Research, Development, and Technology

Turner-Fairbank Highway Research Center

6300 Georgetown Pike

McLean, VA 22101-2296

An FHWA Special Study: Post-Tensioning

Tendon Grout Chloride Thresholds 

PUBLICATION NO. FHWA-HRT-14-039 MAY 2014
FOREWORD

Elevated levels of chloride were recently discovered in a commercially available pre-bagged 

grout product made for post-tensioned (PT) tendons. These chloride values exceed the current 

limits set by all of the domestic and international regulatory committees. However, there is 

limited information reported in the literature pertaining to actual chloride threshold of seven-wire 

PT strands. For this reason, the Federal Highway Administration sponsored a 6-month 

accelerated corrosion testing program to determine chloride threshold(s) of PT strands exposed 

to chloride-contaminated grout. The findings of this study should be useful to bridge owners who 

may be responsible for similarly affected PT bridges. 

This report presents two chloride threshold values determined for the PT strands that are fully 

encased in the commercially available grout described above. The first threshold is 0.4 percent 

chloride concentration by weight of cement, which is the lowest amount to initiate corrosion of 

strands. At this threshold, the testing program demonstrated that rust spots can form, and a small 

number of pits start to form beneath some of the rust spots. The second chloride threshold is 

0.8 percent chloride by weight of cement in association with corrosion propagation. At this 

threshold, the testing program demonstrated that corrosion starts to intensify in terms of number 

of pits and pit depth; therefore, this threshold is considered to be critical. Once chloride 

concentration exceeds the critical threshold, significant corrosion damage can be anticipated, and 

the long-term structural integrity of the PT bridges may be compromised. If PT tendons contain 

carbonated grout, segregated grout, duct cracks, grout voids filled with water with or without 

chloride ions, or free sulfate ions in contact with the strands, corrosion might start below the 

proposed threshold values.

Jorge E. Pagán-Ortiz

Director, Office of Infrastructure

Research and Development

Notice

This document is disseminated under the sponsorship of the U.S. Department of Transportation

in the interest of information exchange. The U.S. Government assumes no liability for the use of 

the information contained in this document. This report does not constitute a standard,

specification, or regulation.

The U.S. Government does not endorse products or manufacturers. Trademarks or 

manufacturers’ names appear in this report only because they are considered essential to the 

objective of the document.

Quality Assurance Statement

The Federal Highway Administration (FHWA) provides high-quality information to serve 

Government, industry, and the public in a manner that promotes public understanding. Standards

and policies are used to ensure and maximize the quality, objectivity, utility, and integrity of its

information. FHWA periodically reviews quality issues and adjusts its programs and processes to

ensure continuous quality improvement.
TECHNICAL REPORT DOCUMENTATION PAGE

1. Report No.

FHWA-HRT-14-039

2. Government Accession No. 3. Recipient’s Catalog No.

4. Title and Subtitle

An FHWA Special Study: Post-Tensioning Tendon Grout Chloride 

Thresholds

5. Report Date

May 2014

6. Performing Organization Code

7. Author(s)

Seung-Kyoung Lee and James Zielske

8. Performing Organization Report No. 

9. Performing Organization Name and Address

Center for Advanced Infrastructure and Transportation

Rutgers University, The State University of New Jersey

100 Brett Road

Piscataway, NJ 08854-8058

10. Work Unit No.

11. Contract or Grant No.

DTFH61-08-C-00005

12. Sponsoring Agency Name and Address

Office of Infrastructure Research and Development

Federal Highway Administration

6300 Georgetown Pike

McLean, VA 22101-2296

13. Type of Report and Period Covered

Final Report

January 2012–September 2013

14. Sponsoring Agency Code

15. Supplementary Notes

The Contracting Officer’s Technical Representatives (COTRs) were Y.P. Virmani and Hamid Ghasemi, HRDI-60.

16. Abstract

Elevated levels of chloride were recently discovered in a commercially available pre-bagged grout product made for 

post-tensioned (PT) tendons. Chloride concentrations were reported to be as high as 5.27 percent by weight of cement. 

These numbers exceed the current limits set by all of the domestic and international regulatory committees. For 

example, the chloride limits for prestressed concrete imposed by American Concrete Institute committees are either 

0.06 percent water-soluble chloride by weight of cement or 0.08 percent acid-soluble chloride by weight of cement. 

Chloride threshold is defined as a minimum chloride concentration needed to induce corrosion. However, there is 

limited information reported in the literature pertaining to actual chloride threshold of seven-wire PT strands. For this 

reason, the Federal Highway Administration sponsored a 6-month accelerated corrosion testing program to determine 

chloride threshold(s) of PT strands exposed to chloride-contaminated grout.

Based on the test results obtained through the 6-month accelerated corrosion testing program and subsequent autopsy 

of the specimens, two chloride threshold values were determined for the PT strands fully encased in normal grout. The 

first threshold is 0.4 percent chloride concentration by weight of cement, which is the lowest amount needed to initiate 

corrosion of strands. At this threshold, rust spots can be formed, and a small number of pits start to form beneath some 

of the rust spots. The second critical chloride threshold is found in association with corrosion propagation. At this 

threshold, corrosion starts to intensify in terms of the number of pits and pit depth. Test data and autopsy results 

obtained in this study indicated that 0.8 percent chloride by weight of cement is the critical threshold. Once chloride 

concentration exceeds the critical threshold, significant corrosion damage can occur rapidly, and structural integrity of 

the PT bridges may be compromised in the near future. The actual deterioration rate will be influenced by many 

factors. These threshold values are applicable to normal grout condition only. They may not be adequate in other 

conditions such as carbonated grout, segregated grout, duct cracks, grout voids filled with water with or without 

chloride ions, or free sulfate ions in contact with the strands. In these circumstances, corrosion should start below the 

proposed threshold values.

17. Key Words

Chloride threshold, Chloride, Corrosion initiation, Corrosion 

propagation, Seven-wire strand, Post-tensioning, Grout, 

Autopsy, Accelerated corrosion testing, Sulfate ions, Void

18. Distribution Statement

No restrictions. This document is available to the 

public through the National Technical Information 

Service, Springfield, VA 22161.

19. Security Classif. (of this report)

Unclassified

20. Security Classif. (of this page)

Unclassified

21. No of Pages

196

22. Price

Form DOT F 1700.7 (8-72) Reproduction of completed pages authorized
SI* (MODERN METRIC) CONVERSION FACTORS 

APPROXIMATE CONVERSIONS TO SI UNITS

Symbol When You Know Multiply By To Find Symbol 

LENGTH

in inches 25.4 millimeters mm 

ft feet 0.305 meters m 

yd yards 0.914 meters m 

mi miles 1.61 kilometers km 

AREA 

in2 square inches 645.2 square millimeters mm2

ft2 square feet 0.093 square meters m2

yd2 square yard 0.836 square meters m2

ac acres 0.405 hectares ha

mi2 square miles 2.59 square kilometers km2

VOLUME 

fl oz fluid ounces 29.57 milliliters mL 

gal gallons 3.785 liters L 

ft3 cubic feet 0.028 cubic meters m3 

yd3 cubic yards 0.765 cubic meters m3 

NOTE: volumes greater than 1000 L shall be shown in m3

MASS oz ounces 28.35 grams g

lb pounds 0.454 kilograms kg

T short tons (2000 lb) 0.907 megagrams (or "metric ton") Mg (or "t") 

TEMPERATURE (exact degrees) o

F Fahrenheit 5 (F-32)/9 Celsius o

C 

or (F-32)/1.8 

ILLUMINATION 

fc foot-candles 10.76 lux lx

fl foot-Lamberts 3.426 candela/m2 cd/m2

FORCE and PRESSURE or STRESS 

lbf poundforce 4.45 newtons N 

lbf/in2 poundforce per square inch 6.89 kilopascals kPa 

APPROXIMATE CONVERSIONS FROM SI UNITS 

Symbol When You Know Multiply By To Find Symbol 

LENGTH mm millimeters 0.039 inches in 

m meters 3.28 feet ft 

m meters 1.09 yards yd 

km kilometers 0.621 miles mi 

AREA mm2 square millimeters 0.0016 square inches in2 

m2 square meters 10.764 square feet ft2 

m2 square meters 1.195 square yards yd2 

ha hectares 2.47 acres ac

km2 square kilometers 0.386 square miles mi2 

VOLUME 

mL milliliters 0.034 fluid ounces fl oz 

L liters 0.264 gallons gal 

m3 cubic meters 35.314 cubic feet ft3 

m3 cubic meters 1.307 cubic yards yd3 

MASS 

g grams 0.035 ounces oz

kg kilograms 2.202 pounds lb

Mg (or "t") megagrams (or "metric ton") 1.103 short tons (2000 lb) T 

TEMPERATURE (exact degrees) o

C Celsius 1.8C+32 Fahrenheit oF 

ILLUMINATION 

lx lux 0.0929 foot-candles fc 

cd/m2 candela/m2 0.2919 foot-Lamberts fl

FORCE and PRESSURE or STRESS 

N newtons 0.225 poundforce lbf 

kPa kilopascals 0.145 poundforce per square inch lbf/in2

*SI is the symbol for the International System of Units. Appropriate rounding should be made to comply with Section 4 of ASTM E380. 

(Revised March 2003)

ii
TABLE OF CONTENTS

CHAPTER 1. INTRODUCTION................................................................................................ 1

CHAPTER 2. OBJECTIVE AND SCOPE OF WORK ............................................................ 5

TASK 1: LITERATURE REVIEW ON CHLORIDE THRESHOLD FOR 

PRESTRESSING STRANDS ................................................................................................ 5

TASK 2: ACCELERATED LABORATORY CORROSION TESTING ......................... 5

CHAPTER 3. EXPERIMENTAL PROCEDURE..................................................................... 7

MATERIALS .......................................................................................................................... 7

ENVIRONMENTAL CHAMBERS...................................................................................... 9

INTRODUCTION TO ACCELERATED CORROSION TESTING PROGRAM ....... 11

SPECIMEN TYPE AND FABRICATION PROCESS ..................................................... 20

Task 2.1: Single-Wire Specimens in Aqueous Solutions................................................. 20

Task 2.2: Single-Strand Specimens in Grout.................................................................... 22

Task 2.3: Multi-Strand Specimens in Grout ..................................................................... 30

CORROSION PERFORMANCE TESTS AND DATA COLLECTION........................ 34

Task 2.1: Electrochemical Testing of Single-Wire Specimens in Aqueous 

Test Solutions.................................................................................................................... 34

Task 2.2: Single-Strand Specimens .................................................................................. 36

Task 2.3: Multi-Strand Specimens.................................................................................... 38

DUCT CRACKING.............................................................................................................. 41

WATER RECHARGING EVENT...................................................................................... 41

INTRODUCTION OF AIR HOLES................................................................................... 44

AUTOPSY PROCEDURE................................................................................................... 45

Task 2.2: Single-Strand Specimens .................................................................................. 45

Task 2.3: Multi-Strand Specimens.................................................................................... 48

CORROSION DAMAGE ASSESSMENT......................................................................... 53

CHAPTER 4. TEST RESULTS AND DISCUSSION ............................................................. 55

TASK 2.1: ELECTROCHEMICAL TESTING OF SINGLE-WIRE SPECIMENS 

IN AQUEOUS TEST SOLUTIONS.................................................................................... 55

ADMIXED VERSUS EXTRACTED CHLORIDE CONTENTS .................................... 57

ENVIRONMENTAL DATA................................................................................................ 62

POTENTIAL DATA............................................................................................................. 63

PSE Versus ACE............................................................................................................... 63

TASK 2.2: CORROSION POTENTIAL OF SINGLE-STRAND SPECIMENS ........... 65

Fully Grouted Specimens Versus Voided Specimens ...................................................... 70

TASK 2.3: CORROSION POTENTIALS AND POLARIZED POTENTIALS OF 

MULTI-STRAND SPECIMENS......................................................................................... 75

CORROSION RATE AND MACRO-CELL CORROSION CURRENT DENSITY 

DATA ..................................................................................................................................... 83

Task 2.2: Corrosion Rate Data of Single-Strand Specimens............................................ 83

Task 2.3: Corrosion Current Density and Driving Voltage Data of Multi-Strand 

Specimens......................................................................................................................... 96

iii
APPARENT GROUT RESISTIVITY AND GROUT RESISTANCE DATA.............. 108

Task 2.2: Apparent Grout Resistivity of Single-Strand Specimens................................ 108

Task 2.3: Grout Resistance of Multi-Strand Specimens................................................. 116

CHARACTERIZATION OF CORROSION MORPHOLOGIES ................................ 118

Task 2.2: Single-Strand Specimens ................................................................................ 118

Task 2.3: Multi-Strand Specimens.................................................................................. 125

CORROSION DAMAGE MEASUREMENT DATA ..................................................... 152

Task 2.2: Voided Single-Strand Specimens.................................................................... 155

Task 2.3: Void/Grout Interface of Multi-Strand Specimens........................................... 156

Task 2.2: Fully Grouted Single-Strand Specimens......................................................... 159

Task 2.3: Fully Grouted Sections of Multi-Strand Specimens....................................... 160

Role of Void/Grout Interface .......................................................................................... 164

SUMMARY OF MAJOR FINDINGS .............................................................................. 166

CHAPTER 5. CHLORIDE THRESHOLDS.......................................................................... 169

CHAPTER 6. MISCELLANEOUS WORK........................................................................... 171

CORROSION RISK ASSESSMENT................................................................................ 171

SERVICE LIFE PREDICTION MODEL........................................................................ 172

CHAPTER 7. CONCLUSIONS............................................................................................... 175

APPENDIX A. EXAMPLES OF CORROSION PROBLEMS CAUSED BY 

INSUFFICIENT GROUT COVER......................................................................................... 177

REFERENCES.......................................................................................................................... 181

iv
LIST OF FIGURES

Figure 1. Illustration. Typical PT anchorage hardware for strand tendons .................................... 1

Figure 2. Photo. Chloride-contaminated grout in the straddle caps for Carbon Plant Road 

bridge .............................................................................................................................................. 2

Figure 3. Photo. Environmental chambers...................................................................................... 9

Figure 4. Photo. Inside the small environmental chamber............................................................ 10

Figure 5. Photo. Inside the large environmental chamber ............................................................ 10

Figure 6. Photo. Data collection terminal cabinet......................................................................... 10

Figure 7. Graph. Major characteristics of active-passive metals presented in a potential-log 

current density (E-log i) diagram.................................................................................................. 13

Figure 8. Graph. Effect of chloride ions on passivity................................................................... 14

Figure 9. Illustration. Pitting corrosion mechanism...................................................................... 15

Figure 10. Equation. Oxidation reaction at anode ........................................................................ 15

Figure 11. Equation. Reduction reaction at cathode ..................................................................... 15

Figure 12. Illustration. Crevice corrosion mechanism.................................................................. 16

Figure 13. Equation. Chemical reaction involving a ferrous ion and two chloride ions .............. 16

Figure 14. Equation. Chemical reaction for acidification............................................................. 16

Figure 15. Equation. Chemical reaction involving ferric oxide formation................................... 16

Figure 16. Photo. Example of PT tendon failure due to macro-cell corrosion—Niles Channel 

bridge in Florida after 13 years in service .................................................................................... 17

Figure 17. Photo. Example of PT tendon failure due to macro-cell corrosion—Sunshine 

Skyway bridge in Florida after 13 years in service....................................................................... 17

Figure 18. Photo. Example of PT tendon failure due to macro-cell corrosion—Verina-Enon 

bridge in Virginia after 17 years in service................................................................................... 18

Figure 19. Illustration. Macro-cell corrosion mechanism in PT tendon....................................... 18

Figure 20. Graph. E–log i diagram describing different anodic and cathodic behaviors ............. 19

Figure 21. Illustration. Task 2.1 test setup for stressed wires....................................................... 20

Figure 22. Photo. Test cell for stressed wires............................................................................... 21

Figure 23. Photo. Task 2.1 test setup for unstressed wires........................................................... 21

Figure 24. Illustration. Task 2.2 loading frame for single-strand specimens ............................... 23

Figure 25. Illustration. Single-strand test cells ............................................................................. 24

Figure 26. Illustration. Pseudo-reference electrode (PSE) probe ................................................. 24

Figure 27. Photo. Actual PSE probe ............................................................................................. 25

Figure 28. Illustration. PSE calibration cylinders......................................................................... 25

Figure 29. Photo. Stressing of single-strand specimens ............................................................... 26

Figure 30. Photo. Stressing of multi-strand specimens................................................................. 27

Figure 31. Photo. Fully assembled single-strand specimens ........................................................ 27

Figure 32. Photo. Grout mixing .................................................................................................... 28

Figure 33. Photo. Grout pumping into a single-strand specimen ................................................. 28

Figure 34. Photo. In-situ grout testing .......................................................................................... 29

Figure 35. Illustration. Task 2.3 multi-strand specimens.............................................................. 31

Figure 36. Illustration. Details of multi-strand specimens............................................................ 31

Figure 37. Illustration. Segment designation of multi-strand specimens ..................................... 32

Figure 38. Photo. Fully assembled multi-strand specimens ......................................................... 33

Figure 39. Photo. Grout pumping into a multi-strand specimen................................................... 34

v
Figure 40. Equation. Calculation of surface area.......................................................................... 34

Figure 41. Illustration. LPR measurement for single-strand specimens....................................... 37

Figure 42. Illustration. Grout resistivity measurement for single-strand specimens.................... 37

Figure 43. Equation. Surface area of six outer wires.................................................................... 38

Figure 44. Equation. Surface area of the center wire.................................................................... 38

Figure 45. Equation. Total surface area of seven wires................................................................ 38

Figure 46. Illustration. Polarized potential measurement for multi-strand specimens................. 39

Figure 47. Illustration. Macro-cell corrosion current measurement for multi-strand 

specimens...................................................................................................................................... 39

Figure 48. Illustration. Corrosion potential measurement for multi-strand specimens ................ 40

Figure 49. Illustration. Grout resistance measurement for multi-strand specimens..................... 40

Figure 50. Photo. Cracked duct of 0.8 percent chloride multi-strand specimen before repair..... 41

Figure 51. Photo. Cracked duct of 0.8 percent chloride multi-strand specimen after repair........ 41

Figure 52. Photo. Water charging into a multi-strand specimen .................................................. 42

Figure 53. Photo. Water charging into a single-strand specimen ................................................. 42

Figure 54. Photo. Multi-strand specimen 1 h after water charging .............................................. 43

Figure 55. Photo. Single-strand specimen 1 h after water charging ............................................. 43

Figure 56. Graph. Relationship between water charging and macro-cell corrosion 

current 

jump .............................................................................................................................................. 44

Figure 57. Photo. Drilled single-strand specimen......................................................................... 45

Figure 58. Photo. Drilled multi-strand specimen with 0.8 percent chloride................................. 45

Figure 59. Photo. Grout powder sampling from a single-strand specimen .................................. 46

Figure 60. Photo. Splitting a single-strand specimen using a circular saw................................... 46

Figure 61. Photo. Single-strand specimen longitudinally cut in half............................................ 47

Figure 62. Photo. Example of superficial rust on the extracted wires.......................................... 47

Figure 63. Photo. Example of heavy rust on the extracted wires ................................................. 47

Figure 64. Photo. Cleaning individual wires using a scrubbing pad ............................................ 47

Figure 65. Photo. Acid cleaning of moderately to severely corroded wires................................. 48

Figure 66. Photo. Removal of the upper anchor bearing plate of a multi-strand specimen.......... 49

Figure 67. Photo. Grout powder sampling for chloride analysis from a multi-strand 

specimen ....................................................................................................................................... 49

Figure 68. Photo. Cutting the clear duct to expose grout.............................................................. 49

Figure 69. Photo. Chipping grout to expose PT strands............................................................... 50

Figure 70. Photo. Retrieved grout pieces from multi-strand specimens....................................... 50

Figure 71. Photo. Cutting PT strands near the lower anchor bearing plate .................................. 50

Figure 72. Photo. Extracting a multi-strand bundle from a loading frame ................................... 51

Figure 73. Photo. Storing the retrieved multi-strand samples in a chamber................................. 51

Figure 74. Photo. Inspecting PT strands removed from a multi-strand specimen on an 

autopsy table ................................................................................................................................. 51

Figure 75. Photo. Several 12-inch-long cut-out segments exhibiting various corrosion 

conditions...................................................................................................................................... 52

Figure 76. Photo. Cleaning and visual inspection of 7-ft-long unstressed strands....................... 53

Figure 77. Photo. Pit depth measurement using a digital pit gauge.............................................. 53

Figure 78. Photo. Section loss measurement in a severely corroded area .................................... 54

Figure 79. Graph. Corrosion potentials of center wires in different test environments................ 56

vi
Figure 80. Graph. Corrosion rates of center wires in different test environments........................ 56

Figure 81. Graph. Relationship between admixed chloride contents and actual 

concentrations in the trial grout mixes extracted by ASTM water-soluble and AASHTO 

acid-soluble methods .................................................................................................................... 58

Figure 82. Graph. Mean acid-soluble chloride contents in grout powder samples calculated 

by three conversion methods ........................................................................................................ 59

Figure 83. Equation. Example of calculating acid-soluble chloride concentration in the 

hardened grout powder sample ..................................................................................................... 59

Figure 84. Equation. Example of converting chloride concentration by weight of grout 

sample to chloride concentration by weight of cement plus weight of mixing water .................. 60

Figure 85. Equation. Example of converting chloride concentration by weight of grout 

sample to chloride concentration by weight of cement without weight of mixing water............. 60

Figure 86. Graph. Mean chloride extraction rates based on three conversion methods............... 61

Figure 87. Graph. Mean extracted acid-soluble chloride concentrations at different 

locations of multi-strand specimens.............................................................................................. 62

Figure 88. Graph. Sequence of exposure cycles and temperature changes during 

accelerated testing......................................................................................................................... 62

Figure 89. Graph. Recorded mean RH readings in the environmental chambers......................... 63

Figure 90. Graph. Corrosion potential versus time for three PSEs embedded in the 

calibration cylinders measured with respect to ACE.................................................................... 64

Figure 91. Graph. Mean corrosion potentials of PSEs versus ACE in three exposure 

conditions...................................................................................................................................... 65

Figure 92. Graph. Corrosion potential versus time for 0 percent chloride single-strand 

specimens...................................................................................................................................... 66

Figure 93. Graph. Corrosion potential versus time for 0.08 percent chloride single-strand 

specimens...................................................................................................................................... 66

Figure 94. Graph. Corrosion potential versus time for 0.2 percent chloride single-strand 

specimens...................................................................................................................................... 67

Figure 95. Graph. Corrosion potential versus time for 0.4 percent chloride single-strand 

specimens...................................................................................................................................... 67

Figure 96. Graph. Corrosion potential versus time for 0.6 percent chloride single-strand 

specimens...................................................................................................................................... 68

Figure 97. Graph. Corrosion potential versus time for 0.8 percent chloride single-strand 

specimens...................................................................................................................................... 68

Figure 98. Graph. Corrosion potential versus time for 1.0 percent chloride single-strand 

specimens...................................................................................................................................... 69

Figure 99. Graph. Corrosion potential versus time for 2.0 percent chloride single-strand 

specimens...................................................................................................................................... 69

Figure 100. Graph. Mean corrosion potential of fully grouted single-strand specimens in 

initial ambient condition ............................................................................................................... 71

Figure 101. Graph. Mean corrosion potential of fully grouted single-strand specimens in 

ambient condition.......................................................................................................................... 71

Figure 102. Graph. Mean corrosion potential of fully grouted single-strand specimens in

H & H condition............................................................................................................................ 72

Figure 103. Graph. Mean corrosion potential of fully grouted single-strand specimens in

F & D condition ............................................................................................................................ 72

vii
Figure 104. Graph. Mean corrosion potential of voided single-strand specimens in initial 

ambient condition.......................................................................................................................... 73

Figure 105. Graph. Mean corrosion potential of voided single-strand specimens in ambient 

condition ....................................................................................................................................... 73

Figure 106. Graph. Mean corrosion potential of voided single-strand specimens in H & H 

condition ....................................................................................................................................... 74

Figure 107. Graph. Mean corrosion potential of voided single-strand specimens in F & D 

condition ....................................................................................................................................... 74

Figure 108. Graph. Potential versus time for 0 percent chloride multi-strand specimen ............. 76

Figure 109. Graph. Potential versus time for 0.08 percent chloride multi-strand specimen ........ 76

Figure 110. Graph. Potential versus time for 0.2 percent chloride multi-strand specimen .......... 77

Figure 111. Graph. Potential versus time for 0.4 percent chloride multi-strand specimen .......... 77

Figure 112. Graph. Potential versus time for 0.6 percent chloride multi-strand specimen .......... 78

Figure 113. Graph. Potential versus time for 0.8 percent chloride multi-strand specimen .......... 78

Figure 114. Graph. Potential versus time for 1.0 percent chloride multi-strand specimen .......... 79

Figure 115. Graph. Potential versus time for 2.0 percent chloride multi-strand specimen .......... 79

Figure 116. Graph. Mean corrosion potential of stressed strands in multi-strand specimens...... 80

Figure 117. Graph. Mean corrosion potential of unstressed strands in multi-strand specimens.. 81

Figure 118. Graph. Mean polarized potential of multi-strand specimens..................................... 81

Figure 119. Graph. Overall mean potential of multi-strand specimens........................................ 82

Figure 120. Graph. Corrosion rate versus time for 0 percent chloride single-strand 

specimens...................................................................................................................................... 84

Figure 121. Graph. Corrosion rate versus time for 0.08 percent chloride single-strand 

specimens...................................................................................................................................... 84

Figure 122. Graph. Corrosion rate versus time for 0.2 percent chloride single-strand 

specimens...................................................................................................................................... 85

Figure 123. Graph. Corrosion rate versus time for 0.4 percent chloride single-strand 

specimens...................................................................................................................................... 85

Figure 124. Graph. Corrosion rate versus time for 0.6 percent chloride single-strand 

specimens...................................................................................................................................... 86

Figure 125. Graph. Corrosion rate versus time for 0.8 percent chloride single-strand 

specimens...................................................................................................................................... 86

Figure 126. Graph. Corrosion rate versus time for 1.0 percent chloride single-strand 

specimens...................................................................................................................................... 87

Figure 127. Graph. Corrosion rate versus time for 2.0 percent chloride single-strand 

specimens...................................................................................................................................... 87

Figure 128. Graph. Mean corrosion rate of fully grouted single-strand specimens in initial 

ambient condition.......................................................................................................................... 88

Figure 129. Graph. Mean corrosion rate of fully grouted single-strand specimens in ambient 

condition ....................................................................................................................................... 89

Figure 130. Graph. Mean corrosion rate of fully grouted single-strand specimens in H & H 

condition ....................................................................................................................................... 89

Figure 131. Graph. Mean corrosion rate of fully grouted single-strand specimens in F & D 

condition ....................................................................................................................................... 90

Figure 132. Graph. Mean corrosion rate of voided single-strand specimens in initial ambient 

condition ....................................................................................................................................... 90

viii
Figure 133. Graph. Mean corrosion rate of voided single-strand specimens in ambient 

condition ....................................................................................................................................... 91

Figure 134. Graph. Mean corrosion rate of voided single-strand specimens in H & H 

condition ....................................................................................................................................... 91

Figure 135. Graph. Mean corrosion rate of voided single-strand specimens in F & D 

condition ....................................................................................................................................... 92

Figure 136. Graph. Magnified corrosion rate for initial ambient condition of fully grouted 

specimens...................................................................................................................................... 93

Figure 137. Graph. Magnified corrosion rate for initial ambient condition of voided 

specimens...................................................................................................................................... 93

Figure 138. Graph. Magnified corrosion rate for ambient condition of fully grouted 

specimens...................................................................................................................................... 94

Figure 139. Graph. Magnified corrosion rate for ambient condition of voided specimens.......... 94

Figure 140. Graph. Magnified corrosion rate for H & H condition of fully grouted specimens.. 94

Figure 141. Graph. Magnified corrosion rate for H & H condition of voided specimens............ 95

Figure 142. Graph. Overall mean corrosion rates of single-strand specimens............................. 95

Figure 143. Graph. Overall mean corrosion rates of single-strand specimens grouped by 

stress level..................................................................................................................................... 95

Figure 144. Graph. imacro-cell and driving voltage versus time for 0 percent chloride 

multi-strand specimen................................................................................................................... 96

Figure 145. Graph. imacro-cell and driving voltage versus time for 0.08 percent chloride 

multi-strand specimen................................................................................................................... 97

Figure 146. Graph. imacro-cell and driving voltage versus time for 0.2 percent chloride 

multi-strand specimen................................................................................................................... 97

Figure 147. Graph. imacro-cell and driving voltage versus time for 0.4 percent chloride 

multi-strand specimen................................................................................................................... 98

Figure 148. Graph. imacro-cell and driving voltage versus time for 0.6 percent chloride 

multi-strand specimen................................................................................................................... 98

