implementing a program to repair portions of liner experiencing corrosion, recoat,

and inspect on an annual basis until a performance data base is established.

The balance of reinforced concrete structures for which the baseline inspections indicated

that no further evaluation or remedial action was required will be examined under the initial

program requirements.

4.4 COMMENTARY ON INSPECTION PROGRAMS

Regular and systematic inspections constitute an integral part in maintaining the safety and

serviceability of the safety-related concrete structures by providing a link between the environments

these structures experience and the manner in which they perform with time. This has been

recognized through the "Maintenance Rule" that requires power reactor licensees to monitor the

performance or condition of structures, systems, and components (SSCs) against licenseeestablished goals in a manner sufficient to provide reasonable assurance that the SSCs are capable

of fulfilling their intended functions (§ 50.65 to Ref. 4).

157 NUREG/CR-6424
Development of an inspection program to evaluate NPP safety-related reinforced concrete

structures basically entails the selection of critical components, application of systematic inspection

routines (including data recording and handling procedures), determination of the current structural

condition, and assessing the ability of the structure in question to continue to meet functional and

performance requirements. Methodologies for selection of components important to aging and

conducting condition assessments have been developed, or are fairly well established. Although

some background information was provided previously on criteria for use in condition

assessments, development of acceptance criteria relating the impact of degradation to the

performance characteristics of the affected structure in large measure require development.

Reinforced concrete structures in NPPs present special challenges for development of acceptance

criteria because of their massive size, limited accessibility in certain areas, stochastic nature of past

and future loads, randomness in strength, uncertainty in material changes due to aging and

possibly degradation, and qualitative nature of many nondestructive evaluation techniques. The

most effective approach at present to managing aging of these structures is to conduct periodic

inspections, implement maintenance measures to prevent or slow deterioration, and repair defects

before they propagate to effect a loss of serviceability.

NUREG/CR-6424 158
Table 4.1. Safety significance ranking criteria.a

Safety Significance Criteria Ranking

1. Non-Category I structures 0

2. Category I structures whose relation to safety is due to their 2

failure consequence alone

3. Category I structures performing one safety function in addition 4

to "failure consequence" (typically related to environmental

protection, etc.)

4. Category I structures performing two safety functions of greater 6

importance than "failure consequence" (i.e., secondary

containment, radiation shielding, etc.)

5. Category I structures performing at least three safety functions of 8

greater importance than "failure consequence" and which are

required for primary containment, support of NSSS components,

etc.

6. Category I structures performing four or more safety functions 10

simultaneously (i.e., the primary containment pressure boundary)

aIntermediate values between those noted above may be used based on actual safety function of

the structure. Non-Category I structures are included for completeness.

159 NUREG/CR-6424
Table 4.2. Environmental exposure categories and ratings.

Exposure Condition Rating

1. Subterranean (below grade) 10

2. Natural environment (direct exposure) 6

3. Natural environment (indirect exposure) 4

4. Continuous fluid exposure (without liner) 7

5. Fluid/pressure retaining (liner provided) 5

6. Inside primary containment 7

7. Controlled interior environment (secondary containment, 2

auxiliary building, etc.)

Reference 259 provides more complete descriptions of the exposure conditions

and criteria for assignment of a rating number.

NUREG/CR-6424 160
Table 4.3. Relative weightsa assigned to criteria affecting degradation factor gradings.

Criteria

Rate Inspectability Range of DegradaMaterial Degradation of and Ultimate tion Factor

System Factor Deterioration Early Identification Repairability Impact Grading Values

Concrete Chemical attack Low to high Low to moderate Moderate Low to high 6-10

Freeze/thaw cycling Low Low Low Low 2-6

Thermal exposure/thermal Moderate Moderate High Moderate 6-10

cycling

Irradiation Low Moderate High Moderate 4-8

Abrasion/erosion/cavitation Low Low Low Low 2-6

Fatigue/vibration Low Moderate High Moderate 4-8

Mild Steel Corrosion High Moderate Moderate High 6-10

Reinforcing Elevated temperature Low Low Moderate Low 2-6

Irradiation Low Low Moderate Low 2-6

Fatigue Low Moderate Low to high Low to moderate 4-8

Prestressing Corrosion High Moderate Low to moderate High 6-10

Elevated temperature Low Low Low to moderate Low 2-6

Irradiation Low Low Low to moderate Low 2-6

Fatigue Low Moderate Low to moderate Low 2-6

Stress Relaxation Low Moderate Low to moderate Moderate 4-8

Liner Plate/ Corrosion Moderate Low Moderate High 6-10

Structural Steel Fatigue Low to moderate Moderate Moderate Low to moderate 4-8

I-.

0'

I-.

aHigh = Criterion creates a significant limiting condition or has a major adverse effect that can significantly impact the structure's serviceability or

integrity following initiation and may be difficult to identify.

Moderate = Criterion creates a minor impact on structures and requires careful inspection.

CLow = Criterion creates either very low or no impact on structures or may be readily observed via inspection.

t-I

-I
(7)

0'j

.I- Table 4.4. Prioritizationa of consequent net effects by generic subelements.

I-.

0'

r'3

Subelement (Relative Importance Value)b

Consequent Net Effect Shear

(relative importance Shell Foundation Floor Beam Columnc Wall

multiplier value) (10) (10) (7) (7) (9) (8)

1. Loss of monolithic 10.0 10.0 7.0 7.0 9.0 8.0

behavior (1.0)

2. Damageto steel 10.0 10.0 7.0 7.0 9.0 8.0

reinforcing system (1.0)

3. Loss of concrete section

a. Gross loss (0.8) 8.0 8.0 5.6 5.6 7.2 6.4

b. Minor loss (0.7) 7.0 7.0 4.9 4.9 6.3 5.6

4. Decreased structural 6.0 6.0 4.2 4.2 5.4 4.8

material performance

(0.6)

aprioritization of consequent net effects values in table for each subelement represents product of relative importance

multiplier value for particular consequent net effect (0-1) and relative importance value of the particular subelement (0-10).

bNumber represents relative importance value assigned to corresponding subelement according to Ref. 16. These values

are not the same as the importance factors assigned in Tables 2.2 and 2.3.

CColumn or interior partition wall (concrete).
Table 4.5. Range of values for degradation factor grading in terms of subelement type.

a. Concrete Materials

STRUCTURAL ELEMENT CONCRETE DEGRADATION FACTORS

Chem. Attack Freeze/Thaw Thermal Irradiation Abrasion Fatigue

1. CONTAINMENT VESSEL

a. Foundation 6-10 2-6 0-4 - 0-6

b. Tendon Galleries 6-10 .....

c. Vertical Walls 6-8 2-6 3-9 0-4 0-4 2-8

d. Dome 5-9 2-6 2-6 - - 0-4

e. Suppression Chamber (BWR) 5-9 - 2-6 0-6 - 0-6

f. Polar Crane Support 2-5 - 2-6 0-4 0-4 4-10

g. Ring Girder (PWR) 5-8 2-6 2-6 - - 2-6

2. CONTAINMENT INTERNAL STRUCTURES

a. Bottom Slab 6-10 - 2-6 0-8 2-6 3-6

b. Reactor Pedestal (BWR) 6-10 - 4-9 2-8 - 4-8

c. Primary Reactor Shield 6-10 - 4-10 5-10 - 4-7

d. Floor Slabs 2-7 - 2-7 2-7 0-4 0-4

e. Walls 4-8 - 2-6 2-8 0-4 2-6

f. Columns 4-8 - 4-8 2-8 0-6 2-6

g. Beams 2-6 - 2-6 0-6 - 2-6

h. Fuel Pool Slabs/Walls 4-8 - 2-8 2-8 0-4 4-8

3. SECONDARY CONTAINMENT

STRUCTURE

a. Foundations 6-10 0-6 - - 0-4

b. Slabs 2-6 - 0-4 0-4 0-4 0-4

c. Walls 4-8 2-6 0-6 0-4 0-4 -

d. Columns 4-8 2-6 0-4 0-6 0-6 -

e. Beams 2-4 0-4 0-4 - - 0-4

4. AUXILIARY STRUCTURES

a. Foundations 6-10 - 0-4 - 0-4 0-4

b. Walls and Columns 2-8 0-8 0-4 - 0-6 0-6

c. Slabs/Beams/Roof Slabs 4-9 0-6 0-4 - 2-8 0-6

0- d. Cable Ducts/Pipe Tunnels 5-9 - 0-4 - - -

0 e. Stacks/Cooling Towers 3-7 6-9 0-4 - 0-4 0-4

f. Concrete Intake Piping/Cribs 5-10 - - - 4-8 0-4

a' g. Tanks 2-6 2-6 0-4 - 0-4 0-4

h. Miscellaneous Structures 4-8 6-10 0-6 - 4-10 0-4 (Water Management)

________________________________________________ L S U U I A.
I'.

a,o

..-

0'

Table 4.5. (Cont'd)

b. Metallic Materials

METALLIC DEGRADATION FACTORS

MILD REINFORCEMENT, LINER

STRUCTURAL ELEMENT PLATE AND STRUCTURAL STEEL PRESTRESSING REINFORCEMENT

Corrosion Thermal Irradiation Fatigue Corrosion Thermal Irradiation Fatigue Relaxation

1. Containment Vessel

a. Foundation 6-10 -- 0-10 0-4 - 0-4 0-4

b.Tendon Galleries 6-10 - - - - - - - -

c. Vertical Walls 6-10 2-6 0-4 3-8 7-10 3-6 0-4 2-6 2-8

d. Dome 6-10 0-4 - 2-6 7-10 0-4 0-4 2-6 2-8

e. Suppression Chamber (BWR) 6-10 2-6 0-4 0-4 - - - - -

f. Polar Crane Support 6-10 2-6 0-4 2-6 - - - - -

g. Ring Girder (PWR) 6-10 0-4 - 0-4 7-10 0-4 0-4 2-6 2-8

2. Containment Internal Structures

a. Bottom Slab 6-10 2-6 2-6 0-6 - - - - -

b. Reactor Pedestal (BWR) 4-10 4-8 4-8 4-10 - - - - -

c. Primary Reactor Shield 4-10 2-6 4-9 2-6 - - - - -

d. Floor Slabs 4-10 2-6 0-4 2-6 - - - - -

e. Walls 6-10 2-6 2-6 2-6 7-10 0-6 2-6 0-6 2-8

f. Columns 6-10 2-6 0-4 3-8 - - - - -

g. Beams 4-10 2-6 0-4 2-6 - - - - -

h. Fuel Pool Slabs/Walls 6-10 0-4 2-6 4-10 7-10 0-6 2-6 0-6 2-8

3. Secondary Containment Structure

a. Foundations 6-10 - - - - - - - -

b. Slabs 4-10 0-4 - 2-6 - - - - -

c. Walls 6-10 0-4 - 0-4 - - - - -

d. Columns 6-10 0-4 - 0-6 - - - - -

e. Beans 4-10 0-4 - 0-4 -. - - - -

4. Auxiliary Structures

a. Foundations 6-10 - - 0-6 - - - - -

b. Walls and Columns 6-10 2-6 0-4 0-4 - - - - -

c. Slabs/Beams/Roof Slabs 4-10 0-4 0-4 0-6 - - - - -

d. Cable Ducts/Pipe Tunnels 6-10 - - - - - - -

e. Stacks/Cooling Towers 6-10 0-4 - 0-4 - - - - -

f. Concrete Intake Piping/Cribs 6-10 - - - - - - -

g. Tanks 6-10 0-4 - - - - - -

h. Miscellaneous Structures (Water 6-10 - - 0-4 - - - - -

Management)
Table 4.6. Standards for use in environmental assessments.

Medium Parameter ASTM

Candidate Test Method

Air Acidity D 1654, G 50, G 92

Carbon Dioxide Content Standard Methods

Humidity D 4230, E 337

Temperature Range Standard Methods

Soil Corrosivity/pH G 51

Oxygen Content D 888, D 4646

Micro-organisms/Bacteria D 4412

Sulfide/Chloride Content D 4542

Resistivity G 57

Moisture Content D 2216, D 3017

Groundwater Water Table Elevation/Sampling D 512, D 1293, D 4448

Corrosivity D 1067, D 1293, E 70

Hydrostatic Pressure Standard Methods

Dissolved Oxygen Content D 888

165 NUREG/CR-6424
Table 4.7. Classification and rating of cracks and surface damage developed

by RILEM 104-DDC. 5 8

a. Cracks

Type Rating Appearance

Diagonal 1 (very slight) < Inmm in width

Longitudinal 2 (slight) 1-10 mm in width

Transverse 3 (moderate) 10-20 mm in width

4 (severe) 20-25 mm in width

5 (very severe) > 25 mm in width, spalling and/or faulting

Craze 1 (very slight) barely noticeable

Pattern 2 (slight) clearly visible-no raveling

Checking 3 (moderate) clearly visible-some raveling

Plastic 4 (severe) cracks raveled over substantial area

Plastic 5 (very severe) cracks severely raveled or spalled

Corner crack 1 (very slight) < 1 mm in width

2 (slight) 1-10 mm in width

3 (moderate) 10-20 mm in width

4 (severe) 20-25 mm in width

5 (very severe) > 25 mm in width, spalling and/or faulting

D cracking 1 (very slight) crack width < 1 mm, effective width < 150 mm

from joint or crack

2 (slight) effective width < 250 mm from joint or crack, no

spalling

3 (moderate) as above but with moderate spalling

4 (severe) as above but with severe spalling

5 (very severe) as above but with very severe spalling

NUREG/CR-6424 166
Table 4.7. (Cont'd)

b. Surface Damage

Type Rating Appearance

Chalking

Delamination

Dusting

Exudation

Blistering

Cavitation

Peeling

Exfoliation

Formation of a loose powder resulting from the

disentegration of a surface of concrete or of applied

coating

Separation along a plane parallel to a surface

Development of a powdered material at the surface of

hardened concrete

Liquid or viscous gel-like material discharged through

concrete surface defect

1 (very slight)

3 (moderate)

5 (very severe)

noticeable

thickness of damage < 10 mm

thickness of damage > 10 mm

Popouts 1 (very slight)

2 (slight)

3 (moderate

4 (severe)

5 (very severe)

Scaling 1 (very slight)

2 (slight)

3 (moderate)

4 (severe)

5 (very severe)

barely noticeable

noticeable

holes up to 10 mm in diameter

holes between 10 and 50 mm in diameter

holes > 50 mm in diameter

noticeable

loss of surface mortar without exposure of coarse

aggregate

loss of surface mortar 5 to 10 mm in depth with

exposure of coarse aggregate

loss of surface mortar 10 to 20 mm in depth

surrounding coarse aggregate

loss of coarse aggregate and mortar to a depth in

excess of 20 mm

barely noticeable

clearly noticeable

holes larger than popout of coarse aggregate

holes 150 mm in diameter and at least 150 mm deep

holes larger than 150 mm

Spalls 1 (very slight)

2 (slight)

3 (moderate)

4 (severe)

5 (very severe)

Loss of coarse aggregate 1 (very slight)

2 (slight)

3 (moderate)

4 (severe)

5 (very severe)

barely noticeable

noticeable

pock- marked appearance

closely spaced pock-marks

surface has raveled appearance

167 NUREG/CR-6424
Table 4.8. Influence of moisture state on durability processes.

Effective relative Process*

humidity Carbonation Corrosion of steel Frost Chemical

In In chloride attack attack

carbonated contaminated

concrete concrete

Very low (< 45%) 1 0 0 0 0

Low (45-65%) 3 1 1 0 0

Medium (65-85%) 2 3 3 0 0

High (85-98%) 1 2 3 2 1

Saturated (> 98%) 0 1 1 3 3

0 = insignificant risk; 1 = slight risk; 2 = medium risk; 3 = high risk.

Source: Comit6 Euro-International du B6ton (CEB), Durable Concrete Structures-Design

Guide, published by Thomas Telford Services Ltd., London, England, 1989; reprinted with

permission from CEB and Thomas Telford Services Ltd.

NUREG/CR-6424 168
Table 4.9. Relationship between environmental conditions and

aggressivity to reinforced concrete.

Exposure Environmental conditions

Class

Dry environment; e.g.,

- interior of buildings for normal habitation or offices - exterior components not exposed to wind and weather or soil or water

- localities with higher relative humidity only for a short period of the year

(e.g., > 60% RH for less than 3 months per year)

a Humid environment without frost;* e.g.,

- interior of buildings where humidity is high

- exterior components exposed to wind and weather but not exposed to frost

- components in non-aggressive soil and/or water not exposed to frost

2 b Humid environment with frost;* e.g.,

- exterior components exposed to wind and weather or non-aggressive soil

and/or water and frost

3 Humid environment with frost* and de-icing agent; e.g.,

-- exterior components exposed to wind and weather or non-aggressive soil

and/or water and frost and de-icing chemicals

a Seawater environment; e.g., - components in splash zone or submerged in seawater with one face exposed

to air - components in saturated salt air (direct coast area)

b Seawater environment with frost;* e.g.,

- components in splash zone or submerged in seawater with one face exposed

to air

- Components in saturated salt air (direct coast area)

The following classes may occur alone or in combination with the above classes Ja Slightly aggressive chemical environment (gas, liquid, or solid)

5t b Moderately aggressive chemical environment (gas, liquid, or solid)

c _ Highly aggressive chemical environment (gas, liquid, or solid)

t

Under moderate European conditions.

See International Standards Organization (ISO) classification of chemically aggressive

environmental conditions affecting concrete. The ISO standard is still to be established. See

also Table 4.11.

Source: ComitM Euro-International du B6ton (CEB), Durable Concrete Structures-Design

Guide, published by Thomas Telford Services Ltd., London, England, 1989; reprinted with

permission from CEB and Thomas Telford Services Ltd.

169 NUREG/CR-6424
Table 4.10. Exposure classes for steel reinforcement-related

environmental conditions.

Exposure Environmental conditions

Class

Dry environment: generally dry localities of fairly constant humidity where the

1 relative humidity only infrequently exceeds 70%; e.g., interiors of buildings for

normal habitation or offices.

a Environments with infrequent major variations in relative humidity, giving only

occasional risk of condensation.

2

b Environments with frequent major variations in humidity, giving frequent risks

of condensation.

3 Humid environment with frost* and de-icing agents; e.g., exterior components

exposed to wind and weather or non-aggressive soil and/or water and frost and deicing chemicals

4 Seawater environment; e.g.,

- components in splash zone or submerged in seawater with one face exposed

to air

- Components in saturated salt air (direct coast area)

* Under moderate European conditions.

Source: Comit6 Euro-International du BMton (CEB), Durable Concrete Structures-Design

Guide, published by Thomas Telford Services Ltd., London, England, 1989; reprinted with

permission from CEB and Thomas Telford Services Ltd.

NUREG/CR-6424 170
Table 4.11. Assessment of the degree of

waters and soils containing

chemical attack of concrete by

aggressive agents.

Exposure Exposure

class* 5a class* 5b Exposure class* 5c

Type of attack Weak Moderate Strong Very Strong

Attack Attack Attack Attack

Water

pH value 6.5-5.5 5.5-4.5 4.5-4.0 < 4.0

Agressive GO 2: 15-30 30-60 60-100 > 100

mg CO 2/ý

Ammonium: 15-30 30-60 60-100 > 100

mg NH 4+/i

Magnesium: 100-300 300-1500 1500-3000 > 3000

mg Mg2+/f

Sulphate: 

mg S04 2-/j 200-600 600-3000 3000-6000 > 6000

Soil

Degree of acidity > 20 Xt Xt

according to

Baumann-Gully

Sulphate: 2000-6000 6000-12000 12000 Xt

mg SO 4

2 -/kg of

air-dry soil

* See Table 4.9

f X = conditions of attack that are not found in practice.

Source: Comit6 Euro-Intemational du B6ton (CEB), Durable Concrete Structures-Design

Guide, published by Thomas Telford Services Ltd., London, England, 1989; reprinted with

permission from CEB and Thomas Telford Services Ltd.

171 NUREG/CR-6424
Table 4.12. Results of application of structural aging assessment methodology:

PWR prestressed large-dry plant*

Cumulative

Primary Structure Rank Function .0,........... o oo........... ................................. ................. °........... ,oo.... .............. ,............................ .......... °................... ............... ,°...,......... o.... ......... ........... .. .

Containment Vessel 171 The containment vessels provide structural support and

radiation shielding and containment for safe operation of the

PWR. This containment is of sufficient volume to support

pressure requirements of a pipe break accident. The vessel

supports all internal equipment and transfers loading into the

underlying soil. These structures were all cast-in-place. ..................................................................................................................................................................................................... °.................................

Environment Safety Subelement ....S.u~b.e.lIem.e.n.t ...................................... mp2or~tMaSn.ce .Exposu re .................... S...g..i.fic.a.n.c.e ............. R.a.n.k ..................