Figure 149. Graph. imacro-cell and driving voltage versus time for 0.8 percent chloride 

multi-strand specimen................................................................................................................... 99

Figure 150. Graph. imacro-cell and driving voltage versus time for 1.0 percent chloride 

multi-strand specimen................................................................................................................... 99

Figure 151. Graph. imacro-cell and driving voltage versus time for 2.0 percent chloride 

multi-strand specimen................................................................................................................. 100

Figure 152. Graph. Mean imacro-cell data of multi-strand specimens by exposure condition........ 101

Figure 153. Graph. Overall mean imacro-cell data of multi-strand specimens................................ 102

Figure 154. Graph. imacro-cell behavior responding to temperature variation ............................... 102

Figure 155. Graph. imacro-cell changes after water recharging ...................................................... 103

Figure 156. Photo. Void condition of 0 percent chloride specimen ........................................... 104

Figure 157. Photo. Void condition of 0.08 percent chloride specimen ...................................... 104

Figure 158. Photo. Void condition of 0.2 percent chloride specimen ........................................ 105

Figure 159. Photo. Void condition of 0.4 percent chloride specimen ........................................ 105

Figure 160. Photo. Void condition of 0.6 percent chloride specimen ........................................ 106

Figure 161. Photo. Void condition of 0.8 percent chloride specimen ........................................ 106

Figure 162. Photo. Void condition of 1.0 percent chloride specimen ........................................ 107

Figure 163. Photo. Void condition of 2.0 percent chloride specimen ........................................ 107

ix
Figure 164. Graph. Apparent grout resistivity versus time for 0 percent chloride 

single-strand specimens.............................................................................................................. 108

Figure 165. Graph. Apparent grout resistivity versus time for 0.08 percent chloride 

single-strand specimens.............................................................................................................. 109

Figure 166. Graph. Apparent grout resistivity versus time for 0.2 percent chloride 

single-strand specimens.............................................................................................................. 109

Figure 167. Graph. Apparent grout resistivity versus time for 0.4 percent chloride 

single-strand specimens.............................................................................................................. 110

Figure 168. Graph. Apparent grout resistivity versus time for 0.6 percent chloride 

single-strand specimens.............................................................................................................. 110

Figure 169. Graph. Apparent grout resistivity versus time for 0.8 percent chloride 

single-strand specimens.............................................................................................................. 111

Figure 170. Graph. Apparent grout resistivity versus time for 1.0 percent chloride 

single-strand specimens.............................................................................................................. 111

Figure 171. Graph. Apparent grout resistivity versus time for 2.0 percent chloride 

single-strand specimens.............................................................................................................. 112

Figure 172. Graph. Mean apparent grout resistivity of fully grouted single-strand 

specimens in initial ambient condition ....................................................................................... 112

Figure 173. Graph. Mean apparent grout resistivity of fully grouted single-strand 

specimens in ambient condition.................................................................................................. 113

Figure 174. Graph. Mean apparent grout resistivity of fully grouted single-strand 

specimens in H & H condition.................................................................................................... 113

Figure 175. Graph. Mean apparent grout resistivity of fully grouted single-strand 

specimens in F & D condition .................................................................................................... 114

Figure 176. Graph. Mean apparent grout resistivity of voided single-strand specimens 

in initial ambient condition ......................................................................................................... 114

Figure 177. Graph. Mean apparent grout resistivity of voided single-strand specimens 

in ambient condition ................................................................................................................... 115

Figure 178. Graph. Mean apparent grout resistivity of voided single-strand specimens 

in H & H condition ..................................................................................................................... 115

Figure 179. Graph. Mean apparent grout resistivity of voided single-strand specimens 

in F & D condition ...................................................................................................................... 116

Figure 180. Graph. Mean grout resistances between P1 and P2 of multi-strand specimens...... 117

Figure 181. Graph. Mean grout resistances between P3 and P4 of multi-strand specimens...... 117

Figure 182. Graph. Overall mean grout resistances of multi-strand specimens......................... 118

Figure 183. Illustration. Condition mapping of 0 percent chloride single-strand 

specimens.................................................................................................................................... 120

Figure 184. Illustration. Condition mapping of 0.08 percent chloride single-strand 

specimens.................................................................................................................................... 120

Figure 185. Illustration. Condition mapping of 0.2 percent chloride single-strand specimens.. 121

Figure 186. Illustration. Condition mapping of 0.4 percent chloride single-strand specimens.. 121

Figure 187. Illustration. Condition mapping of 0.6 percent chloride single-strand specimens.. 122

Figure 188. Illustration. Condition mapping of 0.8 percent chloride single-strand specimens.. 122

Figure 189. Illustration. Condition mapping of 1.0 percent chloride single-strand specimens.. 123

Figure 190. Illustration. Condition mapping of 2.0 percent chloride single-strand specimens.. 123

Figure 191. Photo. Stressed single-strand specimen with void and 0 percent chloride.............. 124

x
Figure 192. Photo. Stressed single-strand specimen with void and 0.08 percent chloride......... 124

Figure 193. Photo. Stressed single-strand specimen with void and 0.2 percent chloride........... 124

Figure 194. Photo. Stressed single-strand specimen with void and 0.4 percent chloride........... 124

Figure 195. Photo. Stressed single-strand specimen with void and 0.6 percent chloride........... 125

Figure 196. Photo. Stressed single-strand specimen with void and 0.8 percent chloride........... 125

Figure 197. Photo. Stressed single-strand specimen with void and 1.0 percent chloride........... 125

Figure 198. Photo. Stressed single-strand specimen with void and 2.0 percent chloride........... 125

Figure 199. Photo. Exposed exterior side of lower anchor plate ................................................ 126

Figure 200. Photo. Exposed interior side of lower anchor plate................................................. 126

Figure 201. Illustration. Condition mapping of 0 percent chloride multi-strand specimen........ 127

Figure 202. Illustration. Condition mapping of 0.08 percent chloride multi-strand specimen... 128

Figure 203. Illustration. Condition mapping of 0.2 percent chloride multi-strand specimen..... 129

Figure 204. Illustration. Condition mapping of 0.4 percent chloride multi-strand specimen..... 130

Figure 205. Illustration. Condition mapping of 0.6 percent chloride multi-strand specimen..... 131

Figure 206. Illustration. Condition mapping of 0.8 percent chloride multi-strand specimen..... 132

Figure 207. Illustration. Condition mapping of 1.0 percent chloride multi-strand specimen..... 133

Figure 208. Illustration. Condition mapping of 2.0 percent chloride multi-strand specimen..... 134

Figure 209. Photo. Exposed strand at the void/grout interface of 0.08 percent chloride 

multi-strand specimen................................................................................................................. 135

Figure 210. Photo. As-extracted condition of four stressed strands near the void/grout 

interface of 0.08 percent chloride multi-strand specimen........................................................... 135

Figure 211. Photo. Exposed strand at the interface of 0.8 percent chloride multi-strand 

specimen ..................................................................................................................................... 136

Figure 212. Photo. As-extracted condition of four stressed strands at the void/grout interface 

of 0.8 percent chloride multi-strand specimen............................................................................ 136

Figure 213. Photo. Strands at the void/grout interface of 0 percent chloride multi-strand 

specimen ..................................................................................................................................... 137

Figure 214. Photo. Close-up plan view of the void/grout interface (top section)....................... 137

Figure 215. Photo. Cross section of the void/grout interface (top section) ................................ 138

Figure 216. Photo. As-extracted condition of the stressed strands at the void/grout 

interface of 0 percent chloride multi-strand specimen................................................................ 138

Figure 217. Photo. As-extracted condition of a stressed segment passed through the 

contaminated void/grout interface of 0 percent chloride multi-strand specimen ....................... 139

Figure 218. Photo. As-extracted condition of another stressed segment passed through 

the contaminated void/grout interface of 0 percent chloride multi-strand specimen.................. 139

Figure 219. Photo. Severely corroded strand with three broken wires at the void/grout 

interface of 0.4 percent chloride multi-strand specimen............................................................. 140

Figure 220. Photo. Close-up view of the severely corroded strand shown in figure 219........... 141

Figure 221. Photo. Close-up view of the broken wires shown in figure 220 ............................. 141

Figure 222. Photo. Top side grout piece exhibiting bleed channel and porous grout removed 

from 0.4 percent chloride specimen............................................................................................ 142

Figure 223. Photo. Cross section of the grout piece shown in figure 222 .................................. 142

Figure 224. Photo. Defective grout along a bleed channel......................................................... 143

Figure 225. Photo. Normal grout ................................................................................................ 143

Figure 226. Photo. Another exposed strand at the void/grout interface of 0.4 percent 

chloride multi-strand specimen................................................................................................... 144

xi
Figure 227. Photo. Severely corroded strand with one broken wire revealed at the void/

grout interface of 2.0 percent chloride multi-strand specimen ................................................... 146

Figure 228. Photo. Close-up view of severely corroded strand with one broken wire 

revealed at the void/grout interface of 2.0 percent chloride multi-strand specimen................... 146

Figure 229. Photo. Active corrosion of a stressed strand inside the 2.0 percent chloride 

grout 70 inches from the lower anchorage.................................................................................. 147

Figure 230. Photo. Corrosion of unstressed strands in 2.0 percent chloride multi-strand 

specimen ..................................................................................................................................... 147

Figure 231. Photo. Carbonation testing results of 0.6 percent chloride multi-strand 

specimens.................................................................................................................................... 149

Figure 232. Photo. Carbonation testing results of 0.8 percent chloride multi-strand 

specimens.................................................................................................................................... 149

Figure 233. Photo. Wet and soft grout found in 0.8 percent chloride multi-strand specimen.... 150

Figure 234. Photo. Wet and soft grout found in 1.0 percent chloride multi-strand specimen.... 150

Figure 235. Photo. Grout fragment found inside an interstice surrounded by corroding 

wires............................................................................................................................................ 151

Figure 236. Photo. Heavily rust-stained grout fragment that fell from the strand shown in 

figure 235.................................................................................................................................... 151

Figure 237. Photo. Stressed and voided 0.4 percent chloride single-strand specimen 

(pit depth < 2 mil)....................................................................................................................... 152

Figure 238. Photo. Stressed and fully grouted 0.8 percent chloride single-strand specimen 

(pit depth ~2 mil) ........................................................................................................................ 152

Figure 239. Photo. Stressed and fully grouted 0.8 percent chloride single-strand specimen 

(pit depth = 4.7 mil).................................................................................................................... 152

Figure 240. Photo. 0 percent chloride multi-strand specimen at the void/grout interface 

(pit depth = 2–4 mil)................................................................................................................... 153

Figure 241. Photo. 0.08 percent chloride multi-strand specimen near the interface 

(pit depth = 2–10 mil)................................................................................................................. 153

Figure 242. Photo. 0.6 percent chloride multi-strand specimen in the grout 

(pit depth = 2–14 mil)................................................................................................................. 153

Figure 243. Photo. Stressed and voided 1.0 percent chloride single-strand specimen 

(pit depth = 2–18 mil)................................................................................................................. 153

Figure 244. Photo. 1.0 percent chloride multi-strand specimen in the grout 

(pit depth = 2–23 mil)................................................................................................................. 154

Figure 245. Photo. 0 percent chloride multi-strand specimen at the interface 

(pit depth = 2–36 mil)................................................................................................................. 154

Figure 246. Photo. 0.4 percent chloride multi-strand specimen near the void 

(pit depth = 2–50 mil)................................................................................................................. 154

Figure 247. Photo. Crevice corrosion between two stressed wires on 0.4 percent chloride 

concentration multi-strand specimen (mean pit depth = 4.5 mil and maximum pit depth = 

8.0 mil)........................................................................................................................................ 154

Figure 248. Photo. Crevice corrosion between two stressed wires on 0.8 percent chloride 

concentration multi-strand specimen (mean pit depth = 4.5 mil and maximum pit depth = 

6.0 mil)........................................................................................................................................ 155

Figure 249. Graph. Mean and maximum pit depths on the voided single-strand specimens..... 156

xii
Figure 250. Graph. Condition of stressed segments at the void/grout interface of 

multi-strand specimens ............................................................................................................... 157

Figure 251. Graph. Number of pits on the interface segments of multi-strand specimens 

measured from the upper anchor plate........................................................................................ 158

Figure 252. Graph. Mean pit depths on the interface segments of multi-strand specimens 

measured from the upper anchor plate........................................................................................ 158

Figure 253. Graph. Maximum pit depths on the interface segments of multi-strand 

specimens measured from the upper anchor plate ...................................................................... 159

Figure 254. Graph. Mean and maximum pit depths on the fully grouted single-strand 

specimens.................................................................................................................................... 160

Figure 255. Graph. Physical condition of stressed in-grout segments in multi-strand 

specimens.................................................................................................................................... 161

Figure 256. Graph. Number of pits observed on the stressed strands in multi-strand 

specimens measured from the upper anchor plate ...................................................................... 161

Figure 257. Graph. Mean pit depth measured on stressed strands in multi-strand 

specimens measured from the upper anchor plate ...................................................................... 162

Figure 258. Graph. Number of pits observed on the in-grout strands in multi-strand 

specimens.................................................................................................................................... 163

Figure 259. Graph. Mean pit depths of the in-grout stressed strands in multi-strand 

specimens.................................................................................................................................... 163

Figure 260. Graph. Maximum pit depths measured on the stressed strands in multi-strand 

specimens measured from the upper anchor plate ...................................................................... 164

Figure 261. Graph. Number of pits measured on the stressed strands in some of the 

multi-strand specimens ............................................................................................................... 165

Figure 262. Graph. Mean pit depth measured on the stressed strands in some of the 

multi-strand specimens ............................................................................................................... 166

Figure 263. Graph. Example of a time-to-failure plot for a corroding strand at different 

penetration rates.......................................................................................................................... 173

Figure 264. Photo. Failed external PT tendon of Verina-Enon bridge in Virginia..................... 177

Figure 265. Photo. Other matching end of the retrieved tendon sample shown in figure 264 ... 178

Figure 266. Photo. Corroding PT tendon of Verina-Enon bridge in Virginia ............................ 178

Figure 267. Photo. External PT tendon of Verina-Enon bridge in Virginia ............................... 179

Figure 268. Photo. External PT tendon of Mid-Bay bridge in Florida ....................................... 179

Figure 269. Photo. External PT tendon of Ringling Causeway bridge in Florida ...................... 180

xiii
LIST OF TABLES

Table 1. Chemical analysis results.................................................................................................. 3

Table 2. Sodium chloride contents for task 2.1 test solutions ........................................................ 7

Table 3. Chemical compositions for task 2.1 test solutions............................................................ 8

Table 4. Sodium chloride weights for tasks 2.2 and 2.3 specimen grout mixes............................. 8

Table 5. 28-day compressive strength data of grout mixes .......................................................... 29

Table 6. Test surface area used in this study ................................................................................ 35

Table 7. Corrosion rate criteria ..................................................................................................... 36

Table 8. Different chloride concentration units reported in this study ......................................... 59

Table 9. Summary of corrosion condition observed on single-strand specimens....................... 119

Table 10. Sulfate ion concentrations in the samples................................................................... 145

Table 11. Condition summary of interface segments of multi-strand specimens....................... 148

Table 12. Condition summary of in-grout strand segments of multi-strand specimens............. 148

Table 13. Summary of the lowest chloride concentrations observed in the present study ......... 169

Table 14. Suggested chloride threshold values for normal grout ............................................... 170

Table 15. Adjusted chloride threshold values to be used for field grout powder samples......... 170

Table 16. Example of corrosion risk management plan with the suggested chloride 

threshold values .......................................................................................................................... 171

xiv
CHAPTER 1. INTRODUCTION

The number of prestressed concrete bridge structures utilizing high-strength seven-wire strands

has steadily increased since the 1970s. The prestressing strands can be used in pre-tensioned and 

post-tensioned (PT) structures. The latter utilizes plastic or metal ducts that are initially empty 

tubes that allow for a bundle of the seven-wire strands to be fed through after the ducts are 

embedded in the concrete. The strands are fixed at both end anchorage points by a wedge plate 

and steel wedges that grip each strand. Ultimately, they are pulled to a pre-determined level of 

stress using a specialized hydraulic jack. The assembly of the prestressed seven-wire strands and 

the duct is called a PT tendon. In this report, a prestressed seven-wire strand will be simply 

referred to as a “PT strand.” Figure 1 shows schematic of a typical PT tendon system.

(1)

Figure 1. Illustration. Typical PT anchorage hardware for strand tendons.

(1)

The PT tendon is divided into two types: (1) unbonded tendons typically filled with grease or 

wax and (2) bonded tendons filled with cementitious material called grout. For bonded PT

tendons and cables, the grout functions as the last layer of corrosion protection for the highly 

stressed PT strands by providing a high pH environment to form a protective oxide film on the 

strand surface and acting as a physical barrier to water and oxygen. Corrosion of strands initiates 

when the protective oxide film is compromised due to chloride attack or carbonation of the 

surrounding grout upon exposure to water and air. Once corrosion initiates, corrosion propagates 

at a certain rate controlled by many factors such as oxygen availability, moisture content, 

electrical resistance, grout pH, and chloride concentration. 

The literature review for this study has shown that among several causes in earlier tendon 

corrosion failures, any one or a combination of some of the following deficiencies can trigger 

active corrosion with presence of moisture and oxygen: voids, micro-cracks, sedimentation 

leading to variable water-cement ratio, bleed water, and contamination with chloride or sulfate 

ions.

(2,3) Uneven grout cover ranging from very thick to virtually no cover within a typical duct 

system can also result in corrosion in the thin and no cover sections where PT strands in a tendon

 

 

1
have a greater chance of getting exposed to water and oxygen if the tendon fails to maintain its

water/air tightness. Some examples caused by this type of corrosion in the field tendons are 

provided in appendix A. The prestressing strand is less tolerant to corrosive environments than 

ordinary reinforcing steel because of its much higher load-bearing requirement per unit area.

While some tendon corrosion problems were observed in the grout defects filled with water and 

oxygen, various amounts of chloride ions were found in other incidents. A couple of 

investigative reports claimed that sulfate ions could be responsible for tendon corrosion as 

well.

(2,4) In particular, a laboratory study conducted by the Florida Department of Transportation 

(FDOT) demonstrated that aggressive nature of free sulfate ions can result in intensive

corrosion.

(4) Excessive free sulfate ions found in segregated grout were also a suspect of 

corrosion failure of some tendon specimens employed in the present study. Recognizing the 

importance of having high-quality grout in the PT system concrete bridges and cable stayed 

bridges, commercially available pre-packaged grouts were introduced to the construction market 

to minimize quality variations that may occur in the construction sites. In addition, enhanced 

grout installation guidelines and training certification programs were developed. These changes 

have been beneficial and improved the quality of in-place grouts. 

A recent discovery of grout with elevated levels of chloride used in a PT concrete straddle cap 

located in Corpus Christi, TX, led to a full investigation to better understand the extent of the 

potential corrosion problem that may be caused by the use of chloride-contaminated grout.

(5)

The grout in question was a commercially available pre-packaged product. According to an

investigative report, the reported chloride concentration was as high as 5.27 percent (52,700 ppm)

by weight of cement, and the chloride concentration in the retained bags was as high as

0.5 percent by weight of cement.

(5,6) Figure 2 shows a 4-month-old segregated grout in question, 

and table 1 shows its corresponding chloride concentration data. Label A indicates normally 

hardened grout, and label B indicates defective grout that contained numerous air voids.

Figure 2. Photo. Chloride-contaminated grout in the straddle caps for Carbon Plant Road 

bridge.(5)

 

2
Table 1. Chemical analysis results.

(5)

Sample Instrument

Chloride 

Percent by 

Weight* ppm

Tendon 1A inlet “A”

Scanning electron 

microscope (SEM)/

energy dispersive 

spectroscopy (EDS)

N/A N/A

X-ray fluorescence 5.27 52,700

Ion chromatography 2.13 21,330

Tendon 1A, outlet “B” SEM/EDS 0.54 5,400

Tendon 1B, inlet “B” SEM/EDS 0.41 4,100

Tendon 2B, inlet “A” SEM/EDS 0.57 5,700

Tendon 2B, inlet “B” SEM/EDS 0.22 2,200

*Chloride percent by weight reported using SEM/EDS is for total chlorides.

N/A = Not available.

It is apparent that the confirmed chloride concentrations exceeded the past and current limits set 

by all of the domestic and international regulatory committees such as American Concrete 

Institute (ACI) 201, 318, 222R, and British Code CP 110. Because the existing limits are 

based on either weight percent of cement or weight percent of grout, a conversion between the 

two units has to be made with certain assumptions regarding the fraction of cement in a given 

weight or volume of a grout sample. According to the current chloride limit by ACI committees, 

maximum water-soluble chloride content by weight of cement of 0.06 percent (600 ppm) is

allowed for the prestressed concrete. The specified limit for the acid-soluble by weight of cement 

is 0.08 percent (800 ppm). (See references 7–10.) The former indicates chloride ions currently 

available for corrosion, while the latter indicates total amount of chloride ions potentially 

available for future corrosion. The Virginia Department of Transportation (VDOT) also limits 

total (acid-soluble) chloride ions by weight of cementitious material to a maximum 0.08 percent

(800 ppm), whereas FDOT Standard Specification Section 934 limits chloride content to a 

maximum of 0.40 lb/yd3

.

(11) As noted in the ACI 222R-01 committee report, other international 

codes have different specified chloride limits for pre-hydrated grouts despite lack of rationale for 

the established values.

(10) The same report also introduces a dimensionless ratio of [Cl-

]/[OH-],

which can be used as a chloride threshold in relation to corrosion initiation of reinforcing steel. 

More information can be found in the literature review report.

(3)

Dr. David Whiting stated in his article, Origins of Chloride Limits for Reinforced Concrete, that:

“... “limits” refers to the amounts of chloride which are permitted in fresh 

concrete by code authorities. This is not necessarily equal to the so-called 

“threshold” limit for corrosion, which is the actual amount of chloride needed to 

initiate corrosion under a given set of conditions. The former term “limits” is thus 

an artificial criterion set by man; the “threshold” is set by nature. Thus, while the 

“limits” must be set, the “threshold” must be investigated.”(pg. 3)(12)

This statement points out that even though the code specifications have built in a factor of safety, 

the allowable chloride limits are not adequate to predict corrosion behavior of the PT concrete 

3
bridges subjected to corrosive environments. Corrosion process of the PT strands and cables 

consists of two steps: (1) corrosion initiation and (2) corrosion propagation. Corrosion initiates 

when a protective oxide film is compromised due to chloride attack or carbonation (lowing pH) 

of the surrounding grout. An FDOT study demonstrated that corrosion can initiate on the 

unstressed strands exposed to fresh water mixed with a chloride concentration of 500 ppm in a 

simulated corrosion study.

(13) Once corrosion begins, the corrosion propagation stage follows a 

kinetic process in the given condition. The rate of corrosion is influenced by many factors 

including electrical resistance, temperature, and dissolved oxygen. In addition, a limited 

laboratory experiment performed at the Federal Highway Administration’s (FHWA) TurnerFairbank Highway Research Center for the internal San Francisco-Oakland Bay Bridge tendon 

corrosion study suggested that the corrosion rate of the prestressing wires may be related to 

chloride concentration, pH, and stress level. Therefore, it is imperative to take into account stress 

level, concentration of actual chloride in the grout, and pH at the strand/grout interface to 

determine a single chloride threshold or a range of chloride threshold values for any PT 

tendons and cables. 

4
CHAPTER 2. OBJECTIVE AND SCOPE OF WORK

The objective of this study was to determine chloride threshold values of PT strands exposed to 

chloride-contaminated grout. For this, a comprehensive investigation has been conducted over 

12 months to complete the following two tasks:

• Task 1: Literature review on chloride threshold for prestressed strands.

• Task 2: Accelerated corrosion testing to determine chloride threshold values to initiate 

corrosion and critical threshold values for recognizable corrosion-induced damage. This 

task had three subtasks employing three types of specimens as follows:

o Task 2.1: Single-wire specimens.

o Task 2.2: Single-strand specimens.

o Task 2.3: Multi-strand specimens.

Based on the study findings, additional work was performed to present current corrosion risk 

assessment and estimate overall long-term safety of the PT strands exposed to the chloridecontaminated grout.

TASK 1: LITERATURE REVIEW ON CHLORIDE THRESHOLD FOR

PRESTRESSING STRANDS

A thorough literature review was conducted as far back as the 1950s to disseminate historical 

chloride limit data applicable to the prestressed strand. Also, the PT tendon corrosion failure 

reports were gathered and analyzed to extract pertinent information for this study. The complete 

literature review report was published as an FHWA summary report.

(3)

TASK 2: ACCELERATED LABORATORY CORROSION TESTING

Accelerated laboratory corrosion testing was performed to determine chloride threshold values 

for the prestressed strands through investigating interacting effects among multiple parameters 

including chloride concentration, presence of voids, stress level, temperature, relative humidity

(RH), and pH using new 0.6-inch, 270-ksi low-relaxation grade strands. The strands employed in 

this research study were handled in the same way as delivered without additional cleaning and 

installed in the actual bridges. After they were delivered to the laboratory, they were stored in a 

clean and dry condition to prevent atmospheric corrosion.

5

CHAPTER 3. EXPERIMENTAL PROCEDURE

MATERIALS

The 0.6-inch, 270-ksi low-relaxation grade PT strand was used in this study. Great care was 

taken not to contaminate the surface of the strands during the handling and storage. More than 

50 center (i.e., king) wires were extracted from 5-ft-long PT strands by untwisting and removing 

the outer six wires. These king wires were used for the electrochemical testing of task 2.1, which 

called for electrochemical testing of instantaneous corrosion rate and corrosion potential of 

king wires exposed to different aqueous test solutions having 10 chloride concentrations and 

2 pH values. The sodium chloride weight in a test solution was equivalent to the chloride 

concentration in a grout mix expressed in terms of weight percent of cement that was calculated 

based on the cement content of 33.3 lb per grout bag (50 lb) and 12.5 lb of distilled water 

for each bag. The following calculations exemplify a chloride concentration equivalent to 

1.0 percent chloride by weight of cement:

• 1.0 percent of chloride by weight of cement required where 33.3 lb × 0.01 lb = 0.33 lb of 

chloride ions for a 50-lb bag.

• Since 60.7 percent of sodium chloride is chloride, 1.0 percent of chloride by weight of 

cement required is 0.33 lb ÷ 0.607 = 0.55 lb of sodium chloride for a 50-lb bag. 

• The fraction of sodium chloride for one unit weight of mixing water is 0.55 lb ÷12.5 lb = 

0.044.

• Hence, 1.6 oz (0.044 × 35.27 oz (i.e., 1,000 g) = 1.6 oz) of sodium chloride should be 

added to 0.26-gal of a test solution containing chloride ions equivalent to 1.0 percent 

chloride by weight of cement. 

Table 2 lists 10 required amounts of sodium chloride in the 0.26-gal test solutions. 

Table 2. Sodium chloride contents for task 2.1 test solutions.

Chloride Concentration by 

Weight of Cement (percent)

Weight of Sodium Chloride 

in 0.26-gal Water (oz)

0.00 0.000

0.04 0.063

0.08 0.123

0.20 0.308

0.40 0.616

0.60 0.924

0.80 1.232

1.00 1.540

2.00 3.077

3.00 7.693

7
Table 3 lists the chemical compositions for making solutions with a pH of 9.0 and 13.6, 

respectively, in the 0.26-gal test solutions.(14) These solutions were prepared to simulate 

carbonated grout and concrete (pH 9.0) and high alkaline environment like uncarbonated grout 

and concrete (pH 13.6). These represent typical pH values observed in the field.

Table 3. Chemical compositions for task 2.1 test solutions.

Solution with pH 9.0 Solution with pH 13.6

Sodium 

Carbonate

Sodium 

Bicarbonate

Calcium 

Hydroxide

Sodium 

Hydroxide

Potassium 

Hydroxide

0.147 oz 0.875 oz 0.070 oz 0.291 oz 0.816 oz

A total of 64 50-lb grout bags were acquired from a local distributor of the same manufacturer 

that produced the grout containing the elevated chloride. It is claimed to be a non-shrink, nonbleed, high flow, sand-free, cementitious grout. Upon receiving them, three bags were randomly 

selected for trial mixes to cast 10 4-by-8-inch cylinders for determining background chloride and 

confirming that the calculated weights of sodium chloride were correct to produce the target 

chloride concentrations. For this, seven chloride concentration mixes and three chloride-free 

mixes were cast. Table 4 provides final sodium chloride weights needed to produce the target 

chloride concentrations in the full batches based on the following:

• The weight of cement contained in a 50-lb grout bag is 33.3 lb. 

• The weight of mixing water is 12.5 lb per manufacturer’s recommendation.

• Six bags of grout were mixed for specimen fabrication.

Table 4. Sodium chloride weights for tasks 2.2 and 2.3 specimen grout mixes.

Chloride 

Concentration 

by Weight of 

Cement 

(percent)

Weight of 

Chloride Ions 

Required for 

One 50-lb 

Grout Bag (lb)

Weight of 

Chloride Ions 

Required for 

Six 50-lb 

Grout Bags 

(lb)

Weight of 

Sodium 

Chloride 

Required for 

Six-Bag Grout 

Mix (lb)*

Weight of 

Water 

Required for 

Six-Bag 

Grout Mix 

(lb)**

Total Weight 

of Six-Bag 

Grout Mix 

(lb)***

0.00 0.00 0.00 0.00 75.00 375.00

0.08 0.03 0.16 0.26 75.00 375.30

0.20 0.07 0.40 0.66 75.00 375.70

0.40 0.13 0.80 1.32 75.00 376.30

0.60 0.20 1.20 1.98 75.00 377.00

0.80 0.27 1.60 2.64 75.00 377.60

1.00 0.33 2.00 3.30 75.00 378.30

2.00 0.67 4.00 6.59 75.00 381.60

*60.7 weight percent of sodium chloride is chloride ions.

**12.5 lb of water per bag.

***6 bags × 50 lb per bag + 75 lb of water + sodium chloride.

8
Proportional amounts of sodium chloride were admixed for the trial mixes as shown in table 4. 

After they were cured for 10 days in ambient condition, acid-soluble and water-soluble chloride 

analyses were made with the grout powder samples obtained by drilling the hardened grout

cylinders. The former was done by American Association of State Highway and Transportation 

Officials (AASHTO) T260 and the latter by ASTM C1218.

(15,16) The entire acid-soluble chloride 

test results reported in this report was done according to AASHTO T260 standards. The chloride 

test results are presented in chapter 4 of this report.

A perfect agreement existed between admixed chloride concentrations and extracted chloride 

concentrations in the trial mixes. The grout mixing and pumping was performed in an industrial 

grade grout mixing system. 

ENVIRONMENTAL CHAMBERS 

To accelerate corrosion of fabricated PT specimens, one large and one small walk-in 

environmental chamber were employed by converting two commercial grade walk-in freezers. 

The smaller chamber had an interior footprint of 6 ft, 4 inches × 12 ft, 4 inches, and the larger 

one had an interior footprint of 11 ft, 4 inches × 12 ft, 4 inches. The interior height of both 

chambers was 6 ft, 11.5 inches. In addition to a standard freezer evaporator and refrigeration 

unit, each chamber had infrared heat lamps, a humidifier, a dehumidifier, and a digital thermostat 

to control the temperature between -10 and 120 °F and RH between 30 and 100 percent. Figure 3

through figure 6 show exterior and interior views of the chambers, respectively. A data collection 

terminal cabinet and fabricated specimens are also shown in figure 4 through figure 6.

Figure 3. Photo. Environmental chambers.

9
Figure 4. Photo. Inside the small environmental chamber.

Figure 5. Photo. Inside the large environmental chamber.

Figure 6. Photo. Data collection terminal cabinet.

10
INTRODUCTION TO ACCELERATED CORROSION TESTING PROGRAM

An accelerated corrosion testing protocol was developed to produce discernible test results 

within 6 months. Because corrosion reaction is greatly influenced by temperature and moisture 

level (RH), cyclic temperature and humidity variations simulating outdoor environmental 

condition to which the PT bridges are exposed in field were maintained inside the 

environmental chambers. 

Fabricated specimens were exposed for a full cycle of 8 weeks in the following sequence: 

• Sub-cycle 1: Ambient environment (77 °F + 60 percent RH) for 2 weeks.

• Sub-cycle 2: Hot and humid (H & H) environment (104 °F + 90 percent RH) for 

2 weeks.

• Sub-cycle 3: Ambient environment (77 °F + 60 percent RH) for 2 weeks.

• Sub-cycle 4: Freezing and dry (F & D) environment (14 °F + 40 percent RH) for 

2 weeks. 

Therefore, the specimens were subjected to three full-cyclic exposures during the 6-month 

accelerated corrosion testing program. As will be discussed later, the last F & D cycle 

was replaced with another H & H cycle because corrosion activity during the F & D cycles

was insignificant. 

Temperature and RH in the chambers were recorded continuously using universal serial busbased monitoring sensors. Ambient temperature and temperature/RH in the chambers were also 

manually measured at the time of performance data collection. At the same time, specimen 

interior temperature was measured using thermocouples placed in the two specimens. The 

experimental data were collected twice per cycle: the first one in the middle (end of the first 

week) and the second one at the end of the exposure cycle (end of the second week). After the 

second data collection was completed, the exposure cycle was switched to the next scheduled 

one. Data collection from the single-strand control specimens was made bi-weekly at end of the 

second week as they did not show noticeable changes in the corrosion activity due to their 

exposure to only room temperature. Condition of strands exposed in the void were also 

occasionally inspected and photographed. More discussion will be made in the Corrosion 

Performance Tests and Data Collection section in this chapter.

Weekly corrosion performance of the accelerated corrosion testing specimens was monitored

with two types of electrochemical measurements. These non-destructive data are commonly 

used to characterize corrosion behavior of metallic specimens embedded in cementitious 

materials non-destructively subjected to various levels of chloride contamination and 

environmental loading. 

The first experimental dataset collected in this study was corrosion potential. It indicates the 

thermodynamic tendency of the specimens to determine whether corrosion initiation is possible 

or not. ASTM C876 provides three corrosion potential criteria with respect to copper/copper 

11
sulfate reference electrode (CSE) to determine corrosion tendency of steel embedded in concrete 

as follows:

(17)

• More positive than -200 mV versus CSE: 90 percent of probability having no corrosion.

• More negative than -350 mV versus CSE: 90 percent of probability having corrosion.

• Between -200 and -350 mV versus CSE: uncertain state.

These criteria are not applicable in many circumstances, but they are still useful references to 

evaluate unknown thermodynamic state of metallic objects in cementitious medium. Its 

usefulness increases when it is used along with corrosion rate information. 

The second experimental dataset in this study reviewed the instantaneous corrosion rate of 

single-strand specimens. It provided kinetic information about the intensity of corrosion once 

corrosion started. This information is vital to predict remaining life of a system or a structure by 

estimating how fast corrosion takes place. As the word “instantaneous” implies, it determines the 

rate of corrosion at the moment of measurement, and it can change anytime depending on 

external and internal conditions including temperature and moisture content. It is calculated from

experimentally determined polarization resistance (Rp) with the assumption that corrosion occurs 

uniformly over the entire surface area. Corrosion rate is usually expressed by mils per year. In 

reality, active-passive metals like PT strands exposed to highly alkaline environment are

damaged at finite fixed spots. This form of localized corrosion includes pitting corrosion and 

crevice corrosion. Although the underlying assumption of uniform corrosion is incorrect, the 

present study uses the collected nominal corrosion rate data with caution because there are no 

other alternatives to provide the corrosion rate information. 

The next section briefly discusses electrochemistry of localized corrosion caused by a breakdown 

of passive film on reinforcing steel and strand exposed to uncarbonated concrete, mortar, or 

grout. The passive film is formed during anodic polarization, as shown in figure 7. 

The anodic polarization process of a metal involves moving electrical potential of the metal 

(working electrode of an electrochemical test cell) toward more a positive direction from its 

reversible potential, as illustrated by the green arrows in figure 7. One way of achieving anodic 

polarization is through a potentiodynamic polarization technique using advanced electrochemical 

instruments. They enable researchers to conduct specific anodic polarization experiments with 

great precision and predetermined polarization limits, scan rates, etc. During the initial upward 

anodic polarization, corrosion current density increases (i.e., corrosion rate of the metal 

increases). This region is defined as the active zone as indicated in figure 7. When continuous 

anodic polarization process makes current density exceed a critical value (icritical), current density 

drops substantially to a passive current density level (ipassive). At this current density level, 

passive film starts to build up on the metal surface in the presence of oxygen, and, consequently,

corrosion rate of the metal decreases. With further anodic polarization in the positive potential 

direction, quality of the passive film is increasingly enhanced, and corrosion rate remains low. 

This region is called the passive zone, as indicated in figure 7. When the anodic potential reaches 

the pitting potential (Epit) and beyond, pits start to form on the metal surface and grow as 

function of anodic potential. Above a certain potential, oxygen gas also evolves by water 

12
dissociation. This region is labeled the transpassive zone, as indicated in figure 7. A cyclic 

potentiodynamic anodic polarization technique can be also employed to investigate pitting 

corrosion behavior of a particular metal. Basically, it is a reversed anodic polarization process 

once the anodic polarization curve reaches a pre-set limit in the transpassive zone. This step is 

shown as the blue line and blue arrows in figure 7. The area surrounded by the hysteresis loop 

indicates the degree of susceptibility to pitting corrosion. A larger area is more susceptible to 

pitting corrosion. The potential where the reversed anodic polarization curve and the upward 

anodic polarization curve intersect is called the protection potential (Eprot) or repassivation 

potential. No pitting occurs below Eprot, whereas pits can nucleate and grow above Epit. Between 

Eprot and Epit, no pits can form, but existing pits can grow.

Figure 7. Graph. Major characteristics of active-passive metals presented in a potential-log 

current density (E-log i) diagram.

Figure 8 illustrates the effect of chloride ions on passivity of steel. As the chloride concentration 

increases, the passive zone shrinks, and both ipassive and icritical increase due to weakened or 

compromised passive film. Consequently, a much higher corrosion rate should be expected at 

any corrosion potential for steel exposed to chloride contaminated concrete, mortar, and grout.

13
Figure 8. Graph. Effect of chloride ions on passivity.

The passive metals and alloys, including steel embedded in cementitious materials, are most 

susceptible to localized corrosion which can have three forms: pitting corrosion, crevice 

corrosion, and intergranular corrosion. They occur in two stages: corrosion initiation and 

corrosion propagation. Pitting corrosion, the first and most common localized corrosion, can 

initiate by four mechanisms, but this report only discusses one of them—passive film breakdown. 

This is because passive film breakdown is mainly responsible for chloride-induced corrosion of 

steel and strand in concrete, mortar, and grout. Pitting corrosion driven by this mechanism 

initiates when a corrosion cell is established between a small active area having damaged passive 

film and a large passive area covered with intact passive film. Schematic of such a corrosion cell 

is shown in figure 9. In the illustration, the following notations are used:

• OH-

: Hydroxyl ion.

• H+: Hydrogen ion.

• O2: Oxygen molecule.

• 2e-: Two electrons.

• Fe2+: Ferrous ion.

• Cl-: Chloride ion.

14
Figure 9. Illustration. Pitting corrosion mechanism.

Two electrochemical reactions occur simultaneously as follows:

Figure 10. Equation. Oxidation reaction at anode.

Figure 11. Equation. Reduction reaction at cathode.

The equations in figure 10 and figure 11 show that the corrosion process liberates numerous 

positively charged ferrous ions (Fe2+) into the pits and negatively charged hydroxyl ions 

(2OH-) elsewhere.

Crevice corrosion, the second form of localized corrosion, initiates at geometrically confined 

areas where electrolytes are stagnant. This condition can establish a differential aeration cell 

when dissolved oxygen in the crevice is depleted by figure 11, whereas electrolytes outside the 

crevice in contact with bulk electrolyte contain abundant oxygen. As a result, the former 

becomes anodic to the latter, and corrosion progresses in the crevice. Again, surface area ratio of 

the anode and cathode can influence the magnitude of the corrosion damage at the anode. An 

anodic reaction in the crevice is encouraged to liberate ferrous ions and provide electrons to the 

cathodic area outside the crevice. A specific case of crevice corrosion related to the PT strand is 

illustrated in figure 12.

Fe  Fe2+ + 2e1/2O2 + H2O + 2e-  2OH15
Figure 12. Illustration. Crevice corrosion mechanism.

During the corrosion propagation stage, the increased concentration of ferrous ions by metal 

dissolution within the pit or crevice attracts negatively charged anions, mainly chloride ions, into 

the pit or crevice to maintain charge neutrality, as shown by figure 13 as follows:

Figure 13. Equation. Chemical reaction involving a ferrous ion and two chloride ions.

The ferrous chloride is then hydrolyzed by water to ferrous hydroxide and hydrochloric acid 

as follows:

Figure 14. Equation. Chemical reaction for acidification.

The acidification process in figure 14 generates a highly corrosive local environment by lowing

the pH within the pit or crevice and producing the acid product via an autocatalytic process. The 

outer corrosion product, ferrous hydroxide, can be converted to ferric hydroxide in contact with

more dissolved oxygen as follows: 

Figure 15. Equation. Chemical reaction involving ferric oxide formation.

As illustrated in figure 8, the accumulated chloride ions further compromise the passive film and 

lead to very active localized corrosion.

For multi-strand specimens, macro-cell corrosion current density data were collected (instead of 

corrosion rate data), as they also provide information about corrosion kinetics. Macro-cell 

corrosion current measurement is the other popular experimental technique to quantify the 

intensity of corrosion by measuring corrosion current from anode to cathode. The word “macro”

Fe2+ + 2Cl-  FeCl2

FeCl2 + 2H2O  Fe(OH)2 + 2HCl

Fe(OH)2 + 1/2H2O + 1/4O2  Fe(OH)3

16
indicates that this type of corrosion involves physically separated anodes and cathodes as 

opposed to conventional micro-cell corrosion (or local action-cell corrosion). It was developed 

based on the field observations that reinforcing steel in concrete bridge decks corrodes by a 

macro-cell corrosion mechanism between salt-exposed top mat steel (macro-anode) and chloridefree bottom steel (macro-cathode). Numerous laboratory experiments have adopted this 

technique to simulate realistic corrosion of steel in concrete using small-scale macro-cell 

corrosion concrete slabs. It has been observed in the field PT bridges that macro-cell corrosion 

can induce serious damage to PT strands in a relatively short period of time. Figure 16 through 

figure 18 show three examples of corrosion-failed PT tendons by this corrosion mechanism. In 

the PT tendon cases, macro-cell corrosion can be established between macro-anode at cracks and 

holes in the ducts and infiltrated water in the grout voids and macro-cathode in the rest of the 

tendon. Figure 19 illustrates a specific macro-cell corrosion mechanism applicable to PT tendons. 

A macro-anode is connected to a nearby macro-cathode via metallic path (strands) and 

electrolyte (grout). This condition fulfills four components required for any operating macro-cell 

corrosion, that is, macro-anode, macro-cathode, metallic path, and electrolyte.

Credit: FDOT

Figure 16. Photo. Example of PT tendon failure due to macro-cell corrosion—Niles 

Channel bridge in Florida after 13 years in service.

Credit: FDOT

Figure 17. Photo. Example of PT tendon failure due to macro-cell corrosion—Sunshine 

Skyway bridge in Florida after 13 years in service.

17
Figure 18. Photo. Example of PT tendon failure due to macro-cell corrosion—Verina-Enon 

bridge in Virginia after 17 years in service.

Figure 19. Illustration. Macro-cell corrosion mechanism in PT tendon.

Although the electrochemical reactions discussed are the same for any corroding steel in an 

aerated environment, the macro-anode and macro-cathode are physically separated, and their 

surface areas are much larger than ordinary micro-cell corrosion that consists of numerous tiny 

alternating anodes and cathodes. Consequently, intense corrosion can take place at the macroanode by figure 10 and reduction of electrons at the macro-cathode by figure 11.

An Evans diagram (or E-log i diagram) illustrates how an anodic polarization diagram of a metal 

intersects with its cathodic polarization diagram. The intersection of two polarization diagrams 

determines the metal’s corrosion potential and corrosion current density. Figure 20 shows four

E-log i plots that describe several corrosion potential-corrosion current density relationships of 

active-passive metals, such as PT strands, depending on different anodic and cathodic behaviors. 

The four plots can be described as follows:

• Case (1) represents a non-corroding PT strand in normal grout. The passive anodic 

polarization curve (solid green line) intersects with the cathodic polarization curve

18
(solid blue line). In this case, relatively positive corrosion potential and low corrosion 

current density are observed. 

• Case (1') also represents a non-corroding PT strand in normal grout, but more oxygen is 

supplied. The passive anodic polarization curve (solid green line) intersects with the more 

positively shifted cathodic polarization curve (dashed blue line). In this case, more positive 

corrosion potential and the same low corrosion current density are observed. 

• Case (2) represents a corroding PT strand in defective grout. The active anodic polarization 

curve (solid magenta line) intersects with the cathodic polarization curve (solid blue line). In 

this case, negative corrosion potential and higher corrosion current density are observed. 

• Case (2') also represents a corroding PT strand in defective grout, but more oxygen is 

supplied. The active anodic polarization curve (solid red line) intersects with the more 

positively shifted cathodic polarization curve (dashed blue line). In this case, slightly more 

positive corrosion potential and significantly higher corrosion current density than case (2) 

are observed. 

Figure 20. Graph. E–log i diagram describing different anodic and cathodic behaviors.

For macro-cell corrosion, cases (1) and (1') can relate to macro-cathode, while cases (2) and (2') can 

be related to macro-anode. The potential difference between any of them determines the driving 

force for macro-cell corrosion. In theory, electrical coupling between a macro-cathode in case (1') 

and a macro-anode in case (2') would yield the largest corrosion-induced damage as magnitude of 

E

log i

+

-

Fe  Fe2+ + 2e-

(Anodic polarization diagram 

of active metal)

1⁄2O2 + H2O + 2e- 2OH-

(Cathodic polarization diagrams)

Fe  Fe2+ + 2e-

(Anodic polarization diagram 

of active-passive metal)

(Ecorr )’

passive/macro-cathode

(Ecorr )

active/macro-anode

(icorr )

passive/macro-cathode

(icorr )

active/macro-anode

(1)’

(2)

(1)

(2)’

(icorr )’

active/macro-anode

(Ecorr )’

active/macro-anode

(Ecorr )

passive/macro-cathode

19
corrosion current density determines level of damage, and the coupled condition yields the highest 

macro-cell current density ((icorr)'active/macro-anode − (icorr)passive/macro-cathode). More severe corrosion 

damage at the macro-anode can occur if surface area ratio of the macro-cathode to the macro-anode 

becomes large (i.e., small anode-large cathode situation).

In addition, apparent grout resistivity and grout resistance data were also collected to provide 

additional information about corrosion environment. Less grout resistance allows the corrosion 

current to travel further distances. As a result, a larger macro-cathode surface area can be involved 

in the corrosion process.

SPECIMEN TYPE AND FABRICATION PROCESS

There were three types of test specimens employed in this study for each of three subtasks. They 

were designed to produce specific test results and, when analyzed together, provide comprehensive

information regarding chloride threshold values.

Task 2.1: Single-Wire Specimens in Aqueous Solutions

The purpose of this subtask was to evaluate the effect of chloride concentration, stress level, and pH 

on corrosion of the PT strand. For this, corrosion potential and instantaneous corrosion rate of 

center wires exposed to various experimental conditions were measured. 

A specially designed test cell was installed in the middle section of a 5-ft-long wire as shown in 

figure 21 and figure 22. 

Figure 21. Illustration. Task 2.1 test setup for stressed wires.

20
Figure 22. Photo. Test cell for stressed wires.

The original wire surface condition including lubricants applied during the manufacturing process 

was untouched to produce the corrosion data from the strands in the as-received condition. Some 

wires were tested under no stress, and the others were prestressed at 60 percent of guaranteed 

ultimate tensile strength (GUTS). For the stressed wires, a custom-made loading frame equipped 

with a load cell and hydraulic pump was used to apply the target stress. The loading frame is shown 

in figure 21 and figure 22. For the unstressed wires, a wooden frame was used to hold the test cell 

in place vertically as shown in figure 23.

Figure 23. Photo. Task 2.1 test setup for unstressed wires.

Test cell

Test cell

21
The following test variables were employed to characterize electrochemical behaviors of stressed 

and unstressed wires at ambient temperature:

• Temperature: Ambient temperature was used.

• Stress level: 0 percent (unstressed) and 60 percent of GUTS (applied load of 35 kips based 

on nominal cross-sectional area of 0.032 inches2).

• Chloride concentration: 0, 0.04, 0.08, 0.2, 0.4, 0.6, 0.8, 1.0, 2.0, and 5.0 percent by weight 

of cement (calculated for equivalent concentrations in aqueous solutions).

• pH: 13.6 (uncarbonated case) and 9.0 (carbonated case).

A combination of the test variables required 40 center wires (stress level × chloride concentration ×

pH = 2 × 10 × 2 = 40). However, some repeated measurements had to be made due to outliers. 

Consequently, more wires were tested during the actual experiment.

Task 2.2: Single-Strand Specimens in Grout

This specimen type was employed to understand the effect of chloride concentration, stress level, 

void, and crevice formed by adjacent wires on corrosion of single PT strands in chloridecontaminated grout. It has been observed that when actual PT strands corrode in the field, 

interstitial sites formed by twisted outer wires can encourage crevice corrosion.

(14) Prestressing a 

strand also tightens all the interstitial spaces by stretching the wires and promoting capillary 

movement of water, thus enhancing corrosion activities there. A 12-inch single-strand specimen

type was developed to measure corrosion rate on a short PT strand encased in hardened grout and a 

counter electrode. 

Four rigid-loading frames were constructed to accommodate three 48-inch-long PT strands in each 

frame. A loading frame contained two stressed strands and one unstressed strand, as shown in 

figure 24.

22
Figure 24. Illustration. Task 2.2 loading frame for single-strand specimens.

Each strand had two mock-up clear polyvinyl chloride (PVC) ducts in the middle, as shown in 

figure 24. Therefore, a total of six mock-up tendon specimens (four stressed and two unstressed)

were tested per loading frame. The upper duct specimens simulated tendons containing a grout 

void, and the lower ones simulated fully grouted tendons. Later, the lower and upper ducts were 

filled with a grout mix containing the same chloride concentration. 

Details of the mock-up specimens are shown in figure 25. Inner diameter and length of each duct 

were 2.0 and 12 inches, respectively. The inner wall of the duct was surrounded with a cylindricalshaped grade 316 stainless steel mesh, which served as the counter electrode for the linear 

polarization resistance (LPR) measurement. Such a confined test cell design was intended to 

capture a very low measurement current (i.e., nano-ampere (10-9 A) range) in (cathodic direction) 

and out (anodic direction) of the strand during the LPR measurement.

Loading frame

Stressed strands

Unstressed strand 4 ft.

Specimens without 

grout void

Specimens with 

grout void

23
Figure 25. Illustration. Single-strand test cells.

The duct also had four resistivity probes at 2-inch spacings. The probes were made out of 0.25-inch

diameter and 1-inch-long mixed metal oxide titanium, and they were installed as illustrated in 

figure 26 and figure 27. 

Figure 26. Illustration. Pseudo-reference electrode (PSE) probe.

24
Figure 27. Photo. Actual PSE probe.

The probes were used to measure apparent grout resistivity using the Wenner four-pin method. The 

second probe from the duct bottom also served as the PSE for the corrosion potential measurement. 

The potential readings made with the PSE were converted to those with respect to a standard silversilver chloride reference electrode (ACE). This is schematically presented in figure 28. Eight blank 

grout cylinders were used to measure corrosion potentials of PSEs in eight chloride concentrations. 

Figure 28. Illustration. PSE calibration cylinders.

An indirect potential measurement via a PSE was necessary to eliminate possible problems during 

the exposure testing when direct contact was made between grout and tip of the fragile ACE 

containing a liquid solution in a glass body. 

The following variables were employed to characterize corrosion behaviors of stressed and 

unstressed PT strands:

• Grout: Pre-bagged grout product was used.

• Stress level: 0 and 60 percent of GUTS.

25
• Grout void: None and 2-by-2-inch cylindrical-shaped void.

• Temperature and RH: 77 °F + 60 percent RH, 104 °F + 90 percent RH, and 14 °F + 

40 percent RH.

• Chloride concentrations: 0, 0.08, 0.2, 0.4, 0.6, 0.8, 1.0, and 2.0 percent by weight 

of cement.

• Ingress of water and oxygen into some voided specimens in the middle of exposure testing.

Combination of test variables required 40 single-strand specimens as follows: 

• Stressed single-strand specimens: Grout void (2) × chloride concentrations (8) = 16.

• Unstressed single-strand specimens: Grout void (2) × four chloride concentrations (0, 0.2, 

0.6, and 1.0 percent by weight of cement) = 8.

• Extra unstressed single-strand specimens: Grout void (2) × chloride concentrations (8) 

= 16.

After the counter electrodes and resistivity probes were installed in the ducts, the assembled duct 

specimens were placed at the fix positions along the strands, and the bottom openings between the 

strands and PVC caps were sealed with a silicon caulking material. The top openings were used as 

air vents during grout pumping. The strands were then installed in the loading frame. For the 

strands designated as stressed specimens, prestressing was done at 60 percent of GUTS from the 

upper end of the strands. For the unstressed ones, the strands were gently placed onto the loading 

frame using steel wedges. The target stress level was selected to avoid possibilities of overstressing 

while reflecting a typical working stress in the field PT bridges. After grout pumping, the top

openings of the ducts were sealed with hot glue. Figure 29 and figure 30 show the stressing work,

and figure 31 shows task 2.2 specimens prepared for grout installation.

Figure 29. Photo. Stressing of single-strand specimens.

26
Figure 30. Photo. Stressing of multi-strand specimens.

Figure 31. Photo. Fully assembled single-strand specimens.

The grouting work was completed in a single day. For each grout mix, six bags of the grout 

material, a predetermined amount of sodium chloride as listed in table 4, and 75 lb of distilled 

water were prepared in advance. The sodium chloride was dissolved in the mixing water just 

before mixing. 

After pouring the dry grout powder into the grout mixer, mixing water was added (see figure 32). 

Once desirable grout consistency was achieved, the grout mix was pumped to the mock-up tendons 

(see figure 33). For the specimens without void, the duct was completely filled with the grout until 

it came out of the vent hole slightly. For those containing a void, 10 inches of the duct from the 

bottom was filled with the grout, and the remaining top 2 inches were left empty to simulate an 

entrapped air void. For each chloride concentration grout mix, casting was done with the 

same batch. 

27
Figure 32. Photo. Grout mixing.

Figure 33. Photo. Grout pumping into a single-strand specimen.

28
Random grout samples were taken from each mix to perform standard grout tests (see figure 34). 

Figure 34. Photo. In-situ grout testing.

The 28-day grout compressive strength data are listed in table 5. Batch #9 was the only batch 

producing 28-day compressive strength below 8,000 psi, the advertised 28-day strength by the 

grout manufacturer.

Table 5. 28-day compressive strength data of grout mixes.

Batch ID

Admixed 

Chloride 

Concentration by 

Weight of 

Cement (percent)

Cube Compressive Strength (psi)

1 2 3

Mean

(psi)

Batch #1 0 10,040 10,290 10,400 10,243

Batch #2* 0 10,910 10,540 11,280 10,910

Batch #3 0.08 8,780 10,140 10,580 9,833

Batch #4 0.2 8,800 7,810 8,300 8,303

Batch #5 0.4 10,060 9,220 9,080 9,453

Batch #6 0.6 8,760 9,090 9,030 8,960

Batch #7 0.8 9,420 9,500 9,020 9,313

Batch #8 1.0 9,500 8,860 9,060 9,140

Batch #9 2.0 6,350 5,810 7,350 6,503

*Repeated casting due to grout leaking problem.

Batch #9 exhibited normal fluidity (7 s) and wet density (126 lb/ft3). Mix and ambient temperatures 

were also within the recommended guidelines. Batch #9 was also the only one with specimens 

29
cured in plastic molds instead of cast iron molds. Plastic molds are known to reduce compressive 

strength compared to cast iron molds. 

When the specimens were cured for 10 days, two baseline data collections were made at ambient 

condition for all 40 single-strand specimens. After that, 24 specimens (16 stressed and 8 unstressed) 

designated for the accelerated corrosion testing were placed in the smaller environmental chamber. 

The other 16 unstressed single-strand specimens were kept in the ambient condition to serve as 

controls as well as backup specimens, if necessary. In order to collect the data efficiently without 

disturbing the temperature and RH inside the chamber, all the lead wires from the specimens were 

organized in several bundles and were run to a data collection panel mounted on the exterior wall of 

the chamber (see figure 4 through figure 6). 