Dome 8 6 10 178

Composition

Prestressing Reinforcement, Key Degradation Factors Rating

Deformed Reinforcement, Portland Chemical Attack 6

Cement Concrete, Liner Plate, Freezem ltaw 5 Structural SteelFrzeTa5

Thermal Exposure 3

Fatigue 3

Reinforcing Corrosion 9

Reinforcing Therm. Exposure 3

Prestressing Corrosion 10

Prestressing Fatigue 2

Prestressing Relaxation 7

Environment Safety Subelement ..................................................................... Subelement ......................................... Importance xposure .x .o~.u..e. ........ ..... S.S.i.g gnificane ~ . !........n.. •..e.... ............. Rank R.a.n..k.. ................... .

Vertical Walls (including 8 7 10 178

buttresses)

Composition ....... ..... ..... oo..°...°.., ..... ........ ,.. ... .. .............

Prsrssing Re ssn ei infor f r ceme em nt. n ,....K ............. .... Kev.Dearadation Xe ...... W .D.e.r..a.d..tio........F..a...t..o...r.... Factors ........................................ Ratin ........

Deformed Reinforcement, Portland

Cement Concrete, Liner Plate, Freeze/Thaw 4 Structural Steel, Waterproofing Thermal Exposure 5

Fatigue 4

Reinforcing/Liner Corrosion 10

Reinforcing Therm. Exposure 4

Prestressing Corrosion 10

Prestressing Fatigue 2

Prestressing Relaxation 8

Abrasion 3

* Results are presented only for the post-tensioned concrete containment. Results for other safety-related reinforced

concrete structures are provided in Ref. 259.

NUREG/CR-6424 172
Table 4.12. (Cont'd)

Subelement

Mat Foundation

Importance Environment

Exposure

Safety

Sienificance

Subelement

........................................... ......................................... ....................................

Rank

10 10 10 200

Composition

Deformed Reinforcement, Portland

Cement Concrete, Waterproofing,

(prestressing reinforcement - local)

Key Degradation Factors Rating

Chemical Attack

Thermal Exposure

Irradiation

Reinforcing Corrosion

Prestressing Corrosion

Prestressing Relaxation

10

4

3

10

6

4

.......................................................................................................................................................................................................................................

Environment

Exposure

Safety

Significance

Subelement

Subelement Rank r ................................... tgG 

Ring Girder

Importance ......... .............................................................

9 6 10 182

Composition

Prestressing Reinforcement

(anchorages), Deformed Reinforcement,

Portland Cement Concrete

Key .......... Degradation .............................................

Factors RatinE.........

Chemical Attack

FreezefIhaw

Fatigue

Reinforcing Corrosion

Prestressing Corrosion

Prestressing Relaxation

5

4

3

9

10

7

Environment Safety Subelement ...S..ub m tm ... ................. o tr......S.i.n.e.................................... RýTp. ...................... i, ificance Rank

Tendon Gallery 3 10 4 118

Composition

Deformed Reinforcement, Portland ...Ke..e-.......a..'....to....Key Degradation Factors R ating Cement Concrete, Waterproofing, Ceia tak1

Structural Steel Chemical Attack 10

Reinforcing Corrosion 10

Cumulative Primary Structure Rank Function

Containment-Internal 144 Provide structural support and radiation shielding for NSSS

Structures equipment for all operating conditions. Provides human

access to primary components and support for refueling

operations. These structures are massive in section and were

cast-in-place.

173 NUREG/CR-6424
IDENTIFY SAFETYRELATED OR CATEGORY I

STRUCTURES

ul

II.-

0

U'

IU'

U0

4x

LU

0.

LUM.

f - DIVIDE INTO

SUBELEMENTS AND

MATERIAL CONSTITUENTS

SUBELEMENT IAI

MATERIAL

CONSTITUENTS

CONCRETE

REBAR

IDENTIFY COMPONENT

CLASSIFICATION

VALUES FOR EACH

SUBELEMENT U d

+

IDENTIFY SITE-SPECIFIC

DEGRADATION FACTORS

E

ENVIRONMENTAL

-- EXPOSURE

EVALUATE INFLUENCE /

DETERMINE GRADINGS FOR

DEGRADATION FACTORS

- II I1COMBINEIAPPLY WEIGHTING

FACTORS TO COMPUTE

SUBELEMENT &

STRUCTURE RANK

-A

I

INSPECTABILITY AND

EARLY IDENTIFICATION I

TIME RATE OF

DETERIORATION I

REPAIRABIUTY II

i J

DEVELOP LISTING OF

CRITICAL

SUBELEMENTS/

STRUCTURES

ULTIMATE IMPACT

I

OTHER FACTORS

II I

Fig. 4.1. Structural aging assessment methodology flow diagram.

NUREG/CR-6424 174
0

0

2~

So

S.

S

0

0

.5

Q

t

Ci-> 1

or

Carbonation to

Depth of Steel

AL

0.4 < C-1

or

Carbonation Depth

Approaching Steel

Structure

requires remedial action

Structure

requires further

evaluation

Structure

acceptable

as-is without

additional

assessment

0< CU<- 0.4

or

Uncarbonated

No visible degradation I or cracking

0<w<0.4mmx

Concrete cracking with or

w/o staining

0.4 < w < 1.0 mm

Concrete cracking,

staining, and spalling

w> 1.0mm

Damage State in Terms of Crack Width, w

Fig. 4.2 Damage state chart relating environmental exposure,, crack width,

and necessity for additional evaluation or repair.

175 NUREG/CR-6424
A

or

-Carbonation to

Depth of Steel

SAL Structure

= 0 •requires

O• .4< C< 1 remedial action

or

Carbonation Depth

C •Approaching Steel

Structure

A- requires further T evaluation

0 < C'< OA Structure0.

or acceptable

Uncarbonated as-is without 0

additional _+_ assessment

0 - 0.001 - 0.002 - 0.003 - 0.004

Half-Cell Potential Reading, volts (c.s.e.)

No visible degradation Concrete crackingj Concrete cracking,

or cracking w']-with or w/o stainingw--staining, and spalling"

Common visual observations

Fig. 4.3 Damage state chart relating environmental exposure, half-cell potential reading,

and necessity for additional evaluation or repair

NUREG/CR-6424 176
5. RELIABILITY-BASED METHODOLOGY FOR CURRENT AND FUTURE

CONDITION ASSESSMENTS

Numerous uncertainties are encountered in trying to assess the current condition and likely

future performance of nuclear power plant (NPP) reinforced concrete structures. Such

uncertainties arise from the randomness in the initial material properties, operating and

environmental loads, material changes and possibly degradation due to aging, mechanistic models

used to predict time-dependent structural behavior, and many other factors. Structural reliability

methods model the uncertainties due to the above sources with probability distributions and

provide a mechanism for dealing with the numerous sources of uncertainty consistently and

explicitly. Such methods have been used to develop a first-generation of limit state design codes,

in which the safety checks are associated with specific limit state probability or reliability. 275

-277

This previous work took into account the stochastic nature of resistance and loads. However, it

did not consider the effect of time-dependent changes in the structural properties on reliability of

components and systems.

In recent years, however, structural reliability techniques have advanced to the point where

it is possible to make a quantitative evaluation of the remaining service life of a reinforced concrete

structure based on knowledge of the condition of the structure when built, its service history, its

present condition, and projected use during a defined continued service period. The reliabilitybased methodologies accomplish this through integration of information on design requirements,

material and structural degradation and damage accumulation, environmental factors, and

nondestructive testing technology into a decision tool that provides a quantitative measure of

structural reliability under projected future service conditions. 278-2 8 1 Time-dependent reliability

analysis methods provide a framework for performing condition assessments of existing structures

and for determining whether in-service inspection (ISI) and repair are required to maintain

reliability and performance at the desired regulatory level. These methods can also be utilized to

identify optimum ISI/repair strategies that minimize cost while ensuring that the structure maintains

a sufficient level of reliability for public safety during a prescribed service life.

5.1 RELIABILITY-BASED METHODS FOR CONDITION ASSESSMENT

The performance of a structure in service is assessed by comparing its state of behavior to

one of several limit states. When a structure or structural component becomes unfit for its intended

purpose, it is said to have reached a limit state. In the reliability-based condition assessment

methodology developed subsequently, structural performance in the presence of uncertainty is

measured in terms of a limit state probability. Ultimate limit states relate to safety under extreme

conditions. A concrete structure is considered to have failed when a load effect (e.g., axial force,

moment, shear or some combination of these effects) to which it is subjected exceeds its available

capacity at that time. Such an event should have a low probability of occurrence. Serviceability

limit states relate to disruption of function under conditions of normal usage. Excessive deflections

or crack widths under normal service loads are examples of serviceability limit states in concrete

structures.

Aging and durability issues introduce the time factor into limit states design. One cannot

deal with durability issues rationally without introducing the notion of design or service life. 2 82

The limit state probability must be determined with respect to a service life in order to be useful as a

decision variable. The service life is defined as that period during which the structure is able to

withstand all loads safely. A design service life (or maintenance interval) is that period during

which the probability of the structure performing its intended functions is acceptable.

177 NUREG/CR-6424
5.1.1 Degradation Mechanisms

Time-dependent reliability analysis and service life predictions for reinforced concrete

structures require time-dependent stochastic models of the structural strength. In concept,

stochastic strength models can be derived from: (1) mathematical models describing the effects of

the aging process resulting from service and environmental factors on steel and concrete materials

and component geometry; (2) accelerated life testing; or (3) a combination of the two. At the

current state of the art, these effects are often known qualitatively; however, quantitative models

that describe material degradation processes often are empirical in nature. 113 The service life

determinations often require that these models be extrapolated outside the range of experimental

data. Primary degradation mechanisms that can impact the NPP reinforced concrete structures

were described in Sect. 2.3.2 along with information on models for service life estimations.

5.1.2 Statistical Data on Loads and Resistance

The condition assessment and time-dependent reliability analysis require probabilistic

descriptions of loads and structural strengths. Some of the statistical data needed to support this

methodology have been collected as part of previous research to develop probability-based design

requirements for ordinary building construction and for NPP structures. 275- 277 Load and

resistance models and data have been analyzed in detail283 and are summarized below.

5.1.2.1 Structural loads

The occurrence and variation in intensity of a structural load, X(t), in space and in time is

modeled as a stochastic process. It is convenient to classify these loads in a general sense

according to the way in which they vary in time, as illustrated by the typical sample functions in

Fig. 5.1. Two sets of probability density functions (pdf) are also shown in Fig. 5.1; fx(x)

describes the intensity of the load at any time, while fz(x) describes the maximum load within the

interval of time (0, t). Permanent loads [Fig. 5.1(a)] are random in intensity but do not vary

significantly in time. The probability density function of the load at any point in time, fx(x), is

sufficient to describe the statistical characteristics of the load process, and is identical to fz(x).

Loads with significant variation in time may act essentially continuously [Fig. 5.1(b)] or

intermittently [Fig. 5. 1(c)]. The probability density function at any point in time is insufficient to

describe these load processes statistically or to determine fz(x); information concerning the timevariation of the load process is required as well.

A relatively simple model of the overall temporal variation in structural loads is provided by

assuming that the changes in the load magnitude occur in time according to a Poisson process.

Under this assumption, the number of load changes, N(t), in the interval (0, t) is described by the

probability function,

P [N(t) = n] = ( .,t)n • exp(-At) n! ;n=0,1,2, n0 ,2.. (5.1) 51

in which P[-] = probability of the event in the brackets and the parameter X = mean rate of

occurrence of the events. The duration of one load event, Ti, also is a random variable. If the

events occur according to a Poisson process with parameter X and the load is a continuous variable

load [Fig. 5.1(b)], the duration has an exponential distribution, with mean value E[Ti] = r = 1/.

If the load is intermittent X(t) = 0 for significant periods of time [Fig. 5.1 (c)], the mean occurrence

rate is X = p/r, in which p = probability that the load is nonzero at any point in time.

NUREG/CR-6424 178
If the load intensity varies negligibly or slowly during each load event, its effect on a

structure is essentially static. For reliability analysis purposes, then, the load intensity can be

treated as constant during any event. The actual time-varying load thus is replaced with the

sequence of load pulses illustrated by the dashed lines in Figs. 5.1 (b) and 5.1 (c). It is customary

to assume that the pulse magnitudes are identically distributed and statistically independent random

variables, each with cumulative distribution function (cdf), Fx(x). Such a load process is referred

to as a Poisson pulse process.284 ,

285 The parameters X, t, and p and F(x) provide a complete

description of the micro-variation of the load in time.

It is assumed that significant structural loads can be modeled as a sequence of such load

pulses. Such a simple load process has been shown to be an effective model for extreme loads on

structures, since normal service loads challenge the structure to only a small fraction of its strength.

Load statistics used in the subsequent time-dependent reliability analysis are summarized in Table

5.1. These statistics are believed to be reasonable and conservative. Only limited data on NPP

loads are available; more detailed descriptions are given elsewhere. 283

5.1.2.2 Structural resistance

The strength, R, of a reinforced concrete component is described by2 86

R = B. Rm(Xj, X2, ... , Xm), (5.2)

in which X1, X2, ... are basic random variables that describe yield strength of reinforcement,

compressive or tensile strength of concrete, and structural component dimensions or section

properties. The function Rm(o) describes the strength based on principles of structural mechanics.

Modeling assumptions invariably must be made in deriving Rm(°), and the factor B describes

errors introduced by modeling and scaling effects. The probability distribution of B describes bias

and uncertainty that are not explained by the model Rm(-) when values of all variables Xi are

known. The probability distribution of B can be assumed to be normal.2 87 A more accurate

behavioral model leads to a decrease in the variability in B and thus in R.

Probability models for R usually must be determined from the statistics of the basic

variables, Xi, since it seldom is feasible to test a sufficient sample of structural components to

determine the cumulative distribution function of R directly. Typical statistical data on material

strengths and dimensions of reinforced concrete structural elements are summarized in

Table 5.2.283,286,287 The material strengths presented are based on "static" rates of load. Longterm strength changes in the concrete or steel due to maturity of concrete, environmental stressors,

and possible corrosion of reinforcement are not reflected in these statistics. When data on the

strength of concrete or steel can be obtained by limited in situ sampling as part of a condition

assessment of a specific structure, that data should be utilized.

Typical means and coefficients of variation (cov) in strength for reinforced concrete

structural elements are summarized in Table 5.3. These estimates were obtained by Monte Carlo

simulation using the strength and dimensional variabilities in Table 5.2. The modeling uncertainty

was described by a normal distribution, with a mean .tB = 1.05 and c.o.v. VB = 0.06 for flexure

and compression and with yB = 1.15 and VB = 0.15 for shear. In all cases, the nominal flexural

179 NUREG/CR-6424
strength for concrete structural members, Mu, the nominal shear strength, Vu, and the nominal

axial strength, Ru, were computed according to American Concrete Institute (ACI) Standards

31816 and 349.17

5.1.3 Time-Dependent Reliability Analysis

The failure probability of a structural component can be evaluated as a function of (or an

interval of) time if the stochastic processes defining the residual strength and the probabilistic

characteristics of the loads at any time are known. The strength, R(t), of the structure and applied

loads, S(t), both are random functions of time. At any time, t, the margin of safety, M(t), is

M(t) = R(t) - S(t). (5.3)

Making the customary assumption that R and S are statistically independent random variables, the

(instantaneous) probability of failure is,

Pf(t) = P[M(t)< 0] = f"FR(x) fs(x) dx. (5.4)

0

in which FR(X) and fs(x) are the cumulative distribution function of R and probability density

function (pdf) of S. Equation (5.4) provides an instantaneous quantitative measure of structural

reliability, provided that Pf(t) can be estimated and/or validated. 288

For service life prediction and reliability assessment, one is more interested in the

probability of satisfactory performance over some period of time, say (O,t), than in the snapshot of

the reliability of the structure at a particular time provided by Eq. (5.4). Indeed, it is difficult to use

reliability analysis for engineering decision analysis without having some time period (say, an inservice maintenance interval) in mind. The probability that a structure survives during interval of

time (0,t) is defined by a reliability function, L(0,t). If, for example, n discrete loads S1, S 2,

Sn occur at times tj, t2, ..., tn during (0,t), the reliability function becomes,

L(0,t) = P[R(t,)> Sl,...,R(tn) > SD]. (5.5)

If the load process is continuous rather than discrete, an analogous expression derived from an

upcrossing analysis 289 can be used to obtain an approximation to Eq. (5.5).

The conditional probability of failure within time interval (t), given that the component has

survived during (0,t), is defined by the hazard function:

h(t) = -d In L(0,t)/dt. (5.6)

Solving for L(O,t) yields,

L(Ot) = exp[- h(x)dx]. (5.7)

The hazard function is especially useful in analyzing structural failures due to aging or

deterioration. For example, the probability that time to structural failure, Tf, occurs prior to a

future maintenance operation scheduled at t + At, given that the structure has survived to t, can be

evaluated as,

NUREG/CR-6424 180
P[Tf < t + AtITf > t] = 1- exp[-f t+At h(x)dx]. (5.8)

In turn, the structural reliability for a succession of inspection periods is

L(Ot) HII L(ti_1, ti) exp( f h(x)dx (5.9) t, ti

in which ti-1 = 0 when i = 1. Note that failures in successive intervals, (ti-1 , ti) generally are not

statistically independent events.

The hazard function for pure chance failures is constant with time. When structural aging

occurs and strength deteriorates, h(t) characteristically increases with time. ISI and repair impact

the reliability analysis through the hazard function, causing it to change discontinuously at the time

an inspection/repair operation is performed. Undegrading and degrading structural components

can be distinguished in a time-dependent reliability analysis by their hazard functions. Much of the

challenge in structural reliability analysis involving deteriorating structures lies in relating the

hazard function to specific degradation mechanisms, such as corrosion.

With the assumption that loads can be described by a Poisson pulse process, the reliability

function becomes

L(Ot) 0f exp[-Xtl - t 1 fFs[rg(x)dx]fR (r)drJ]' (5.10)

in which fR(r) is the pdf of initial strength, R(0), and g(t) equals the mean of R(t)/R(O), a function

describing the degradation of strength in time (see Fig. 5.2). The limit state probability, or

probability of failure during (0,t), is F(t) = I - L(0,t); F(t) is not the same as Pf(t) in Eq. (5.4).

5.1.4 Illustrations of Service Life Prediction

5.1.4.1 Reinforced concrete slab

Time-dependent reliability concepts are illustrated with a simple conceptual example of a

concrete slab drawn from recent research on aging of reinforced concrete structures. 279 The slab

was designed using the requirements for flexure strength found in ACI Standard 31816

0.9 Rn = 1.4 Dn + 1.7 Ln, (5.11)

in which Rn is the nominal or code resistance, and Dn and Ln are the code-specified dead and live

loads, respectively. The strength of the slab changes in time, initially increasing as the concrete

matures and then decreasing due to (unspecified) environmental attack. This situation is illustrated

conceptually by the sample functions r(t) and s(t) for strength and load in Fig. 5.2. Two other

situations are also illustrated in Fig. 5.2, one in which the strength degrades linearly to 90% of

initial strength at 40 years and one in which the strength remains constant with time. In general,

the behavior of resistance over time must be obtained from mathematical models describing the

degradation mechanism(s) present (see Sect. 2.3.2). The statistics used in this example are

contained in Tables 5.1-5.3.

181 NUREG/CR-6424
Figure 5.3 compares the limit state probabilities [F(t) = 1 - L(t) in Eq. (5.10)] obtained for

the three degradation models considered in Fig. 5.2 for service lives (0,t) ranging up to 60 years.

When R(t) = R(0) and no degradation of strength occurs, a result is obtained analogous to what

has been done in probability-based code work to date. Neglecting strength degradation entirely in

a time-dependent reliability assessment can be quite unconservative, depending on the timedependent characteristics of strength.

5.1.4.2 Reinforced concrete shear wall

As shown by the previous example, the failure probability of a structural component under

stationary random loading can be evaluated as a function of time if the strength degradation and the

probabilistic characteristics of the initial strength are known. It was assumed that strength

degradation at any section was caused by one randomly occurring defect of random intensity.

Such a model is reasonable when the degradation is such that at most one defect or zone of damage

is likely to occur within a given cross section. The strength degradation of a reinforced concrete

beam or column due to corrosion of reinforcement can be estimated by such modeling. However,

there are cases where several defects or zones of damage may contribute in reducing strength. For

example, the strength of a reinforced concrete wall in flexure and/or shear might degrade due to the

effects of sulfate attack occurring at several points along a given cross section of the wall. The

evaluation of the (random) residual strength of the wall requires that the cumulative effect of

defects in a cross section be considered. Recent research has provided a method whereby the

impact of randomly occurring multiple defects on structural capacity can be considered.290 Some

results are summarized in the following.