Task 2.3: Multi-Strand Specimens in Grout

The last specimen type was a large-scale multi-strand PT mock-up tendon. It was introduced to 

investigate the effects of chloride concentration, void, charging water, and macro-cell corrosion on 

PT strand corrosion. In this sub-task, reproduction of the worst PT corrosion problem caused by 

macro-cell corrosion was attempted using an innovative specimen design. Such a unique approach 

was necessary because conventional specimen configurations could not mimic the real macro-cell 

corrosion situation due to experimental limitations such as lab space and complexities involving

fabricating long stressed tendon specimens in the confined environmental chambers. The multistrand specimens were intended to simulate the principle of macro-cell corrosion mechanism as 

close as practical: macro-anode passing through a grout void with rechargeable water and macrocathode embedded in normal grout. The intended macro-cathode was made with unstressed short 

strands surrounded by the intended macro-anode that was made with four stressed strands. Nearly

equal surface areas were adopted for macro-anode and macro-cathode, thus surface effect (small 

macro-anode and large macro-cathode) could not be investigated in this study. This configuration 

also allowed comparing corrosion behaviors of stressed strands versus unstressed ones. This type of 

specimen could create more positive corrosion potential at the intended macro-cathode (case (1) or 

case (1') in figure 20) compared to the intended macro-anode (case (2) or case (2') in figure 20) and 

large driving force between them. 

Each multi-strand specimen was 10 ft long and 6 inches in diameter. They were supported by a 

25-degree inclined wooden support structure to simulate typical inclined tendons in the field. They 

contained four 10-ft stressed strands that served as a macro-anode and that were partially buried in 

the grout and five 7-ft unstressed strands that served as a macro-cathode and that were completely 

embedded in the grout. Both groups of strands were electrically isolated from each other with the 

exception of being coupled via an external toggle switch. Sufficient clearance among nine strands 

was maintained by three plastic circular plates with nine holes for the strands to run through. Four 

resistivity titanium probes identical to those used in task 2.2 specimens were installed at the 12-inch 

spacing along the 2 o’clock line starting from the lower anchor plate. The titanium probes were also 

used as the PSEs to measure the strand potentials at the probe locations. Figure 35 and figure 36

show specimen configuration and some of its details, respectively. 

30
Figure 35. Illustration. Task 2.3 multi-strand specimens.

Figure 36. Illustration. Details of multi-strand specimens.

Figure 37 identifies the position of 12-inch-long strand segments with respect to the upper 

anchor plate. To monitor temperature inside specimens, two thermocouples were installed in the 

Lower 

anchorage

block

Macro-cathode

(five unstressed strands)

Macro-anode

(four stressed strands)

Clear HDPE tube filled with chloride 

contaminated grout (Electrolyte)

Air

void

Charged water introduced later

A

B

B

[Section A – A]

A

Plastic spacer

Number of Specimens

•Stressed Specimens @ 60% GUTS: 8 EA 

(Chloride concentrations: 0, 0.08, 0.2, 

0.4, 0.6, 0.8, 1.0, and 2.0 % by weight of 

cement) 

[Section B – B]

Upper 

anchorage

block

[Section C – C]

Air

void

Resistivity probe & 

pseudo-reference 

electrode @ 2 o’clock

C

C

31
0 percent chloride specimen placed in the smaller chamber and 1.0 percent chloride specimen 

placed in the larger chamber. The strand orientation was indicated as follows:

• Top-left strand (TL).

• Top-right strand (TR).

• Bottom-left strand (BL).

• Bottom-right strand (BR).

Figure 37. Illustration. Segment designation of multi-strand specimens.

A total of eight specimens were fabricated based on the following test variables:

• Grout: Pre-bagged grout product.

• Stress level: 60 percent of GUTS.

• Temperature and RH: 104 °F + 90 percent RH, 77 °F + 60 percent RH, and 14 °F + 

40 percent RH.

• Chloride concentrations: 0, 0.08, 0.2, 0.4, 0.6, 0.8, 1.0, and 2.0 percent by weight 

of cement.

• Ingress of water and oxygen into the simulated air void.

[Section A – A]

TL TR

BL BR

Grout pumping stops at this point

32
Figure 38 shows some multi-strand specimens prepared for grouting. 

Figure 38. Photo. Fully assembled multi-strand specimens.

As described for task 2.2, eight chloride contaminated grout mixes were made per table 4. The 

fresh grout mix was pumped into the inclined duct until the horizontal void/grout interface reached 

16 inches down from the upper anchor plate. This stopping point is indicated with a cross mark in 

figure 37. In this way, five unstressed strands were buried completely in the grout, and four stressed 

ones were partially embedded in the grout such that the top 18.5-inch section of two upper stressed 

strands (TL and TR in figure 37) and the top 13.5-inch section of two lower stressed strands (BL 

and BR in figure 37) were exposed in the void. Figure 39 shows grouting work in progress. It 

should be mentioned that the first specimen containing 0 percent chloride started to leak in the 

bottom grout cap area when about 30 percent of fresh grout was pumped into the duct. As a result, 

the first batch was discarded, the leaky cap was replaced, and new batch of fresh grout was installed 

in the specimen.

As explained in task 2.2, three data collections following the 10-day curing were made at ambient 

condition before they were placed in the environmental chambers to commence the first H & H 

cycle. The smaller chamber housed two specimens having the lowest chloride concentrations, and 

the larger chamber had the other six specimens (see figure 4 through figure 6). 

33
Figure 39. Photo. Grout pumping into a multi-strand specimen.

CORROSION PERFORMANCE TESTS AND DATA COLLECTION

Task 2.1: Electrochemical Testing of Single-Wire Specimens in Aqueous Test Solutions

When an experiment was about to begin for a king wire in a particular test condition, a test cell 

mounted on the wire was filled with an aqueous test solution so that the immersed test area was 

1.78 inches2

, as listed in table 6. The test area is calculated as follows:

Figure 40. Equation. Calculation of surface area.

Where:

S = Surface area.

π= Constant (3.14).

d = Wire diameter.

l = Immersion depth.

S = π × d × l

34
Table 6. Test surface area used in this study.

Category

Single PT Strand

Task 2.1 

(Center Wire)

Task 2.2 

(Single-Strand Specimens)

Task 2.3 

(Multi-Strand Specimens)

Diameter 

(inch)

Surface 

Area (inch2) Full Void Macro-Anode Macro-Cathode

Outer 

six wires

0.198

Center 

(king) wire

0.199

Calculated 

test surface 

area

52.3 3.14 × 0.199 inch 

× 2.85 inch = 

1.8 inches2

1 strand × 

1 ft/strand × 

52.3 inches2

/ft = 

52.3 inches2

1 strand × 

0.83 ft/strand × 

52.3 inches2

/ft = 

43.6 inches2

4 strands × 

8.7 ft/strand × 

52.3 inches2

/ft = 

1,813.1 inches2

5 strands × 7 

ft/strand × 

52.3 inches2

/ft = 

1,830.5 inches2

Note: Blank cells indicate no data were relevant to the cell. 35
After 24 h of initial conditioning in the quiescent test solution, the three-electrode LPR test was 

performed using the experimental setup shown in figure 21 through figure 23. The first step of a 

LPR measurement determined a corrosion potential of a wire specimen. Once the stable corrosion 

potential was obtained, Rp was determined through a series of step-wise potentiostatic polarizations. 

Finally, Rp was used to calculate an instantaneous nominal corrosion rate. For steel embedded in 

mortar or concrete, intensity of corrosion is usually classified into four corrosion rates, either by 

corrosion current density (μA/cm2) or by mils per year. These criteria are listed in table 7. Since 

test medium for task 2.1 is aqueous solutions, direct application of the criteria listed in table 7

is not feasible. 

Table 7. Corrosion rate criteria.

Degree of 

Corrosion

Corrosion Rate Criteria

Corrosion 

Current Density 

( A/cm2)

Corrosion 

Penetration Rate 

(mils/year)

Negligible ˂ 0.1 ˂ 0.046

Low 0.1–0.5 0.046–0.230

Moderate 0.5–1.0 0.230–0.460

High ˃ 1.0 ˃ 0.460

1 cm2 = 0.155 inch2

Task 2.2: Single-Strand Specimens

Corrosion performance of single-strand specimens was nondestructively monitored once a week 

using the following methods:

1. Corrosion potential measurement.

2. Instantaneous corrosion rate measurement by LPR method.

3. Apparent grout resistivity.

The condition of the strands in the void was occasionally inspected. Corrosion potential and 

instantaneous corrosion rate were measured using the setup shown in figure 41. Apparent grout 

resistivity data were collected as supplementary information using the Wenner four-pin method 

with 2-inch pin spacings as shown in figure 42.

μ

36
Figure 41. Illustration. LPR measurement for single-strand specimens.

Figure 42. Illustration. Grout resistivity measurement for single-strand specimens.

37
Since test surface area should be known in order to calculate instantaneous corrosion rate, the 

following method was used to calculate PT strand surface area per unit length. The diameter of the 

outer wire was 0.198 inch, and the diameter of the center wire was 0.199 inch, as seen in figure 43.

Figure 43. Equation. Surface area of six outer wires.

As such, the following calculation was used to determine the surface area of the center wire:

Figure 44. Equation. Surface area of the center wire.

Where:

Sc = Surface area of the center wire.

dc = Diameter of the center wire.

Strictly speaking, the length of the outer wires should be longer than that of the center wire, but the 

additional length is ignored in this study to simplify the calculation.

Figure 45. Equation. Total surface area of seven wires.

As listed in table 6, surface areas of 52.3 and 43.6 inches2 were adopted for fully grouted specimens 

and those having a 2-inch void, respectively. 

Task 2.3: Multi-Strand Specimens

Corrosion performance of multi-strand specimens was nondestructively monitored using the 

following methods once a week:

1. Polarized potential measurement.

2. Macro-cell corrosion current (Imacro-cell) measurement.

3. Corrosion potential measurement.

4. Grout resistance measurement.

Condition of strands in the void were also occasionally inspected. When Imacro-cell was flowing 

between macro-anode and macro-cathode, both electrodes polarized away from their natural 

corrosion potentials and established a mixed potential somewhere between the corrosion potentials. 

A corrosion cell’s relative proximity to a particular electrode’s corrosion potential can indicate if it

is under cathodic or anodic control. The polarized potential was measured with respect to a PSE, as 

illustrated in figure 46. Imacro-cell quantifies intensity of corrosion between the macro-anode and the 

macro-cathode at the time of measurement. In order to avoid interruption in the circuit, an electrical 

connection was made between the macro-anode and the macro-cathode through a toggle switch. If 

S6 = π × d × l × 6 = 3.14 × 0.198 × 12 × 6 = 44.8 inches2

Sc = π × dc× l = 3.14 × 0.199 × 12 = 7.5 inches

2

S7 = S6 + Sc = (44.8 + 7.5) inch2 = 52.3 inches2

38
current interruption occurred, it would have taken a significant amount of time to regain 

electrochemical stability within the corrosion cell. 

Figure 46. Illustration. Polarized potential measurement for multi-strand specimens.

Therefore, Imacro-cell was measured after a zero resistance ammeter (ZRA) was temporarily inserted 

in the circuit and then the toggle switch was turned off. This step is shown in figure 47. 

Figure 47. Illustration. Macro-cell corrosion current measurement for multi-strand 

specimens.

Later, the measured Imacro-cell was divided by macro-anode surface area of 1,813.19 inches2 from 

table 5 to calculate the macro-cell corrosion current density (imacro-cell) that is a normalized form of 

corrosion intensity. After the polarized potential and Imacro-cell were measured, ZRA was removed,

and the electrical connection was left disconnected for several hours until the macro-anode and 

macro-cathode returned to their natural corrosion potentials. Once stable corrosion potentials were 

achieved, they were measured with respect to four PSEs. This step is shown in figure 48. 

Pseudo-RE

V

+ _

Air

void

s/w

From macro-anode 

(four stressed strands) 

From macro-cathode 

(five unstressed strands) 

Air

void

Macro-anode 

(four stressed strands) 

Macro-cathode 

(five unstressed strands) ZRA

+ _

s/w

off

+I

+I

39
Figure 48. Illustration. Corrosion potential measurement for multi-strand specimens.

Grout resistance data were collected as supplementary information. Grout resistance was measured 

using four resistivity probes. The internal duct diameter with respect to resistivity probe spacing 

was too small to measure a valid apparent resistivity. As a result, the resistivity probes were used in 

two pairs to measure grout resistance in the upper and lower sections of the grouted duct as 

described in figure 49. 

Figure 49. Illustration. Grout resistance measurement for multi-strand specimens.

V

+ _

Air

void

s/w

off

From macro-anode 

(four stressed strands) 

From macro-cathode 

(five unstressed strands) 

(A total of eight corrosion potentials are measured)

P1

P2

P3

P4

Air

void

s/w

R E1 E2

I1 I2

R E1 E2

I1 I2

P1

P2

P3

P4

40
DUCT CRACKING

The PVC duct of a 0.8 percent chloride multi-strand specimen was abruptly cracked upon minor 

impact. It happened when a researcher tried to inspect the void condition of the specimen during the 

second F & D cycle. Emergency repair was made using a special wrapping tape to minimize the 

impact of cracks. Figure 50 and figure 51 show the specimen before and after repair. 

Figure 50. Photo. Cracked duct of 0.8 percent chloride multi-strand specimen before repair.

Figure 51. Photo. Cracked duct of 0.8 percent chloride multi-strand specimen after repair.

WATER RECHARGING EVENT

In order to replicate the water collecting problem from external sources into the voided area of duct 

in the field PT bridges, a small volume of water was added to the void space in five single-strand

specimens and all eight multi-strand specimens after approximately 4 months of accelerated 

corrosion testing as follows: 

• Stressed voided single-strand specimens: 0.08, 0.4, 0.8, and 2.0 percent chloride.

41
• Unstressed voided single-strand specimen: 0.6 percent chloride.

• Multi-strand specimens: All eight chloride concentrations.

This was done to duplicate the PT corrosion problem reported in the field. A total of 0.17 fl oz of

distilled water was injected into each of five void single-strand specimens after a 0.25-inch hole 

was drilled through the clear duct in the void space. For multi-strand specimens, 5.1 fl oz of

distilled water was fed through a pre-introduced hole in the void space. Figure 52 through figure 55

show this step. The filling holes were left open for the remainder of testing so that oxygen 

could enter. 

Figure 52. Photo. Water charging into a multi-strand specimen.

Figure 53. Photo. Water charging into a single-strand specimen.

42
Figure 54. Photo. Multi-strand specimen 1 h after water charging.

Figure 55. Photo. Single-strand specimen 1 h after water charging.

Researchers at the University of South Florida and FDOT also performed a similar laboratory study 

to investigate the same problem using a macro-cell corrosion test setup between PT anchorage 

assemblies and strands.

(18) They were able to produce marked current increases as shown in 

figure 56 upon repeated water charging events. 

43
Figure 56. Graph. Relationship between water charging and macro-cell corrosion current 

jump.

(18)

INTRODUCTION OF AIR HOLES 

Right after water was added to the specimen voids, 4 0.5-inch-diameter holes were introduced 

to each of the 6 voided single-strand specimens, and 24 0.5-inch-diameter holes were introduced

to one of the multi-strand specimens that was cracked and repaired. The specimens included

the following:

• Stressed voided single-strand specimens: 0.08, 0.4, and 0.8 percent chloride (all three 

concentrations had water and air holes).

• Unstressed voided single-strand specimens: 0.08, 0.4, and 0.8 percent chloride, which were 

the control and kept in room temperature.

• Multi-strand specimen: 0.8 percent chloride (had water and air holes).

Figure 57 and figure 58 show a single-strand specimen and a multi-strand specimen that were 

introduced with 0.5-inch holes, respectively. This effort was to see if more oxygen could enhance 

corrosion activities of the strands. 

44
Figure 57. Photo. Drilled single-strand specimen.

Figure 58. Photo. Drilled multi-strand specimen with 0.8 percent chloride.

AUTOPSY PROCEDURE

At the completion of the 6-month data collection period, the specimens were taken out of the 

environmental chambers, and an autopsy was carried out. One day before performing the autopsy, 

final data collection was performed at ambient condition. Following the data collection, all of the 

stressed specimens were detensioned, and destructive autopsy work began. 

Task 2.2: Single-Strand Specimens

The detensioned and originally unstressed single-strand specimens were cut out from their loading 

frames followed by the autopsy. As the first step, three grout powder samples along the length were 

extracted for chloride analysis at 0- to1-inch depth from each specimen (see figure 59). A total of 

72 samples were taken from 16 stressed and 8 unstressed specimens. Then, a right angle grinder 

equipped with a diamond blade was used to split each specimen open (see figure 60). Every strand 

was extracted, and their grout/strand interfaces were examined and photographed (see figure 61). 

Some of the strands exhibited minor corrosion (see figure 62), and the others exhibited heavy 

corrosion (see figure 63). The as-extracted condition was documented on a mapping sheet. The 

extracted PT strands were dismantled, and the individual wires were cleaned with a scrubbing pad 

(see figure 64). Superficial rust stains could be removed by this method, but those having stubborn 

45
corrosion products associated with severe corrosion damage had to be cleaned further with an acid 

cleaning solution per ASTM G1 (see figure 65).

(19) The cleaned wires were visually examined with 

occasional use of a magnifier. 

Figure 59. Photo. Grout powder sampling from a single-strand specimen.

Figure 60. Photo. Splitting a single-strand specimen using a circular saw.

46
Figure 61. Photo. Single-strand specimen longitudinally cut in half.

Figure 62. Photo. Example of superficial rust on the extracted wires.

Figure 63. Photo. Example of heavy rust on the extracted wires.

Figure 64. Photo. Cleaning individual wires using a scrubbing pad.

47
Figure 65. Photo. Acid cleaning of moderately to severely corroded wires.

Task 2.3: Multi-Strand Specimens

After stress in a multi-strand specimen was released, the upper anchor plate was removed (see 

figure 66). Following it, 12 grout power samples per specimen were taken along the 3 and 9 o’clock 

lines by drilling at two depths (0 to 1 and 1 to 2 inches) and three locations (14, 48, and 96 inches)

from the lower anchor plate (see figure 67). As a result, a total of 96 grout powder samples were 

extracted to determine spatial distribution of the admixed chloride in the multi-strand specimens. 

As soon as grout sampling was completed, the destructive dismantling process started by cutting 

the clear duct along the 3 and 9 o’clock lines (see figure 68), removing the two duct halves, and 

chipping the grout away (see figure 69). Initially, great caution was exercised in anticipation of 

residual stress in the strands embedded in grout, but the stress dissipated quickly as the grout 

chipping work progressed from one specimen end. Some large grout pieces in direct contact 

with the strands were retrieved from the upper, middle, and lower sections of each specimen 

(see figure 70). These chunks were later used to collect powder samples at the strand imprints 

by carefully removing portions of the grout with a hammer and chisel to determine chloride 

concentrations at the grout/strand interface. Some grout fragments were also used to examine grout 

quality. After grout was completely removed, the strands were cut one by one near the lower anchor 

plate (see figure 71), and they were taken out of the loading frame (see figure 72). The removed 

strands were preserved in the smaller chamber which maintained a RH lower than 50 percent 

(see figure 73). After all the destructive work was completed, examination of the multi-strands 

started on a specially constructed autopsy table (see figure 74).

48
Figure 66. Photo. Removal of the upper anchor bearing plate of a multi-strand specimen.

Figure 67. Photo. Grout powder sampling for chloride analysis from a multi-strand specimen.

Figure 68. Photo. Cutting the clear duct to expose grout.

49
Figure 69. Photo. Chipping grout to expose PT strands.

Figure 70. Photo. Retrieved grout pieces from multi-strand specimens.

Figure 71. Photo. Cutting PT strands near the lower anchor bearing plate.

50
Figure 72. Photo. Extracting a multi-strand bundle from a loading frame.

Figure 73. Photo. Storing the retrieved multi-strand samples in a chamber.

Figure 74. Photo. Inspecting PT strands removed from a multi-strand specimen on an autopsy 

table.

51
The as-extracted condition of each strand was carefully inspected and documented on a mapping 

sheet. As indicated in figure 37, the reference point in the mapping sheet was 16 inches, the mean 

distance to the void/grout interface from the upper anchor plate. From the reference point moving 

toward the lower anchor plate, 12-inch-long segments were marked on the stressed and unstressed 

strands. This is also depicted in figure 37. Every 12-inch segment that exhibited rust spots was

identified on the mapping sheet. With this marking system, four stressed strand segments passed 

through the void/grout interface: two 4-to-16-inch segments of the BL and BR strands and two 

16- to 28-inch segments of the TL and TR strands as labeled in figure 37. A total of eight strand 

segments close to the upper anchor plate were situated near the void/grout interface, and all the 

other stressed and unstressed strand segments were contained in the grout. It was observed during 

the autopsy that some of the interface segments exposed to unintentional sulfate ions exhibited 

more corrosion damage compared to the rest. This situation resembled severe corrosion failures

observed in the defective tendons in service. 

Many segments of the stressed strands exhibited signs of corrosion, and they were cut out for 

further examination (see figure 75). Individual wires removed from the cut-out segments were 

cleaned with a scrubbing pad first. Similar to single-strand specimens, the wires with stubborn 

corrosion products were cleaned further with an acid cleaning solution. The unstressed strands 

exposed to grout with moderate to high levels of chloride contamination often showed rust stains 

in the localized areas. Each unstressed strand with this condition was dismantled as one 7-ft-long 

piece, and the wires were also cleaned with a scrubbing pad. No significant corrosion damage 

requiring acid cleaning was observed on any of the unstressed wires (see figure 76).

Figure 75. Photo. Several 12-inch-long cut-out segments exhibiting various corrosion 

conditions.

52
Figure 76. Photo. Cleaning and visual inspection of 7-ft-long unstressed strands.

CORROSION DAMAGE ASSESSMENT

The cleaned wires were first examined visually to classify them in two groups. The minor corrosion 

group included the wires exhibiting superficial corrosion or shallow pits (less than 2 mil deep). The 

wires that appeared to contain pits deeper than 2 mil were placed in the moderate to severe 

corrosion group. Most of the acid cleaned wires fell into the latter group. 

Figure 77 shows a pit depth measurement in progress. The particular spot showed a pit depth of 

13.5 mil. A digital pit depth gauge capable of measuring the lowest depth of 1.0 mil was used. A 

custom-made magnet-based measurement bed was a convenient means to hold the curved wire 

being measured steady in any orientation. Excessive section loss was measured with a digital 

micrometer as shown in figure 78. 

Figure 77. Photo. Pit depth measurement using a digital pit gauge.

53
Figure 78. Photo. Section loss measurement in a severely corroded area.

54
CHAPTER 4. TEST RESULTS AND DISCUSSION

This chapter presents electrochemical data that were collected weekly, other laboratory test results, 

autopsy results, and discussion with the outcomes of the indepth data analyses. The first set of test 

results is presented with experimental data versus time plots to show data changes as a function of 

time. In each plot shown in figure 88 through figure 90, figure 108 through figure 115, figure 120 

through figure 127, figure 144 through figure 151, figure 154, figure 155, and figure 164 through 

figure 171, the duration of individual H & H and F & D cycles is highlighted with red and blue 

columns, respectively. White columns indicate either initial ambient or ambient cycles. The second 

set of test results shown in figure 100 through figure 107 is related to individual mean values of 

experimental datasets collected over time to show major trends associated with admixed chloride 

concentration, stress level, existence of void, and/or exposure conditions (i.e., initial ambient, 

ambient, H & H, and F & D). The first three datasets collected during the first ambient cycle were 

treated separately as the initial ambient cycle because they indicated more active electrochemical 

behavior than those collected in the other ambient cycles at later times. It is thought that most 

strands experienced active corrosion during the initial ambient exposure as they were first in contact 

with fresh grout and then started to corrode. The initial corrosion process continued until the 

formation of protective oxide layers (passive film) on the strands was completed. Once the stable

passivity was established, the strands maintained a low corrosion state as discussed in figure 7. The 

third set of test results shows photographs that were taken throughout the study. Lastly, corrosion 

damage of the autopsied specimens is documented with pit depth measurements, photographs, and 

condition mapping sheets. 

From all of the results, a comparative analysis of the collected non-destructive data and the 

observed physical corrosion damage data was performed to determine chloride threshold values for 

PT tendons in chloride-contaminated grout.

TASK 2.1: ELECTROCHEMICAL TESTING OF SINGLE-WIRE SPECIMENS IN 

AQUEOUS TEST SOLUTIONS

Preliminary tests were conducted to become familiar with experimental setup and select the initial 

conditioning parameters for king wires prior to performing actual electrochemical testing. These 

parameters were maintained throughout the task 2.1 experiment. Physical aspects of the experiment 

setup such as installing test cells and applying target stress in the wires were finalized after several 

trials. The immersion time in the test solutions and solution agitation were two main issues. After 

reviewing the preliminary test results, initial conditioning of 24-h immersion time without agitating 

solution was selected for actual specimens. Figure 79 and figure 80 present corrosion potential data 

and corrosion rate data of king wires, respectively. 

55
Figure 79. Graph. Corrosion potentials of center wires in different test environments.

Figure 80. Graph. Corrosion rates of center wires in different test environments.

As expected, the wires immersed in the 13.6 pH solutions were protected by the passive film 

formed in the high pH environment. Their performance was characterized by relatively positive 

corrosion potentials, indicating an inactive thermodynamic state for corrosion and low corrosion 

rates indicating slow metal dissolution rate. In the high pH environment, the corrosion rate was 

virtually 0 mil/year up to an equivalent chloride concentration of 0.6 percent by weight of cement 

and then slightly increased at higher chloride concentrations. Corrosion products observed on the 

-700

-600

-500

-400

-300

-200

-100

0

0 0.04 0.08 0.2 0.4 0.6 0.8 1 2 5

Corrosion potential (mVAg/AgCl)

Chloride concentration (percent by weight of cement)

Stressed pH 13.6 Unstressed pH 13.6 Stressed pH 9 Unstressed pH 9 

56
tested wires and in the solutions were also negligible. The effect of stress was not apparent in a low 

chloride concentration up to 0.6 percent, but the stressed wires exposed at 0.8 and 1.0 percent 

chloride concentration showed slightly higher corrosion rates than the unstressed counterparts in the 

same test conditions. However, conflicting data from corrosion behavior at 2.0 and 5.0 percent 

chloride concentrations indicated that there was no trend. Additional tests are suggested to yield 

statistically significant results in the future.

For the specimens tested in the pH 9.0 solutions containing an equivalent chloride concentration 

of 0.04 percent by weight of cement and higher concentrations, corrosion potentials became

progressively more negative as chloride concentration increased. Conversely, the matching 

corrosion rates went up sharply compared to the electrochemical data obtained in the chloride-free 

9.0 pH solutions and all 13.6 pH solutions. The corrosion products deposited on the tested wires 

and bottom of the test cells were much more visible and abundant compared to the high pH 

counterparts. Also, color of the solutions became darker by dissolved corrosion products as 

corrosion rate increased. The observed reduction of corrosion rate at 5.0 percent chloride 

concentration is thought to be caused by a lack of dissolved oxygen in the low pH solutions due to 

excessive chloride ions. Again, there was no consistency observed regarding the effect of stress on 

corrosion rate regardless of chloride concentration.

The observed passive behavior of king wires in the high pH solutions signifies the importance of 

maintaining a high pH environment to prevent corrosion of PT strands. Formation of sound passive 

film is only possible on PT strands completely surrounded by high-quality grout that offers the high 

pH environment. The protective film will eventually become compromised when chloride ions 

exceed threshold values. The electrochemical data in figure 79 and figure 80 suggest that PT strands

can tolerate chloride contamination without significant corrosion up to 0.6 percent by weight of 

cement in carbonation-free (high pH) grout, whereas PT strands with as low as 0.04 percent 

chloride by weight of cement can initiate active corrosion in the carbonated (low pH) grout. This is 

below the AASHTO specified chloride limit of 0.08 percent by weight of cement. Any air getting 

through the grout voids and cracked duct/grout will likely expose PT strands to a lower pH 

environment through carbonation process. Once corrosion starts in the low pH environment, the 

level of corrosion damage will depend on chloride concentration.

ADMIXED VERSUS EXTRACTED CHLORIDE CONTENTS 

Figure 81 shows chloride analysis results of the 10-day trial mixes. Mean background acid-soluble 

chloride content in the grout powder was determined to be 0.009 percent (90 ppm) by weight of 

sample. It was small enough to consider the acquired grout chloride-free. The 0.009 percent by 

weight of sample is equivalent to 0.017 percent by weight of cement. The conversion was made 

based on known cement weight of 33.33 lb per 50-lb grout bag and 12.5 lb of mixing water 

added per bag. 

57
Figure 81. Graph. Relationship between admixed chloride contents and actual concentrations 

in the trial grout mixes extracted by ASTM water-soluble and AASHTO acid-soluble 

methods.

(15,16)

As shown in figure 81, mean extracted acid-soluble chloride concentrations were virtually identical 

to the admixed chloride concentrations in that a linear regression line runs nearly over the line of 

equality with almost perfect coefficient of determination (R2 = 0.9997). This means that nearly all 

admixed sodium chloride contents were extracted through the acid-soluble titration method. It is 

also true that the calculated sodium chloride weights listed in table 2 were accurate. For watersoluble chloride data, the extracted amounts were somewhat less compared to the acid-soluble 

results, and the difference increased as chloride concentration increased. Nonetheless, the watersoluble data also yielded R2 = 0.9996. After confirming adequacy of the mix proportions, actual 

grout mixes were made to cast tasks 2.2 and 2.3 specimens.