The wall considered is a low-rise wall with a height-to-width ratio equal to unity. It is

subjected to vertical load, D, that is uniformly distributed on the top of the wall, and in-plane lateral

load, V, that is concentrated at the top of the wall. The shear strength of concrete walls can be

estimated from empirical models. 16 ,

29 1 These models are not sufficient to analyze the strength of

deteriorating low-rise shear walls. Although finite-element analysis is versatile and able to provide

detailed information on the shear resistance mechanisms, it requires lengthy computational effort,

especially when adapted to reliability analysis. A recent theoretical approach for evaluating shear

strength of reinforced concrete components 292-294 determines the ultimate shear strength as the

sum of the forces sustained by a truss mechanism, Vt, and by an arch mechanisms, Va. It is

assumed that the wall fails if all the reinforcing bars yield in tension and the concrete arch crushes

in compression. According to the lower bound theorem of plasticity,2 95 this approach provides a

conservative estimate of the shear strength. These models have been modified for the reliability

analysis of a degrading low-rise concrete shear wall. 290 Figure 5.4 shows that the strength

predicted by this method compares well to experimental tests of low-rise shear walls.296,

297

Wall in Shear A wall subjected to sulfate attack or attack by strong acids can suffer a

loss of concrete section. If the wall is not heavily reinforced in the transverse direction, the

contribution of the truss mechanism is small. Thus, it can be assumed that only the strength of the

arch mechanism decreases due to the loss of concrete section while the strength attributed to the

truss mechanism is independent of the degradation. If the wall is reinforced in the longitudinal

direction, the vertical reaction is sustained by the longitudinal reinforcement, and degradation of

concrete outside the concrete strut in the arch mechanism can be neglected. Assume that the stress

in the concrete strut is uniform. Then the degradation function of the shear wall can be given by

G(t) = Vt + Va(t) _ Vt + Ga(t) Va(0) (5.12) vu0 Vu0

NUREG/CR-6424 182
in which Vuo is the initial shear strength of the wall, Va(t) is the shear strength of the arch

mechanism at time t, and Ga(t) is the degradation function of the shear strength of the arch

mechanism.

Wall in Flexure and Compression The ultimate flexure capacity of a cross section is

expressed as

b _ b-kC)+C b-d (5.13)

in which Ts and Cs are the total force transferred to reinforcement in the tension and compression

zone, respectively, dc is the concrete cover, cu is the distance from the compressive face to the

neutral axis, and k2cu locates the compressive resultant Cc.

For illustration, assume that

" The wall is subjected to time-invariant dead load, D, that is uniformly distributed on

the wall, and intermittent lateral load V, that is concentrated at the top of the wall

and may act either in-plane or out-of-plane.

• The wall is designed for in-plane shear based on the current design requirement 17

0.9Rn = Ess (5.14)

in which Rn is the nominal shear strength and Ess is the structural action due to

safe-shutdown earthquake. The statistical characteristics of the earthquake load and

shear strength are shown in Tables 5.1 and 5.3, respectively. It is assumed that Ess

-- 3D = 3.21MN.

* The mean initiation rate of local damage per unit surface area, v u, due to chemical

attack is time invariant and is 0. l/m2/year.

* The defect intensity is modeled as,

Y(t) = C(t - TI)2 (5.15)

in which C is a time-invariant random variable described by a lognormal

distribution with mean value, mc, of 2.22 x 10-6/year and coefficient of variation,

VC, of 0.5. This value results in an average defect size that is large enough after

several years following its initiation to be found by visual inspection.

* The 28-day specified compressive strength of concrete equals 27.6 MPa. The

corresponding mean compressive strength at 28 days is 28.7 MPa.287 The

specified yield strength of the reinforcement is 414 MPa and the mean is 465 MPa.

183 NUREG/CR-6424
* Compressive strength of the concrete increases during the first 10 years but does

not change thereafter. The mean compressive strength (units of MPa) at time t is

estimated based on information provided in Ref. 134 according to* { 15.51 + 3.95 In t t < 10 years fc(t) = 47.91 t > 10 years'

in which t is in days. The concrete section area decreases with time as damage

accumulates. Other engineering properties of the wall are assumed to be timeinvariant.

The mean degradation in shear strength of the wall in which chemical attack is occurring is

illustrated in Fig. 5.5, assuming vu = 0.1/m 2/year. The mean degradation in shear strength

evaluated ignoring the cumulative effect of multiple defects in a section on the strength degradation

of the wall is also illustrated in the figure. The gain in shear strength due to the continuous

hydration of concrete more than compensates for the strength degradation due to the loss of section

area up to about 50 years. Ignoring the cumulative effect of defects provides an overly optimistic

degradation function.

The failure probabilities and the hazard functions associated with the strength degradation

illustrated in Fig. 5.5 are presented in Figs. 5.6 and 5.7, respectively. The increase in failure

probability due to the strength degradation is small because of the large variability in earthquake

load intensity. 29 8 However, the hazard function increases rapidly after about 50 years when the

cumulative effect of defects is considered.

The mean degradation in flexure/compression strength of the wall is more sensitive to the

loss of the outer part of the cross section area than is the shear strength, as shown in Fig. 5.8.

Since the loss of the outer part of the wall leads to a reduction in the internal moment arm, the

flexural strength degrades more rapidly than the shear strength, which decreases linearly as a

function of the loss of cross-section area. Thus, if the governing limit state of the wall is flexure,

special attention should be given to the potential for degradation when performing a condition

assessment.

5.2 ROLE OF INSPECTION/REPAIR IN MAINTAINING RELIABILITY

Forecasts of time-dependent reliability (e.g., Fig. 5.3) enable the analyst to determine the

time period beyond which the desired reliability of the structure can not be assured. At such time,

the structure should be inspected and its condition evaluated. Intervals of inspection and repair that

may be required as a condition for continued operation can be determined from the time-dependent

reliability analysis. Figure 5.9 presents a conceptual illustration of the effect of inspection and

repair on the failure rate. The effect of the inspection/repair is to remove larger defects from the

structure and upgrade its strength, thus reducing its conditional failure rate. As the structure ages,

the failure rate increases until another inspection/repair operation occurs. Such inspection/repair

strategies should be designed so that the integrated effect of h(t) in Fig. 5.9 remains at or below an

established target value during its required service period. ISI and repair are a routine part of

managing aging and deterioration in many engineered facilities; 299 work is already underway to

develop reliability-based policies for offshore platforms,300 and aircraft.30 1

* This expression should not be used "a priori" to estimate concrete strength as a function of time for reasons cited in

Chapter 2.

NUREG/CR-6424 184
5.2.1 Degradation Function Based on Individual Damage Intensities

The damage intensity is modeled as a state variable taking a value within the interval [0,1];

the values 0 and 1 indicate no damage and no residual strength, respectively. Examples of this

state variable describing damage would include the ratio of area of reinforcement lost due to

corrosion to the original area, the ratio of strength loss of concrete due to sulfate attack to the

strength of undegraded concrete, and the ratio of the depth of crack in a member to the member

thickness. The following assumptions are made:

(a) Initiation of damages in a component is described by a Poisson process in which

the expected number of damages in time interval (t, t + At) (t > 0) is f •'t v(T)dr.

The damage initiation rate v(t) is dependent on the surface area or volume of the

component.

(b) Damages initiate homogeneously over the surface area or volume of the

component.

(c) Once damage initiates at location j, it grows according to

X3(t) = a " j (5.17)

in which Xj(t) j = 1, 2, ... is the intensity of damage at time t, T. j = 1, 2, ... are

the random initiation times of damage, Cj's are damage growth rates that are

identically distributed and statistically independent random variables described by a

cdf Fc(c), and a is an deterministic parameter. The assumption of independent

Cj's provides a conservative estimate of failure probability. Parameters C and a

depend on the degradation mechanism (e.g., Refs. 113 and 119). If damage

grows linearly, a = 1.

(d) The degradation function G(t) for a component is defined in terms of damage

intensities as

G(t) = 1- max{oX(t)}. (5.18)

all j

Consider damages that initiate within interval (tj, t). Assume first that the number of these

damages, Nj(t1, t), is n. From assumption (c), the cdf of Xj(t), FX. (x; tj, t), is expressed as

Fxj (x; tl, t) P[Ci(t - TI.)a <x] (5.19)

f t (t --X JfT )d'r (5.20)

185 NUREG/CR-6424
where fTlj ('T) is the pdf of T j. Given N1(tl, t) = n, the rank-ordered initiation times, TI1, T12,

Tim are n order statistics of random variables W11, W12 ' .... Win that are statistically

independent and identically distributed with pdf expressed as 30 2

v(w)

fWj(W) =

0

;tI w <w t

(5.21)

; Otherwise

Therefore, the cdf of the intensity of an arbitrary damage, X(t), that initiates within (tj, t) is,

FX(x t, t) =(r)d. _tc~ T)cxOfI (5.22)

Note that fwI (w) is an unconditional pdf. Since the Cj's and W1j's are statistically independent of

one another, the Xj(t)'s are also statistically independent. Accordingly, the cdf of Xma(tl, t) -

max{Xj(t) that initiated within (tj, t]} is,

FXmax (x; tj, tjNl(t1, t) = n) = [Fx(x; t,, t)]n.

Removing the condition that N1 (t1, t) = n,

(5.23)

FXmax (X; t1, t) = = [Fx(X; tl, t)]n

n~=O

jt] v('r)dr) Jexp({', v(,r)d~rj

n! (5.24)

= exp[ -fi v(r)d-r{1-Fx(X, t) .

From assumption (d), the mean and variance of the degradation function are evaluated by

E[G(t)] = E[1- Xmax(O,t)]

= 1-J;[1-Fx(x;0,t)]dx,

Var[G(t)] = Var[Xmax(O;t)]

(5.25)

and

= J

1 2x[l-FXm(x;0,t)jdx-{E[G(t)]}2 . (5.26)

NUREG/CR-6424 186
In the course of the analysis, it was found that the variability in G(t) has a secondary effect on the

time dependent reliability of a component and, thus, the reliability can be evaluated considering

only the mean of G(t), defined at g(t). 30 3

5.2.2 Degradation Function After Repair

No nondestructive evaluation (NDE) method can detect a given defect with certainty. The

imperfect nature of NDE methods must be described in statistical terms. Figure 5.10 illustrates

conceptually the probability, d(x), of detecting a defect of size x. Such a relation exists, at least

conceptually, for each ISI technology. The strength following repair depends on both the

detectability and repair strategy adopted.

5.2.2.1 Full inspection/repair

Assume that during inspection/repair the entire component is inspected, that all detected

damages are repaired immediately and completely, and that the repaired parts of the component are

restored to their initial strength levels. Then the effect of inspection/repair on g(t) depends on the

detectability function, d(x), associated with the NDE method. The inspection with higher

d(x) makes repair more likely and, accordingly, leads to higher values of the degradation function,

g(t). In the limit, if an inspection is perfect (i.e., d(x) = 1 for x > 0) then the component is restored

to its original condition by the repair.

First assume that the detectability function, d(x), is defined as,

0 ; O !ý < Xth

d(x) = 1 (5.27)

1 Xth < x< 1

where Xth is the minimum detectable value of damage (see Fig. 5.10). The same detection

threshold values are assumed for all inspections. Following m inspections at tR = {tR .... , tRm 1,

some of the damages are repaired and the cdf describing X(t) and the number of damages existing

at time t > tRm, N(t), changes. The intensities of damages that initiate after tRn are independent

of repair, and only the pdf of the intensities of damages initiating before tRm is updated. Let us

consider damages that exist at time t and initiated with (A) (0, tR ] and (B) (tRm, t]. The number

of damages left unrepaired after tRm, N(A), can be described by a filtered Poisson process with a

parameter p. v(w) where,

p = P[A damage is not repaired by tRm] = Fx(xth, 0, tRm), (5.28)

while the number of damages initiating within (tRm, t], N(B), is described by a Poisson process

with a parameter v(w). In other words, the number of defects existing at time t can be described

by a nonstationary Poisson process within a parameter v"(w) given by

187 NUREG/CR-6424
{ p v(w) v"(w) = P

Sv(w)

;O <W tRm

;tRm <W<t

(5.29)

Therefore, the procedure to estimate the degradation function for a component before an

inspection/repair can be used to estimate the function after multiple inspection/repair, by replacing

v(w) by v"(w), and Fx(x; 0, t) by the updated cdf F"x (x; tR, t).

By the theorem of total probability, F"X (x; tRm, t) can be expressed as,

F "x(X; tRm, t) = FX(A)(x; tRm, t)'P(A) + FX(B)(x; tRm, t)'P(B), (5.30)

in which FX(A) (x, tRm, t) and FX(B) (x; tRm, t) are the cdf of intensity of damages in group (A),

X(A)(t), and intensity of damages in group (B), X(B)(t), respectively, and P(A) and P(B) are the

probabilities that a defect belongs to group (A) or (B),

P(A) = P[w -'Rm] = fJ1Rm (w)dw

P(B) = 1- P(A),

(5.31)

(5.32)

in which fw1 (w) is evaluated by Eq. (5.21) replacing v(w) with v"(w). FX(A) (X, tRm, t) is

expressed as,

FX(A) (x; tRm' t) = P[X(t) < x X(tRm ) < Xth] (5.33)

(5.33)'

The cdf FXmax(B) (x; t)is given by,

FXmax(B) (x; t) = FX(X;,R t). -534 (5.34)

NUREG/CR-6424 188
In general, the detectability function, d(x), is not a step function but rather a non-decreasing

function of damage intensity (see Fig. 5.10). Procedures for dealing with this more general

detectability function and for partial inspections have been developed and described elsewhere. 303

Examples Illustrating Effect of Inspection/Repair Operations

Simple conceptual examples are provided below to illustrate the effect of single

inspection/repair approaches. The examples consider a beam in flexure designed according to

Eq. (5.11) with the load and resistance statistics given in Tables 5.1 and 5.3, respectively.

Single Inspection/Repair Consider the following

a. Every part of the structure is fully inspected and all detected damages are repaired

completely.

b. The initiation of damages is described by a stationary Poisson process with a

parameter v = 5/yr that is dependent on the surface area or volume of the structure.

c. Damage grows linearly as a function of time as described by Eq. (5.17) with (x = 1.

d. The degradation rate, C, is lognormally distributed with mean value,

gC = 0.00125, that corresponds to E[X(40)ITI = 0] = 0.05, and with a coefficient

of variation, V. = 0.5.

The effect on the mean degradation function of inspection/repair described by several

detectability functions is illustrated in Fig. 5.11. The first detectability function considered is a

step detectability function in which Xth = 0.03; in the second, Xth is uniformly distributed [i.e.,

d(x) is linear between Xmin and x,=a, where d(xmin) = 0 and d(xmax) = 1]; in the third and fourth,

Xth is lognormally distributed with mean, gXth' equal to 0.03, and coefficient of variation, VXth,

equal to 0.3 or 0.5. It is assumed that inspection/repair is carried out at tRm = 20 years. The mean

degradation function decreases as Vxth increases (that would result in lower reliability); however,

the effect of the general shape of d(x) (Fig. 5.10) is not significant and decreases with time elapsed

since inspection. This insensitivity of the mean degradation to the choice of detectability function

suggests that a general detectability function might be approximated for practical purposes by a step

function with Xth = Lxth. This would be advantageous for NDE technologies currently used for

reinforced concrete structures because information on /uxth may be more readily available than

information on d(x).

Multiple Inspection/Repair The effect of multiple inspection/repair and the mean

degradation function is illustrated in Fig. 5.12, assuming a step detectability function and the same

assumptions as used in the previous example. Inspection/repairs are carried out at 20, 30, 40, and

50 years with xth = 0.05 when E[X(40)ITI =0] = 0.05. For comparison, the mean degradation

function for a component without repair and for a component with one repair at 30 years with

Xth = 0.01 is also presented in the figure. With multiple inspections/repairs, the mean degradation

function can be kept within a narrow range during the service life of the structure. This suggests

the existence of an optimum inspection/repair strategy in which the failure probability of the

189 NUREG/CR-6424
component is kept below an established target probability during its service life and the total

expected cost, defined as the sum of the cost of inspections/repairs and expected cost (loss) due to

failure, is minimized.

5.2.2.2 Partial inspection/repair

In many instances, only a small part of the structure or component is inspected to assess its

condition because of cost or accessibility constraints. Inherent in this procedure is the assumption

that any part of the component is representative of the component as a whole, and the rest of the

component is amenable to a statistical representation. 304 At each time when a partial inspection is

carried out, we gain some information about the strength and degradation of the component and

make a decision whether or not to repair the component, depending on the result of the inspection.

Assume that each inspection is carried out using an NDE method with detectability described by a

step function. A decision policy at each inspection can be established as follows:

1. Inspect a% of the component (stage I).

2. If the maximum intensity of damages detected at stage I, Xmaj(tR) is less than the

predetermined critical value, Xcr, then no further inspection is performed until the

next scheduled inspection.

3. If Xmax(tjR) Ž Xcr, then inspect another a% of the component (stage II).

4. If the maximum intensity of damages detected at stage II, XmalJ(tR), is less than

Xcr, then no further inspection is performed until the next scheduled inspection.

5. If Xmaxjj(tR) > Xcr, then inspect the entire component and repair all detected

damages (state III).

6. If a stage III inspection is carried out at tRj, then during the rest of its service life

the component will be fully inspected and all detected damages will be repaired.

According to this policy, the parameters of the degradation model are updated after each

partial inspection (stage I and stage II), but the component is not repaired. Since the portion of a

component partially inspected is generally small (10% or less), the impact of not repairing the

detected damages on the degradation function and on the failure probability of the component is

assumed to be small.

A series of partial inspections using this policy leads to a decision tree as shown in

Fig. 5.13. Although other policies can be envisioned (e.g., Ref. 305), this policy is consistent, in

a general sense, with acceptance criteria currently in use for weldments and certain other critical

structures. The total number of branches in the decision tree after the mth inspection is

2m + 2m-1 + 1.

Besides the assumptions (a)-(d) made in Sect. 5.2.2.1, assume that the mean damage

initiation rate, v, and the mean of the damage growth rate, C, are random variables jointly

distributed with pdf, fvý(4, 77). Furthermore, assume that the cdf of the maximum intensity of

damages existing at time t without repair can be evaluated approximately by Eqs. (5.21, 5.22, and

5.24) by substituting v(r) = gv and C = uc, where !v and 4uC are the means of y and C,

respectively.

NUREG/CR-6424 190
After each inspection uv and luc are updated depending on the decision made at the

inspection (i.e., the path to be taken in Fig. 5.13). Based on Bayes theorem, 306 the posterior joint

density function of v and C, vl(•, ii), is,

vVi(•, n) = kL(7, "I')fv(•, rj), (5.35)

in which k is a normalizing constant, L(4, 7711) is the likelihood function of taking a path given

v = 4 and Cý = r7, and fvZý (4, 17) is the prior joint density of v and C. Then the parameters of

the degradation model are updated as, 11

Y 1v = J0 04f fvll(, '7)drid4, and (5.36)

11orln f zl (4, TI)drld4 (5.37)

At an inspection, one of three decisions is made. The probabilities of making each decision

are,

P[StageI] = Fxma(Xcr;O, tR) (5.38)

P[Stage nI] = [1-FFxmx(xcr;O, tR)]Fxmax(xcr;O, tR) (5.39)

P[Stage1II] = [1-FXmax(Xcr;0,tR)]2 (5.40)

in which FXmax(x; 0, t) is evaluated by Eq. (5.24) substituting v = a!.v /100 and/.C M = C.

Therefore, the likelihood function of taking any path is,

L(4, 7111) = -' P[Stage I at tRiV=,-=v ]

• IP[Stage H at tRiV= , C=v 7] (5.41)

P[Result of the kth inspectionlv = 4, C =

Example Illustrating Partial Inspection/Repair Operation

The effect of partial inspection/repair on the failure probability of a component is illustrated

in Fig. 5.14. The component is designed for flexure by Eq. (5.11). The statistics of the initial

resistance and of the loads are the same as used in the previous example. Partial inspections are

scheduled at 20, 30, 40, and 50 years with 5% of the component inspected at each partial

inspection. As an additional constraint, if a Stage III inspection is carried out at 20 years, the

component also is fully inspected/repaired at 40 years. However, if a Stage III inspection is

carried out at 30 years or later, the component is not inspected/repaired during the rest of its service

life. The critical value for decision at partial inspection, Xcr, and the threshold value of damage

detection for full inspection are the same at all inspections and equal 0.03 and 0.05. For purpose

191 NUREG/CR-6424
of comparison, the results of the full inspection/repair strategy (multiple) are also included in the

figure. These partial inspection/repair strategies are compatible with full inspection/repair strategies

in terms of failure probability for a lifetime of 60 years. Thus, the choice of a strategy should be

based on total cost associated with the particular strategy.

5.3 OPTIMUM INSPECTION/REPAIR STRATEGIES

Costs of inspection and repair may be a significant part of the overall lifetime cost of a

concrete structure. These costs should be balanced by the benefits to be gained, both in economic

and reliability terms. There are tradeoffs between the extent and accuracy of inspection, required

level of reliability, and cost. One can perform this tradeoff by defining an objective function that

takes into account the cost of failure, cost of inspection, and costs of repair. The failure probability

of a component during its service life, F(t), provides one of several constraints on the

optimization.

To design an optimum inspection/repair program, the following optimization problem must

be solved:

Minimize CT

Subject to F(tL)<PfT, (5.42)

in which PfT is an established target failure probability during the service life, tL, of a

component/structure and CT is the total cost of inspection/repair plus expected losses due to failure

of a structure. Other possible constraints might include the minimum and maximum time intervals

between inspections, and the minimum threshold value of detection of the NDE method that is

available.

Some studies have been done to determine optimal inspection/repair strategies for metallic

structures subjected to fatigue, assuming that a component is replaced if the intensity of detected

damage exceeds a critical value.305 The cost of repair (i.e., cost of replacement) was assumed to

be constant in these studies. However, a component may not be replaced; instead, only detected

damages might be repaired. In this case, the cost of repair would be a function of damage

intensities to be repaired. This aspect also should be considered in designing an optimum strategy.

Optimum inspection/repair strategies are illustrated using several parametric representations

of relative cost of inspection, repair, and losses due to structural failure. In order to study a simple

case, let us assume that:

1. Initiation of damages in a component is described by a stationary Poisson process

with mean initiation rate v = 5/year.

2. Once damage initiates, it grows linearly with time as described by Eq. (5.17) with

a=1.

3. The degradation rate, C, is lognormally distributed with mean value,

lIc = 0.00125, that corresponds to E[x(40)ITI = 0] = 0.05, and a coefficient of

variation, VC = 0.5.

4. The detectability of damages is described by Eq. (5.27).

NUREG/CR-6424 192
5. All detected damages are repaired.

Assume that the total cost can be expressed as

CT Cins + Crep + Cf. F(tL), (5.43)

in which Cins is expected inspection cost, Crep is expected repair cost, and Cf is the loss due to

structural failure. Assume further that Cf is constant, and Cins and Crep are evaluated by the

following formulas: m[ b

Cins = ains. IE[ Ainsj - Xthj

j=1 (5.44)

= czins(1- Xth)I j=E[Ainsj]

erep = (rep,. YE[Arepj] E [Xmx (tRj) (5.45)

in which Cains and Otrep are constants, Ainsj, and Arepj are the normalized area (or volume) of a

component to be inspected and repaired at the jth inspection, respectively, and Xmax( tRj) is the

maximum damage intensity prior to the repair at tRj. In Eqs. (5.44 and 5.45), it is assumed that

Cins depends on the quality and extent of inspection, while Crep is a linear function of the area to be

repaired and a nonlinear function of maximum damage intensity to be repaired. The parameter, b,

in Eq. (5.44) is set equal to 20 so that Cinslxth /Cinslxth = i0.01 0.15. For strategies involving full

inspections,

E[Ainsj E[Aepj ] for 1 ...,m. (5.46)

Future inspection and repair costs have not been discounted to present worth in the interest of

simplicity.

The component is designed for flexure by Eq. (5.11), assuming that Ln = Dn. The initial

resistance, RO, is described by a lognormal distribution, with mean of 1. 15Mn and c.o.v. of 0.15.

The probabilistic models of load events are presented in Table 5.1. No data could be located to

define the costs of inspection, repair, and failure in absolute terms. Thus, several cases were

considered in which the relative costs were varied to determine the sensitivity of the optimal

strategy to these relative costs. These relative costs of inspection, repair, and losses due to failure

determined by Cins, Oarep, and Cf in Eqs. (5.43-5.45) are summarized in Table 5.4. PfT is

assumed to equal 4 x 10 -4 in 60 years.

193 NUREG/CR-6424
The gradient-based nonlinear optimization program MINOS 3 0 7 was employed to perform

the minimum cost optimization. Since the mathematical programming problem expressed by

Eq. (5.42) is nonconvex, the solution depends on the starting point and might be locally rather

than globally optimum. To test the validity of a solution, several different starting points were

selected. In the following, "optimum solution" means a locally optimum solution.

5.3.1 Full-Inspection Survey

The decision variables in the optimization are the times at which inspections/repairs are

carried out, tR = { tR1 ,..., tRm }, and the threshold value of detection for inspections, XthI = ... =

Xthm = Xth. The number of inspections, m, during the service life of the structure is assumed to

be given. The influence of m can be taken into account by solving the optimization problem for a

number of different values of m to identify that value for which CT is minimum. With this

approach, the more complicated integer programming problem can be avoided.

Besides the constraint on failure probability in Eq. (5.42), additional constraints are

introduced by assuming that performance requirements of the structure preclude inspection more

often than once every four years and that Xth for the best available NDE procedure, Xthmin, is

either 0.05 or 0.005

AtRj > 4 years, j = .... m+1, (5.47)

Xth Ž_ Xthmin, (5.48)

in which AtRj = tRj - tRj.1 ; tR0 = 0; and tRm+l = tL.

The optimum solutions for policies involving full inspection/repair with given values of m

are shown in Tables 5.5-5.7, and the expected costs associated with these solutions and the mean

degradation function associated with the optimum policy are presented in Figs. 5.15-5.17. In the

first two examples, the ratios among ains, arep, and Cf are different. When the repair is relatively

expensive (Table 5.5 and Fig. 5.15), tRj tends to shift toward the early period of the service life.

With this strategy, the component should be repaired when it is lightly damaged, where the cost of

repair can be reduced. However, if the service life of the component were to be extended beyond

tL = 60 years, a large repair cost would be expected. Because there is some time between the last

scheduled inspection/repair and tL, the component would be degraded relatively heavily at tL

[Fig. 5.15(b)]. If it is considered possible at the time of the reliability analysis that the service life

might be extended, an additional constraint in the optimization, such as a lower bound of g(t),

might be imposed. Since the inspection is relatively inexpensive, Xth has little effect on the total

cost, and the optimal xth equals its lower bound [Eq. (5.48)]. Moreover, since only the minimum

of repairs is performed to meet the constraint in terms of failure probability, the failure probability

equals its upper bound [Eq. (5.42)], here 4 x 10-4 in 60 years. The optimum number of

inspections/repairs is three; however, Fig. 5.15(a) shows that the total cost is insensitive to m if

2 <m 5.

When the total expected cost is dominated by the expected losses due to structural failure

(Table 5.6 for m = 1 . . ., 5 and Fig. 5.16), inspection/repair should be carried out with the best

available NDE method of damage detection at nearly uniform intervals during the lifetime of the

NUREG/CR-6424 194
component. With this strategy, the maximum effect of repair on reducing failure probability can be

obtained. The optimum full inspection policy for m > 6 is to perform inspections/repairs at

uniform intervals during the service life with the best available method of damage detection. The

expected total cost of these strategies is also presented in Fig. 5.16(a). Since the failure probability

can be reduced by increasing the number of inspection/repairs, the optimum number of

inspections, mopt, reaches its upper bound [Eq. (5.47)] of 14 in 60 years. However, the reduction

of CT by increasing m is small for m greater than about 7. With this strategy, the mean strength of

the component can be kept near its original state throughout its service life [cf. Fig. 5.16(b)].

In the third example [Table 5.7 and Figs. 5.17(a) and 5.17(b)], the damage growth rate and

the relative cost are the same as that of example 1, but Xthmin is increased to 0.05. The

optimization problem is solved for m > 3 because the constraint on failure probability cannot be

met when m < 2. The optimum times of inspection shift toward the later stage of service life

because an NDE method with relatively large detection threshold does not work effectively if the

damage intensity is small. As a result, the intensities of detected damages are relatively large, that

leads to expensive repair. The optimum number of inspections/repairs is three; however, CT is

insensitive to m for 3 < m < 5.

The intervals between inspections/repairs are nearly uniform in these examples regardless

of damage accumulation. Results presented elsewhere298 suggest that if the damage growth rate is

nonlinear, inspection intervals remain nearly uniform, but the optimal number of inspections may

not be the same. It is assumed that damage initiation is stationary and that the threshold value of

damage detection is the same in all cases. Since most rapidly growing damages that are overlooked

at an inspection initiate after the previous inspection, the cdf of damage remaining immediately

following an inspection is nearly identical to the cdf of damage remaining immediately following

the previous inspection/repair. These factors cause the maintenance intervals to be nearly uniform.

5.3.2 Partial Inspection Strategy

In the optimization involving a partial inspection strategy, there exists a number of decision

variables, such as the number of partial inspections to be scheduled, mp, critical value of damage

intensity at the ith partial inspection below which damage may not be repaired, xcr , times at which

the ith partial inspection is carried out, tp., the number of full inspections after a stage III (see Sec.

5.2.2.2) inspection is performed at tpi (branch i), mi, the threshold value of the jth

inspection/repair in branch i, tRij. Because extensive computational effort is required to optimize

all decision variables, the following relations among variables are assumed:

Xcr, = Xcr, (5.49)

Xth,, = Xth, (5.50)

Xth = Xcr, (5.51)

tp1 = t'j + (i - 1)Atp, (5.52)

tR& = tRil + (j- 1)AtRi, (5.53)

AtRi = aR.- tPi, (5.54)

195 NUREG/CR-6424
Mi=MP-i+l; for j =1... mi and i = 1,....mp (5.55)

in which Atp and AtRi = respectively, the intervals between partial inspections and those between

full inspections/repairs after a stage III inspection is carried out. It was observed in the analysis of

optimum full inspection/repair strategies that the optimum value of Xth converges toward the

minimum value when inspection is inexpensive relative to repair and failure costs. Since this

usually is the case in practice, it is assumed that Xth = Xthmin. Thus, the problem is reduced to one

having three decision variables, t/j, Atp, and aR.

The optimum partial inspection strategies for example 1 (see Table 5.4) are presented in

Table 5.8. Because of the large number of branches, extensive computational effort was required

to obtain the optimum solution by MINOS for mp = 4, and the optimum solutions were obtained

by solving with several combinations of (tlý, Atp, aR). The optimum full inspection/repair

strategy is also presented in the last line of Table 5.8 for comparison. The optimum partial

inspection strategy (mp = 4) is slightly more cost-efficient than the optimum full inspection/repair

strategy. The benefit of partial inspection/repair would increase further if the tp. and tRj in

Eq. (5.52) and Eq. (5.53) were optimized rather than tjq, Atp, and aR.

5.3.3 Example Illustrating Development of Optimum Inspection/Repair Strategy

The methodology is illustrated below through its application to a simply-supported

reinforced concrete slab subjected to corrosion as a result of carbonation of the cover concrete.

5.3.3.1 Description of slab

The dimensions of the slab and the nominal material strengths are shown in Table 5.9.

Assume that

1. The slab is subjected to time invariant dead load, D, and intermittent live load L,

both of which are distributed uniformly on the beam. The load effect at location u,

S(u) is given by

S(u) = q(u).(D+ L), (5.56)

in which q(u) = 4(ul - u2)/12 ; and max{S(u)} = S(12) = D + L.

2. The slab is underreinforced and only fails due to flexure if all reinforcing bars

within a given cross section yield.

3. Compressive strength of the concrete changes with time due to continuous

hydration of cement, while the section area of the reinforcement decreases with time

due to corrosion. Other engineering properties of the beam are time-invariant.

NUREG/CR-6424 196
The slab is designed based on the current design requirement expressed by Eq. (5.11) with

Ln = Dn. The nominal load effects of dead and live load are both 87.6 kN.m. The nominal

flexural capacity 16 is Mn = 301.6 kN.m. The initial resistance of the slab, RO, is described by a

lognormal distribution with mean value, RR = 1.1 Mn, and coefficient of variation 0.13.287 The

time-dependent stochastic models are presented in Table 5.1.

5.3.3.2 Time-dependent strength in flexure

Reinforcing bars are generally ductile, and so assumption 1 implies that the slab can be

modeled as a strictly parallel system composed of ductile identical components. The reinforcing

bars are assumed to be supplied by the same manufacturer and the initial yield strengths and the

section areas of reinforcing bars are assumed to be perfectly correlated. The initial flexural

strength, RO, of the slab is

R0 = ao(1 - 0.59ao)bd2 fc 28 , (5.57)

in which

coO= Ofy, (5.58)

fc28

= nsAs = reinforcement ratio, (5.59)

bd

fc28 = compressive strength of concrete at 28 days; ns = number of reinforcing bars; and fy and

As = yield strength and section area of a reinforcing bar, respectively. Assume for the moment

that N(t) = n corroded areas exist at locations U = u = { u 1 • •., Un) at time t. Let us define the

intensity of damage at location uj at time t as

Xj~t)= l As(t, As uj , (5.60)

in which As(t, uj) = residual cross section area of a reinforcing bar at uj at time t. The flexural

strength at uj at time t , R(t, uj), is expressed as

R(t, uj) =a•(t, uj)[ l- 0.59a( t, uj)]bd fc( t), (5.61)

in which

uj P(t, f()t, uj)fy (5.62)

fc(t)

pb t, uj) nsAs(t uj) (5.63a) bd

197 NUREG/CR-6424
bd ' (5.63b)

in which fc(t) = compressive strength of concrete at time t. In the absence of environmentally

assisted degradation, fc(t) generally is larger than fc28 due to continued hydration of the concrete.

The degradation function for the slab is defined by

G(t) =mrain rain q( , , (5.64)

in which Ggain(t) = ratio of the strength of the component without any damage at time t to the initial

strength, given by

1 -0.59 nsAsfy

Ggain(t) = bdfc(t) (5.65)

1 - 0.59 nsAsfy

bdfc28

Then the cdf of G(t) can be expressed as2 8 1

FG(z) = 1 - exp[-t v(,r) d1 - J fv P[ •q( u) > z]du; [0 Lt V V Roq(u)

0• z < E[Ggain(t)], (5.66a)

FG(z) = 1; z > E[Ggain(t)]. (5.66b)

5.3.3.3 Determination of optimum strategy

The initiation of corrosion following carbonation of the outer portion of concrete is

described by a Poisson process with parameter v(t) expressed as

0, for w <t

v(w) =, (5.67)

v, for w>t*

for purposes of the example, assume that

•t* is deterministic and equal to 10 years;

SAt a given cross section, corrosion initiates in the reinforcing bars at the same time;

however, the point along the span at which corrosion initiates is random;

SThe mean initiation rate of corrosion, v, is 0.2/year;

The intensity of a damage due to corrosion is described by

NUREG/CR-6424 198
x(t) =4RC (t - T), (5.68)

in which Rc = corrosion rate; ds = diameter of a reinforcing bar; and T1 = initiation

time of the corrosion; Eq. (5.68) is valid if the depth of corrosion is small relative to

the diameter of the reinforcing bar;

Rc is a time-invariant random variable described by a lognormal distribution with

mean value, gRC, of 50 gm/year and coefficient of variation, VRc, of 0.5.308 Then

the damage growth rate, C, in Eq. (5.17) defined by

C = 4Rc (5.69) d'

is also a time-invariant random variable described by a lognormal distribution with

VC = 0.5. The mean of the damage growth rate, gC, equals 156 gim/year for D =

32 rnm;

" The 28-day specified compressive strength of concrete equals 27.6 MPa. The

corresponding mean of the compressive strength at 28 days is 28.7 MPa.2 87 The

specified yield strength of the reinforcement is 414 MPa and the mean is 465 MPa;

and

" Concrete matures with time; the compressive strength increases during the first 10

years and does not change thereafter. The mean compressive strength (in

megapascals) at time t is evaluated by Eq. (5.16).

The mean degradation functions of the slab subjected to reinforcement corrosion are

presented in Fig. 5.18. Three cases are illustrated - the slab is fully inspected/repaired: (1) At tR

= 35 years with Xth = 0.05; (2) at tR = 20, 30, 40, and 50 years with Xth = 0.15; and (3) not

inspected/repaired. For comparison, the mean degradation function of a slab with continuous

hydration of cement but without corrosion is also presented. Without corrosion, the strength of the

slab increases about 5% over 60 years, while it decreases 22% at 60 years due to reinforcement

corrosion if repairs are not made.

The failure probabilities of the slab with mean strength degradation illustrated in Fig. 5.18

are presented in Fig. 5.19 along with the failure probability with g(t) = 1. Since the latter

probability can be considered as that intended for design, it would be the target failure probability

for developing an inspection/repair strategy if degradation occurs. Since F(40) without repair is

smaller than F(40) with g(t) = 1, inspection/repair would not be necessary if the intended service

life is 40 years or less. If the intended service life is 60 years, some inspections/repairs are

required. Both strategies described in the figure are feasible and compatible in terms of failure

probability.

The optimum inspection/repair strategies for this slab for tL = 60 years are presented in

Table 5.10 and Fig. 5.20 for full inspection/repair strategies. The total cost is evaluated by

Eqs. (5.43-45) with ains; (rep; Cf= 1:500:104 and with Xthmin = 0.05. In this example, the

199 NUREG/CR-6424
optimum strategy is to perform three inspection/repair operations at 19.6, 24.8, and 30.7 years

using an NDE method with a capability of detecting defects of a size causing 5% loss of flexural

capacity.

5.4 COMMENTARY ON RELIABILITY-BASED CONDITION ASSESSMENTS

A methodology has been developed that can be used in assessments of current and future

structural reliability and performance of concrete structures in NPPs. The effects of aging and

environmental stressors that might diminish the ability of these structures to withstand future

operating, extreme environmental, or accident conditions have been taken into account. The

overall condition assessment methodology is summarized in Fig. 5.21.

Basic concepts for time-dependent reliability assessment of a single component or system

of components were developed. Time-dependent strength models were convolved with the

stochastic load models to determine limit state probabilities and hazard functions. The sensitivity

of the reliability to various parameters describing load occurrence and strength degradation was

evaluated using conceptual models. Parametric analyses showed that the time-dependent system

reliability is sensitive to the choice of mean initial component strength, mean strength degradation

rate, and general stochastic characteristics of the time-varying load. Less sensitivity was shown to

dependence in component strengths within a system. Limit state probability and hazard functions

developed for the conceptual models both show a relatively sharp increase in magnitude shortly

after 40 years. This indicates that under aggressive environments, evidence of past acceptable

performance may not be sufficient to ensure future acceptable performance. Therefore, periodic inservice inspections may be required to ensure future acceptable performance.

The role of in-service inspection and repair on the time-dependent reliability was

investigated. The quality of an inspection was described by a detectability function, d(x). As a

result of an in-service inspection/repair activity, the distribution function of damage intensity can be

updated using a Bayesian analysis. The resulting posterior distribution can be used to update the

time-dependent strength. This information, when combined with the time-dependent reliability

analysis, can be used to evaluate the effect of inspection intervals and NDE detection capabilities on

the failure probabilities of a component.

The degradation in strength of a structure with random damage can be evaluated based on a

growth model for individual damages that can be obtained from experimental data. This method is

combined with the time-dependent reliability analysis to design optimum strategies for inspection

and maintenance that minimize the expected future cost, while maintaining the limit state probability

of the structure at or below an established target failure probability. Optimum inspection/repair

strategies are sensitive to the relative cost of inspection, repair, and failure as well as to the

threshold value of damage detection. If the consequences of failure are high, a policy requiring

inspection and maintenance at nearly uniform intervals over the service life of the structure results

in near-minimum total cost. Additional data on damage initiation, NDE capabilities and the cost of

inspection, repair and failure are required to complete the development of an optimum

inspection/repair strategy.

Several areas for future work were identified. They include:

1. Supporting Data - Time-dependent reliability analysis provides a quantitative

framework for condition assessment of concrete structures. The methodology is

data-intensive in comparison to the subjective rating procedures that often are used

in the in-service evaluation of ordinary civil structures. Implementation and

extension of the method to realistic condition assessment is difficult due to a lack of

NUREG/CR-6424 200
supporting quantitative data on strength degradation models, including initiation and

rate of damage growth, the mean occurrence rate of load events and cdf of the

intensity of time-varying loads. The threshold value of defect detection associated

with each NDE method must be identified to evaluate the effect of inspection/repair

on structural failure probability. Information about relative cost of inspection,

repair, and losses due to failure is required to design an optimum inspection

strategy.

2. Interactions between Degradation Mechanisms - A structural component may

degrade due to more than one environmental factor. In such a case, a damage

model reflecting the interactions between degradation mechanisms must be

developed. With such a model combined with time-dependent reliability analysis

developed in this study, the failure probability of a component can be evaluated.