Figure 82 shows acid-soluble chloride analysis results, expressed in three different units, of 

grout powder samples taken from single-strand specimens and multi-strand specimens after the 

accelerated corrosion testing was terminated. It can be seen that the extracted acid-soluble chloride 

contents were virtually the same between single-strand specimens and multi-strand specimens. This 

finding confirms that the admixed sodium chloride was uniformly dispersed in the fresh grout, and 

both types of specimens were tested in the identical chloride concentrations. 

58
Figure 82. Graph. Mean acid-soluble chloride contents in grout powder samples calculated by 

three conversion methods.

It can be also seen in figure 82 that three different chloride concentration units yielded three 

different extracted concentrations for a given admixed chloride concentration (see table 8).

Table 8. Different chloride concentration units reported in this study.

Notation

Description of Acid-Soluble (Total) Chloride 

Concentration Units

[Cl-](cement + water) Percent by weight of cement plus weight of mixing water

[Cl-]cement Percent by weight of cement excluding weight of mixing water

[Cl-]sample Percent by weight of grout powder sample

Mean extracted acid-soluble chloride concentrations expressed by [Cl-

](cement + water) accounted for 

almost 100 percent of the admixed chloride concentrations. Those expressed by [Cl-

]cement and by 

[Cl-]sample resulted in approximately 75 and 50 percent of the admixed chloride concentrations, 

respectively. The following example exemplifies the 1.0 percent admixed chloride concentration 

case to demonstrate why different concentration units yield different concentration values.

As listed in table 2, a six-bag grout mix containing 1.0 percent chloride by weight of cement

requires 2.0 lb of chloride ions (60.6 percent of 3.3 lb sodium chloride) dissolved in 75 lb distilled 

water and six 50-lb grout bags. Theoretical acid-soluble chloride concentration in the hardened 

grout powder sample should be as follows:

Figure 83. Equation. Example of calculating acid-soluble chloride concentration in the 

hardened grout powder sample.

[Cl-]

acid-soluble = 2.0 lb

(300 lb + 75 lb + 3.3 lb)

= 0.005287 = 0.5287 percent = 5,287 ppm by weight of sample

59
Mean acid-soluble chloride concentration in the grout powder samples taken from 1.0 percent 

chloride multi-strand specimen was 0.5235 percent, or 5,235 ppm by weight of sample. This is 

close to the theoretical value and is also about 50 percent of the original chloride concentration by 

weight of cement.

There are two correction factors to convert the chloride concentration [Cl-]sample back to either

[Cl-](cement + water) or [Cl-]cement. If weight of mixing water is considered, use a correction factor of 

1.875 as follows (see figure 84):

• Weight of cement in six 50-lb grout bags = 2/3 × 300 lb = 200 lb.

• Weight percentage of cement in six 50-lb grout bag and mixing water = 200 lb/(300 lb + 

75 lb) = 53.33 percent by weight of cement plus water.

Figure 84. Equation. Example of converting chloride concentration by weight of grout sample 

to chloride concentration by weight of cement plus weight of mixing water.

This can be considered 100 percent of the original chloride concentration by weight of cement. If 

weight of mixing water is not considered, use a correction factor of 1.5 as follows (see figure 85):

• Weight of cement in six 50-lb grout bags = 2/3 × 300 lb = 200 lb.

• Weight percent of cement in six 50-lb grout bags = 200 lb/300 lb = 66.67 percent.

Figure 85. Equation. Example of converting chloride concentration by weight of grout sample 

to chloride concentration by weight of cement without weight of mixing water.

This can be treated approximately 75 percent of the original chloride concentration by weight 

of cement.

Even though any of three chloride concentration units may be used interchangeably for hardened 

grout powder samples, each unit is suitable for different circumstances. Specifically, a chloride 

concentration expressed by [Cl-

](cement + water) is the best if precise mix proportion including weight of 

mixing water is known like the present study. This is because it represents almost 100 percent of 

acid-soluble chloride concentration in the hardened grout. For the particular grout product used in 

this study, a correction factor of 1.875 should be used. If weight of cement is known and weight of 

mixing water is unknown, concentration expressed by [Cl-

]cement can be used; however, it accounts 

for approximately 75 percent of acid-soluble chloride concentration in the hardened grout. In the 

same grout case, use a correction factor of 1.5. If none is known, concentration expressed by

[Cl-]sample is the only choice. In practice, the last option may be the most frequently used and easy to 

[Cl-]

acid-soluble = 0.5235 percent ×

1

0.5333 = 

0.5235 percent × 1.875 = 0.9816 percent by weight of cement plus water

[Cl-]

acid-soluble= 0.5235 percent ×

1

0.6667

= 0.5235 percent × 1.5 =

0.7853 percent by weight of cement without water

60
use because no conversion is necessary. However, it is important to indicate that a chloride 

concentration expressed by this unit represents only 50 percent of true acid-soluble chloride 

concentration in the hardened grout. 

To illustrate differences in these concentration units, figure 86 plots fractions of mean acid-soluble 

chloride data presented in figure 82 to their respective admixed chloride concentrations. Fractions 

of 0.08 and 0.2 percent data exceeded those of the higher chloride concentrations because 

background chloride concentration was significant for the two lowest chloride concentrations. For 

the other commercially available pre-bagged grout products, similar conversion relationships can be 

developed provided that exact weight of cement in a bag is known.

Figure 86. Graph. Mean chloride extraction rates based on three conversion methods.

Figure 87 provides detailed chloride data analysis results of multi-strand specimens. It shows mean 

chloride data grouped by sampling location and depth. It can be seen in the figure that mean 

extracted chloride concentrations were identical within each sample group except for those at the 

grout/strand interface being slightly lower. The adherence of chloride ions to the steel surface might

be responsible for the lower chloride concentration at the interface. The same trend is typical for 

reinforcing steel bar/concrete interface as well. From this dataset, the admixed chloride ions were 

confirmed to be evenly distributed in each grout mix, and no biased test results were originated 

from uneven chloride distribution. The single-strand specimens had the same level of even chloride 

distribution at each target concentration. 

61
Figure 87. Graph. Mean extracted acid-soluble chloride concentrations at different locations 

of multi-strand specimens.

ENVIRONMENTAL DATA

Figure 88 shows temperature variations in the environmental chambers and interior of two multistrand specimens during the accelerated corrosion testing. 

Figure 88. Graph. Sequence of exposure cycles and temperature changes during accelerated 

testing.

Figure 89 shows variations of mean RH readings in the environmental chambers. It can be seen that 

target temperatures were fairly well maintained, whereas target RHs were somewhat difficult to 

0.0 

0.2 

0.4 

0.6 

0.8 

1.0 

1.2 

1.4 

1.6 

1.8 

2.0 

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Mean extracted acid-soluble chloride concentration 

(percent by weight of cement without water wt.)

Admixed chloride concentration (percent by weight of cement)

Grout/strand Interface Lower section (0.5 in. deep) Lower section (1.5 in. deep) Mid section (0.5 in. deep)

Mid section (1.5 in. deep) Upper section (0.5 in. deep) Upper section (1.5 in. deep) Bulk overall mean

0

20

40

60

80

100

120

140

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Temperature (F)

Time (day)

Mean temperature in large chamber Temperature by a portable thermometer in large chamber

Temperature inside #7 specimen Mean temperature in small chamber

Temperature by a portable thermmometer in small chamber Temperature inside #1 specimen

1st full cycle 2nd full cycle 3rd full cycle

62
control. Overall, environmental loading was thought to be adequate enough to expedite the 

corrosion process fairly well.

Figure 89. Graph. Recorded mean RH readings in the environmental chambers.

POTENTIAL DATA

PSE Versus ACE

The portable ACE and CSE are the most widely used and commercially available reference 

electrodes. They have finite potential differences at any given temperature with respect to the 

standard hydrogen electrode, a specially constructed reference electrode for all other reference 

electrodes. The potential readings taken with one type of reference electrode can be converted to 

those measured with other types as inherent potential difference between the two reference 

electrodes is known.

The PSE used in this study was a custom-made project-specific reference electrode used to collect 

potential data for the strand specimens. For the purpose of potential conversion to ACE, corrosion 

potentials of PSEs embedded in the calibration blocks containing eight chloride concentrations were 

measured weekly. These calibration blocks were subjected to the same temperature/RH cycles as 

the strand specimens in the environmental chambers. Figure 90 shows exemplary corrosion 

potential data of three PSEs versus ACE. The other PSE potential data fell within the ranges

exhibited by these PSEs. It can be seen that 0 percent chloride PSE tended to show more 

temperature sensitive behavior than the others, especially during the F & D cycles. Also, 

1.0 percent PSE exhibited nearly -220 mV versus ACE at an early measurement. The reason 

for the negative potential reading could not be identified.

30

40

50

60

70

80

90

100

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Relative humidity (percent)

Time (day)

RH in the large chamber RH in the small chamber

63
Figure 90. Graph. Corrosion potential versus time for three PSEs embedded in the calibration 

cylinders measured with respect to ACE.

Figure 91 shows PSE mean corrosion potentials for each chloride concentration calculated from 

individual corrosion potential data for 6 months in ambient, H & H, and F & D cycles. All exhibited 

relatively positive corrosion potentials during the F & D cycles. Since PSE potential can be

influenced by many factors, it would be difficult to come up with an accurate, universal conversion 

factor. For the sake of simplifying the data analysis, a mean value of -30 mV versus ACE was 

chosen for this study as a single conversion factor regardless of chloride concentration and exposure 

condition. The number was selected based on a dominant trend observed during ambient and H & H 

cycles in figure 91. Since the potential data collected in ambient and H & H cycles account for 

majority of the potential data and corrosion activity observed in F & D was insignificant, use of the 

conversion factor should be considered reasonable. The converted potential values were used to 

compare them to the existing corrosion potential criteria given in ASTM C876.

(18) With the -30 mV 

conversion factor, -100 mV versus PSE is equivalent to -130 mV versus ACE or -230 mV versus

CSE. Conversely, -350 mV (90 percent probability of corrosion per ASTM C876) and -200 mV 

(90 percent probability of no corrosion per ASTM C876) versus CSE are equivalent to -220 and 

-70 mV versus PSE, respectively.

-220

-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0

20

40

60

80

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Corrosion potential vs. Ag/AgCl RE (mV)

Time (day)

0 percent chloride 0.4 percent chloride 1.0 percent chloride

64
Figure 91. Graph. Mean corrosion potentials of PSEs versus ACE in three exposure 

conditions.

TASK 2.2: CORROSION POTENTIAL OF SINGLE-STRAND SPECIMENS

Figure 92 through figure 99 present corrosion potentials versus time plots for single-strand

specimens. The first measurements after receiving water and/or air holes are indicated in the 

relevant plots. Most specimens in the environmental chambers exhibited fluctuating potentials as 

they were subjected to temperature and RH changes. The majority of corrosion potential data stayed 

between -70 and -220 mV versus PSE or -200 and -350 mV versus CSE. This range corresponds to 

uncertain corrosion state according to ASTM C876 criteria.(18)

While a voided specimen containing 0.08 percent chloride showed little change upon water 

recharging, the other specimens containing 0.4, 0.6, 0.8, and 2.0 percent chloride concentrations 

experienced a potential drop in the negative direction as soon as water entered the voids. Since 

more negative corrosion potential means more thermodynamically susceptible to corrosion, the 

added water should have made these voided specimens more corrosion active. The most negative 

corrosion potential, -380 mV versus PSE, was exhibited by a stressed specimen containing 

2.0 percent chloride concentration (see figure 99), which is equivalent to approximately -410 mV 

versus ACE or -510 mV versus CSE. Such a negative potential is considered very active corrosion 

even for ordinary reinforced concrete. 

Most control specimens in room temperature remained in the relatively positive potential range 

throughout the study, indicating enhanced passive behavior regardless of chloride concentration. 

Introduction of air holes (no water added) to 0.08, 0.4, and 0.8 percent chloride specimens did not 

influence their corrosion potential trends. One exception was noted for 1.0 percent chloride voided 

control specimen. This specimen exhibited a sudden potential drop to nearly -200 mV versus PSE 

or -330 mV versus CSE at the 177th day as shown in figure 87, indicating that corrosion might have 

just initiated in the specimen containing the highest chloride concentration among the voided 

specimens. Comparing corrosion potential data of the accelerated corrosion testing specimens to 

-60

-40

-20

0

20

40

60

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion potential vs. Ag/AgCl RE (mV)

Chloride concentration (percent by weight of cement)

Ambient mean H & H mean F & D mean

65
those exposed in the room temperature reveals that alternating temperature and RH was effective to 

expedite strand corrosion in the former group.

Figure 92. Graph. Corrosion potential versus time for 0 percent chloride single-strand

specimens.

Figure 93. Graph. Corrosion potential versus time for 0.08 percent chloride single-strand

specimens.

-400

-350

-300

-250

-200

-150

-100

-50

0

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Corrosion potential (mV vs. Pseudo-RE)

Time (day)

Stressed - Full (0%) Stressed - Void (0%) Unstressed - Full (0%)

Unstressed - Void (0%) Control ambient - Full (0%) Control ambient - Void (0%)

66
Figure 94. Graph. Corrosion potential versus time for 0.2 percent chloride single-strand

specimens.

Figure 95. Graph. Corrosion potential versus time for 0.4 percent chloride single-strand

specimens.

-400

-350

-300

-250

-200

-150

-100

-50

0

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Corrosion potential (mV vs. Pseudo-RE)

Time (day)

Stressed - Full (0.2%) Stressed - Void (0.2%) Unstressed - Full (0.2%)

Unstressed - Void (0.2%) Control ambient - Full (0.2%) Control ambient - Void (0.2%)

67
Figure 96. Graph. Corrosion potential versus time for 0.6 percent chloride single-strand

specimens.

Figure 97. Graph. Corrosion potential versus time for 0.8 percent chloride single-strand

specimens.

68
Figure 98. Graph. Corrosion potential versus time for 1.0 percent chloride single-strand

specimens.

Figure 99. Graph. Corrosion potential versus time for 2.0 percent chloride single-strand

specimens.

-400

-350

-300

-250

-200

-150

-100

-50

0

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Corrosion potential (mV vs. Pseudo-RE)

Time (day)

Stressed - Full (1.0%) Stressed - Void (1.0%) Unstressed - Full (1.0%)

Unstressed - Void (1.0%) Control ambient - Full (1.0%) Control ambient - Void (1.0%)

69
Fully Grouted Specimens Versus Voided Specimens

Figure 100 through figure 103 summarize mean corrosion potentials calculated for the stressed and 

unstressed strands in fully grouted specimens per chloride concentration in initial ambient, ambient, 

H & H, and F & D cycles, respectively. Similar plots for the voided specimens are presented in 

figure 104 through figure 107. Because water was added to some of the voided specimens after 

initial ambient and F & D cycles were completed, mean corrosion potential data related to postwater recharging event are presented in figure 105 (ambient) and figure 106 (H & H) only. In 

addition, control specimen data were also included in relevant initial ambient and ambient plots.

In general, mean corrosion potentials of both specimen types were gradually more negative as 

chloride concentration increased. As a result, the 2.0 percent chloride specimens exhibited the most 

negative mean corrosion potentials in each exposure condition except for F & D condition. Adding 

0.17 fl oz of distilled water to the 0.08 percent chloride specimen did not change its mean corrosion 

potentials as shown in figure 105 and figure 106. Conversely, the voided specimens containing 

0.4 to 2.0 percent chloride resulted in more negative mean corrosion potentials upon water 

recharging relative to those measured before adding water. The risk of corrosion is elevated with 

increasing chloride concentration if water enters voids in the grout containing higher acid-soluble 

chloride concentration than 0.08 percent by weight of cement, which is the current chloride limit 

specified by many design codes and specifications.

(1)

The control specimens did not exhibit noticeable changes in corrosion potentials upon air hole 

introduction. Conversely, the specimens that received recharging water became active regardless 

of air holes. Therefore, it is concluded that presence of air holes alone did not impact corrosion 

potentials of the voided single-strand specimens.

The effect of stress on mean corrosion potential was not apparent for fully grouted specimens. 

For voided specimens after water recharging, stressed specimens containing 0.4 and 0.8 percent 

chloride experienced larger potential drops than a single unstressed specimen containing 0.6 percent 

chloride (see figure 105 and figure 106). The limited data suggest that stressed strands may be more 

susceptible to corrosion compared to the unstressed ones if the grout void is filled with water. 

70
Figure 100. Graph. Mean corrosion potential of fully grouted single-strand specimens in 

initial ambient condition.

Figure 101. Graph. Mean corrosion potential of fully grouted single-strand specimens in 

ambient condition.

-400

-350

-300

-250

-200

-150

-100

-50

0

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion potential (mV vs. Pseudo RE)

Chloride concentration (percent by weight of cement)

Stressed - Full Unstressed - Full Control (ambient) - Full

-400

-350

-300

-250

-200

-150

-100

-50

0

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion potential (mV vs. Pseudo RE)

Chloride concentration (percent by weight of cement)

Stressed - Full Unstressed - Full Control (ambient) - Full

71
Figure 102. Graph. Mean corrosion potential of fully grouted single-strand specimens in 

H & H condition.

Figure 103. Graph. Mean corrosion potential of fully grouted single-strand specimens in 

F & D condition.

-400

-350

-300

-250

-200

-150

-100

-50

0

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion potential (mV vs. Pseudo RE)

Chloride concentration (percent by weight of cement)

Stressed - Full Unstressed - Full

-400

-350

-300

-250

-200

-150

-100

-50

0

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion potential (mV vs. Pseudo RE)

Chloride concentration (percent by weight of cement)

Stressed - Full Unstressed - Full

72
Figure 104. Graph. Mean corrosion potential of voided single-strand specimens in initial 

ambient condition.

Figure 105. Graph. Mean corrosion potential of voided single-strand specimens in ambient 

condition.

-400

-350

-300

-250

-200

-150

-100

-50

0

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion potential (mV vs. Pseudo RE)

Chloride concentration (percent by weight of cement)

Stressed - Void Unstressed - Void Control (ambient)- Void

73
Figure 106. Graph. Mean corrosion potential of voided single-strand specimens in H & H 

condition.

Figure 107. Graph. Mean corrosion potential of voided single-strand specimens in F & D 

condition.

-400

-350

-300

-250

-200

-150

-100

-50

0

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion potential (mV vs. Pseudo RE)

Chloride concentration (percent by weight of cement)

Stressed - Void Unstressed - Void

74
TASK 2.3: CORROSION POTENTIALS AND POLARIZED POTENTIALS OF MULTISTRAND SPECIMENS

Figure 108 through figure 115 present corrosion potentials of stressed strands (macro-anode) and

unstressed strands (macro-cathode) versus time plots for multi-strand specimens. The plots also 

contain their polarized potentials. Polarization in electrochemistry is defined as a potential shift 

corresponding to a directional current flow. Anodic polarization occurs in the positive potential 

direction when current leaves the electrode into electrolyte, while cathodic polarization occurs in 

the negative potential direction if current enters the electrode from the electrolyte. Flow of electrons 

is opposite to the current flow by the known sign convention. The polarized potential is often 

called “mixed potential” because macro-anode is polarized from its corrosion potential in the 

positive direction, and macro-cathode is polarized from its corrosion potential in the negative 

direction until their polarized potentials intersect at a mid-point where steady state macro-cell 

current is maintained.

Corrosion potentials of macro-anode and macro-cathode of multi-strand specimens containing 

0 to 0.2 percent chloride remained above -150 mV versus PSE, thus their potential difference was 

small (roughly less than 100 mV). Their data are shown in figure 108 through figure 110. When 

chloride concentration increased to between 0.4 and 1.0 percent, the majority of stressed strand

corrosion potentials became more negative than -200 mV versus PSE. Since they were equivalent to 

more negative than -350 mV versus CSE, active corrosion of the stressed strands was evident. 

These data are shown in figure 111 through figure 114. The majority of unstressed strands corrosion 

potentials still remained above -150 mV versus PSE. As a result, potential difference became larger 

than 150 mV in many cases. A larger potential difference means a greater driving force for macrocell corrosion current to flow. Subsequently, when acid-soluble chloride concentration was higher 

than 0.4 percent by weight of cement, multi-strand specimens developed large driving voltage 

between macro-anode (stressed strands) and macro-cathode (unstressed strands) to facilitate active 

macro-cell corrosion. Multi-strand specimens were intended to create intensive macro-cell 

corrosion by design, and actual data supported that the specimen design worked to produce realistic 

macro-cell corrosion damage on the stressed strands.

When 5.1 fl oz of distilled water was added in the voids, most specimens (0.08, 0.2, 0.4, 0.6, 1.0, 

and 2.0 percent chloride) experienced negative potential shifts immediately. The others experienced 

either no change (0 percent chloride in figure 108) or positive potential shifts (0.8 percent chloride 

in figure 113) upon water recharging. All three kinds of potentials of the 0.8 percent chloride 

specimen remained in the positive range afterwards. This specimen’s duct cracked during the 

second F & D cycle, and emergency repair was made just before water recharging (see figure 50

through figure 55). In addition, 24 0.5-inch-diameter holes were introduced over the repaired duct 

along with the water recharging event to see whether more oxygen can diffuse to the strands (see 

figure 57 and figure 58). This is the only multi-strand specimen that received the air holes. As will 

be discussed later, the upward potential shifts might have been caused by increased oxygen levels at 

the strands.

Corrosion potential data of macro-cathode in 2.0 percent chloride specimen started to corrode 

approximately 1 month after accelerated corrosion testing began (see figure 115). Such an early 

corrosion of macro-cathode shifted its corrosion potential steadily toward the more negative 

direction approaching corrosion potential of macro-anode. Figure 115 shows that both corrosion 

75
potentials became similar from the 106th day onward. Consequently, the intended macro-cell 

corrosion setup of 2.0 percent chloride specimen became less effective with time in association 

with the shrinking driving voltage.

Figure 108. Graph. Potential versus time for 0 percent chloride multi-strand specimen.

Figure 109. Graph. Potential versus time for 0.08 percent chloride multi-strand specimen.

-400

-350

-300

-250

-200

-150

-100

-50

0

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Potential vs. Pseudo RE (mV)

Time (day)

Stressed corrosion potential Unstressed corrosion potential Polarized potential

Water added

-400

-350

-300

-250

-200

-150

-100

-50

0

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Potential vs. Pseudo RE (mV)

Time (day)

Stressed corrosion potential Unstressed corrosion potential Polarized potential

Water added

76
Figure 110. Graph. Potential versus time for 0.2 percent chloride multi-strand specimen.

Figure 111. Graph. Potential versus time for 0.4 percent chloride multi-strand specimen.

77
Figure 112. Graph. Potential versus time for 0.6 percent chloride multi-strand specimen.

Figure 113. Graph. Potential versus time for 0.8 percent chloride multi-strand specimen.

-400

-350

-300

-250

-200

-150

-100

-50

0

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Potential vs. Pseudo RE (mV)

Time (day)

Stressed corrosion potential Unstressed corrosion potential Polarized potential

Water added

78
Figure 114. Graph. Potential versus time for 1.0 percent chloride multi-strand specimen.

Figure 115. Graph. Potential versus time for 2.0 percent chloride multi-strand specimen.

Figure 116 through figure 118 present mean corrosion potentials per chloride concentration and 

exposure condition of stressed strands, unstressed strands, and mean polarized potentials, 

respectively. As discussed earlier, stressed strands (macro-anode) exhibited more negative mean 

corrosion potentials than unstressed ones (macro-cathode). Comparing all the plots in the figures, it 

79
appears that the stressed strands exhibited the most negative potentials at any chloride concentration, 

irrespective of exposure condition and water in the void. It is unclear if presence of stress or 

localized corrosion at the void/grout interface or both factors made the entire stressed strands active.

Although water recharging made the corrosion potentials of the stressed strands more negative, 

potential changes before and after water recharging were less than 50 mV in most chloride 

concentrations. Once chloride concentration increased to 0.4 percent, mean corrosion potentials of 

stressed strands fell into the 90 percent corrosion probability threshold, -220 mV versus PSE 

(-350 mV versus CSE), independent of exposure condition as shown in figure 116. The same 

situation occurred for unstressed strands only after chloride concentration reached 2.0 percent as 

shown in figure 117.

Figure 116. Graph. Mean corrosion potential of stressed strands in multi-strand specimens.

80
Figure 117. Graph. Mean corrosion potential of unstressed strands in multi-strand specimens.

Figure 118. Graph. Mean polarized potential of multi-strand specimens.

81
Figure 119 shows overall mean corrosion potentials of stressed and unstressed strands and overall 

mean polarized potentials as a function of chloride concentration. Overall mean corrosion potentials 

of stressed strands decreased more than 120 mV when chloride concentration increased to 

0.4 percent by weight of cement. Such a large decrease suggests that the stressed PT strands 

initiated active corrosion at 0.4 percent chloride concentration. At chloride concentrations between 

0.4 and 1.0 percent, overall mean polarized potential was closer to overall mean stressed corrosion 

potentials instead of being near the overall mean unstressed corrosion potentials. This behavior 

suggests that the macro-cell corrosion process in these multi-strand specimens was under cathodic 

control. Under this condition, oxygen availability must have controlled the most common cathodic 

reaction given in figure 11.

Figure 119. Graph. Overall mean potential of multi-strand specimens.

Figure 116 through figure 119 show data humps on 0.8 percent chloride specimen data. It was 

mentioned earlier that the PVC duct of 0.8 percent chloride specimen was cracked, and 24 artificial 

holes were introduced afterwards. These irregular potential jumps were only related to the data 

collected after the integrity of the PVC duct was compromised. It is speculated that the positive 

potential shifts were caused by more oxygen available at the strands that would move potentials in 

the positive direction by concentration polarization process. This phenomenon can be described 

with corrosion potential changes from case (1) to case (1') and from case (2) to case (2') in figure 20. 

This theory also suggests the opposite situation for the other multi-strand specimens, which were

encased by tightly sealed ducts that created less oxygen environment.

As will be discussed in the Characterization of Corrosion Morphologies section, it was confirmed 

that 0 percent chloride concentration specimen corroded due to excessive free sulfate ions at the 

void/grout interface. In normal conditions without the unintended corrosion, the specimen should 

have experienced no corrosion, and its potential values should have increased in the positive 

82
direction so that their data are placed in more positive positions in the plots than the 0.08 percent 

chloride potential data. 

If the duct cracking problem and excessive sulfate ions in 0 percent chloride concentration 

specimen as discussed above had not occurred, the data trend lines would have looked more like 

linear as a function of chloride concentration.

CORROSION RATE AND MACRO-CELL CORROSION CURRENT DENSITY DATA

Task 2.2: Corrosion Rate Data of Single-Strand Specimens

Figure 120 through figure 127 present corrosion rates versus time plots for single-strand specimens. 

The first measurements after receiving water and/or introducing air holes are indicated in the 

relevant plots. Total surface area of a PT strand in contact with grout was used to calculate the 

nominal corrosion rate. The surface area information is listed in table 6. Since the actual size of 

corroding areas by pitting corrosion and crevice corrosion was relatively small compared to the 

total surface area, the calculated corrosion rates were low in all chloride concentrations. The 

corrosion rate data obtained in this study can fall into the negligible range if the corrosion rate 

criteria listed in table 7 are strictly applied. Since there is little published information available 

regarding corrosion rate of PT strands measured by the same technique, it is difficult to interpret the 

experimental data as they were collected. For this reason, data analysis in this section takes into 

account actual conditions of the extracted strands during the autopsy.

A general trend observed in figure 120 through figure 127 is that specimens subjected to accelerated 

corrosion testing experienced higher corrosion rates during the H & H cycles and much lower 

corrosion rates during the F & D cycles. Intermediate corrosion rates were observed in initial 

ambient and ambient cycles. The observed data confirmed the corrosion rate’s dependency on 

temperature. However, the corrosion rates also gradually decreased with time. The 1.0 and 

2.0 percent chloride specimens depict this trend well because it is easy to see their higher corrosion 

rates compared to the lower concentration specimens. As observed in corrosion potential data 

of the voided single-strand specimens, corrosion rate increased when water was added in the 

voids. As chloride concentration increased, the magnitude of the corrosion rate increase became 

proportionally larger. The lowest chloride concentration to induce noticeable increase was 

0.4 percent (see figure 123). Increased corrosion rates decreased again in the subsequent 

measurements with time. Introduction of air holes did not change the corrosion rate.

All control specimens showed steadily decreasing corrosion rates with time as their corrosion 

potentials moved upward steadily in the more positive direction. Again, formation of passive film 

on the strand surface was thought to be responsible for the corrosion rate reduction under the mild 

room temperature environment in every chloride concentration.

83
Figure 120. Graph. Corrosion rate versus time for 0 percent chloride single-strand specimens.

Figure 121. Graph. Corrosion rate versus time for 0.08 percent chloride single-strand

specimens.