3. Verification - The reliability models established for condition assessment should

be validated through application to laboratory and prototypical structures. In

laboratory tests, the effects of environmental factors can be evaluated under

controlled conditions. Selecting an existing structure with available design

documentation as a prototype, the analysis should be run forward to the present

time and the current predicted condition should be compared with the estimate of

actual present condition. The results can be utilized to modify the theoretical

models.

4. User-Oriented Guideline - The condition assessment and life prediction

methodology may not be suitable for routine use in practice without simplification.

User-oriented requirements and guidelines for in-service evaluation of concrete

structures should be developed.

201 NUREG/CR-6424
Table 5.1. Statistical properties of NPP loads.

Load Rate of Duration, Mean+ cov pdf Occurrence, X IT

Dead n/a n/a I.ODn 0.07, 0.10 Normal

Live 0.5/yr 3 mo 0.4Ln 0.50 Type I

Earthquake 0.05/yr 30 sec 0.078 Ess 0.85* Type ]I**

Equipment n/a 40 yr 1.0Qn 0.04 Normal

Acc. Pressure 1/10000 yr 20 min 0.8Pa 0.20 Type I

Acc. Temp. 1/10000 yr 20 min 0.9Ta 0.08 Type I

+Dn, Ln, Qn, Pa, and Ta are nominal loads;

* Es is safe-shutdown earthquake load.

**For the pdf of annual maximum values, FEann (x).

Table 5.2. Statistical data for reinforced concrete.

Parameter Mean cov pdf

Material Strength

fc, psi 960 + 0.8Fc 0.12 Normal

ft, psi 6.4 7-c 0.18 Normal

fy (ASTM A615/Gr.60) 67 ksi 0.11 Lognormal

fu (ASTM A416/A421) 270 ksi 0.04 Lognormal

Dimensions

Overall dimensions (in.) Nominal, h 0.41h Normal

Placement of reinf. (in.) Nominal, d 0.6/d Normal

NUREG/CR-6424 202
Table 5.3. Probabilistic descriptions of resistance of concrete components.

Limit State Mean* coy pdf

Beam, flexure 1.1 lMn 0.13 Lognormal

Beam, shear 1.22Vn 0.18 Lognormal

Wall, shear 1.35Vn 0.18 Lognormal

Short wall, shear 1.70Vn 0.18 Lognormal

Slabs, flexure 1.12Mn 0.14 Lognormal

Short column, compression 1.1 3Pn 0.14 Lognormal

Short column, tension 1.12P 1 0.13 Lognormal

*The nominal values Mn, Vn, and Pn are the capacities that would be computed from the

ACI Standards 31816 or 349.17

Table 5.4. Models analyzed in examples.

Example Xthmin Relative cost EXins: arep: Cf

1 0.005 1:5000:104

2 0.005 1:1000:107

3 0.05 1:5000:104

Table 5.5. Optimum solutions for full-inspection strategies for

Example 1 in Table 5.4.

F(60) tRi

m (10-4) Xth 1 2 3 4 5

1 4.00 0.005 21.8 - - - -

2 4.00 0.005 10.8 20.6 - - -

3 4.00 0.005 7.9 13.8 20.3 - -

4 4.00 0.005 5.8 10.3 15.0 20.3 -

5 4.00 0.005 4.1 8.1 12.1 16.1 22.2

203 NUREG/CR-6424
Table 5.6. Optimum solutions for full-inspection

Example 2 in Table 5.4.

strategies for

F(60)

m (10-4) xth 1 2 3 4 5

1 3.43 0.005 30.7 - - - -

2 2.52 0.005 20.3 39.6 - - -

3 2.21 0.005 15.2 29.2 44.0 - -

4 2.05 0.005 12.5 23.7 35.3 47.3 -

5 1.97 0.005 10.0 18.6 27.5 37.4 48.2

Table 5.7. Optimum solutions for full-inspection strategies for

Example 3 in Table 5.4.

F(60) tRi

m (10-4) Xth 1 2 3 4 5

3 4.00 0.005 22.0 31.1 41.0 - -

4 4.00 0.005 4.0 22.3 31.6 40.9 -

5 4.00 0.005 4.0 8.0 22.1 31.1 41.0

Table 5.8. Optimum solutions for

Example 1 in

partial inspection strategies for

Table 5.4.

m F(60) Xcr tp1 Atp CaR CT Cins Crep CfF(60) (10-4)

2 4.00 0.005 13.2 4.0 0.7 13.2 1.2 7.9 4.0

3 4.00 0.005 9.1 4.0 0.6 11.5 1.9 5.5 4.0

4a 4.00 0.005 5.8 4.0 0.5 11.4 2.2 5.2 4.0

Full 4.00 - - - - 11.5 2.7 4.8 4.0

aOptimum partial-inspection policy.

NUREG/CR-6424 204
Table 5.9. Engineering properties of slab.

Parameter Value

Width, b

Span, I

Effective depth, d

Height, h

Concrete cover

Diameter of reinforcing bar, d,

Distance between reinforcing bars, s

Number of reinforcing bars, ns

fc (specified at 28 days)

fy (minimum specified)

300 mm

7m

500 mm

550 mm

50 mm

32 mm

100mm

2

27.6 MPa

414 MPa

Table 5.10. Optimum solutions for full-inspection strategies for slab.

F(60) tRi

m (10-4) Xth 1 2 3 4 5

1 1.20 0.05 31.2 - - - -

2 1.20 0.05 22.8 30.8 - - -

3 1.20 0.05 19.6 24.8 30.7 - -

4 1.20 0.05 17.0 21.0 25.7 30.7 -

5 1.04 0.05 16.6 20.6 24.6 28.6 32.6

205 NUREG/CR-6424
X

fz(x)

t

(a) Permanent loads.

x

Yzxx)

Z=max{X( t) }

t

(b) Continuous variable loads.

Y x )

( i-p t

(c) Intermittent variable loads.

Fig. 5.1 Typical sample functions of structural loads.

NUREG/CR-6424 206
100

II

1.0

0.9

0•

0 10 20 30 40 50 60

Time, years

Fig. 5.2 Mean degradation functions of one-way slab.

207 NUREG/CR-6424
x 10-5

-20

10

W2 H Linear degradation,

S-. Nondegrading, g(40) 1 -A Strength increases, t

I I I I I

0 10 20 30 40 5

Time, years

Fig. 5.3 Failure probability of a one-way slab.

) 60

NUREG/CR-6424 208
250 1 1 1

• ii

0 Ref. 296

200r' Ref.'297-

1 5 0ef ! ............. .......... 2 9 .. .......................... ................. .................. ..

PO 150 ......

Elj

100

.................................................. ............... ......................... 500

0

0-1

0 50 100 150 200 250

Calculated

Fig. 5.4 Comparison of measured and calculated shear strength

of low-rise reinforced concrete wall.

209 NUREG/CR-6424
0

0

1.15

1.1

1.05

1

0.95

0.9

0.85

0.8

0 10 2C1 30 40

Time, years

50 60

Fig. 5.5 Mean degradation function of wall in shear without repair.

NUREG/CR-6424 210
1-ý

0.008

0.007

0.006

0.005

0.004

0.003

0.002

0.001

0

0 10 20 30 40 50

Time, years

60

Fig. 5.6 Failure probability of wall in shear without repair.

211 NUREG/CR-6424
PO

P.

CuA

0.0003

0.00025

0.0002

0.00015

0.0001

0.00005

0

_ _ _ _ _ _ _ _ _

-E-) Based on Multiple Defects .... B 1--• sed n SigleDefet [............ ................... .

-8 -Based on Single Defect

*---- No Degradation .... ...... ........ . ..... ................... ............ J............i ................ /

.................

i......................... ....... ....... ! .........

... ...

. . ..................... • ........................ :........................ .................. °..o•... ...................................... .

0 10 20 30 40 50 60

Time, years

Fig. 5.7 Hazard function of wall in shear without repair.

NUREG/CR-6424 212
1.15

-. 1.037

• 0.925 .

gZ 0.8125 .... Based on Local Surface Loss

-B - No degradation

0.7

0 10 20 30 40 50 60

Time, years

Fig. 5.8 Mean degradation function of wall in flexure/compression.

213 NUREG/CR-6424
Inspection/Repair

Activities 4 *j

C)

LL

Vtime

Fig. 5.9 Role of inspection/repair in controlling failure rate.

0

/o ,

00

0

0 Xmin Xth Xmax

Defect Size

Fig. 5.10 Probability of detecti e of size x.

NUREG/CR-6424 214
1.00

0.95

__ -- Step, xth=0.03 . 0.90

"0** Uniform,n, =0.5 aVX th

..... Lognormal, VX = 0.3

0.85 Lognormal, VX th= 0.5

No repair

0.80 I I III

0 10 20 30 40 50 60

Time, years

Fig. 5.11 Effect of several detectability functions on mean

degradation function of inspection/repair.

215 NUREG/CR-6424
1.00

0.95

40

: 0.90

.. . ... ... x oR . .0.85 th = 0.01 at t = 30 years

---------- X th = 0.05 at tR = 2 0 , 30, 40, 50 years

No repair

0.80 I I I II

0 10 20 30 40 50

Time, years

Fig. 5.12 Effect of multiple inspection/repairs on mean degradation function.

60

NUREG/CR-6424 216
r,0

j-.

100

cJQ

CD2

0

C-,

0'

p.

N.,

p.
0.001

4 0.0001

0

0.00001

0.001

• 0.0001

0

.0

,0

0.00001

0 10 20 30 40

Time, years

(a). E [X(40) I TI = 0 ]=.05

50 60

Linear l:amage Grovith

03, v 5

at tR = 20,30, 40, 50

Full: xth = 0.08 at tR 20, 30, 40, 50

0 10 20 30 40

Time, years

(b). E [X(40) I TI = 0 ]=.10

50 60

Fig. 5.14 Failure probability of a component with partial inspection.

NUREG/CR-6424 218
25

20

15

0

10

5

0

0 1 2 3 4 5 6

Number of Inspections/Repairs

(a) Cost Function at Optimum Solution.

1

0

=

0

6

2

0.95

0.9

0.85

0.8

0 10 20 30 40 50 60

Time, years

(b) Mean Degradation Function with Optimum Policy.

Fig. 5.15 Optimum solutions for full-inspection strategies for example 1 in Table 5.4.

219 NUREG/CR-6424
0

Q

4000

3500

3000

2500

2000

1500

1000

500

0

I I I lI I Ii I I rI I I * i ai I I I 1 II I

ip L . o ......................................................... ............................................ 0....... ..... ....... ......................... •.....................................

l I I I I f I I I l I I I I I I I | I j a i i i , i i

0 5 10 15

Number of Inspections/Repairs

(a) Cost function at optimum solution.

....... -- @---i h u u ............................... e t arpi a e yI................................ ..... ..............................

...... , .............. .... . ............. . . . . . . .............

-e- Optimum Strategy

-aWithout Repair

1.05

1

.• 0.95

0.9

0.85

0.8

0 10 20 30 40

Time-Dependent Degradation Function

(b) Mean degradation function for with optimum policy.

50 60

Fig. 5.16 Optimum solutions for full-inspection strategies for example 2 in Table 5.4.

NUREG/CR-6424 220
100 T-r-l T-r-T !- 1 1 1 1 1 1

8 0 . .................. .......................... ....................... ................................................ ........................ ........................ ..................... .

6 0 . .................. ........................ ........................ ....................... ........................... • ..................... ..................................................

-e- Total Cost

4 0 . .................. ......................... ........................ ........................ ............- C s o f I p e t n

40 -.- B -Cost of Inspection

-Cost of Repair

X - - Cost of Failure

2 0 ..............-. .............. ....................................... ........................ ........................ •-................................... 20 ~~ ~~ ~~ ~ .......... ............. ........................

0 L 1 1 . . . 1... .. | I I I r " I.. = * , I - i * '1=' , , 1 , , I 1

2 2.5 3 3.5 4 4.5 5 5.5 6

Number of Inspections/Repairs

(a) Cost function at optimum solution.

1.05 ... L Without Reair

I E= - - Optimum StrategyI = ............................ ................................. ............................... ..........

.0.9

0.8

0 10 20 30 40 50 60

Number of Inspections/Repairs

(b) Mean degradation function with optimum policy.

Fig. 5.17 Optimum solutions for full-inspection strategies for example 3 in Table 5.4.

221 NUREG/CR-6424
1.1

•a1.05 ....................... nn -1. ..... -..-....... .......... 3 -. • .. • ................... .. .

0.9-

.................. ....... ................ •.. ... ............................ ... . ........ . F:......... ..t"i ........... "" ' ,...

..... .... .... ..... .... .... .. ... ... ... ... ... .. ............ ........... ...........

0.85 e-- Without Repair --- 13 - No Corrosion

-0- - xth = 0.05 at tR = 35 0.8 . ....... x- -X--th = 0.15 at tR = 20, 30, 40, 50 ............................. ! .......

0.75 . . . . i . . . . i . . 0 10 20 30 40 50

Time, years

Fig. 5.18 Mean degradation function of slab subject to rebar corrosion.

60

10-3

10-4

.0

0

aa-

............. .............

e---- Without Repair

-6• - No Degradation

- xth =0.05 at tR 35

-- X-- xth = 0.15 at tR 20, 30, 40,50

-- +-- g(t)=1

10- 6

0 10 20 30 40 50 60

Time, years

Fig. 5.19 Failure probability of slab with mean strength degradation in Fig. 5.18.

NUREG/CR-6424 222
10

8

6

0

4

2

0

0 1 2 3 4 5 6

Number of Inspections/Repairs

(a) Cost function at optimum solution.

1.1

=

•0

a-J

1.05

1

0.95

0.9

0.85

0.8

0.75

0.7

0 10 20 30 40 50 60

Time, years

(b) Mean degradation function with optimum policy.

Fig. 5.20 Optimum solutions for full-inspection strategies for slab.

223 NUREG/CR-6424
Analyze Data:

* Structural Computations

* Structural Drawings

* Inspection and Construction Data

* Code Information

Run Models/Update Data:

* Load History Data/Models

" Damage Accumulation Models

" Inspection Data

" Repair History

Analyze Data:

* Structural Survey

* Load Tests

* Destructive Evaluation

* Nondestructive Evaluation

* Structural Analysis

Run Models:

* Load Models

" Damage Accumulation Models

* Future Inspection Procedures

* Repair Reliability

Initial Condition, X(O)

I •

Model Verification

-] IPresent Condition, X(t)

I -

Owl

dd

r I_________ .1I

uture Condition, X (t + D t)

Structural Capacity

Additional Useable Life

Fig. 5.21 System reliability approach to condition assessment.

NUREG/CR-6424 224
6. SUMMARY, CONCLUSIONS, AND RECOMMENDATIONS

6.1 SUMMARY

The main objective of the Structural Aging (SAG) Program was to prepare a document that

provides the U. S. Nuclear Regulatory Commission (USNRC) license reviewers with

(1) identification and evaluation of the structural degradation processes; (2) issues to be addressed

under NPP continued service reviews, as well as criteria, and their bases, for resolution of these

issues; (3) identification and evaluation of relevant ISI or structural assessment programs in use,

or needed; and (4) quantitative methodologies for assessing current, or estimating future structural

safety margins. Results developed under the SAG Program and summarized in this document

provide an improved basis for the USNRC staff to evaluate NPPs for continued service. Potential

regulatory applications of this research include (1) improved predictions of long-term material and

structural performance and available safety margins at future times, (2) establishment of limits on

exposure to environmental stressors, (3) reduction in total reliance by licensing on limited

available inspection and surveillance data through development of a methodology that will enable

the integrity of structures to be assessed, and (4) potential for improvements in damage inspection

methodology through incorporation of program results into national standards [e.g., American

Concrete Institute (ACI) 349]230 that could be referenced by Standard Review Plans.

Activities under the SAG Program were conducted under three technical task areas:

(1) materials property data base, (2) structural component assessment and repair technologies,

and (3) quantitative methodology for continued service determinations. Results provided in this

report meet the objectives of the SAG Program noted above as well as provide a compendium of

knowledge for use in developing a program for nuclear power plant (NPP) reinforced concrete

structures to demonstrate that the effects of aging are being effectively managed. Where feasible,

examples have been utilized to demonstrate pertinent aspects in several key areas of a life

management program (e.g., condition assessments of NPP reinforced concrete structures,

utilization of results contained in the Structural Materials Information Center (SMIC), and

development of optimum ISI/repair strategies).

Primary SAG Program developments included the following

* Structural Materials Information Center containing data and information on the time

variation of material properties under the influence of pertinent environmental

stressors and aging factors for 144 materials;

* Aging assessment methodology that uses ranking criteria to identify structural

components and degradation factors of primary importance to aging management;

" Guidelines and evaluation criteria for use in condition assessments of NPP

reinforced concrete structures; and

* Reliability-based methodology for current condition assessments and estimations of

future performance of NPP reinforced concrete structures.

In addition, the SAG Program conducted in-depth evaluations of several technologies. The

primary purpose of these evaluations was to develop guidance on their applicability to NPP

reinforced concrete structures. Reviews in the form of state-of-the-art reports, were provided on

* Data bases for concrete and concrete-related materials;

225 NUREG/CR-6424
SIn-service inspection (ISI) (destructive and nondestructive) and condition

assessment techniques, and methodologies for their application;

* Corrosion of metals embedded in concrete, including criteria for use of cathodic

protection systems and an assessment of the potential for occurrence of stray

electrical current-induced corrosion; and

* Remedial measures strategies, repair materials and techniques, and performance

characteristics (durability).

6.2 CONCLUSIONS

Throughout this report, conclusions are provided in the form of commentary. This

commentary is provided at the end of each section that addresses a pertinent aspect related to aging

management of NPP reinforced concrete structures. Summarized below are major conclusions that

resulted from this program.

1. The performance of the reinforced concrete structures in NPPs has been good, with

the majority of the identified problems initiating during construction and being

corrected at that time. However, as these structures age, incidences of degradation

due to environmental stressor effects are likely to increase to potentially threaten

their durability. Items of note would be corrosion of steel reinforcement due to

carbonation of the concrete or presence of chloride ions, excessive loss of

prestressing force, leaching of concrete, and leakage of post-tensioning system

corrosion inhibitor through cracks in the concrete.

2. Techniques for detecting the effects of environmental stressors are sufficiently

developed to provide qualitative data. Areas of concern include massive members

that contain large quantities of steel reinforcement such as the basemat and members

that are inaccessible, such as portions of the steel pressure boundary that are

embedded in concrete. Also, detectability functions that relate flaw characteristics

to a probability of detection require development. Despite the limitations associated

with many of the techniques, their proper use and application provides vital input

for assessing the structural condition of reinforced concrete members. Frequently,

increased confidence in results can be provided by using a combination of methods

in tandem.

3. Methods for conducting condition assessments of reinforced concrete structures are

fairly well established and generally start with a visual examination of the

structure's surfaces. Condition assessments provide an effective aging

management tool in that when a discontinuity is detected, a maintenance activity can

be implemented to prevent the discontinuity from becoming a defect that requires a

major repair. To be of most use, the condition assessments should be conducted at

regular intervals. Established condition assessment methods, however, have been

application specific (e.g., parking structure decks). Few standards or criteria are

available for interpreting the results obtained from the condition assessments.

Current inspection requirements for NPP reinforced concrete structures are fairly

limited in that they only address the unbonded tendon systems in post-tensioned

concrete containments and a general visual inspection in conjunction with the leakrate tests. With the adoption of American Society of Mechanical Engineers (ASME)

Section XI Subsection IWL, requirements will increase somewhat. Also, ACI

Committee 349230 has developed guidelines for inspection of safety-related

NUREG/CR-6424 226
concrete structures other than containments. Due to the importance of condition

assessments in effectively managing aging of structures and the likelihood for

incidences of degradation to increase as the NPP structures age, it seems prudent

that condition assessments of these structures should be conducted periodically.

Structures identified to be of high safety significance and potentially at risk should

receive the most detailed and frequent inspections. The inspection interval for

future inspections could be increased based on a proven performance history.

Structures having accessibility constraints for visual examinations or conduction of

other non-destructive evaluations could start with an indirect approach such as

monitoring the structure's ambient environment to determine if it is potentially

aggressive. More detailed examinations would be required if the environment is

found to be potentially aggressive.

4. Techniques for repair of concrete structures are well established and when properly

selected and applied are effective. At present no codes or standards are available for

repair of reinforced concrete structures, although some are being developed.

Criteria that may be used to determine when a repair action should be implemented

are not available (e.g., parameters that relate damage state such as crack width to

environmental exposure). Data on the long-term effectiveness or durability of

remedial measures is required. Effective implementation of a repair strategy

requires knowledge of the degradation mechanisms, the environment of the

structure at the macro and micro level, proper preconditioning of the structure to be

repaired, correct choice of repair technique and material, and quality workmanship.

5. A reliability-based methodology has been developed that can be used to facilitate

quantitative assessments of current and future structural reliability and performance

of reinforced concrete structures in NPPs. The methodology is able to take into

account the nature of past and future loads, and randomness in strength and in

degradation resulting from environmental factors. The methodology can be used as

a basis for selecting appropriate periods for continued service and/or determining

optimum intervals and extent of inspection and repair activities. Inspection/repair

strategies can be developed to minimize expected future cost while keeping the

failure probability of the structure at or below an established target failure

probability during its anticipated service period. Implementation and extension of

the method to realistic condition assessments is difficult due to a lack of supporting

quantitative data on strength degradation models, including initiation and rate of

damage growth, the mean occurrence rate of local events and cumulative density

function (cdf) of the intensity of time-varying loads. The reliability models

established for condition assessments have not been validated through application to

laboratory or prototypical structures.

6.3 RECOMMENDATIONS

A number of recommendations have been made throughout this report with respect to such

things as use of existing data and service life models to estimate longevity of NPP concrete

structures, capabilities of ISI techniques, methods for conducting condition assessments and

criteria for interpretation of results, selection of repair materials and techniques, and development

of ISI/repair strategies. Presented below is a summary of major recommendations for additional

activities.

1. Durability assessments of reinforced concrete structures require an improved

understanding of the degradation mechanisms, improved characterization of service

environments, the development of advanced service life models, and the

227 NUREG/CR-6424
development of guidelines and standards for acceptance of service life estimations.

Information and data on the estimation of service life of in-service concrete should

be investigated in more detail, particularly where two or more degradation factors

may be occurring simultaneously.

2. The SMIC is the most comprehensive data base that has been developed to date for

concrete and concrete-related materials. As more data and information become

available as a result of activities related to continued service assessments of NPPs,

it is recommended that this data continue to be incorporated into SMIC. Also, as

SMIC has the capability to add other structural materials of importance to aging of

NPPs, it is recommended that SMIC be expanded to include data and information

on these materials. Furthermore, based on experience gained during development

of SMIC, advances in personal computer hardware capabilities, and corresponding

developments in software tools for building customized data bases, it is

recommended that the data base be completely redesigned to address the

shortcomings of the current data base management system.

3. Nondestructive evaluation techniques were found in large measure to be more

qualitative than quantitative and require the development of correlation curves for

interpretation of results. Detectability functions for these techniques are not

presently available. Developments in non-destructive evaluation techniques are

required with respect to two specific areas related to inspection of NPP reinforced

concrete structures. No technique was found to be capable of providing reliable

information when inspecting massive, heavily-reinforced concrete structures such

as basemats. Non-destructive evaluation techniques capable of inspecting

inaccessible regions of the pressure boundary such as where it is embedded in

concrete also require development.

4. Post-tensioning systems should be investigated in more detail to investigate the

cause(s) of larger than estimated losses of prestressing force that are being

experienced by several of the older prestressed concrete containments. The

significance of leakage of corrosion inhibitor that is occurring through cracks in the

concrete of several of the prestressed concrete containments should be investigated

with respect to its potential effects on the properties of the concrete and structural

performance of the containment. Results of evaluation of a limited data set indicate

that the use of lift-off loads to indicate prestressing forces in the concrete

containments may overestimate the actual prestressing forces. This should be

investigated in more detail to assess the significance of this difference, particularly

with respect to long-term aging considerations.

5. Although basic approaches are available for conducting condition assessments of

reinforced concrete structures, information is required for use in interpretation of

results. Criteria for interpretation of crack parameters (e.g., width, depth, and

length) with respect to environmental factors and the impact of observed

degradation on structural performance in large measure, require development.

Additional guidance should be developed that provides acceptance criteria for use in

conjunction with condition assessments of NPP reinforced concrete structures. In

association with the condition assessments, more definitive guidelines are required

for application of repair strategies that cover the entire repair spectrum (i.e., repair

requirements, selection of repair materials and techniques, preparation of the

NUREG/CR-6424 228
structure, application of repair methods, and curing). Additional information is

required with respect to methods for use in evaluation of the effectiveness of a

remedial measure, and durability characteristics of the various methods.

6. Several areas should be pursued in more detail with respect to the time-dependent

reliability analysis. Additional data should be developed so that the method can be

extended to make more realistic condition assessments (e.g., strength degradation

models; mean occurrence rate of load event and cumulative distribution function of

the intensity of time-varying loads; threshold level of defect detection for

nondestructive evaluation methods; and costs of inspection, repair, and loss due to

structural failure). A damage model reflecting interactions between degradation

mechanisms is required for use with the time-dependent reliability analysis to

evaluate the failure probability of a component. The reliability models developed

should be evaluated through application to laboratory and prototypical structures.

Also, user-oriented guidelines for use in the condition assessment and life

estimation methodology should be developed.

7. Finally, the effect of aging on the structural margins of safety-related reinforced

concrete structures such as containments should be evaluated.

229 NUREG/CR-6424

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231 NUREG/CR-6424
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NUREG/CR-6424 232
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32. "Low-Alloy Steel Deformed Bars for Concrete Reinforcements," Annual Book of ASTM

Standards, ASTM A706, Ibid.

33. "Deformed Billet Steel Bars for Concrete Reinforcement with 60,000 psi Minimum Yield

Strength (superseded)," ASTM A432, Ibid.

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Rails, Wheels, and Tires; Bearing Steel; Steel Forgings; and Ferrous Filler Metal,

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36. "Steel Welded Wire Fabric, Deformed, for Concrete Reinforcement," Annual Book of

ASTM Standards, ASTM A 497, Ibid.

37. "Steel Wire, Deformed, for Concrete Reinforcements," Annual Book of ASTM Standards,

ASTM A 496, Ibid.

38. "Fabricated Deformed Steel Bar Mats for Concrete Reinforcement," Annual Book of A STM

Standards, ASTM A 184, Ibid.

39. "Uncoated Stress-Relieved Steel Wire for Prestressed concrete," Annual Book of ASTM

Standards, ASTM A 421, Ibid.

40. "Steel Strand, Uncoated Seven Wire Stress-Relieved for Prestressed Concrete," Annual

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233 NUREG/CR-6424
41. "Uncoated High-Strength Steel Bar for Prestressing Concrete," Annual Book of ASTM

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NUREG/CR-6424 234
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66. Standard Test Method for Potential Alkali Reactivity of Carbonate Rocks for Concrete

Aggregates (Rock Cylinder Method), ASTM C 586, Ibid.

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Concrete Institute, 74(12), Detroit, Michigan, December 1977.

68. Standard Test Method for Evaluation of Frost Resistance of Coarse Aggregates in AirEntrained Concrete by Critical Dilatation Procedures, ASTM C 682, Vol. 04.02,

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213. "Measuring Corrosion Rate of Reinforced Concrete," SHRP Product-2001, Strategic

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214. T. M. Refai and M. K. Lim, In-Service Inspection and Structural Integrity Assessment

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215. F. K. Brocklehurst et al., Evaluation of the Use of Laser Ultrasonics for the Rapid,

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221. K. A. Snyder, J. R. Clifton, and N. J. Carino, Nondestructive Evaluation of the InPlace Compressive Strength of Concrete Based Upon Limited Destructive Testing,

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NUREG/CR-6424 246
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Detroit, Michigan, May-June 1984.

225. ACI Committee 207, "Practices for Evaluation of Concrete in Existing Massive Structures

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226. ACI Committee 437, "Strength Evaluation of Existing Concrete Structures," pp. 707-710 in

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227. ACI Committee 311, "Guide for Concrete Inspection," pp. 81-85 in Concrete

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228. W. F. Perenchio, "The Condition Survey," pp. 59-62 in Concrete International, 11(1),

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261. PRA Procedures Guide, NUREG/CR-2300, Vols. 1 and 2, U.S. Nuclear Regulatory

Commission, Washington, D.C., 1983.

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263. P. Emilius, Jr., "Close-Range Photogrammetry Documents As-Built Structures," Power,

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264. C. J. Hookham, In-Service Inspection Guidelines for Concrete Structures in Nuclear

Power Plants, ORNL/NRC/LTR-95/14, Lockheed Martin Energy Systems, Inc., Oak

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265. J. R. Dougan, Evaluation of Inservice Inspections of Greased-Prestressing Tendons,

NUREG/CR-2719 (ORNL/TM-8278), Oak Ridge National Laboratory, Oak Ridge,

Tennessee, September 1982.

249 NUREG/CR-6424
266. An International Survey of In-Service Inspection Experience with Prestressed

Concrete Pressure Vessels and Containments for Nuclear Reactors, FIP/3/5, F~drration

Internationale de la Prrcontrainte, Wexham Springs, Slough, England, April 1982.

267. "ASTM Protective Coatings Standards for Use in Nuclear Power Plants," American Society

for Testing and Materials, Philadelphia, Pennsylvania, 1990.

268. Manual on Maintenance of Coatings for Nuclear Power Plants, Manual MNL 8,

American Society for Testing and Materials, Philadelphia, Pennsylvania, 1990.

269. Concrete-Performance Production, Placing, and Compliance Criteria, (Draft Document)

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270. M. Regourd, "Physico-Chemical and Biological Processes Related to Concrete," in

Durability of Concrete Structures, Comit6 Euro-Intemational du Baton, Bulletin

d'Information 152, Leusanne, Switzerland, 1984.

271. "Load Carrying Capacity of Structural Members Subjected to Alakli-Silica Reactions,"

Danish Ministry of Transport, Copenhagen, Denmark, October 1990.

272. E. Koehn and A. Kao, Evaluation of Existing Condition Rating Procedures for Civil

Works Structures and Facilities, Technical Report REMR-OM-1, Department of Army,

U.S. Army Corps of Engineers, Champaign, Illinois, May 1986.

273. R. Stowe and H. Thornton, Jr., Engineering Condition Survey on Concrete in Service,

Technical Report REMR-CS-1, Department of Army, U.S. Army Corps of Engineers,

Vicksburg, Mississippi, May 1989.

274. R. Bullock, A Rating System for the Concrete in Navigation Lock Monoliths, Technical

Report REMR-OM-4, Department of Army, U.S. Army Corps of Engineers, Vicksburg,

Misissppi, May 1989.

275. E. Ellingwood et al., "Probability Based Load Criteria: Load Factors and Load

Combinations," pp. 978-997 in Journal of Structural Division, 108(5), American Society

of Civil Engineers, New York, New York, 1982.

276. T. V. Galambos et al., "Probability Based Load Criteria: Assessment of Current Design

Practice," pp. 957-977, Ibid.

277. H. B. Hwang et al., "Probability-Based Design Criteria for Nuclear Plant Structures,"

pp. 925-942 in Journal of Structure Design, 113(5), American Society of Civil

Engineers, New York, New York, 1987.

278. B. Ellingwood and Y. Mori, "Probabilistic Methods for Condition Assessmentand Life

Prediction of Concrete Structures in Nuclear Plants," pp. 155-160 in Nuclear Engineering

and Design, 142, Elsevier Science Publishers, Amsterdam, The Netherlands, 1993.

279. Y. Mori and B. Ellingwood, "Reliability-Based Service Life Assessment of Aging Concrete

Structures," pp. 1600-1621 in Journal of Structural Engineering, 119(5), American

Society of Civil Engineers, New York, New York, 1993.

NUREG/CR-6424 250
280. Y. Mori and B. Ellingwood, "Maintaining Reliability of Concrete Structures I: Role of

Inspection/Repair," pp. 824-845 in Journal of Structural Engineering, 120(3), American

Society of Civil Engineers, New York, New York, 1994.

281. Y. Mori and B. Ellingwood, "Maintaining Reliability of Concrete Structures II: Optimum

Inspection/Repair Strategies," pp. 846-862 in Journal of Structural Engineering, 120(3),

American Society of Civil Engineers, New York, New York, 1994.

282. G. Somerville, "The Design Life of Concrete Structures," pp. 60-71 in The Structural

Engineer, 64A(2), with "Discussion" in pp. 233-241 of The Structural Engineer,

64A(9), London, England, 1986.

283. B. Ellingwood and Y. Mori, Condition Assessment and Reliability-Based Life

Prediction of Concrete Structures, ORNL/NRC/LTR-92/4, Martin Marietta Energy

Systems, Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee, 1992.

284. R. Larrabee and C. A. Cornell, "Combinations of Various Load Processed," pp. 223-239

in Journal of Structural Design, 107(1), American Society of Civil Engineers, New York,

New York, 1981.

285. T. H. Pearce and Y. K. Wen, "Stochastic Combinations of Load Effects," pp. 1613-1629

in Journal of Structural Engineers, 110(7), American Society of Civil Engineers, New

York, New York, 1984.

286. B. Ellingwood and H. Hwang, "Probabilistic Descriptions of Resistance of Safety-Related

Structures in Nuclear Plants," pp. 169-178 in Nuclear Engineering and Design, 88(2),

Elsevier Science Publishers, Amsterdam, The Netherlands, 1985.

287. J. G. MacGregor, A. Mirza, and B. Ellingwood, "Statistical Analysis of Resistance of

Reinfored and Prestressed Concrete Members," pp. 167-176 in Journal of American

Concrete Institute, 80(3), Detroit, Michigan, May/June 1983.

288. B. Ellingwood, "Probabilistic Risk Assessment," pp. 89-116 in Engineering Safety,

McGraw-Hill Book Co., Ltd., London, England, 1992.

289. D. Veneziano et al., "Vector Process Models for System Reliability," pp. 441-460 in

Journal of Engineering Mechanics, 103(3), American Society of Civil Engineers, New

York, New York, 1977.

290. Y. Mori and B. Ellingwood, Reliability Assessment of Degrading Concrete Shear Walls,

ORNL/NRC/LTR-94/6, Martin Marietta Energy Systems, Inc., Oak Ridge National

Laboratory, Oak Ridge, Tennessee, April 1994.

291. F. Barda, J. M. Hanson, and W. G. Corley, "Shear Strength of Low-Rise Walls with

Boundary Element," in Reinforced Concrete Structures in Seismic Zones, ACI SP-53,

American Concrete Institute, Detroit, Michigan, 1977.

292. I. Shiraishi et al., "Macroscopic Models for R/C Shear Walls," pp. 271-280 in Proceedings

of Structural Congress, Structural Design, Analysis, and Testing, American Society of

Civil Engineers (ASCE), New York, New York, 1989.

251 NUREG/CR-6424
293. R. Shohara, N. Shirai, and H. Noguchi, "Verification of Macroscopic Models for R/C

Walls," pp. 281-290, Ibid.

294. F. Watanabe and T. Ichinose, "Strength and Durability Design of RC Members Subjected to

Combined Bending and Shear," pp. 429-438 in Concrete Shear in Earthquake,

T. C. C. Hus and S. T. Mau (eds.), Elsevier Applied Science, New York, New York,

1992.

295. W. F. Chen and D. J. Han, "Plasticity for Structural Engineers," Springer-Vartag, New

York, New York, 1988.

296. F. P. Vecchio and M. P. Collins, "The Modified Compression-Field Theory for Reinforced

Concrete Elements Subjected to Shear," pp. 219-231 in Journal of American Concrete

Institute, 83(2), Detroit, Michigan, March-April 1986.

297. A. E. Cardenas et al., "Design Provisions for Shear Walls," pp. 221-230 in Journal of

American Concrete Institute, 70(3), Detroit, Michigan, March 1973.

298. Y. Mori, "Reliability-Based Condition Assessment and Life Prediction of Concrete

Structures," thesis presented to the Johns Hopkins University, Baltimore, Maryland, in

partial fulfillment of the requirements for the degree of Doctor of Philosophy, 1992.

299. Reliability-Based Inspection-Development of Guidelines, NUREG/GR-0005, U.S.

Nuclear Regulatory Commission, Washington, D.C., 1992.

300. H. Madsen, J. D. Sorenson, and R. Olesen, "Optimal Inspection Planning for Fatigue

Damage of Offshore Structures," pp. 2099-2106 in Proceedigs of ICOSSAR '89,

Vol. III, ASCE, New York, New York, 1989.

301. J. Yang, "Application of Reliability Methods to Fatigue, Quality Assurance, and

Maintenance," pp. 3-18 in Proceedings of ICOSSAR '93, Vol. I, A. A. Balkema,

Rotterdam, The Netherlands, 1994.

302. H. M. Taylor and S. Karlin, "An Introduction to Stochastic Modeling," Academic Press,

Orlando, Florida, 1984.

303. Y. Mori and B. Ellingwood, Methodology for Reliability Based Condition AssessmentApplication to Concrete Structures, NUREG/CR-6052, U.S. Nuclear Regulatory

Commission, Washington, D.C., August 1993.

304. 0. J. V. Chapman, "A Statistical Approach to the Analysis of ISI Data Using Bayes

Method," in Proceedings of the 7th International Conference on Structural Mechanics in

Reactor Technology, Vol. K, Chicago, Illinois, 1983.

305. P. Thoft-Christensen and J. D. Sorensen, "Optimal Strategy for Inspection and Repair on

Structural Stystems," pp. 94-100 in Civil Engineering Systems, 4, June 1987.

306. J. R. Benhamin and C. A. Cornell, Probability, Statistics, and Decision for Civil

Engineering, McGraw-Hill Publishers, New York, New York, 1970.

NUREG/CR-6424 252
307. B. A. Urtagn and M. A. Saunders, MINOS 5.1 Users Guide, Technical Report SOL 83-

20R, System Optimization Laboratory, Department of Operation Research, Stanford

University, Stanford, California, 1987.

308. K. Tutti, Corrosion of Steel in Concrete, Report 4-82, Swedish Cement and Concrete

Research Institute, Stockholm, Sweden, 1982.

253 NUREG/CR-6424

APPENDIX A

U.S. Commercial Nuclear Power Reactor Summaryl, 21

Nuclear Docket Power Type* Type* Construction Operating License Commercial

Unit Number MW(e), Net Plant Containment Permit Issue Expiration Operation

Arkansas 1

Arkansas 2

Beaver Valley 1

Beaver Valley 2

Bellefonte I

Bellefonte 2

Big Rock Point 1

Braidwood I

Braidwood 2

Browns Ferry 1

Browns Ferry 2

Browns Ferry 3

Brunswick 1

Brunswick 2

Byron 1

Byron 2

Callaway 1

Calvert Cliffs 1

Calvert Cliffs 2

Catawba I

Catawba 2

Clinton 1

Comanche Peak 1

Comanche Peak 2

Cook 1

Cook 2

Cooper Station

Crystal River 3

50-313

50-368

50-334

50-412

50-438

50-439

50-155

50-456

50-457

50-259

50-260

50-296

50-325

50-324

50-454

50-455

50-483

50-317

50-318

50-413

50-414

50-461

50-445

50-446

50-315

50-316

50-298

50-302

836

858

810

820

1212

1212

69

1120

1120

1065

1065

1065

790

790

1120

1120

1180

845

845

1145

1145

950

1150

1150

1030

1100

778

825

PC-D

PC-D

RC-SA

RC-SA

PC-C

PC-C

S-PREMK

PC-D

PC-D

S-MKI

S-MKI

S-MKI

RC-MKI

RC-MKI

PC-D

PC-D

PC-E

PC-B

PC-B

S-IC

S-IC

RC-MKIII

RC-LD

RC-LD

RC-IC

RC-IC

S-MKI

PC-B

1968

1972

1970

1974

1974

1974

1960

1975

1975

1967

1967

1968

1970

1970

1975

1975

1976

1969

1969

1975

1975

1976

1974

1974

1969

1969

1968

1968

1974

1978

1976

1987

1962

1987

1988

1973

1974

1976

1976

1974

1985

1987

1984

1974

1976

1985

1986

1987

1990

1993

1974

1977

1974

1977

2014

2018

2016

2027

2000

2026

2027

2013

2014

2016

2016

2014

2024

2026

2024

2014

2016

2024

2026

2026

2030

2033

2014

2017

2014

2016

1974

1980

1976

1987

1963

1988

1988

1974

1975

1977

1977

1975

1985

1987

1984

1975

1977

1985

1986

1987

1990

1975

1978

1974

1977

4Ž.

tSources: 1.

2.

Information Digest, U.S. Nuclear Regulatory Commission, Office of the Controller, NUREG-1350, Vol. 7, Washington, D.C., March 1995.