84
Figure 122. Graph. Corrosion rate versus time for 0.2 percent chloride single-strand

specimens.

Figure 123. Graph. Corrosion rate versus time for 0.4 percent chloride single-strand

specimens.

85
Figure 124. Graph. Corrosion rate versus time for 0.6 percent chloride single-strand

specimens.

Figure 125. Graph. Corrosion rate versus time for 0.8 percent chloride single-strand

specimens.

86
Figure 126. Graph. Corrosion rate versus time for 1.0 percent chloride single-strand

specimens.

Figure 127. Graph. Corrosion rate versus time for 2.0 percent chloride single-strand

specimens.

87
Fully Grouted Specimens Versus Voided Specimens

Figure 128 through figure 131 summarize mean corrosion rates calculated for the stressed and 

unstressed strands in fully grouted specimens per chloride concentration in initial ambient, ambient, 

H & H, and F & D cycles, respectively. The data for control specimens were also included in initial 

ambient and ambient plots. Similar plots for voided specimens are presented in figure 132 through 

figure 135. In each exposure condition, mean corrosion rates per chloride concentration were 

grouped by stress versus no stress. Because water was added to some of the voided specimens after 

initial ambient and F & D cycles were completed, mean corrosion rate data related to post-water 

recharging event are presented in figure 133 (ambient) and figure 134 (H & H) only. 

To maintain the same scale in each plot, maximum corrosion rate was set at 0.2 mil/year, close to 

the highest corrosion rate reported in figure 127. As a result, it is hard to see subtle differences 

among different sets of corrosion rate data. There were no clear trends observed by stress level and 

presence of void except that voided specimens exhibited increased mean corrosion rates upon water 

recharging, and this trend is more pronounced at higher chloride concentrations as shown in 

figure 133 and figure 134. The effect of drilling air holes on corrosion rate was not observed. 

Figure 128. Graph. Mean corrosion rate of fully grouted single-strand specimens in initial 

ambient condition.

0.000

0.050

0.100

0.150

0.200

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion rate (mpy)

Chloride concentration (percent by weight of cement)

Stressed - Full Unstressed - Full Control (ambient) - Full

88
Figure 129. Graph. Mean corrosion rate of fully grouted single-strand specimens in ambient 

condition.

Figure 130. Graph. Mean corrosion rate of fully grouted single-strand specimens in 

H & H condition.

0.000

0.050

0.100

0.150

0.200

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion rate (mpy)

Chloride concentration (percent by weight of cement)

Stressed - Full Unstressed - Full Control (ambient) - Full

0.000

0.050

0.100

0.150

0.200

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion rate (mpy)

Chloride concentration (percent by weight of cement)

Stressed - Full Unstressed - Full

89
Figure 131. Graph. Mean corrosion rate of fully grouted single-strand specimens in 

F & D condition.

Figure 132. Graph. Mean corrosion rate of voided single-strand specimens in initial ambient 

condition.

0.000

0.050

0.100

0.150

0.200

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion rate (mpy)

Chloride concentration (percent by weight of cement)

Stressed - Full Unstressed - Full

0.000

0.050

0.100

0.150

0.200

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion rate (mpy)

Chloride concentration (percent by weight of cement)

Stressed - Void Unstressed - Void Control (ambient) - Void

90
Figure 133. Graph. Mean corrosion rate of voided single-strand specimens in ambient 

condition.

Figure 134. Graph. Mean corrosion rate of voided single-strand specimens in 

H & H condition.

91
Figure 135. Graph. Mean corrosion rate of voided single-strand specimens in 

F & D condition.

Figure 136 through figure 141 present enlarged portions of mean corrosion rate plots related to 

initial ambient (see figure 128 and figure 132), ambient (see figure 129 and figure 133), and H & H 

(see figure 130 and figure 134) conditions. Since corrosion activity diminished in the F & D 

condition, their data were not included in figure 136 through figure 141. The plots in figure 136, 

figure 138, and figure 140 contain mean corrosion rate data of fully grouted specimens, and the 

plots in figure 137, figure 139, and figure 141 contain the same data for voided specimens. To 

capture subtle trends for undisturbed conditions, corrosion rate data after water recharging were 

removed from the original plots, and the maximum scale for the plots was set at 0.03 mil/year. 

Because the key point of the plots was to understand corrosion rate behaviors corresponding to 

chloride concentrations between 0 and 0.8 percent, losing some of the 2.0 percent chloride 

specimen data in the enlarged plots was accepted. Two more enlarged plots are presented in 

figure 142 and figure 143. The former is related to overall mean corrosion rates per chloride 

concentration, and the latter is related to mean corrosion rates grouped by stress level per chloride 

concentration. Careful examination of figure 136 through figure 143 leads to the following findings:

• No difference in mean corrosion rate was observed between fully grout specimens and 

voided specimens.

• There was no discernible difference between stressed specimens and unstressed specimens.

• Mean corrosion rates in ambient condition were slightly lower than those collected in initial 

ambient and H & H conditions.

0.000

0.050

0.100

0.150

0.200

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion rate (mpy)

Chloride concentration (percent by weight of cement)

Stressed - Void Unstressed - Void

92
• Mean corrosion rates of the accelerated corrosion testing specimens became almost the same 

as those of control specimens as low corrosion rates measured during the F & D cycles 

offset higher corrosion rates obtained during the H & H cycles.

• Mean corrosion rates of 0, 0.08, and 0.2 percent specimens were nearly the same. They were 

thought to be in sound passive state. At 0.4 percent chloride concentration, mean corrosion 

rates started to increase. 

• According to the autopsy results, the lowest chloride concentration to initiate pitting 

corrosion was also the 0.4 percent chloride. Therefore, there is good agreement between 

corrosion morphology and the lowest chloride concentration that began to increase 

corrosion rate beyond the passive state. 

Figure 136. Graph. Magnified corrosion rate for initial ambient condition of fully grouted 

specimens.

Figure 137. Graph. Magnified corrosion rate for initial ambient condition of voided 

specimens.

0.000

0.010

0.020

0.030

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion rate (mpy)

Chloride concentration (percent by weight of cement)

Stressed - Full Unstressed - Full Control (ambient) - Full

0.000

0.010

0.020

0.030

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion rate (mpy)

Chloride concentration (percent by weight of cement)

Stressed - Void Unstressed - Void Control (ambient) - Void

93
Figure 138. Graph. Magnified corrosion rate for ambient condition of fully grouted 

specimens.

Figure 139. Graph. Magnified corrosion rate for ambient condition of voided specimens.

Figure 140. Graph. Magnified corrosion rate for H & H condition of fully grouted specimens.

0.000

0.010

0.020

0.030

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion rate (mpy)

Chloride concentration (percent by weight of cement)

Stressed - Full Unstressed - Full Control (ambient) - Full

0.000

0.010

0.020

0.030

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion rate (mpy)

Chloride concentration (percent by weight of cement)

Stressed - Void Unstressed - Void Control (ambient) - Void

0.000

0.010

0.020

0.030

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion rate (mpy)

Chloride concentration (percent by weight of cement)

Stressed - Full Unstressed - Full

94
Figure 141. Graph. Magnified corrosion rate for H & H condition of voided specimens.

Figure 142. Graph. Overall mean corrosion rates of single-strand specimens.

Figure 143. Graph. Overall mean corrosion rates of single-strand specimens grouped by stress 

level.

0.000

0.010

0.020

0.030

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion rate (mpy)

Chloride concentration (percent by weight of cement)

Stressed - Void Unstressed - Void

0.000

0.010

0.020

0.030

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion rate (mpy)

Chloride concentration (percent by weight of cement)

Stressed - Full Stressed - Void Unstressed - Full

Unstressed - Void Control (ambient) - Full Control (ambient) - Void

0.000

0.010

0.020

0.030

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Corrosion rate (mpy)

Chloride concentration (percent by weight of cement)

Mean stressed Mean unstressed

95
Task 2.3: Corrosion Current Density and Driving Voltage Data of Multi-Strand Specimens

Figure 144 through figure 151 present macro-cell corrosion current density (imacro-cell) data and the 

corresponding driving voltage data versus time plots for multi-strand specimens. The driving 

voltage is the difference of corrosion potentials between macro-anode and macro-cathode. 

The magnitude of imacro-cell increases proportionally to that of driving voltage. Figure 144

through figure 146 show a small driving voltage (nearly zero macro-cell corrosion current 

density relationship (i.e., less than 50 mV and less than 0.01 μA/cm2)) for 0, 0.08, and 

0.2 percent chloride specimens. Figure 149 and figure 150 provide examples of a large driving 

voltage (substantial macro-cell corrosion current density relationship (i.e., over 100 mV and

0.10 μA/cm2)) for 0.8 and 1.0 percent specimens. The relationships are particularly pronounced 

during the H & H cycles. As discussed previously in conjunction with figure 115, the 2.0 percent 

chloride specimen data shown in figure 151 indicated that unstressed strands were already 

corroding in the highest chloride contaminated grout mix. This situation resulted in reduction 

of driving voltage between macro-anode and macro-cathode. Such a reduced driving voltage

attributed to the gradual reduction of imacro-cell to nearly zero and even a small current reversal at 

the end of testing. 

Figure 144. Graph. imacro-cell and driving voltage versus time for 0 percent chloride multistrand specimen.

0.00

0.03

0.06

0.09

0.12

0.15

0.18

0.21

0.24

-400

-350

-300

-250

-200

-150

-100

-50

0

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Macro-cell current density (μA/cm2)

Driving voltage (mV)

Time (day)

Driving voltage Macro-cellcurrent density

96
Figure 145. Graph. imacro-cell and driving voltage versus time for 0.08 percent chloride multistrand specimen.

Figure 146. Graph. imacro-cell and driving voltage versus time for 0.2 percent chloride multistrand specimen.

0.00

0.03

0.06

0.09

0.12

0.15

0.18

0.21

0.24

-400

-350

-300

-250

-200

-150

-100

-50

0

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Macro-cell current density (μA/cm2)

Driving voltage (mV)

Time (day)

Driving voltage Macro-cell current density

0.00

0.03

0.06

0.09

0.12

0.15

0.18

0.21

0.24

-400

-350

-300

-250

-200

-150

-100

-50

0

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Macro-cell current density (μA/cm2)

Driving voltage (mV)

Time (day)

Driving voltage Macro-cell current density

97
Figure 147. Graph. imacro-cell and driving voltage versus time for 0.4 percent chloride multistrand specimen.

Figure 148. Graph. imacro-cell and driving voltage versus time for 0.6 percent chloride multistrand specimen.

0.00

0.03

0.06

0.09

0.12

0.15

0.18

0.21

0.24

-400

-350

-300

-250

-200

-150

-100

-50

0

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Macro-cell current density (μA/cm2)

Driving voltage (mV)

Time (day)

Driving voltage Macro-cell current density

0.00

0.03

0.06

0.09

0.12

0.15

0.18

0.21

0.24

-400

-350

-300

-250

-200

-150

-100

-50

0

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Macro-cell current density (μA/cm2)

Driving voltage (mV)

Time (day)

Driving voltage Macro-cell current density

98
Figure 149. Graph. imacro-cell and driving voltage versus time for 0.8 percent chloride multistrand specimen.

Figure 150. Graph. imacro-cell and driving voltage versus time for 1.0 percent chloride multistrand specimen.

0.00

0.03

0.06

0.09

0.12

0.15

0.18

0.21

0.24

-400

-350

-300

-250

-200

-150

-100

-50

0

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Macro-cell current density (μA/cm2)

Driving voltage (mV)

Time (day)

Driving voltage Macro-cell current density

0.00

0.03

0.06

0.09

0.12

0.15

0.18

0.21

0.24

-400

-350

-300

-250

-200

-150

-100

-50

0

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Macro-cell current density (μA/cm2)

Driving voltage (mV)

Time (day)

Driving voltage Macro-cell current density

99
Figure 151. Graph. imacro-cell and driving voltage versus time for 2.0 percent chloride multistrand specimen.

Figure 152 presents mean imacro-cell data grouped by exposure condition for each chloride 

concentration. It is clear that imacro-cell started to increase substantially once chloride concentration 

exceeded 0.4 percent. The H & H cycles produced the highest mean imacro-cell at all chloride 

concentrations in different environmental conditions: mean imacro-cell increased almost linearly as a 

function of chloride concentration up to 1.0 percent. In contrast, the lowest mean imacro-cell data were 

obtained during the F & D cycles independent of chloride concentration. Mean imacro-cell was also 

somewhat insensitive to chloride concentration under ambient exposure condition. Water 

recharging did not increase imacro-cell under the H & H and ambient cycles. Figure 153 summarizes 

overall mean imacro-cell data as a function of chloride concentration. It can be seen again that 

recognizable corrosion accompanied by imacro-cell over 0.010 μA/cm2 began at 0.4 percent chloride. 

Figure 154 shows the compiled imacro-cell changes as a function of time and temperature. Actual 

temperature recording data were overlapped in the figure to show that imacro-cell increased during the 

H & H cycles and decreased during the F & D cycles. The specimens containing more than 

0.4 percent chloride concentration exhibited much higher imacro-cell than those containing lesser 

chloride. However, the differences gradually decreased with time. As discussed before, the 

specimen containing 2.0 percent chloride exhibited less imacro-cell than ones with 0.8 and 1.0 percent 

chloride due to the corroding macro-cathode. Figure 155 shows an enlarged section of the imacro-cell

plot in figure 154, which shows the imacro-cell data before and after charging 5.1 fl oz of distilled 

water in the voids. Even though the 0.8 percent chloride specimen responded with increased

imacro-cell upon water entry, the effect of water recharging was not considered significant compared to 

the data shown in figure 56. Some possible reasons for not being able to observe similar current 

surges in the present study include the following:

0.00

0.03

0.06

0.09

0.12

0.15

0.18

0.21

0.24

-400

-350

-300

-250

-200

-150

-100

-50

0

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Macro-cell current density (μA/cm2)

Driving voltage (mV)

Time (day)

Driving voltage Macro-cell current density

100
• The present study did not use enough water.

• Distilled water was not a strong oxidizer compared to salt water.

• Oxygen was limited in the grout to support cathodic reaction. 

It is recommended to conduct follow-up tests by varying frequency of water recharging and type 

and volume of water used in the voids.

Figure 152. Graph. Mean imacro-cell data of multi-strand specimens by exposure condition.

101
Figure 153. Graph. Overall mean imacro-cell data of multi-strand specimens.

Figure 154. Graph. imacro-cell behavior responding to temperature variation.

0.005

0.001 0.002

0.016

0.033

0.052

0.059

0.040

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Macro-cell corrosion current density (μA/cm2)

Chloride concentration (percent by weight of cement)

102
Figure 155. Graph. imacro-cell changes after water recharging.

Figure 156 through figure 163 show stressed strands in the voids. The photographs were taken 

1 week after recharging water was added to the voids. It was observed even during early days of 

testing that three specimens containing 0 percent chloride (see figure 156), 0.4 percent chloride 

(see figure 159), and 2.0 percent chloride (see figure 163) showed black deposit on the void/grout

interface where stressed strands passed through. The unknown deposit material is indicated with

arrows in the figures. The specimens covered with the deposit layer also appeared to hold water 

longer than specimens without it. This mysterious layer turned out to be important, as the autopsy 

revealed that some strands in contact with the deposit layer suffered from severe corrosion. 

Three wires of 0.4 percent chloride specimen and one wire of 2.0 percent chloride specimen were 

corroded in two during the 6-month accelerated corrosion testing. The 0 percent chloride specimen 

had severely corroded strands at the interface during the same period. The other specimens showed 

only minor corrosion on the stressed strands near the interface. The encountered excessive

corrosion problem was never expected. Consequently, data analysis became complicated as to 

what to include and what to exclude among the weekly collected electrochemical data.

103
Figure 156. Photo. Void condition of 0 percent chloride specimen.

Figure 157. Photo. Void condition of 0.08 percent chloride specimen.

104
Figure 158. Photo. Void condition of 0.2 percent chloride specimen.

Figure 159. Photo. Void condition of 0.4 percent chloride specimen.

105
Figure 160. Photo. Void condition of 0.6 percent chloride specimen.

Figure 161. Photo. Void condition of 0.8 percent chloride specimen.

106
Figure 162. Photo. Void condition of 1.0 percent chloride specimen.

Figure 163. Photo. Void condition of 2.0 percent chloride specimen.

107
APPARENT GROUT RESISTIVITY AND GROUT RESISTANCE DATA 

Task 2.2: Apparent Grout Resistivity of Single-Strand Specimens

Figure 164 through figure 171 present apparent grout resistivity versus time plots for single-strand

specimens. Apparent grout resistivity increased when temperature decreased, and it decreased when 

temperature increased. Such a dependency of grout resistivity on temperature is also responsible for 

increased corrosion rate during the H & H cycles and suppressed corrosion rate during the F & D 

cycles. The control specimens maintained more or less their initial resistivity values at the

ambient condition.

Figure 172 through figure 175 summarize mean apparent grout resistivity of fully grouted 

specimens per chloride concentration in initial ambient, ambient, H & H, and F & D conditions, 

respectively. Similarly, figure 176 through figure 179 summarize mean apparent grout resistivity of 

voided specimens per chloride concentration in initial ambient, ambient, H & H, and F & D 

conditions, respectively. Mean apparent grout resistivity data showed the same trend—the highest 

resistivity during the F & D cycles and the lowest during the H & H cycles. No definite relationship 

was found between apparent grout resistivity and chloride concentration. Grout void and stress did 

not appear to influence the apparent grout resistivity, either. 

Figure 164. Graph. Apparent grout resistivity versus time for 0 percent chloride single-strand

specimens.

0

200

400

600

800

1000

1200

1400

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Apparent grout resistivity (ohm-cm)

Time (day)

Stressed - Full (0%) Stressed - Void (0%) Unstressed - Full (0%)

Unstressed - Void (0%) Control ambient - Full (0%) Control ambient - Void (0%)

108
Figure 165. Graph. Apparent grout resistivity versus time for 0.08 percent chloride singlestrand specimens.

Figure 166. Graph. Apparent grout resistivity versus time for 0.2 percent chloride singlestrand specimens.

0

200

400

600

800

1000

1200

1400

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Apparent grout resistivity (ohm-cm)

Time (day)

Stressed - Full (0.2%) Stressed - Void (0.2%) Unstressed - Full (0.2%)

Unstressed - Void (0.2%) Control ambient - Full (0.2%) Control ambient - Void (0.2%)

109
Figure 167. Graph. Apparent grout resistivity versus time for 0.4 percent chloride singlestrand specimens.

Figure 168. Graph. Apparent grout resistivity versus time for 0.6 percent chloride singlestrand specimens.

110
Figure 169. Graph. Apparent grout resistivity versus time for 0.8 percent chloride singlestrand specimens.

Figure 170. Graph. Apparent grout resistivity versus time for 1.0 percent chloride singlestrand specimens.

0

200

400

600

800

1000

1200

1400

8 22 36 50 64 78 92 106 120 134 148 162 176 190

Apparent grout resistivity (ohm-cm)

Time (day)

Stressed - Full (1.0%) Stressed - Void (1.0%) Unstressed - Full (1.0%)

Unstressed - Void (1.0%) Control ambient - Full (1.0%) Control ambient - Void (1.0%)

111
Figure 171. Graph. Apparent grout resistivity versus time for 2.0 percent chloride singlestrand specimens.

Figure 172. Graph. Mean apparent grout resistivity of fully grouted single-strand specimens 

in initial ambient condition.

0

200

400

600

800

1000

1200

1400

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Apparent grout resistivity (ohm-cm)

Chloride concentration (percent by weight of cement)

Stressed - Full Unstressed - Full Control (ambient) - Full

112
Figure 173. Graph. Mean apparent grout resistivity of fully grouted single-strand specimens 

in ambient condition.

Figure 174. Graph. Mean apparent grout resistivity of fully grouted single-strand specimens 

in H & H condition.

0

200

400

600

800

1000

1200

1400

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Apparent grout resistivity (ohm-cm)

Chloride concentration (percent by weight of cement)

Stressed - Full Unstressed - Full Control (ambient) - Full

0

200

400

600

800

1000

1200

1400

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Apparent grout resistivity (ohm-cm)

Chloride concentration (percent by weight of cement)

Stressed - Full Unstressed - Full

113
Figure 175. Graph. Mean apparent grout resistivity of fully grouted single-strand specimens 

in F & D condition.

Figure 176. Graph. Mean apparent grout resistivity of voided single-strand specimens in 

initial ambient condition.

0

200

400

600

800

1000

1200

1400

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Apparent grout resistivity (ohm-cm)

Chloride concentration (percent by weight of cement)

Stressed - Full Unstressed - Full

0

200

400

600

800

1000

1200

1400

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Apparent grout resistivity (ohm-cm)

Chloride concentration (percent by weight of cement)

Stressed - Void Unstressed - Void Control (ambient) - Void

114
Figure 177. Graph. Mean apparent grout resistivity of voided single-strand specimens in 

ambient condition.

Figure 178. Graph. Mean apparent grout resistivity of voided single-strand specimens in 

H & H condition.

0

200

400

600

800

1000

1200

1400

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Apparent grout resistivity (ohm-cm)

Chloride concentration (percent by weight of cement)

Stressed - Void Stressed - Void after water charging Unstressed - Void

Unstressed - Void after water charging Control (ambient) - Void

0

200

400

600

800

1000

1200

1400

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Apparent grout resistivity (ohm-cm)

Chloride concentration (percent by weight of cement)

Stressed - Void Stressed - Void after water charging

Unstressed - Void Unstressed - Void after water charging

115
Figure 179. Graph. Mean apparent grout resistivity of voided single-strand specimens in 

F & D condition.

Task 2.3: Grout Resistance of Multi-Strand Specimens

Figure 180 and figure 181 summarize mean grout resistances of multi-strand specimens in different 

exposure conditions measured with upper pair (P1 and P2) and lower pair (P3 and P4) of resistance 

probes, respectively. Figure 182 shows overall mean grout resistances. As discussed in the apparent 

grout resistivity section, the F & D cycles yielded the highest grout resistance, and the H & H 

cycles yielded the lowest. The grout resistance’s dependency on temperature should be partially 

responsible for increased macro-cell corrosion current density during the H & H cycles and 

suppressed macro-cell corrosion current density during the F & D cycles. The lower grout section 

associated with the P3 and P4 probes had much lower resistance than the upper section with P1 and 

P2 probes.

0

200

400

600

800

1000

1200

1400

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Apparent grout resistivity (ohm-cm)

Chloride concentration (percent by weight of cement)

Stressed - Void Unstressed - Void

116
Figure 180. Graph. Mean grout resistances between P1 and P2 of multi-strand specimens.

Figure 181. Graph. Mean grout resistances between P3 and P4 of multi-strand specimens.

1

10

100

1000

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Grout resistance (kohm)

Chloride concentration (percent by weight of cement)

Initial Ambient Ambient Ambient after water charging H & H H & H after water charging F & D

0

5

10

15

20

25

30

35

40

45

50

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Grout resistance between P3 - P4 (kohm)

Chloride concentration (percent by weight of cement)

Initial Ambient Ambient Ambient after water charging H & H H & H after water charging F & D

117
Figure 182. Graph. Overall mean grout resistances of multi-strand specimens.

CHARACTERIZATION OF CORROSION MORPHOLOGIES

Task 2.2: Single-Strand Specimens

Table 9 summarizes the number of rust spots and the number of pits counted on single-strand

specimens. Physical condition of every extracted strand was documented schematically using a 

condition mapping sheet. Figure 183 through figure 190 show the mapped conditions of singlestrand specimens. The red marks on the mapping sheets indicate where rust spots were observed. 

Length of the marks indicates relative surface area showing sign of corrosion. Figure 191 through 

figure 198 show as-extracted conditions of voided single-strand specimens. The first rust spot 

appeared on a stressed 0.4 percent chloride specimen with void (see figure 194). As chloride 

concentration increased, the number of rust spots increased. Although the 0.4 percent chloride 

specimen received 0.17 fl oz of distilled water in the void, the added water might not play a role to 

initiate corrosion. This is because other voided specimens containing a lower chloride concentration 

(0.08 percent) and a higher chloride concentration (0.6 percent) did not initiate corrosion even 

though they also received the same volume of distilled water. The fully grouted stressed specimens 

containing 0.8 and 2.0 percent chloride developed two and five rust spots, respectively. The voided

stressed specimens containing the same chloride concentrations also developed the same number 

of rust spots despite having water in the voids. Measurable pits (≥ 2 mil) started to appear at 

0.8 percent chloride concentration. More pits were observed on the strands exposed to higher 

chloride concentrations and voids. Markedly higher number of pits found in the stressed 2.0 percent 

chloride specimen with void suggests that a combination of a void, recharged water, and elevated 

chloride concentration can be detrimental. 

1

10

100

1000

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Grout resistance (kohm)

Chloride concentration (percent by weight of cement)

P1 - P2 P3 - P4

118
Table 9. Summary of corrosion condition observed on single-strand specimens.

Chloride 

Concentration 

(Percent by 

Cement 

Weight)

Number of Rust Spots* Number of Pits

StressedFullStressedVoidUnstressedFullUnstressedVoidStressedFullStressedVoidUnstressedFullUnstressedVoid

0.00 0 0 0 0 0 0 0 0

0.08 0 0 0 0

0.20 0 0 0 0 0 0 0 0

0.40 0 1 0 0

0.60 1 2 1 0 0 0 0 0

0.80 2 2 0 0

1.00 5 4 4 5 0 34 0 26

2.00 5 5 35 102

*Size of individual rust spots varied.

Note: Bolded text indicates voids that were recharged with 0.17 fl oz of distilled water at day 127. Blank cells indicate no data were 

relevant to the cell.

119
Figure 183. Illustration. Condition mapping of 0 percent chloride single-strand specimens.

Figure 184. Illustration. Condition mapping of 0.08 percent chloride single-strand specimens.

F – S 

F – US 

V – S 

V – US 

0 4 8 12

F: full grout V: voided grout S: stressed US: unstressed

F: full grout V: voided grout S: stressed US: unstressed

F – S 

V – S 

0 4 8 12

12

0
Figure 185. Illustration. Condition mapping of 0.2 percent chloride single-strand specimens.

Figure 186. Illustration. Condition mapping of 0.4 percent chloride single-strand specimens.

F – S 

F – US 

V – S 

V – US 

0 4 8 12

F: full grout V: voided grout S: stressed US: unstressed

F – S 

V – S 

0 4 8 12

F: full grout V: voided grout S: stressed US: unstressed : rust spot

121
Figure 187. Illustration. Condition mapping of 0.6 percent chloride single-strand specimens.

Figure 188. Illustration. Condition mapping of 0.8 percent chloride single-strand specimens.

F – S 

F – US 

V – S 

V – US 

0 4 8 12

F: full grout V: voided grout S: stressed US: unstressed : rust spot

F – S 

V – S 

0 4 8 12

F: full grout V: voided grout S: stressed US: unstressed : rust spot

122
Figure 189. Illustration. Condition mapping of 1.0 percent chloride single-strand specimens.

Figure 190. Illustration. Condition mapping of 2.0 percent chloride single-strand specimens.

F – S 

F – US 

V – S 

V – US 

0 4 8 12

F: full grout V: voided grout S: stressed US: unstressed : rust spot

F – S 

V – S 

0 4 8 12

F: full grout V: voided grout S: stressed US: unstressed : rust spot

12

3
Figure 191. Photo. Stressed single-strand specimen with void and 0 percent chloride.

Figure 192. Photo. Stressed single-strand specimen with void and 0.08 percent chloride.

Figure 193. Photo. Stressed single-strand specimen with void and 0.2 percent chloride.

Figure 194. Photo. Stressed single-strand specimen with void and 0.4 percent chloride.

124
Figure 195. Photo. Stressed single-strand specimen with void and 0.6 percent chloride.

Figure 196. Photo. Stressed single-strand specimen with void and 0.8 percent chloride.

Figure 197. Photo. Stressed single-strand specimen with void and 1.0 percent chloride.

Figure 198. Photo. Stressed single-strand specimen with void and 2.0 percent chloride.

Task 2.3: Multi-Strand Specimens

After grout caps were removed from the lower anchor plates, the anchor plates and electrical 

connections inside the caps were inspected for any signs of corrosion that might have affected 

corrosion measurements. Figure 199 and figure 200 show exterior and interior conditions of a 

125
lower anchor plate. Other than yellowish spray-on corrosion inhibitor film and residue of 

expansive foam that was used as filler, everything on the exterior face appeared clean without 

corrosion. Similarly, no corrosion was found on the interior face of the anchor plate. The other 

anchor plates showed similar conditions.

Figure 199. Photo. Exposed exterior side of lower anchor plate.

Figure 200. Photo. Exposed interior side of lower anchor plate.