Commercial Nuclear Power Plants, Edition No. 19, Halliburton NUS, Clearwater, Florida, December 1991.
z

0

a'

-Is

I'-)

~Jl

0'

APPENDIX A (Cont'd)

Nuclear Docket Power Type Type Construction Operating License Commercial

Unit Number MW(e), Net Plant Containment Permit Issue Expiration Operation

Davis-Besse 1

Diablo Canyon 1

Diablo Canyon 2

Dresden 2

Dresden 3

Duane Arnold

Farley 1

Farley 2

Fermi-2

FitzPatrick

Fort Calhoun 1

Ginna

Grand Gulf 1

Haddam Neck

Hatch I

Hatch 2

Hope Creek 1

Indian Point 2

Indian Point 3

Kewaunee

LaSalle 1

LaSalle 2

Limerick 1

Limerick 2

Maine Yankee

McGuire 1

McGuire 2

Millstone 1

Millstone 2

Millstone 3

Monticello

Nine Mile Point 1

50-346

50-275

50-323

50-237

50-249

50-331

50-348

50-364

50-341

50-333

50-285

50-244

50-416

50-213

50-321

50-366

50-354

50-247

50-286

50-305

50-373

50-374

50-352

50-353

50-309

50-369

50-370

50-245

50-336

50-423

50-263

50-220

906

1073

1087

794

794

538

829

829

1093

816

492

470

1250

565

786

795

1067

1007

965

535

1078

1078

1055

1055

840

1180

1180

654

863

1137

536

610

P

P

P

B

B

B

P

P

B

B

P

P

B

P

B

B

B

P

P

P

B

B

B

B

P

P

P

B

P

P

B

B

S-LD

RC-LD

RC-LD

S-MKI

S-MKI

S-MKI

PC-D

PC-D

S-MKI

S-MKI

PC-A

PC-A

RC-MKIII

RC-LD

S-MKI

S-MKI

S-MKI

RC-LD

RC-LD

S-LD

PC-MKII

PC-MKII

RC-MKII

RC-MKII

RC-SA

S-IC

S-IC

S-MKI

PC-D

RC-SA

S-MKI

S-MKI

1969

1968

1970

1966

1966

1970

1972

1972

1972

1970

1968

1966

1974

1964

1969

1972

1974

1966

1969

1968

1973

1973

1974

1974

1968

1973

1973

1966

1970

1974

1967

1965

1977

1984

1985

1969

1971

1974

1977

1981

1985

1974

1973

1969

1984

1967

1974

1978

1986

1973

1976

1973

1982

1984

1985

1989

1973

1981

1983

1970

1975

1986

1971

1969

2017

2008

2010

2006

2011

2014

2017

2021

2025

2014

2013

2009

2022

2007

2014

2018

2026

2013

2015

2013

2022

2023

2024

2029

2008

2021

2023

2010

2015

2025

2010

2009

1978

1985

1986

1970

1971

1975

1977

1981

1988

1975

1974

1970

1985

1968

1975

1979

1986

1974

1976

1974

1984

1984

1986

1990

1972

1981

1984

1971

1975

1986

1971

1969
APPENDIX A (Cont'd)

Nuclear Docket Power Type Type Construction Operating License Commercial

Unit Number MW(e), Net Plant Containment Permit Issue Expiration Operation

Nine Mile Point 2 50-410 1080 B RC-MKII 1974 1987 2026 1988

North Anna 1 50-338 915 P RC-SA 1971 1978 2018 1978

North Anna 2 50-339 915 P RC-SA 1971 1980 2020 1980

Oconee 1 50-269 860 P PC-B 1967 1973 2013 1973

Oconee 2 50-270 860 P PC-B 1967 1973 2013 1974

Oconee 3 50-287 860 P PC-B 1967 1974 2014 1974

Oyster Creek 1 50-219 620 B S-MKI 1964 1969 2009 1969

Palisades 50-255 777 P PC-B 1967 1972 2007 1971

Palo Verde 1 50-528 1270 P PC-E 1976 1985 2024 1986

Palo Verde 2 50-529 1270 P PC-E 1976 1986 2025 1986

Palo Verde 3 50-530 1270 P PC-E 1976 1987 2027 1988

Peach Bottom 2 50-277 1065 B S-MKI 1967 1973 2013 1974

Peach Bottom 3 50-278 1065 B S-MKI 1967 1974 2014 1974

Perry 1 50-440 1205 B S-MKIII 1977 1986 2026 1987

Pilgrim 1 50-293 670 B S-MKI 1968 1972 2012 1972

Point Beach 1 50-266 497 P PC-B 1967 1970 2010 1970

Point Beach 2 50-301 497 P PC-B 1968 1971 2013 1972

Prairie Island 1 50-282 520 P S-LD 1968 1974 2013 1973

Prairie Island 2 50-306 520 P S-LD 1968 1974 2014 1974

Quad Cities 1 50-254 789 B S-MKI 1967 1972 2012 1973

Quad Cities 2 50-265 789 B S-MKI 1967 1972 2012 1973

River Bend 1 50-458 936 B S-MKIII 1977 1985 2025 1986

Robinson 2 50-261 665 P PC-A 1967 1970 2010 1971

St. Lucie 1 50-335 810 P S-LD 1970 1976 2016 1976

St. Lucie 2 50-389 810 P S-LD 1977 1983 2023 1983

Salem 1 50-272 1090 P RC-LD 1968 1976 2016 1977

Salem 2 50-311 1115 P RC-LD 1968 1981 2020 1981

San Onofre 2 50-361 1070 P PC-E 1973 1982 2013 1983

z San Onofre 3 50-362 1080 P PC-E 1973 1983 2013 1984

Seabrook 1 50-443 1150 P RC-LD 1976 1990 2026 1990

t Sequoyahl1 50-327 1150 P S-IC 1970 1980 2020 1981

0'

.N.
APPENDIX A (Cont'd)

a,

0:n

Nuclear Docket Power Type Type Construction Operating License Commercial

Unit Number MW(e), Net Plant Containment Permit Issue Expiration Operation

Sequoyah 2 50-328 1150 P S-IC 1970 1981 2021 1982

Shearon Harris 1 50-400 860 P RC-LD 1978 1987 2026 1987

South Texas 1 50-498 1250 P PC-E 1975 1988 2027 1988

South Texas 2 50-499 1250 P PC-E 1975 1989 2028 1989

Summer 1 50-395 900 P PC-C 1973 1982 2022 1984

Surry 1 50-280 781 P RC-SA 1968 1972 2012 1972

Surry 2 50-281 781 P RC-SA 1968 1973 2013 1973

Susquehanna 1 50-387 1050 B RC-MKII 1973 1982 2022 1983

Susquehanna 2 50-388 1050 B RC-MKII 1973 1984 2024 1985

Three Mile Island 1 50-289 792 P PC-B 1968 1974 2014 1974

Turkey Point 3 50-250 728 P PC-B 1967 1972 2012 1972

Turkey Point 4 50-251 728 P PC-B 1967 1973 2013 1973

Vermont Yankee 1 50-271 514 B S-MKI 1967 1973 2012 1972

Vogtle 1 50-424 1160 P PC-E 1974 1987 2027 1987

Vogtle 2 50-425 1160 P PC-E 1974 1989 2029 1989

Washington Nucl. 2 50-397 1150 B S-MKII 1973 1984 2023 1984

Waterford 3 50-382 1165 P S-LD 1974 1985 2024 1985

Watts Bar 1 50-390 1170 P S-IC 1971

Watts Bar 2 50-391 1170 P S-IC 1971

Wolf Creek 50-482 1150 P PC-E 1977 1985 2025 1985

Zion 1 50-295 1040 P PC-B 1968 1973 2013 1973

Zion 2 50-304 1040 P PC-B 1968 1973 2013 1974

B = Boiling Water Reactor

P = Pressurized-Water Reactor

S-LD = Steel, Large Dry

S-PREMK = Steel, Pre-Mark

S-MKI = Steel, Mark I

S-MKII = Steel, Mark II

S-MKIII = Steel, Mark III

S-IC = Steel, Ice Condenser

PC-A = Prestr. Concrete, No Buttresses

PC-B = Prestr. Concrete, 6 Buttresses/Shallow Dome

PC-C = Prestr. Concrete, 4 Buttresses/Shallow Dome

PC-D = Prestr. Concrete, 3 Buttresses/Shallow Dome

PC-E = Prestr. Concrete, 3 Buttresses/Hemispherical Dome

PC-MKII = Prestr. Concrete, Mark II

RC-LD = Rein. Concrete, Large Dry

RC-MKI = Rein. Concrete, Mark I

RC-MKII = Rein. Concrete, Mark II

RC-MKIII = Rein. Concrete, Mark III

RC-SA = Rein. Concrete, Subatmospheric

RC-IC = Rein. Concrete, Ice Condenser
APPENDIX B

REPORTS AND PAPERS PREPARED UNDER THE SAG PROGRAM

I D. J. Naus, Concrete Component Aging and Its Significance Relative to Life

Extension of Nuclear Power Plants, NUREG/CR-4652 (ORNL/TM-10059),

Martin Marietta Energy Systems, Inc., Oak Ridge National Laboratory, Oak Ridge,

Tennessee, September 1986.*

2. D. J. Naus, "Aging of Concrete Components and Its Significance Relative to Life

Extension of Nuclear Power Plants," pp. 229-234, in Trans. of the 9th Int'l. Conf.

on Str. Mech. in Reactor Tech., Paper D5/1, Lausanne, Switzerland, A. A. Belkema

(Publisher), August 1987.*

3. D. J. Naus and C. E. Pugh, Report of Foreign Travel of C. E. Pugh and D. J.

Naus, Engineering Technology Division, ORNL/FTR-2694, Martin Marietta Energy

Systems, Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee, August 8-

26, 1987 (September 21, 1987).*

4. D. J. Naus, M. F. Marchbanks, and E. G. Arndt, "Evaluation of Aged Concrete

Structures for Continued Service in Nuclear Power Plants," pp. 57-67, in Proc. of

Topical Meeting on Nuclear Power Plant Life Extension, July 31-August 3,

1988, Snowbird, Utah, Session 2, Paper 1, American Nuclear Society, 1988.

5. D. J. Naus et al., Structural Aging Program Five-Year Plan: FY 1988-1992,

ORNLUNRC/LTR-89/1, Martin Marietta Energy Systems, Inc., Oak Ridge National

Laboratory, Oak Ridge, Tennessee, March 1989.

6. M. F. Marchbanks, A Review and Assessment of Materials Property Databases

with Particular Reference to Concrete Material Systems, ORNL/NRC/LTR-89/3,

Martin Marietta Energy Systems, Inc., Oak Ridge National Laboratory, Oak Ridge,

Tennessee, March 1989.

7. D. J. Naus, M. F. Marchbanks, and E. G. Arndt, "Evaluation of Aged Concrete

Structures for Continued Service in Nuclear Power Plants," Proceedings of the

United States Nuclear Regulatory Commission Sixteenth Water Reactor Safety

Information Meeting held at National Institute of Standards and Technology,

Gaithersburg, Maryland, NUREG/CP-0097, Vol. 3, Structural and Seismic

Engineering, March 1989.

8. D. J. Naus et al., "Structural Aging Program to Assess Adequacy of Critical Concrete

Components in Nuclear Power Plants," pp. 109-118 in Transactions of the 10th

International Conference on Structural Mechanics in Reactor Technology,

Session D, Paper 122, Anaheim, California, August 1989.

9. C. B. Oland, M. F. Marchbanks, and D. J. Naus, Plan for Use in Development of

the Structural Materials Information Center, ORNL/NRC/LTR-89/8, Martin

Marietta Energy Systems, Inc., Oak Ridge National Laboratory, Oak Ridge,

Tennessee, September 1989.

*Prepared under NPAR Program.

259 259 NUJRBGA2R-6424
10. D. J. Naus and C. B. Oland, Five-Year Compressive Strength Test Results for

Moist-Cured and Sealed High-Strength Variable Fly Ash Content Concretes,

ORNL/NRC/LTR-89/12, Martin Marietta Energy Systems, Inc., Oak Ridge National

Laboratory, Oak Ridge, Tennessee, November 1989.

11. D. J. Naus et al., "Considerations in the Evaluation of Concrete Structures for

*Continued Service in Aged Nuclear Power Plants," pp. 827-32 in Proceedings of

the American Power Conference, Vol. 51, Illinois Institute of Technology,

Chicago, Illinois, 1989.

12. D. J. Naus et al., Structural Aging (SAG) Program Five-Year Plan: FY 1989 -

1993, ORNL/NRC/LTR-89/15, Martin Marietta Energy Systems, Inc., Oak Ridge

National Laboratory, Oak Ridge, Tennessee, December 1989.

13. D. J. Naus, C. B. Oland, and M. F. Marchbanks, Structural Aging Program Annual

Technical Progress Report for Period October 1, 1988, to September 30, 1989

(FY 1989), ORNL/NRC/LTR-90/1, Martin Marietta Energy Systems, Inc., Oak

Ridge Oak Ridge National Laboratory, Oak Ridge, Tennessee, January 1990.

14. C. B. Oland, Report of Canadian Travel of C. B. Oland, Pressure Vessel

Technology Section, Engineering Technology Division, Letter Report to E. G.

Arndt, U. S. Nuclear Regulatory Commission, Rockville, Maryland, from C. B.

Oland, Martin Marietta Energy Systems, Inc., Oak Ridge Oak Ridge National

Laboratory, Oak Ridge, Tennessee, April 10, 1990.

15. D. J. Naus, Report of Foreign Travel of D. J. Naus, Engineering Technology

Division, June 9-23, 1990, ORNL/FTR-3641, Martin Marietta Energy Systems,

Inc., Oak Ridge Oak Ridge National Laboratory, Oak Ridge, Tennessee, July 9,

1990.

16. C. B. Oland and D. J. Naus, Structural Materials Information Center for

Presentation of the Time Variation of Materials Properties, ORNL/NRC/LTR90/22, Martin Marietta Energy Systems, Inc., Oak Ridge Oak Ridge National

Laboratory, Oak Ridge, Tennessee, November 1990.

17. D. J. Naus, Report of Foreign Travel of D. J. Naus, Engineering Technology

Division, November 3-15, 1990, ORNL/FTR-3827, Martin Marietta Energy

Systems, Inc., Oak Ridge Oak Ridge National Laboratory, Oak Ridge, Tennessee,

November 27, 1990.

18. C. J. Hookham, Structural Aging Assessment Methodology for Concrete

Structures in Nuclear Power Plants, ORNL/NRC/LTR-90/17 (Subcontract Report

11 X-SD343V from Multiple Dynamics Corporation, Southfield, Michigan), Martin

Marietta Energy Systems, Inc., Oak Ridge Oak Ridge National Laboratory, Oak

Ridge, Tennessee, March 1991.

19. D. J. Naus and C. B. Oland, Structural Aging Program Technical Progress Report

for Period October 1, 1989, to December 31, 1990, ORNL/NRC/LTR-91/2, Martin

Marietta Energy Systems, Inc., Oak Ridge Oak Ridge National Laboratory, Oak

Ridge, Tennessee, March 1991.

NURBG/CR-6424
20. D. J. Naus, C. B. Oland, and E. G. Arndt, "Management of the Aging of Critical

Safety-Related Concrete Structures in Light-Water Reactor Plants," pp. 527-552 in

Proceedings of United States Nuclear Regulatory Commission Eighteenth Water

Reactor Safety Information Meeting held at Holiday Inn-Crowne Plaza, Rockville,

Maryland, NUREG/CP-01 14, Vol. 1 - Structural and Seismic Engineering, April

1991.

21. C. B. Oland and D. J. Naus, "Development of the Structural Materials Information

Center," pp. 579-596 in Proceedings of United States Nuclear Regulatory

Commission Eighteenth Water Reactor Safety Information Meeting held at

Holiday Inn- Crowne Plaza, Rockville, Maryland, NUREG/CP-01 14, Vol. 1-

Structural and Seismic Engineering, April 1991.

22. B. R. Ellingwood and Y. Mori, "Probabilistic Methods for Condition Assessment

and Life Prediction of Concrete Structures in Nuclear Power Plants, pp. 553-577 in

Proceedings of United States Nuclear Regulatory Commission Eighteenth Water

Reactor Safety Information Meeting held at Holiday Inn-Crowne Plaza, Rockville,

Maryland, NUREG/CP-01 14, Vol. 1-Structural and Seismic Engineering, April

1991.

23. D. J. Naus, C. B. Oland, and E. G. Arndt, "Aging Management of Safety-Related

Concrete Structures in Nuclear Power Plants," pp. 1-7 in Technical Sessions on

Nuclear Plant Systems/Components Aging Management and Life Extension,

ASME Pressure Vessel and Piping Conference 1991, ASME PVP-Vol. 208, San

Diego, California, June 23-27, 1991.

24. D. J. Naus, C. B. Oland, and E. G. Arndt, "Aging Management of Safety-Related

Concrete Structures to Provide Improved Bases for Continuing the Service of

Nuclear Power Plants," pp. 308-316 in Journal. of Mat'ls and Struct., 24(142), The

International Union of Testing and Research Laboratories for Materials and

Structures, Chapman and Hall, New York, July 1991.

25. B. R. Ellingwood and Y. Mori, "Probabilistic Methods for Life Prediction of

Concrete Structures in Nuclear Power Plants," pp. 291-296 in Transactions of the

11th International Conference on Structural Mechanics in Reactor Technology,

Vol. D, Tokyo, Japan, August 18-23, 1991.

26. C. B. Oland, Report of Foreign Travel of C. B. Oland, Engineering Technology

Division, August 4-16, 1991, ORNL/FTR-4004, Martin Marietta Energy Systems,

Inc., Oak Ridge Oak Ridge National Laboratory, Oak Ridge, Tennessee, August 30,

1991.

27. D. J. Naus, C. B. Oland, and E. G. Arndt, "Aging of Concrete Structures in Nuclear

Power Plants," M91-50997, Minutes, RILEM TC-104 Damage Classification of

Concrete Structures, Technical University, Kosice, Czechoslovakia, August 30,

1991.

28. D. J. Naus, C. B. Oland, and E. G. Arndt, "Aging of Concrete Structures in Nuclear

Power Plants," Proceedings of the RILEM-IMEKO International Conference on

Diagnosis of Concrete Structures, EXPERTCENTRUM CZECHOSLOVAKIA,

September 3-6, 1991, Slovak Technical University, Bratislava, Czechoslovakia,

1991.

261 261 NURF.GiR-6424
29. T. M. Refai and M. K. Lim, Inservice Inspection and Structural Integrity

Assessment Methods for Nuclear Power Plant Concrete Structures,

ORNL/NRC/LTR-90/29 (Subcontract Report 17X-SE611V from Construction

Technology Laboratories, Inc., Skokie, Illinois), Martin Marietta Energy Systems,

Inc., Oak Ridge Oak Ridge National Laboratory, Oak Ridge, Tennessee, September

1991.

30. D. J. Naus, Report of Foreign Travel of D. J. Naus, Engineering Technology

Division, August 26-September 7, 1991 , ORNL/FTR-4029, Martin Marietta

Energy Systems, Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee,

September 23, 1991.

31. J. R. Clifton, Predicting the Remaining Life of In-Service Concrete, NISTIR

4712, National Institute of Standards and Technology, U.S. Department of

Commerce, Gaithersburg, Maryland, November 1991.

32. B. R. Ellingwood and Y. Mori, "Condition Assessment and Reliability-Based Life

Prediction of Concrete Structures in Nuclear Power Plants," ORNLINRC/LTR-92/4

(Subcontract Report 19X-SD684V from The Johns Hopkins University, Baltimore,

Maryland), Martin Marietta Energy Systems, Inc., Oak Ridge National Laboratory,

Oak Ridge, Tennessee, January 1992.

33. M. G. Van Geem and D. J. Naus, Summary of Test Results for Portland Cement

Association Study on Long-Term Properties of Concrete, ORNL/NRC/LTR-91/26

(Subcontract Report 11X-SF71OV from Construction Technology Laboratories, Inc.,

Skokie, Illinois), Martin Marietta Energy Systems, Inc., Oak Ridge National

Laboratory, Oak Ridge, Tennessee, January 1992.

34. D. J. Naus and C. B. Oland, Structural Aging Program Technical Progress Report

for Period January 1, 1991, to December 31, 1991, ORNL/NRC/LTR-92/3, Martin

Marietta Energy Systems, Inc., Oak Ridge National Laboratory, Oak Ridge,

Tennessee, February 1992.

35. D. J. Naus et al., "Progress in Research on Aging of Structures," pp. 91-114 in

Proceedings of United States Nuclear Regulatory Commission Nineteenth Water

Reactor Safety Information Meeting held at Marriott Hotel, Bethesda, Maryland,

NUREG/CP-0 119, Vol. 3, April 1992.

36. C. B. Oland, The Structural Materials Information Center and Its Potential

Applications, ORNL/NRC/LTR-92/8, Martin Marietta Energy Systems, Inc., Oak

Ridge National Laboratory, Oak Ridge, Tennessee, May 1992.

37. G. M. Sabnis and A. T. Kemngang, Development of a Damage Rating and Repair

Prioritization Methodology for Reinforced Concrete Components in Nuclear

Power Plants, Phase I Report, Subcontract No. 1 X-SD343V, Howard University,

Washington, D. C., June 10, 1992.

38. D. J. Naus et al., "An Improved Basis for Evaluating Continued Service of Category

I Concrete Structures in Nuclear Power Plants (NPPs)," pp. 31-39 in Proceedings

of ASME Pressure Vessel and Piping Conference, PVP-Vol. 228, Technical

Session 16 on Nuclear Plant Systems/Components Aging Management and Life

Extension, New Orleans, Louisiana, June 21-25, 1992.

NUREG/CR-6424 262
39. D. J. Naus et al., "Aging of Concrete Containment Structures in Nuclear Power

Plants," Proceedings of Fifth Workshop on Containment Integrity held at

Washington Marriott, Washington, D.C., NUREG/CP-0120 (SAND92-0173), July

1992.

40. K. A. Snyder, J. R. Clifton, and N. J. Carino, Nondestructive Evaluation of the

In-Place Compressive Strength of Concrete Based Upon Limited Destructive

Testing, NISTIR 4874, National Institute of Standards and Technology, U.S.

Department of Commerce, Gaithersburg, Maryland, July 1992.

41. D. J. Naus and E. G. Arndt, "An Overview of the Structural Aging Program,"

pp. 253-275 in Proceedings of NRC Aging Research Information Conference held

at Holiday Inn-Crowne Plaza, Rockville, Maryland, NUREG/CP-0122, Vol. 2,

September 1992.

42. C. B. Oland, "Data Base on Structural Materials Aging Properties," pp. 276-298 in

Proceedings of NRC Aging Research Information Conference held at Holiday InnCrowne Plaza, Rockville, Maryland, NUREG/CP-0122, Vol. 2, September 1992.

43. B. R. Ellingwood and Y. Mori, "Reliability-Based Condition Assessment of

Concrete Structures in Nuclear Power Plants," pp. 299-323 in Proceedings of NRC

Aging Research Information Conference held at Holiday Inn-Crowne Plaza,

Rockville, Maryland, NUREG/CP-0122, Vol. 2, September 1992.

44. M. G. Van Geem, Forty-Two-Year Test Results for PCA Test Series 374 on LongTerm Properties of Concrete, ORNL/NRC/LTR-92/26 (Subcontract Report 37XSK412V from Construction Technology Laboratories, Inc., Skokie, Illinois), Martin

Marietta Energy Systems, Inc., Oak Ridge National Laboratory, Oak Ridge,

Tennessee, September 1992.

45. Y. Mori, "Reliability-Based Condition Assessment and Life Prediction of Concrete

Structures," Dissertation submitted to The Johns Hopkins University in partial

fulfillment of the requirements for the degree of Doctor of Philosophy, Baltimore,

Maryland, September 1992.

46. C. B. Oland, Report of Foreign Travel of C. B. Oland, Engineer, Structural Aging

Program, Engineering Technology Division, September 24-October 2, 1992,

ORNL/FTR-4416, Martin Marietta Energy Systems, Inc., Oak Ridge National

Laboratory, Oak Ridge, Tennessee, October 16, 1992.

47. D. J. Naus et al., "An Update on the Structural Aging Program," Proceedings of

United States Nuclear Regulatory Commission Twentieth Water Reactor Safety

Information Meeting held at Marriott Hotel, Bethesda, Maryland, October 28-30,

1992.

48. D. J. Naus and C. B. Oland, "An Update on the Structural Aging Program,"

Proceedings of the 6th Meeting: RILEM Technical Committee 104-DDC,

Damage Classification of Concrete Structures held 18-20 May 1992 Haifa, Israel,

Published by The National Building Research Institute, Technion-Israel Institute of

Technology, I. Soroka (ed.), Technion City Haifa, Israel, 1992.

263 NUREGA-R-6424
49. P. Dawson and M. J. M. Wilson, Surveillance Data for PCPVs at Wylfa,

Hartlepool and Heysham I Power Stations, Report No. 1302/92/5957 (Subcontract

No. 3 1X-SH469V), Taywood Engineering Ltd., London, England, January 1993.

50. S. J. Widdows, Testing of Aged Concretes from UK Nuclear Power Stations,

Report No. 1303/92/6037 (Subcontract No. 3 1X-SH469V), Taywood Engineering

Ltd., London, England, April 1993.

51. J. Hartley and P. B. Bamforth, Collation of Survey Data and Review of the

Durability Assessment of Reinforced Concrete Structures at Nuclear Power

Stations in the UK, Report No. 1303/92/6163 (Subcontract No. 31X-SH469V),

Taywood Engineering Ltd., London, England, April 1993.

52. W. F. Price,. P. B. Bamforth, and G. K. Glass, Review of European Repair

Practice for Corrosion Damaged Reinforced Concrete, Report No. 1303/91/5823

(Subcontract No. 31X-SH469V), Taywood Engineering Ltd., London, England,

April 1993.

53. W. J. Swiat et al., State-of-the-Art Report - Corrosion of Steel in Concrete,

ORNL/NRC/LTR-93/2 (Subcontract Report 1BX-SK529V from Corrpro

Companies, Inc., Medina, Ohio), Martin Marietta Energy Systems, Inc., Oak Ridge

National Laboratory, Oak Ridge, Tennessee, May 1993.

54. D. J. Naus et al., "Continuing the Service of Safety-Related Concrete Structures in

Nuclear Power Plants," Proceedings of the RILEM International Conference on

Failures of Concrete Structures, EXPERTCENTRUM-BRATISLAVA, Strbske

Pleso, Slovakia, June 14-18, 1993.

55. D. J. Naus, "Report of Foreign Travel of D. J. Naus, Manager, Structural Aging

Program, Engineering Technology Division," Letter DJN/93-53 from D. J Naus,

Martin Marietta Energy Systems, Inc., Oak Ridge National Laboratory, Oak Ridge,

Tennessee, to E. G. Arndt, U.S. Nuclear Regulatory Commission, Washington,

D.C., July 22, 1993.

56. D. J. Naus et al., "Towards Assuring the Continued Performance of Safety-Related

Concrete Structures in Nuclear Power Plants," Technical Sessions on Nuclear Plant

Systems/Components Aging Management and Life Extension, PVP-Vol. 252,

ASME Pressure Vessel and Piping Conference, July 25-29, 1993, Denver,

Colorado, 1993.

57. G. M. Sabnis and A. T. Kemngang, Damage Rating and Repair Prioritization of

Concrete Structural Components in Nuclear Power Plants, Subcontract Report

19X-SJ685V from Howard University, Washington, D.C., to Martin Marietta

Energy Systems, Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee, July

1993.

58. Y. Mori and B. Ellingwood, "Reliability-Based Condition Assessment of Concrete

Structures," Proceedings of the 6th International Conference on Structural Safety

and Reliability, International Association for Structural Safety and Reliability,

Innsbruck, Austria, August 9-13, 1993.

NUREG/CR-642426 264
59. D. J. Naus, C. B Oland and E. G. Arndt, "Continuing the Service of Aging Concrete

Structures in Nuclear Power Plants," pp. 473-478 in Transactions of the 12th

International Conference on Structural Mechanics in Reactor Technology,

Session DH06 - Concrete and Containment Structures, Vol. D, Stuttgart,

Germany, August 15-20, 1993.

60. C. B. Oland, D. J. Naus, and S. Jerath, "A Data Base for Aging of Structural

Materials," pp. 461-466 in Transactions of the 12th International Conference on

Structural Mechanics in Reactor Technology, Session DH06 - Concrete and

Containment Structures, Vol. D, Stuttgart, Germany, August 15-20, 1993.

61. B. R. Ellingwood and Y. Mori, "Role of InService Inspection and Maintenance in

Reliability Assurance of Concrete Structures in Nuclear Plants," pp. 87-92 in

Transactions of the 12th International Conference on Structural Mechanics in

Reactor Technology, Session M - Structural Reliability and Probabilistic Safety,

Vol. M, Stuttgart, Germany, August 15-20, 1993.

62. D. J. Naus and C. B. Oland, Structural Aging Program Technical Progress for

Period January-December 31, 1992, NUREG/CR-6015 (ORNL/TM-12342),

Martin Marietta Energy Systems, Inc., Oak Ridge National Laboratory, Oak Ridge,

Tennessee, July 1993.

63. Y. Mori and B. R. Ellingwood, Methodology for Reliability-Based Condition

Assessment - Application to Concrete Structures in Nuclear Plants,

NUREG/CR-6052 (ORNL/Sub/93-SD684), The Johns Hopkins University,

Baltimore, Maryland, August 1993.

64. D. J. Naus, C. B. Oland, and B. R. Ellingwood, "Structural Aging Program

Approach to Providing an Improved Basis for Aging Management of Safety-Related

Concrete Structures," Proceedings of United States Nuclear Regulatory

Commission Twenty-First Water Reactor Safety Information Meeting held at

Marriott Hotel, Bethesda, Maryland, October 25-27, 1993.

65. Y. Mori and B. Ellingwood, "Time-Dependent System Reliability Analysis by

Adaptive Importance Sampling," Journal of Structural Safety, 1993.

66. Y. Mori and B. Ellingwood, "Reliability-Based Service Life Assessment of Aging

Concrete Structures," pp. 1600-1621, Journal of Structural Engineering, American

Society of Civil Engineers, 119(5), 1993.

67. D. J. Naus et al., "An Overview of the ORNL/NRC Program to Address Aging of

Concrete Structures in Nuclear Power Plants, pp. 327-329 in Nucl. Eng. and

Design, 142, 1993.

68. C. B. Oland, D. J. Naus and E. G. Arndt, "Research and Development at the

Structural Materials Information Center," pp. 179-187 in Nucl. Eng. and Design,

142, 1993.

69. B. Ellingwood and Y. Mori, "Probabilistic Methods for Condition Assessment and

Life Prediction of Concrete Structures in Nuclear Power Plants," pp. 155-166 in

Nucl. Eng. and Design 142, 1993.

265 NUREG/CR-6424
70. G. M. Sabnis and A. T. Kemngang, "Damage Rating and Repair Prioritization of

Concrete Structures in Nuclear Power Plants," Proceedings of the 18th Conference

on Our World in Concrete & Structures, Singapore, August 25-27, 1993.

71. D. J. Naus, Report of Foreign Travel of D. J. Naus March 8-March 15, 1994,

Engineering Technology Division, ORNL/FTR-4924, Martin Marietta Energy

Systems, Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee, March

25,1994.

72. P. D. Krauss, Repair Materials and Techniques for Concrete Structures in

Nuclear Power Plants, ORNL/NRC/LTR-93/28 (Subcontract 90X-SJ024C with

Wiss, Janney, Elstner, and Associates, Inc., Northbrook, Illinois), Martin Marietta

Energy Systems, Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee,

March 1994.

73. B. Ellingwood and Y. Mori, Reliability Assessment of Degrading Concrete Shear

Walls, ORNL/NRC/LTR-94/6, (ORNL/Sub/93-SD684V), (Subcontract 19XSD684V with The Johns Hopkins University, Baltimore, Maryland), Martin Marietta

Energy Systems, Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee, April

1994.

74. J. Pachner, T. Tai, and D. J. Naus, "Concrete Aging Study," Proceedings of the

American Power Conference held at Sheraton Chicago Hotel and Towers April 25-

27, 1994, Chicago, Illinois, April 1994.

75. D. J. Naus, C. B. Oland, and B. R. Ellingwood, "Structural Aging Program to

Evaluate Continued Performance of Safety-Related Concrete Structures in Nuclear

Power Plants," Proceedings of British Nuclear Energy Society 1994 International

Conference on Thermal Reactor Safety Assessment, held at Ramada Hotel,

Manchester, England, May 23-26, 1994.

76. H. Ashar, D. J. Naus, and C. P. Tan, "Prestressed Concrete in U. S. Nuclear Power

Plants: Part 1," pp. 58-61 in Concrete International, 16(5), American Concrete

Institute, Detroit, Michigan, May 1994.

77. H. Ashar, C. P. Tan and D. J. Naus "Prestressed Concrete in U. S. Nuclear Power

Plants: Part 2," pp. 30-34 in Concrete International, 16(6), American Concrete

Institute, Detroit, Michigan, June 1994.

78. D. J. Naus, Report of Foreign Travel of D. J. Naus, May 23-28, 1994,

Engineering Technology Division, ORNL/FTR-4987, Martin Marietta Energy

Systems, Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee, June 8,

1994.

79. B. Ellingwood and Y. Mori, "Maintaining Reliability of Concrete Structures I: Role

of Inspection/Repair, pp. 824-345 in Journal of Structural Engineering, 120(3),

American Society of Civil Engineers, New York, 1994.

80. B. Ellingwood and Y. Mori, "Maintaining Reliability of Concrete Structures II:

Optimum Inspection/Repair Strategies, pp. 846-862 in Journal of Structural

Engineering, 120(3), American Society of Civil Engineers, New York, 1994.

NUREG)CR-6424 266
81. D. J. Naus et al., "Aging Management of Containment Structures in Nuclear Power

Plants," Proceedings of Third International Conference on Containment Design

and Operation held at Toronto Hilton October 19-21, 1994, The Canadian Nuclear

Society, Toronto, Ontario, Canada, 1994.

82. D. J. Naus, Report of Foreign Travel of D. J. Naus, Engineering Technology

Division, October 6-October 11, 1994, ORNL/FTR-5167, Martin Marietta Energy

Systems, Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee, October 31,

1994.

83. D. J. Naus, Report of Foreign Travel of D. J. Naus, Engineering Technology

Division, October 13-October 21, 1994, ORNL/FTR-5168, Martin Marietta Energy

Systems, Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee, November 4,

1994.

84. M. F. Marchbanks and C. B. Oland, Considerations in the Formulation of a

Customized Data Base for Structural Materials, ORNL/NRC/LTR-94/20, Martin

Marietta Energy Systems, Inc., Oak Ridge National Laboratory, Oak Ridge,

Tennessee, November 1994.

85. C. B. Oland and D. J. Naus, Summary of Materials Contained in the Structural

Materials Information Center, ORNL/NRC/LTR-94/22, Martin Marietta Energy

Systems, Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee, November

1994.

86. D. J. Naus and C. B. Oland, Structural Aging Program Technical Progress Report

for Period January 1, 1993, to June 30, 1994, ORNL/NRC/LTR-94/21, Martin

Marietta Energy Systems, Inc., Oak Ridge National Laboratory, Oak Ridge,

Tennessee, November 1994.

87. D. J. Naus, Report of Foreign Travel of D. J. Naus, Engineering Technology

Division, February 20, 1995, ORNL/FTR-5305, Lockheet Martin Energy Systems,

Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee, February 18, 1995.

88. D. J. Naus, Report of Foreign Travel of D. J. Naus, Engineering Technology

Division, April 4-6, 1995, ORNL/FTR-5335, Martin Marietta Energy Systems,

Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee, April 16, 1995.

89. D. J. Naus, Report of Foreign Travel of D. J. Naus, Engineering Technology

Division, April 19-21, 1995, ORNL/FTR-5356, Martin Marietta Energy Systems,

Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee, May 1, 1995.

90. B. R. Ellingwood, "Reliability-Based Service Life Assessment of Concrete

Structures in Nuclear Power Plants: Optimum Inspection and Repair," Transactions

of the 13th International Conference on Structural Mechaincs in Reactor

Technology, Session M - Reliability, Paper 81, Porto Alegre, RS, Brazil, August

1995.

91. D. J. Naus, B. R. Ellingwood, and C. B. Oland, "Extending the Lifespan of Nuclear

Power Plant Concrete Structures," Symposium - Extending the Lifespan of

Structures, International Association for Bridge and Structural Engineers, San

Francisco, California, August 23-25, 1995.

267 NUREG/-R-6424
92. H. T. Hill, Concrete Containment Posttensioning System Aging Study,

ORNL/NRC/LTR-95/13, Lockheed Marietta Energy Systems, Inc., Oak Ridge

National Laboratory, Oak Ridge, Tennessee, August 1995.

93. D. J. Naus, C. B. Oland, and B. R. Ellingwood, "Management of Age-Related

Degradation in Nuclear Power Plant Concrete Structures," Proceedings of

Specialists Meeting on the Effectiveness of Methods for Detection and

Monitoring of Age Related Degradation in Nuclear Power Plants, Comisfon

Nacional de Energfa Atomica of Argentina, Bariloche, Argentina, October 1995.

94. D. J. Naus, Report of Foreign Travel of D. J. Naus, Engineering Technology

Division, September 25-29, 1995, ORNL/FTR-5584, Lockheed Martin Energy

Systems, Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee, October 13,

1995.

95. D. J. Naus, Report of Foreign Travel of D. J. Naus, Engineering Technology

Division, October 14-21, 1995, ORNL/FTR-5607, Lockheed Martin Energy

Systems, Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee, October 27,

1995.

96. C. J. Hookham, In-Service Inspection Guidelines for Concrete Structures in

Nuclear Power Plants, ORNL/NRC/LTR-95/14, Lockheed Martin Energy Systems,

Inc., Oak Ridge National Laboratory, Oak Ridge, Tennessee, December 1995.

97. B. R. Ellingwood, Aging Effects on Probabilistic Risk Assessment, NUREG/CR6425 (ORNL/TM-13149), Lockheed Martin Research Corp., Oak Ridge National

Laboratory, Oak Ridge, Tennessee, January 1996.

98. D. J. Naus, C. B. Oland, and B. R. Ellingwood, Final Report on Aging of

Reinforced Concrete Structures, NUREG/CR-6424 (ORNLfM-13148), Lockheed

Martin Energy Research Corp., Oak Ridge National Laboratory, Oak Ridge,

Tennessee, January 1996.

NUREBGR-6424 268
NUREG/CR-6424

ORNL/TM-13148

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77. R. Mills, Professor of Civil Engineering, University of Toronto, Toronto, Ontario

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271 NUREG/CR-6424

NRC FORM 335 U.S. NUCLEAR REGULATORY COMMISSION 1. REPORT NUMBER

NRCM (2-89( IAslgned by NRC. Add Vol., Supp., Rev., 1102, and Addendum Numbers, If any.) 3201,3202 BIBLIOGRAPHIC DATA SHEET NUREG/CR-6424

(See instructions on the reverse) ORNL/TM-13148

2. TITLE AND SUBTITLE

Report on Aging of Nuclear Power Plant Reinforced Concrete

Structures 3. DATE REPORT PUBLISHED

MONTH "I YEAR

March 1996

4. FIN OR GRANT NUMBER

B0845

5. AUTHOR(S) 6. TYPE OF REPORT

D.J. Naus, C..B. Oland, ORNL

B.R. Ellingwood, JHU

7. PERIOD COVERED (Inclusive Dates)

8. PERFORMING ORGANIZATION - NAME AND ADDRESS (If NRC, provide Division. Office or Region, U.S. Nuclear Regulatory Commission. and mailing address, if contractor. provide

name and mailing address.J

Oak Ridge National Laboratory Subcontractor:

Oak Ridge, Tennessee 37831-6285 The Johns Hopkins University

3400 North Charles Street

Baltimore, Maryland 21218-2699

9. SPONSORING ORGANIZATION - NAME AND ADDRESS (It NRC. type "'Same as above", if contractor provide NRC Divison, Office or Region. U.S. Nuclear Regulatory Comimion,

and mailing address.)

Division of Engineering Technology

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

10. SUPPLEMENTARY NOTES

H. Graves, NRC Project Manager

11. ABSTRACT (200 words or less)

The Structural Aging Program provides the U.S. Nuclear Regulatory Commission with

potential structural safety issues and acceptance criteria for continued service

assessments of safety-related nuclear power plant concrete structures. The program

was organized under four task areas - Program Management, Materials Property Data

Base, Structural Component Assessment/Repair Technology, and Quantitative Methodology for Continued Service Determinations. Under these tasks, over 90 papers and reports

were prepared addressing pertinent aspects associated with aging management of

nuclear power plant reinforced concrete structures. Contained in this report is a

summary of program results in the form of information related to longevity of nuclear

power plant reinforced concrete structures, a data base presenting data and information4

on the time variation of concrete materials under the influence of environmental

stressors and aging factors, in-service inspection and condition assessments techniques, repair materials and methods, evaluation of nuclear power plant reinforced

concrete structures and a reliability-based methodology for current and future

condition assessments. Recommendations for future activities are also provided.

12. KEY WORDS!DESCR!PTORS (List words or nhrases that Will/assist resedrchsr.s in lo-atieq the report 1 13. AVAILABILITY STATEMENT

unlimited

Aging Management Non-Destructive Evaluation 14. SECURITY CLASSIFICATION

Concrete Condition Assessment Prestressed Concrete (This Page)

Concrete Containments Reinforced Concrete

Degradation Models Repair unclassified

Electronic Data Base Service Life f hisReport)

In-Service Inspection Structural Reliability unclassified

Materials Handbook 15. NUMBER OF PAGES

16. PRICE

NRC FORM 335 (2-891


Federal Recycling Program

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