Figure 201 through figure 208 show condition mapping results of multi-strand specimens in a

12-inch grid system. Individual marks indicate where rust spots were observed. They do not 

represent the extent of corrosion. Rather, they show locations of a rust spot or a group of rust 

spots. On the stressed strands, rust spots started to appear at 0 percent chloride concentration and 

increased with chloride concentration. On the unstressed strands, rust spots first appeared at 0.4 

percent chloride, and more appeared proportionally at higher chloride concentrations. In theory, 

the unstressed strands served as a macro-cathode; therefore, they were not supposed to corrode. 

One possibility is that polarized potentials of unstressed strands might place the unstressed 

strands outside the corrosion immune zone of a Pourbaix diagram if sufficient amount of 

chloride ions exists, in this case, 0.4 percent by weight of cement. The Pourbaix diagram 

classifies various thermodynamic states for corrosion as a function of potential and pH.

126
Figure 201. Illustration. Condition mapping of 0 percent chloride multi-strand specimen.

4 16 28 40 52 64 76 88 100 112

TR

TL

BR

BL

0-TR-16 0-TR-28 0-TR-40 0-TR-52 0-TR-64 0-TR-76 0-TR-88 0-TR-100 0-TR-112

0-TL-16 0-TL-28 0-TL-40 0-TL-52 0-TL-64 0-TL-76 0-TL-88 0-TL-100 0-TL-112

0-BR-16 0-BR-28 0-BR-40 0-BR-52 0-BR-64 0-BR-76 0-BR-88 0-BR-100 0-BR-112

0-BL-16 0-BL-28 0-BL-40 0-BL-52 0-BL-64 0-BL-76 0-BL-88 0-BL-100 0-BL-112

C1

C2

C3

C4

Bottom strands/grout C5

interface line 

Top strands/grout

interface line 

[Stressed strands: macro-anode]

[Unstressed strands: macro-cathode]

12

7
Figure 202. Illustration. Condition mapping of 0.08 percent chloride multi-strand specimen.

4 16 28 40 52 64 76 88 100 112

TR

TL

BR

BL

0.08-TR-40 0.08-TR-52 0.08-TR-64 0.08-TR-76 0.08-TR-88 0.08-TR-100 0.08-TR-112

0.08-TL-40 0.08-TL-52 0.08-TL-64 0.08-TL-76 0.08-TL-88 0.08-TL-100 0.08-TL-112

0.08-BR-40 0.08-BR-52 0.08-BR-64 0.08-BR-76 0.08-BR-88 0.08-BR-100 0.08-BR-112

0.08-BL-40 0.08-BL-52 0.08-BL-64 0.08-BL-76 0.08-BL-88 0.08-BL-100 0.08-BL-112

C1

C2

C3

C4

C5

0.08-BL-28

0.08-TR-16

0.08-TL-16 0.08-TL-28

0.08-BR-16 0.08-BR-28

0.08-BL-16

0.08-TR-28

Bottom strands/grout

interface line 

Top strands/grout

interface line 

[Stressed strands: macro-anode]

[Unstressed strands: macro-cathode]

12

8
Figure 203. Illustration. Condition mapping of 0.2 percent chloride multi-strand specimen.

4 16 28 40 52 64 76 88 100 112

TR

TL

BR

BL

0.2-TR-16 0.2-TR-28 0.2-TR-40 0.2-TR-52 0.2-TR-64 0.2-TR-76 0.2-TR-88 0.2-TR-100 0.2-TR-112

0.2-TL-16 0.2-TL-28 0.2-TL-40 0.2-TL-52 0.2-TL-64 0.2-TL-76 0.2-TL-88 0.2-TL-100 0.2-TL-112

0.2-BR-16 0.2-BR-28 0.2-BR-40 0.2-BR-52 0.2-BR-64 0.2-BR-76 0.2-BR-88 0.2-BR-100 0.2-BR-112

0.2-BL-16 0.2-BL-28 0.2-BL-40 0.2-BL-52 0.2-BL-64 0.2-BL-76 0.2-BL-88 0.2-BL-100 0.2-BL-112

C1

C2

C3

C4

Bottom strands/grout C5

interface line 

Top strands/grout

interface line 

[Stressed strands: macro-anode]

[Unstressed strands: macro-cathode]

129
Figure 204. Illustration. Condition mapping of 0.4 percent chloride multi-strand specimen.

4 16 28 40 52 64 76 88 100 112

TR

TL

BR

BL

0.4-TR-16 0.4-TR-28 0.4-TR-40 0.4-TR-52 0.4-TR-64 0.4-TR-76 0.4-TR-88 0.4-TR-100 0.4-TR-112

0.4-TL-16 0.4-TL-28 0.4-TL-40 0.4-TL-52 0.4-TL-64 0.4-TL-76 0.4-TL-88 0.4-TL-100 0.4-TL-112

0.4-BR-16 0.4-BR-28 0.4-BR-40 0.4-BR-52 0.4-BR-64 0.4-BR-76 0.4-BR-88 0.4-BR-100 0.4-BR-112

0.4-BL-16 0.4-BL-28 0.4-BL-40 0.4-BL-52 0.4-BL-64 0.4-BL-76 0.4-BL-88 0.4-BL-100 0.4-BL-112

C1

C2

C3

C4

Bottom strands/grout C5

interface line 

Top strands/grout

interface line 

[Stressed strands: macro-anode]

[Unstressed strands: macro-cathode] 3 wires

corroded 

in two

130
Figure 205. Illustration. Condition mapping of 0.6 percent chloride multi-strand specimen.

4 16 28 40 52 64 76 88 100 112

TR

TL

BR

BL

0.6-TR-16 0.6-TR-28 0.6-TR-40 0.6-TR-52 0.6-TR-64 0.6-TR-76 0.6-TR-88 0.6-TR-100 0.6-TR-112

0.6-TL-16 0.6-TL-28 0.6-TL-40 0.6-TL-52 0.6-TL-64 0.6-TL-76 0.6-TL-88 0.6-TL-100 0.6-TL-112

0.6-BR-16 0.6-BR-28 0.6-BR-40 0.6-BR-52 0.6-BR-64 0.6-BR-76 0.6-BR-88 0.6-BR-100 0.6-BR-112

0.6-BL-16 0.6-BL-28 0.6-BL-40 0.6-BL-52 0.6-BL-64 0.6-BL-76 0.6-BL-88 0.6-BL-100 0.6-BL-112

C1

C2

C3

C4

Bottom strands/grout C5

interface line 

Top strands/grout

interface line 

[Stressed strands: macro-anode]

[Unstressed strands: macro-cathode]

131
Figure 206. Illustration. Condition mapping of 0.8 percent chloride multi-strand specimen.

4 16 28 40 52 64 76 88 100 112

TR

TL

BR

BL

0.8-TR-16 0.8-TR-28 0.8-TR-40 0.8-TR-52 0.8-TR-64 0.8-TR-76 0.8-TR-88 0.8-TR-100 0.8-TR-112

0.8-TL-16 0.8-TL-28 0.8-TL-40 0.8-TL-52 0.8-TL-64 0.8-TL-76 0.8-TL-88 0.8-TL-100 0.8-TL-112

0.8-BR-16 0.8-BR-28 0.8-BR-40 0.8-BR-52 0.8-BR-64 0.8-BR-76 0.8-BR-88 0.8-BR-100 0.8-BR-112

0.8-BL-16 0.8-BL-28 0.8-BL-40 0.8-BL-52 0.8-BL-64 0.8-BL-76 0.8-BL-88 0.8-BL-100 0.8-BL-112

C1

C2

C3

C4

Bottom strands/grout C5

interface line 

Top strands/grout

interface line 

[Stressed strands: macro-anode]

[Unstressed strands: macro-cathode]

132
Figure 207. Illustration. Condition mapping of 1.0 percent chloride multi-strand specimen.

4 16 28 40 52 64 76 88 100 112

TR

TL

BR

BL

1.0-TR-16 1.0-TR-28 1.0-TR-40 1.0-TR-52 1.0-TR-64 1.0-TR-76 1.0-TR-88 1.0-TR-100 1.0-TR-112

1.0-TL-16 1.0-TL-28 1.0-TL-40 1.0-TL-52 1.0-TL-64 1.0-TL-76 1.0-TL-88 1.0-TL-100 1.0-TL-112

1.0-BR-16 1.0-BR-28 1.0-BR-40 1.0-BR-52 1.0-BR-64 1.0-BR-76 1.0-BR-88 1.0-BR-100 1.0-BR-112

1.0-BL-16 1.0-BL-28 1.0-BL-40 1.0-BL-52 1.0-BL-64 1.0-BL-76 1.0-BL-88 1.0-BL-100 1.0-BL-112

C1

C2

C3

C4

Bottom strands/grout C5

interface line 

Top strands/grout

interface line 

[Stressed strands: macro-anode]

[Unstressed strands: macro-cathode]

133
Figure 208. Illustration. Condition mapping of 2.0 percent chloride multi-strand specimen.

4 16 28 40 52 64 76 88 100 112

TR

TL

BR

BL

2.0-TR-16 2.0-TR-28 2.0-TR-40 2.0-TR-52 2.0-TR-64 2.0-TR-76 2.0-TR-88 2.0-TR-100 2.0-TR-112

2.0-TL-16 2.0-TL-28 2.0-TL-40 2.0-TL-52 2.0-TL-64 2.0-TL-76 2.0-TL-88 2.0-TL-100 2.0-TL-112

2.0-BR-16 2.0-BR-28 2.0-BR-40 2.0-BR-52 2.0-BR-64 2.0-BR-76 2.0-BR-88 2.0-BR-100 2.0-BR-112

2.0-BL-16 2.0-BL-28 2.0-BL-40 2.0-BL-52 2.0-BL-64 2.0-BL-76 2.0-BL-88 2.0-BL-100 2.0-BL-112

C1

C2

C3

C4

Bottom strands/grout C5

interface line 

Top strands/grout

interface line 

[Stressed strands: macro-anode]

[Unstressed strands: macro-cathode]

one wire corroded in two

134
Figure 209 shows an intact strand near the void/grout interface of 0.08 percent chloride specimen,

and figure 210 shows the as-extracted condition of four interface segments of the same specimen. 

Figure 209. Photo. Exposed strand at the void/grout interface of 0.08 percent chloride multistrand specimen.

Figure 210. Photo. As-extracted condition of four stressed strands near the void/grout 

interface of 0.08 percent chloride multi-strand specimen.

Since the void/grout interface was sloped at a 25-degree angle, the 4-to-16-inch segments of the BL 

and BR strands and 16-to-28-inch segments of the TL and TR strands were exposed at the interface 

(see figure 37). The labels included in figure 210 provide relative position of each segment with 

respect to the distance from the upper anchor plate and the interface. Only minor corrosion can be 

seen on the interface segments. Figure 211 shows a stressed strand of 0.8 percent chloride specimen 

showing moderate corrosion at the interface. Figure 212 shows the interface segments of the 

same specimen, with BL and BR exhibiting minor corrosion and TL and TR exhibiting

moderate corrosion. 

16 inches Void/grout interface 28 inches

TR

TL

4 inches Void/grout interface 16 inches

BR

BL

135
Figure 211. Photo. Exposed strand at the interface of 0.8 percent chloride multi-strand 

specimen.

Figure 212. Photo. As-extracted condition of four stressed strands at the void/grout interface 

of 0.8 percent chloride multi-strand specimen.

The specimens containing 0.08, 0.8, and 1.0 percent chloride showed overall good condition at the 

interface even after 5.1 fl oz of distilled water was added. Their interface conditions observed 

during the accelerated corrosion testing are shown in figure 157, figure 161, and figure 162, 

respectively. The specimens containing 0.2 and 0.6 percent chloride also showed good condition 

with minor corrosion at the interface. Their interface photographs are shown in figure 158 and 

figure 160, respectively.

Severe corrosion was observed at the interfaces of three other specimens containing 0, 0.4, and 

2.0 percent chloride. Figure 213 shows the void/grout interface of 0 percent chloride specimen upon 

removal of the PVC duct. The interface was covered with a thick black deposit. Figure 214 and 

figure 215 show a top section of the interface and its cross section, respectively. The black deposit 

layer appeared to be porous and contained numerous small black particles. Figure 216 shows 

two pairs of the stressed strand segments that passed through the void/grout interface. 

16 inches 28 inches

16 inches 4 inches

Void/grout interface

Void/grout interface

BL

BR

TR

TL

136
Figure 217 and figure 218 show close-up views of two interface segments shown in figure 216. 

Severe corrosion took place despite a chloride-free condition. 

Figure 213. Photo. Strands at the void/grout interface of 0 percent chloride multi-strand 

specimen.

Figure 214. Photo. Close-up plan view of the void/grout interface (top section).

137
Figure 215. Photo. Cross section of the void/grout interface (top section).

Figure 216. Photo. As-extracted condition of the stressed strands at the void/grout interface of 

0 percent chloride multi-strand specimen.

16 inches Void/grout interface 28 inches

TL

TR

16 inches 4 inches Void/grout interface

BR

BL

138
Figure 217. Photo. As-extracted condition of a stressed segment passed through the 

contaminated void/grout interface of 0 percent chloride multi-strand specimen.

Figure 218. Photo. As-extracted condition of another stressed segment passed through the 

contaminated void/grout interface of 0 percent chloride multi-strand specimen.

139
Figure 219 shows the condition of stressed strands just behind the void/grout interface of the 

0.4 percent chloride specimen. It was suspected in the middle of the detensioning process that some 

wires must have fractured under the tension. Upon grout removal, it was discovered that three wires 

of a stressed strand were indeed severed. The corroded strand is indicated by the red arrow in 

figure 219. Figure 220 and figure 221 show details of the broken wires. It was obvious that they 

failed during the detensioning process caused by excessive stress concentrated in the corroded areas 

with significant section loss. This specimen also had black deposit on the interface as indicated by 

the gray arrows in figure 219. Figure 222 shows a grout piece taken from the 12 o’clock orientation 

near the interface of the 0.4 percent chloride specimen. A bleed channel and poor grout condition 

can be seen. Figure 223 shows a cross section of the same piece, and a porous layer and black 

particles can be seen. They looked identical to the grout fragment of 0 percent chloride specimen 

shown in figure 215. 

Figure 219. Photo. Severely corroded strand with three broken wires at the void/grout 

interface of 0.4 percent chloride multi-strand specimen.

140
Figure 220. Photo. Close-up view of the severely corroded strand shown in figure 219.

Figure 221. Photo. Close-up view of the broken wires shown in figure 220.

141
Figure 222. Photo. Top side grout piece exhibiting bleed channel and porous grout removed 

from 0.4 percent chloride specimen.

Figure 223. Photo. Cross section of the grout piece shown in figure 222.

The porous grout defect is not new. In fact, FDOT, the Texas Department of Transportation, and 

VDOT reported the same “grout segregation” phenomenon pertaining to a particular grout product.

Figure 224 and figure 225, taken by the author of this report, show VDOT’s field trial result of 

two grout products. As observed in the present study, the same grout product used in VDOT’s trial 

142
also exhibited the segregated grout along the bleed channel on the 12 o’clock orientation (see 

figure 224), while the other product yielded a good quality, hardened grout (see figure 225). 

Figure 224. Photo. Defective grout along a bleed channel.

Figure 225. Photo. Normal grout.

Figure 226 shows another stressed strand of the same 0.4 percent chloride specimen that 

experienced similar significant section loss and black deposit, as indicated by the arrows.

143
Figure 226. Photo. Another exposed strand at the void/grout interface of 0.4 percent chloride 

multi-strand specimen.

Discovery of severe corrosion in the absence of chloride triggered a small lab investigation using an 

ion chromatography instrument to analyze water-soluble (free) sulfate ions in the black particles 

removed from the 0 percent chloride specimen and grout fragments retrieved from the void/grout 

interfaces of the 0, 0.2, and 0.4 percent chloride specimens. Some bulk grout powder samples 

collected earlier for chloride analysis and original grout powder samples were also analyzed as 

controls. Table 10 summarizes the analysis results. It was determined that the grout powder 

contained 0.9 percent of water-soluble sulfate ions by weight of cement, whereas the black 

particles contained 2.8 percent water-soluble sulfate ions by weight of cement. The high sulfate 

concentration was also found in the black deposit sample scrapped off the interface of the 

0 percent chloride specimen. The elevated level of free sulfate ions is thought to be responsible for 

the extraordinary corrosion damage observed in the chloride-free grout. The other interface samples

(0.08 and 0.2 percent chloride) and grout powder samples did not reveal high levels of free sulfate 

ions because they did not contain the black particles. Although grout fragments of the 0.4 percent 

chloride specimen was also suspected to suffer the sulfate induced corrosion, they did not contain 

sufficient black deposit material to perform ion chromatography analysis, and it could not be 

confirmed that high concentration of free sulfate ions on its interface was also responsible for 

corrosion failure of three wires. 

144
Table 10. Sulfate ion concentrations in the samples.

Sample ID

Sample 

Weight (oz)

Percent by 

Grout 

Weight

Percent by 

Cement 

Weight

Black particles collected from void/grout 

interface of 0 percent chloride specimen

0.07 1.5 2.8

Top layer scraped from void/grout interface 

of 0 percent chloride specimen

0.175 1.5 2.7

Top layer scraped from void/grout interface 

of 0.2 percent chloride specimen

0.175 0.4 0.7

Top layer scraped from void/grout interface 

of 0.4 percent chloride specimen

0.175 0.1 0.1

Grout powder sample from 0 percent 

chloride specimen

0.07 0.0 0.1

Grout powder sample from 0.08 percent 

chloride specimen

0.07 0.1 0.1

Grout powder sample from 0.4 percent 

chloride specimen

0.07 0.0 0.0

Original grout powder 0.175 0.5 0.9

If proper hydration of grout takes place, the sulfate ions in the dry grout should be chemically 

bound evenly in the cement paste, and little can be leached out in deionized water. This binding 

effect can be confirmed with 0 to 0.1 percent free sulfate concentrations by weight of cement in the 

hardened grout. It is speculated that when grout segregation occurs, free sulfate ions and black 

particles, presumably some sort of carbon black component of silica fume, can float to the top along 

the 12 o’clock bleed channel and form the corrosive deposit layer containing an excessive amount 

of free sulfate ions on top of the segregated and porous void/grout interface. The porous layer also 

appeared to hold water longer, as indicated by arrows in figure 156 and figure 159. It is unknown at 

this time how and why the segregated grout problem occurred only in some specimens even though 

the grouting work for every specimen was done in a consistent way.

Figure 227 and figure 228 show the condition of the exposed stressed strands in 2.0 percent 

chloride specimen after grout was removed near the void/grout interface. A severely corroded 

strand having a broken wire at the interface is shown. The broken wire must have been corroded

for an extended period of time judging by the thick corrosion products formed on the failed section. 

The failed strand had been covered with black corrosion product formed at the interface, as shown 

in figure 163. It is not known whether the corrosion product contained a high level of free sulfate 

ions that could be also responsible for the intensive wire corrosion.

145
Figure 227. Photo. Severely corroded strand with one broken wire revealed at the void/grout 

interface of 2.0 percent chloride multi-strand specimen.

Figure 228. Photo. Close-up view of severely corroded strand with one broken wire revealed 

at the void/grout interface of 2.0 percent chloride multi-strand specimen.

Figure 229 also shows corrosion of a stressed strand developed in grout containing the highest 

chloride concentration. The 1.0 percent or less chloride concentration specimens did not exhibit 

such an active corrosion in the grout-covered section. 

Figure 230 shows three corroding unstressed strands along with two stressed strands in 2.0 percent 

chloride specimen. As will be discussed later, all of the rust spots formed on the unstressed strands 

were superficial even in the highest chloride concentration.

146
Figure 229. Photo. Active corrosion of a stressed strand inside the 2.0 percent chloride grout 

70 inches from the lower anchorage.

Figure 230. Photo. Corrosion of unstressed strands in 2.0 percent chloride multi-strand 

specimen.

Table 11 and table 12 summarize physical conditions of the 12-inch strand segments extracted 

from the void/grout interface (4 to 16 inches and 16 to 28 inches) and those embedded in the 

grout, respectively.

Stressed strand

Unstressed strands

Stressed strand

147
Table 11. Condition summary of interface segments of multi-strand specimens.

Chloride 

Concentration 

(Percent by 

Cement Weight)

Number of Segments Showing 

Superficial Rust (Percent)

Number of Segment Wires Showing 

Heavy Rust Requiring Acid Cleaning 

(Percent)

Stressed: Total 

8 Segments per 

Specimen Unstressed

Stressed: 

56 Wires per 

Specimen (Total 8 

Segments × 7 Wires/

Specimen) Unstressed

0.00 4 (50.0 percent)

None

17 (30.4 percent)

None

0.08 6 (75.0 percent) 1 (1.8 percent)

0.20 6 (75.0 percent) 0 (0 percent)

0.40 7 (87.5 percent) 20 (35.7 percent)

0.60 7 (87.5 percent) 2 (3.6 percent)

0.80 8 (100 percent) 13 (23.2 percent)

1.00 8 (100 percent) 23 (41.1 percent)

2.00 8 (100 percent) 36 (64.3 percent)

Table 12. Condition summary of in-grout strand segments of multi-strand specimens.

Chloride 

Concentration

(Percent by 

Cement Weight)

Number of Segments Showing 

Superficial Rust (Percent)

Number of Segment Wires Showing 

Heavy Rust Requiring Acid Cleaning 

(Percent)

Stressed: Total 

28 Segments per 

Specimen

Unstressed: 

Total 30 

Segments per 

Specimen

Stressed: 

196 Wires per 

Specimen (Total 28 

Segments × 7 Wires/

Specimen)

Unstressed: 210 

Wires per 

Specimen (Total 

30 Wires ×

7 Wires/Segment

0.00 0 (0 percent) 0 (0 percent) 0 (0 percent) 0 (0 percent)

0.08 0 (0 percent) 0 (0 percent) 0 (0 percent) 0 (0 percent)

0.20 2 (7.1 percent) 0 (0 percent) 0 (0 percent) 0 (0 percent)

0.40 10 (35.7 percent) 2 (6.7 percent) 2 (1.0 percent) 0 (0 percent)

0.60 19 (67.9 percent) 11 (36.7 percent) 0 (0 percent) 0 (0 percent)

0.80 22 (78.6 percent) 6 (20.0 percent) 30 (15.3 percent) 0 (0 percent)

1.00 20 (71.4 percent) 16 (53.3 percent) 44 (22.4 percent) 0 (0 percent)

2.00 12 (42.9 percent) 14 (46.7 percent) 34 (17.3 percent) 0 (0 percent)

Because the majority of the interface segments developed rust in every chloride concentration, only 

the in-grout strand conditions presented in table 12 are discussed here. Even though rust first 

appeared on two stressed segments (or 7.1 percent) of 0.2 percent chloride specimen, 0.4 percent 

chloride is considered the lowest concentration with appreciable number of segments exhibiting 

corrosion: 10 stressed segments (or 35.7 percent) and two unstressed segments (or 6.7 percent). 

Figure 231 and figure 232 show random carbonation test results of 0.6 and 0.8 percent chloride 

specimens. Based on the dark pink color, the pH of the grout was confirmed to be above 10.0. 

Therefore, it was concluded that carbonation did not occur during the accelerated corrosion testing.

148
Figure 231. Photo. Carbonation testing results of 0.6 percent chloride multi-strand specimens.

Figure 232. Photo. Carbonation testing results of 0.8 percent chloride multi-strand specimens.

During the grout demolition, localized wet and soft grout was uncovered in 0.8 and 1.0 percent 

chloride specimens. As-discovered conditions are shown in figure 233 and figure 234, respectively. 

Despite the abnormal (wet and soft) grout condition, no corrosion was found in these areas. FDOT 

observed severe corrosion beneath the same type of defective grout. It is unknown at this time how 

the wet grout was formed and why corrosion did not occur.

149
Figure 233. Photo. Wet and soft grout found in 0.8 percent chloride multi-strand specimen.

Figure 234. Photo. Wet and soft grout found in 1.0 percent chloride multi-strand specimen.

When the PT strands were dismantled, it was observed that hardened grout filled some interstices 

formed by the outer six wires and the center wire. Figure 235 shows a hardened grout layer stuck in 

an interstice when an outer wire was removed. Moderately corroded wires can be seen. Figure 236

shows a rust-stained small grout piece fell from the strand shown in figure 235. It has been known 

that water can get into the wire interstices and induce crevice corrosion. These photographs 

150
document that chloride-contaminated fresh grout can also smear into the interstices to induce the 

same type of corrosion there.

Figure 235. Photo. Grout fragment found inside an interstice surrounded by corroding wires.

Figure 236. Photo. Heavily rust-stained grout fragment that fell from the strand shown in 

figure 235.

151
CORROSION DAMAGE MEASUREMENT DATA

The acid-cleaned wires were visually examined, and those that appeared to have pits deeper than 

2 mil were set aside for pit depth measurement. Figure 237 through figure 246 show representative 

pitting corrosion morphologies exhibiting different levels of damage. Also, figure 247 and 

figure 248 show crevice corrosion damage found on a 0.4 percent and 0.8 percent chloride multistrand specimen, respectively. The deepest pit found in this study was 60 mil, which is 30 percent 

of the wire diameter.

Figure 237. Photo. Stressed and voided 0.4 percent chloride single-strand specimen 

(pit depth < 2 mil).

Figure 238. Photo. Stressed and fully grouted 0.8 percent chloride single-strand specimen 

(pit depth ~2 mil).

Figure 239. Photo. Stressed and fully grouted 0.8 percent chloride single-strand specimen 

(pit depth = 4.7 mil).

152
Figure 240. Photo. 0 percent chloride multi-strand specimen at the void/grout interface (pit 

depth = 2–4 mil).

Figure 241. Photo. 0.08 percent chloride multi-strand specimen near the interface

(pit depth = 2–10 mil).

Figure 242. Photo. 0.6 percent chloride multi-strand specimen in the grout 

(pit depth = 2–14 mil).

Figure 243. Photo. Stressed and voided 1.0 percent chloride single-strand specimen 

(pit depth = 2–18 mil).

153
Figure 244. Photo. 1.0 percent chloride multi-strand specimen in the grout 

(pit depth = 2–23 mil).

Figure 245. Photo. 0 percent chloride multi-strand specimen at the interface

(pit depth = 2–36 mil).

Figure 246. Photo. 0.4 percent chloride multi-strand specimen near the void 

(pit depth = 2–50 mil).

Figure 247. Photo. Crevice corrosion between two stressed wires on 

0.4 percent chloride concentration multi-strand specimen 

(mean pit depth = 4.5 mil and maximum pit depth = 8.0 mil).

154
Figure 248. Photo. Crevice corrosion between two stressed wires on 

0.8 percent chloride concentration multi-strand specimen 

(mean pit depth = 4.5 mil and maximum pit depth = 6.0 mil).

For single-strand specimens, a total of 202 pit depth measurements were made on 13 wires removed 

from 5 12-inch segments. As summarized in table 9, measurable pits started to appear at a chloride 

concentration of 0.8 percent by weight of cement. For multi-strand specimens, a total of 3,426 pit 

depth measurements were made on 222 wires removed from 59 12-inch segments. Table 11 and 

table 12 provide detailed physical conditions of interface segments and in-grout segments of 

multi-strand specimens, respectively. The data in both tables were divided in two groups: number 

(percentage) of stressed and number (percentage) of unstressed segments showing superficial rust

and heavy rust requiring acid cleaning. All of the unstressed segments exhibited superficial

corrosion, and the acid-cleaned wires were entirely from the stressed ones.

Task 2.2: Voided Single-Strand Specimens

Figure 249 presents mean and maximum pit depth data of voided single-strand specimens. Each 

figure in parenthesis is the number of pits associated with a particular mean pit depth. No 

measureable pits were found in less than 0.8 percent chloride concentration. At higher chloride 

concentrations (1.0 to 2.0 percent), mean pit depth ranged between 3.6 and 6.2 mil, and maximum 

pit depths were between 7 and 18 mil. 

155
Figure 249. Graph. Mean and maximum pit depths on the voided single-strand specimens.

Task 2.3: Void/Grout Interface of Multi-Strand Specimens

Figure 250 shows fractions of stressed interface segments exhibiting superficial rust and fractions 

of individual wires (after untwisting strand segments) requiring acid cleaning due to heavy rust as a 

function of chloride concentration. Actual numbers are also listed in table 11. The data can be used

to estimate severity of the strand corrosion. Reference photographs describing typical superficial 

rust and heavy rust are shown in figure 62 and figure 63, respectively. Corrosion appeared on some 

or the entire interface segments, regardless of chloride concentration, even at 0 percent chloride. As 

discussed previously, interface segments of 0, 0.4, and 2.0 percent chloride specimens suffered 

from unexpected abnormal corrosion damage due to excessive sulfate ions. If rust data of these 

specimens are excluded, 0.8 percent chloride is the lowest concentration to exhibit heavy corrosion 

products requiring acid cleaning. 

156
Figure 250. Graph. Condition of stressed segments at the void/grout interface of multi-strand 

specimens.

Figure 251 shows the number of pits measured on the interface segments. If pit data of 0 and 

0.4 percent chloride specimens are ignored and the number of pits measured on 0.08 and 

0.6 percent chloride specimens are considered insignificant, 0.8 percent chloride is the lowest 

concentration to have a meaningful number of pits. In other words, severe corrosion can be 

anticipated if stressed strands are exposed at the void/grout interface containing 0.8 percent chloride. 

Pitting corrosion tended to concentrate more on the 16- to 28-inch segments (TL and TR) compared 

to the 4- to 16-inch segments (BL and BR). 

157
Figure 251. Graph. Number of pits on the interface segments of multi-strand specimens

measured from the upper anchor plate.

Figure 252 shows mean pit depth data for the interface segments. If pit data affected by sulfate

contamination are omitted, overall mean pit depth was approximately 6 mil. The largest mean pit 

depth was 11 mil at 2.0 percent chloride concentration.

Figure 252. Graph. Mean pit depths on the interface segments of multi-strand specimens 

measured from the upper anchor plate.

9.7

5.9

10.9

4.7 5.7 5.7 7.2

4.4

8.2

5.8 6.2

11.0

0

10

20

30

40

50

60

70

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Mean pit depth (mils)

Chloride concentration (percent by weight of cement)

4-16 segments 16-28 segments

158
The intensity of pitting corrosion was estimated by maximum pit depths. The results are plotted

in figure 253. If excessive sulfate ions had not induced severe corrosion at the interfaces, it is 

reasonable to assume that maximum pit depth of 0 and 0.4 percent chloride concentrations might 

have been approximately 10 mil because neighboring concentrations of 0.08 and 0.6 percent

chloride yielded the same number. At 0.8 and 1.0 percent chloride concentrations, maximum pit 

depths increased to 18 and 25 mil, respectively. The measured largest pit depth was 60 mil at 

2.0 percent chloride concentration.

Figure 253. Graph. Maximum pit depths on the interface segments of multi-strand specimens 

measured from the upper anchor plate.

Task 2.2: Fully Grouted Single-Strand Specimens

Figure 254 plots mean and maximum pit depth data of fully grouted single-strand specimens. 

Measureable pits began to show at 0.8 percent chloride. Mean pit depth varied between 3.6 and 

4.4 mil, and maximum pit depths were between 5 and 12 mil. These values are somewhat lower 

than those measured on the voided single-strand specimens.

36

10

52

11 11

15

33

14

37

19

25

60

0

10

20

30

40

50

60

70

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Maximum pit depth (mils)

Chloride concentration (percent by weight of cement)

4-16 segments 16-28 segments

159
Figure 254. Graph. Mean and maximum pit depths on the fully grouted single-strand

specimens.

Task 2.3: Fully Grouted Sections of Multi-Strand Specimens

Figure 255 shows fractions of stressed in-grout segments exhibiting superficial rust and fractions 

of individual wires (after untwisting strand segments) requiring acid cleaning due to heavy rust. 

Superficial rust and heavy rust started to appear when chloride concentration was 0.4 and 

0.8 percent, respectively. Interface segments also exhibited heavy rust from 0.8 percent chloride 

concentration. With higher chloride concentrations up to 1.0 percent, more rust was observed in 

both rust types. Low macro-cell corrosion current density of 2.0 percent chloride specimen was

responsible for its reduced number of rust spots. The stressed in-grout strands developed far less 

corrosion at any chloride concentration compared to the stressed interface segments. This suggests 

that grout might have provided some corrosion protection to the strands. Figure 256 shows the 

number of pits measured on the stressed segments versus their distance from the upper anchor 

plate. More pits were found on the segments closer to the interface and also as chloride 

concentration increased.

160
Figure 255. Graph. Physical condition of stressed in-grout segments in multi-strand 

specimens.

Figure 256. Graph. Number of pits observed on the stressed strands in multi-strand 

specimens measured from the upper anchor plate.

Figure 257 shows mean pit depth data of stressed strand segments embedded in grout by distance. 

Their mean pit depths varied between 3.4 and 7.4 mil when chloride concentration was 1.0 percent 

or less. At 2.0 percent chloride concentration, mean pit depth increased to 13 mil. Mean pit depth

was not affected by distance from the upper anchor plate at any chloride concentration.

161
Figure 257. Graph. Mean pit depth measured on stressed strands in multi-strand specimens 

measured from the upper anchor plate.

Figure 258 compares the total number of pits versus chloride concentration data determined over 

the same in-grout length of stressed and unstressed strands. A small number of pits appeared at 

0.4 percent (stressed strands) and 0.6 percent (unstressed strands), but pitting corrosion damage 

deeper than 2 mil (sign of corrosion propagation) was observed only on the stressed strands 

exposed to 0.8 percent chloride and higher concentrations. Heavy rust also developed on the 

stressed strands exposed to the same chloride concentration (see figure 250 and figure 255). 

Figure 259 shows overall mean pit depth versus chloride concentration data over the same in-grout 

length of stressed and unstressed strands. None of the unstressed in-grout strands exceeded the 

2-mil pit depth threshold regardless of chloride concentration. Overall mean pit depth measured on 

the stressed in-grout strands increased from 4.3 to 9.4 mil as the chloride concentration increased 

from 0.4 to 2.0 percent. 

8.0

5.8

7.4

4.8

13.1

3.4

5.6

5.1

0

10

20

30

40

50

60

70

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Mean pit depth (mils)

Chloride concentration (percent by weight of cement)

28 - 40 40 - 52 52 - 64 64 - 76 76 -88 88 - 100 100 - 112

4.3

162
Figure 258. Graph. Number of pits observed on the in-grout strands in multi-strand 

specimens.

Figure 259. Graph. Mean pit depths of the in-grout stressed strands in multi-strand 

specimens.

As shown in figure 260, maximum pit depth tended to increase as chloride concentration increased.

The measured largest pit depth was 36 mil at 2.0 percent chloride concentration. There was no 

strong relationship observed between distance from the upper anchor plate and maximum pit depth. 

163
Figure 260. Graph. Maximum pit depths measured on the stressed strands in multi-strand 

specimens measured from the upper anchor plate.

Role of Void/Grout Interface

It is believed that premature failure of some field PT tendons was caused by a macro-cell corrosion 

mechanism of intensive localized corrosion in a relatively small anodic area of a PT tendon in 

relation to the rest of the tendon. In the present study, employment of the multi-strand specimen 

was intended to reproduce the worst corrosion damage by a simulated macro-cell corrosion setup in 

a relatively short period of time in the laboratory. The intended macro-cell corrosion was created 

between four stressed strands as macro-anode, especially near the void/grout interface, and 

five unstressed strands completely embedded in grout as macro-cathode. The majority of the 

electrochemical data and autopsy results of multi-strand specimens indicate that the macro-cell 

corrosion setup effectively facilitated accelerated corrosion of PT strands. 

Figure 261 shows how macro-cell corrosion damage was influenced by distance with respect to

the void/grout interface in normal grout. The stressed strands in the figure were divided into three 

sections: two interface segments (4–16 and 16–28 inches), a transition segment (28–40 inches), and 

the remaining segments (40–112 inches). Corrosion damage data of 0, 0.4, and 2.0 percent chloride 

concentration specimens are not included in the plot due to their excessive corrosion damage by 

sulfate ions. 

A general trend is that more pits were found on the segments closer to the interface when chloride 

concentration was 0.8 and 1.0 percent. When chloride concentration was between 0.08 and 

0.6 percent, no measurable pits were observed. This finding indicates that stressed strands near the 

interface were subjected to the most intensive corrosion if chloride concentration was 0.8 percent 

chloride. At this chloride concentration level, small surface area at the interface became the most 

anodic, probably in the presence of more oxygen and moisture in the void. The number of pits 

tended to decrease as a segment was getting away from the interface. It is thought that this 

23

13

27

9.0

12

23

36

25

15 14

17

35

6

13

19

11

0

10

20

30

40

50

60

70

0 0.08 0.2 0.4 0.6 0.8 1.0 2.0

Maximum pit depth (mils)

Chloride concentration (percent by weight of cement)

28 - 40 40 - 52 52 - 64 64 - 76 76 -88 88 - 100 100 - 112

164
phenomenon was caused by weakened macro-cell corrosion current at the segments in the deeper 

grout upon presence of higher grout resistance from the interface.

Figure 261. Graph. Number of pits measured on the stressed strands in some of the multistrand specimens.

Figure 262 suggests that mean pit depth data did not follow the same trend. Pit depth was fairly 

uniform around approximately 6 mil along most of the strand length including the interface section.

165
Figure 262. Graph. Mean pit depth measured on the stressed strands in some of the multistrand specimens.

SUMMARY OF MAJOR FINDINGS

The electrochemical testing of center wires conducted under task 2.1 revealed that the corrosion rate

of center wires in pH 13.6 solutions was less than 0.05 mil/year at chloride concentrations up to

0.6 percent and increased at higher chloride concentrations. For the specimens tested in pH 9.0

solutions containing 0.04 percent and higher chloride concentrations, typical active corrosion 

behavior was observed: corrosion potentials became more negative and corrosion rates increased. 

The electrochemical testing results suggest that the PT strand can tolerate chloride contamination 

without significant corrosion up to 0.6 percent by weight of cement in carbonation-free (high pH) 

grout, whereas as low as 0.04 percent chloride by weight of cement can initiate active corrosion in 

the carbonated (low pH) grout. This is below the AASHTO specified chloride limit of 0.08 percent 

by weight of cement. Once corrosion starts in the low pH environment, level of corrosion damage 

will depend on chloride concentration.

The apparent grout resistivity of single-strand specimens and grout resistance of multi-strand 

specimens were sensitive to temperature. High grout resistivity/resistance was measured during 

the F & D cycles, and low resistivity/resistance was measured during the H & H cycles. Chloride 

concentration did not affect the apparent grout resistivity/resistance values.

For single-strand specimens (task 2.2), chloride concentration of 0.4 percent by weight of cement 

was the lowest concentration to make the strands more prone to corrosion in most conditions. Mean 

corrosion potential became gradually negative starting from 0.4 percent chloride in initial ambient, 

ambient, and hot and H & H conditions. Mean corrosion rate became progressively higher starting 

from 0.4 percent chloride, and a dependence of corrosion rate on temperature was observed—

corrosion rate increased during the H & H cycles, and intermediate corrosion rate was observed in 

the ambient condition. In F & D cycles, corrosion of all specimens decreased to a negligible state 

166
regardless of chloride concentration. But, absolute values of the measured corrosion rates were very 

low due to small corroding areas compared to conventional corrosion rate criteria for ordinary 

reinforcing steel. There were no definite trends observed in single-strand specimens regarding stress 

level and presence of void, although limited data suggest that the stressed strands exposed to water 

in the grout void may be more susceptible to corrosion than the unstressed ones if acid-soluble 

chloride concentration is higher than 0.08 percent by weight of cement and the risk of corrosion is 

elevated with increased chloride concentration. All control specimens experienced steadily 

decreasing corrosion rates as their corrosion potential data also indicated the passive behavior.

An autopsy of single-strand specimens revealed that rust began to appear when chloride 

concentration was 0.4 percent. However, measurable pits were observed if chloride concentration 

was 0.8 percent. As chloride concentration increased, the severity of the corrosion got worse, 

particularly for the stressed specimens with void. However, the same observation was not indicated 

by their corrosion rate data. Mean pit depths ranged between 3.6 and 6.2 mil. A combination of a 

void, recharged water, and elevated chloride concentration can be detrimental, evidenced by 

markedly higher number of pits found on the 2.0 percent chloride stressed specimen with void.

For multi-strand specimens (task 2.3), chloride concentration was a more influential parameter for

corrosion potential than exposure condition. As the macro-cell corrosion setup intended, stressed 

strands exhibited more negative corrosion potentials than unstressed ones, making the stressed 

strands anodic to the unstressed ones at any chloride concentration. It is unclear if presence of stress 

or localized corrosion at the void/grout interface or both made the entire stressed strands active. 

When chloride concentration increased to 0.4 percent, mean corrosion potentials of the stressed 

strands fell into the 90 percent corrosion probability. For unstressed strands, the same condition

occurred only after chloride concentration reached 2.0 percent. Overall mean macro-cell corrosion 

current density also began to increase at 0.4 percent chloride. At any chloride concentration, the 

H & H and F & D cycles produced the highest and the lowest mean macro-cell corrosion current 

densities, respectively. Water recharging did not influence the mean corrosion potentials and mean 

macro-cell current densities significantly.

Autopsy of multi-strand specimens revealed that, in general, stressed segments started to show 

superficial rust at 0.4 percent chloride concentration, and heavy rust required acid cleaning at

0.8 percent chloride concentration. This observation could be made with both of the interface 

segments and in-grout segments. Mean pit depths measured on the stressed in-grout strands 

increased from 4.3 to 9.4 mil if chloride concentration increased from 0.4 to 2.0 percent. For the 

interface segments, mean pit depth was approximately 6 mil when chloride concentration was less 

than 1.0 percent. At 2.0 percent chloride concentration, mean pit depth increased to 11 mil. The 

largest measured pit depths were 60 and 36 mil for the interface segments and in-grout segments, 

respectively. For unstressed strands, superficial rust began to show up at 0.6 percent chloride 

concentration, and none of the pits exceeded 2 mil regardless of chloride concentration.

A general trend observed from the multi-strand specimens with normal grout at the interface is that 

more pits were found on the segments closer to the interface when chloride concentration was 0.8 

and 1.0 percent. When chloride concentration was between 0.08 and 0.6 percent, no measurable pits 

were found at all. This finding suggests that stressed strands near the interface were subjected to the 

most intensive macro-cell corrosion if chloride concentration was 0.8 percent chloride. At this level 

of chloride, small surface area of the interface became the most anodic in the presence of abundant 

167
dissolved oxygen and moisture in the void. The number of pits tended to decrease as a segment was 

getting away from the interface. It is thought that this phenomenon was related to weakened macrocell corrosion current in the deeper grout. Mean pit depth data did not follow the same trend in that 

it was fairly uniform in the neighborhood of 6 mil along the most part of the stressed strand length,

including the interface section.

168
CHAPTER 5. CHLORIDE THRESHOLDS

Chloride threshold is defined as the minimum chloride concentration required to induce corrosion. 

By this definition, the lowest chloride concentration creating signs of corrosion seems to be the 

appropriate chloride threshold. Because exact amount of admixed chloride concentrations were 

known in this study, corrosion performance of stressed PT strands exposed to seven different 

chloride concentrations and one chloride-free mix was characterized by their electrochemical data 

collected through 6-month accelerated corrosion testing and subsequent autopsy results. The other 

chloride threshold for corrosion propagation was also sought by determining the lowest chloride 

concentration needed to produce measurable pits. Table 13 lists the lowest acid-soluble chloride 

concentrations experimentally determined for corrosion initiation and corrosion propagation. 

Table 13. Summary of the lowest chloride concentrations observed in the present study.

Criteria

Grout with Void Fully Grouted

Specimen Type

Lowest Chloride 

Concentration 

(Percent by 

Weight of 

Cement) Specimen Type

Lowest Chloride 

Concentration 

(Percent by 

Weight of 

Cement)

Corrosion 

initiation

Single-strand specimens 0.4 Single-strand 

specimens

0.6

Corrosion 

propagation

Multi-strand specimens 

contaminated with 

excessive sulfate ions

0 Multi-strand 

specimens

0.4

Single- and multi-strand 

specimens

0.8 Single- and multistrand specimens0.8

Finally, two chloride threshold values were selected among the numbers in table 13 for the PT 

strands protected by normal grout (see table 14). The first threshold is 0.4 percent by weight of 

cement including mixing water ([Cl-](cement+water)), which is the lowest amount to initiate corrosion of 

strands fully encased in normal grout. At this threshold, a small number of pits can begin to form on 

the strands in association with corrosion initiation process, but the rate of corrosion should be low. 

It is a conservative number, and there is still sufficient time to act against potential corrosion 

problems. Therefore, it can serve as the lower limit in assessing longer-term corrosion risk of PT 

bridges. The second critical chloride threshold is determined in association with corrosion 

propagation. At this threshold, corrosion starts to intensify in terms of number of pits and pit depth. 

Test results obtained in this study suggest that 0.8 percent [Cl-](cement+water) seems to be the critical 

threshold, which can be considered the upper limit in corrosion risk assessment. Once chloride 

concentration exceeds it, it is possible that significant corrosion damage can occur rapidly, and 

structural integrity of the PT bridges will be compromised. However, the actual rate of corrosion is

influenced by many other factors set by individual bridges. 

169
Table 14. Suggested chloride threshold values for normal grout.

Classification [Cl-](cement+water)

Chloride threshold for corrosion initiation 0.4 percent

Critical chloride threshold for corrosion 

propagation

0.8 percent

It should be emphasized that these values are applicable to normal grout conditions only. They may 

not be adequate in other conditions such as segregated grout, duct cracks, grout voids filled with 

excessive water with or without chloride ions, and free sulfate ions in contact with the strands. In 

these circumstances, corrosion can start at different chloride concentrations, most likely below the 

suggested threshold values. 

As shown in table 8, there are three units of chloride concentration to determine whether hardened 

grout powder samples taken from a PT bridge contain acid-chloride concentrations more than the 

chloride thresholds. When performing acid-soluble chloride titration with the field powder samples, 

the results are reported by [Cl-

]sample. For the grout product used in this study, they represent 

approximately 50 percent of actual chloride content included in the grout powder samples. 

Therefore, 50 percent of the established chloride thresholds should be used—0.2 percent for 

corrosion initiation and 0.4 percent for corrosion propagation. If construction records of detailed 

grout mix proportions are available, the [Cl-

]sample can be converted to those expressed by [Cl-

](cement+water) using a correction factor that accounts for weight of cement and weight of mixing water 

in a particular grout product. The converted concentrations in the grout samples indicated total 

(100 percent) acid-soluble chloride concentrations present. Therefore, it is easy compare the 

analysis results against the chloride threshold values listed in table 14. If information about precise 

weight of cement is known but weight of mixing water is not, a concentration unit of [Cl-

]cement

should be used with another correction factor. For the same grout product, this conversion method 

yields about 75 percent of the acid-soluble chloride concentrations resided in the grout samples. 

Therefore, 75 percent of the suggested chloride thresholds should be used—0.3 percent for 

corrosion initiation and 0.6 percent for corrosion propagation. Table 15 summarizes the adjusted 

threshold values for three different concentration units.

Table 15. Adjusted chloride threshold values to be used for field grout powder samples.

Classification [Cl-](cement+water) [Cl-]cement

Fraction of

[Cl-](cement+water) [Cl-]sample

Fraction of

[Cl-](cement+water)

Corrosion 

initiation 0.4 percent 0.3 percent 0.75 percent 0.2 percent 0.5 percent

Corrosion 

propagation 0.8 percent 0.6 percent 0.75 percent 0.4 percent 0.5 percent

170
CHAPTER 6. MISCELLANEOUS WORK

CORROSION RISK ASSESSMENT 

It has been shown that PT corrosion can initiate at any chloride concentration depending on grout 

condition and many other external and environmental conditions pertaining to particular sets of PT 

tendons. In high-quality normal grout without any other deficiencies, the severity of pitting 

corrosion induced by chloride ions begins to increase significantly when [Cl-](cement+water) is 

0.8 percent or higher. In performing the corrosion risk assessment of field PT bridges, total

(acid-soluble) chloride concentration in grout powder samples must be determined first through a 

statistically sound grout powder sampling methodology. Once distribution of acid-soluble chloride 

contents in the bridge is determined, the corrosion risk assessment can be performed using the predetermined risk criteria. As an example, table 16 lists three corrosion risk levels and maintenance 

strategies for the affected PT bridges containing various levels of chloride contamination. 

Table 16. Example of corrosion risk management plan with the suggested 

chloride threshold values.

Risk Level 

Classified by 

Different 

Chloride 

Threshold 

Values

If Weight of 

Cement and 

Mixing Water 

are Known

If Cement Weight 

is Known and 

Mixing Water 

Weight is 

Uncertain

If Weight of 

Cement and 

Mixing Water 

are not Known

Recommended Actions

Matching Corrected Chloride Threshold Values for 

Different Concentration Units

[Cl-](cement+water) [Cl-]cement [Cl-]sample

Low ˂ 0.4 percent ˂ 0.3 percent ˂ 0.2 percent Routine biennial bridge 

inspection

Intermediate 0.4–0.8 percent 0.3–0.6 percent 0.2–0.4 percent Routine biennial bridge 

inspection; develop a 

longer-term maintenance 

plan

High ˃ 0.8 percent ˃ 0.6 percent ˃ 0.4 percent Conduct immediate indepth inspection; more 

frequent inspections

The classification of risk levels and proposed actions for bridge inspection and maintenance are 

discussed below based on the chloride threshold values in terms of [Cl-](cement+water). The same 

approach can be adopted for the other sets of chloride threshold values listed in table 15. Depending 

on situation of each bridge, any of these categories may be modified for corrosion risk assessment.

The PT bridges may be classified as low-risk bridges if the mean [Cl-

](cement+water) is less than 

0.4 percent. At this chloride level, the PT tendons are likely to maintain corrosion-free condition;

therefore, regular biennial bridge inspection is sufficient. 

The intermediate risk group bridges contain mean [Cl-

](cement+water) between 0.4 and 0.8 percent. 

Even though rate of corrosion in this group may be slow enough not to cause an immediate concern, 

171
they may progress into the high-risk group if PT seal systems are breached. Therefore, along with 

routine biennial inspection, development of an appropriate longer-term maintenance plan is 

recommended for bridges in this group due to their unpredictable corrosion behaviors. Specifically, 

all of the potentially problematic areas should be identified in the maintenance plan and 

subsequently inspected in detail when time and resources are available.

High-risk bridges refer to ones containing mean [Cl-](cement+water) higher than 0.8 percent. They 

require an immediate initial indepth inspection and subsequent inspections at closer intervals. It is 

safe to assume that these bridges may well undergo intensive pitting corrosion. This assumption is 

particularly true for bridges exposed to warm weather and those already having long postconstruction service life. Every tendon deficiency found in a high-risk bridge needs to be addressed 

immediately or kept on a close watch until appropriate repair measures are developed. 

The important issue here is how to determine statistically sound mean chloride concentrations for 

the whole bridge or various sections depending bridge characteristics. A separate small study is 

recommended to develop a statistically sound sampling methodology for sample size and its 

distribution, confidence interval, and acceptable data variation indicated by standard deviation, 

coefficient of variance, etc. The goal is to make sure that mean chloride concentrations are truly

representative values for the bridge or sections of the bridge without unacceptable deviations. 

Otherwise, corrosion risk may be elevated, as there can be some outliers that are undetected through 

the improper statistical analyses. The outcomes of the study can be incorporated into the existing 

grout sampling guideline.

(20) In addition to level of chloride contamination, annual temperature 

variation needs to be considered as part of the corrosion risk assessment since corrosion rate is 

heavily influenced by temperature. Longer duration of warm weather promotes more intense 

corrosion, as seen by the data obtained in this study.

The preceding section discusses corrosion risk of the PT bridges under the assumption that they are 

surrounded by reasonably good quality grout. As some test results indicated in this study, however, 

chloride threshold values for poor-quality grout having segregation and voids can be meaningless,

as corrosion can initiate without chloride ions under these conditions. It is particularly alarming to 

learn that excessive corrosion damage can occur at much higher rates if the segregated grout 

contains elevated water-soluble free sulfate ions. 

It is critical to assess condition of the strands and thoroughly and accurately locate grout defects. As 

the current state of inspection practices for PT bridges are labor intensive partly due to limited 

inspection tools and techniques, it is highly desirable to develop new technologies or improve the 

existing ones. While dealing with chloride contaminated grout issues, there should be more effort 

on a national level to develop 100 percent reliable non-destructive inspection technologies to 

diagnose corrosion damage in the tendons and identify grout defect areas.

SERVICE LIFE PREDICTION MODEL

This section offers an example of service life prediction modeling with a very simplified approach. 

It is hoped that more sophisticated modeling software will be developed in the near future to 

analyze various field conditions and provide accurate and reliable predictions. 

172
A stressed PT strand will fail by corrosion when stress in the remaining cross-sectional area of the 

strand reaches the 270 ksi GUTS under a constant tensile load. Figure 263 shows an example of 

corrosion damage prediction model outputs for a corroding strand at different penetration rates. The 

following assumptions were made for the modeling:

• Annual loss of cross-sectional area of a wire is the square of annual penetration rate 

expressed by mils per year. For example, if corrosion penetration rate is 6 mil/year, annual 

section loss is 0.000036 inch2 = 0.006 inch2.

• Annual penetration rate is constant (i.e., no change over time).

• Fixed number of wires corrode simultaneously at the same rate. 

• Structural behavior of stressed strands is independent of the number of strands.

• Temperature effect is not considered.

• The tensile stress in the strand is 60 percent GUTS, and it remains the same over time.

Figure 263. Graph. Example of a time-to-failure plot for a corroding strand at different 

penetration rates.

These assumptions may be unrealistic in the real world, but the model outputs can be informative 

because they illustrate the effects of cross section loss and penetration rates on structural integrity. 

The penetration rates used in the example are representative pit depths observed in the autopsied 

strands in the present study. If corrosion intensifies by rapidly losing cross-sectional area, the rate 

of stress buildup in the strand is no longer linear or linear-like, and strand failure occurs 

173
prematurely at higher rates. The test results indicate that corrosion damage of stressed strands 

increased significantly if total chloride concentration exceeded the critical chloride threshold of 

0.8 percent by weight of cement. It is critical to determine present chloride concentrations 

in the affected bridges as soon as possible and take appropriate actions based on level of 

chloride contamination.

174
CHAPTER 7. CONCLUSIONS

Based on the test results obtained from the 6-month accelerated corrosion laboratory study and 

subsequent data analyses, the following conclusions can be made:

• There are two chloride threshold values determined for PT strands. The first threshold is 

0.4 percent [Cl-

](cement+water) which is the lowest concentration to initiate corrosion of strands

fully encased in normal grout. At this threshold, a small number of pits start to form on the 

strands in association with corrosion initiation process. The second “critical” chloride 

threshold of 0.8 percent [Cl-

](cement+water) is determined in association with corrosion 

propagation. At this threshold, corrosion starts to intensify in term of number of pits 

and pit depth. 

• If mean acid-soluble chloride concentration in the grout powder samples from a PT bridge is 

determined to be less than 0.4 percent [Cl-

](cement+water), this bridge belongs to the low risk 

group and regular biennial inspection is sufficient. If its mean acid-soluble chloride 

concentration is higher than 0.8 percent [Cl-](cement+water), it is considered a high risk bridge 

that should require an immediate in-depth inspection and subsequent routine inspections at 

closer intervals. For the PT bridges containing mean acid-soluble chloride concentrations 

between these two sets of threshold values, they should be included in the intermediate risk 

bridge group as there is a chance that they may progress into the high risk group. 

175

APPENDIX A. EXAMPLES OF CORROSION PROBLEMS CAUSED BY INSUFFICIENT 

GROUT COVER

This appendix contains examples of tendon corrosion problems caused by insufficient grout cover 

that were observed in three PT bridges in service. Figure 264 shows a failed externa PT tendon 

of the Verina-Enon bridge in Virginia. Little grout cover is visible between the 6 and 

12 o’clock orientation.

Figure 264. Photo. Failed external PT tendon of Verina-Enon bridge in Virginia.

177
Figure 265. Photo. Other matching end of the retrieved tendon sample shown in figure 264.

Figure 266 shows a corroding PT tendon of the Verina-Enon bridge in Virginia. Corrosion started 

in some areas with no grout cover near a deviation block that was exposed to stagnant water.

Figure 266. Photo. Corroding PT tendon of Verina-Enon bridge in Virginia.

178
Figure 267 shows an external PT tendon of the Verina-Enon bridge in Virginia. Little or no grout 

cover is visible along the free length of the tendon, but no corrosion was observed.

Figure 267. Photo. External PT tendon of Verina-Enon bridge in Virginia.

Figure 268 shows an external PT tendon of the Mid-Bay bridge in Florida. No grout cover 

was discovered.

Figure 268. Photo. External PT tendon of Mid-Bay bridge in Florida.

(21)

Figure 269 shows an external PT tendon of the Ringling Causeway bridge in Florida. Significant 

corrosion was observed in the areas with no grout cover in contact with segregated thin grout cover.

179
Figure 269. Photo. External PT tendon of Ringling Causeway bridge in Florida.

(4)

180
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Federal Highway Administration, Washington, DC.

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Virginia Department of Transportation, Final Report, Exton, PA.

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7. American Concrete Institute. (2011). Building Code Requirements for Structural Concrete and 

Commentary, ACI 318M-11, Farmington Hills, MI.

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HRDI-60/05-14(600)E Recycled

Recyclable