US Army Corps

of Engineers

Construction Engineering

Research Laboratories

USACERL Technical Report 98/81

June 1998

CONSTRUCTION PRODUCTIVITY ADVANCEMENT

RESEARCH (CPAR) PROGRAM

Composite Grids for

Reinforcement of Concrete Structures

by

Piyush K. Dutta, David M. Bailey, Stephen W. Tsai,

David W. Jensen, John R. Hayes Jr., Willie E. McDonald,

Craig W. Smart, Tracy Colwell, Jason S. Earl, and Hong-Ji Chen

Approved for public release; distribution is unlimited.

A Corps/Industry Partnership To Advance

Construction Productivity and Reduce Costs

Cvl
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1. AGENCY USE ONLY (Leave Blank) 2. REPORT DATE

June 1998

4. TITLE AND SUBTITLE

Composite Grids for Reinforcement ofConcrete Structures

3. REPORT TYPE AND DATES COVERED

Final

6. AUTHOR(S)

Piyush K. Dutta, David M. Bailey, Stephen W. Tsai, David W. Jensen, John R. Hayes

Jr., Willie E. McDonald, Craig W. Smart, Trac

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

U.S. Army Construction Engineering Research Laboratories (USACERL)

P.O. Box 9005

Champaign, IL 61826-9005

9. SPONSORING / MONITORING AGENCY NAME(S) AND ADDRESS(ES)

Headquarters, U.S. Army Corps ofEngineers

ATTN: (CEMP-CE)

20 Massachusetts Avenue NW

Washington, DC 20314-1000

5. FUNDING NUMBERS

CPAR

LX4

8. PERFORMING ORGANIZATION

REPORT NUMBER

TR 98/81

10. SPONSORING / MONITORING

AGENCY REPORT NUMBER

11. SUPPLEMENTARY NOTES

Copies are available from the National Technical Information Service, 5285 Port Royal Road, Springfield, VA 22161.

12a. DISTRIBUTION / AVAILABILITY STATEMENT

Approved for public release; distribution is unlimited.

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13. ABSTRACT (Maximum 200 words)

This research investigated a new concept that uses fiber-reinforced plastic (FRP) composite grid to reinforce concrete

structural members. Prefabricated two- and three-dimensional FRP grid structures were investigated as a possible

alternative to conventional one-dimensional steel reinforcement rods. Currently available commercial grid manufacturing

techniques were found to be inadequate due to material flaws, poor fiber volume fraction, and low strength and stiffness.

Through laboratory investigations, significant improvements in fiber volume fraction in orthogrid and isogrid systems were

achieved. Laboratory-scale samples demonstrated excellent results under loading tests.

Concurrent investigations showed that although the FRP grid-reinforced concrete is more flexible than steel-reinforced

concrete, its postfailure deformation was pseudo-ductile, characterized by continuous structural deformation through

multiple low-level brittle failures before the onset of catastrophic failure. It was also found that a combined

concrete/composite reinforcement structure, with a higher volume ofFRP composite fraction in the concrete, would

substantially increase stiffness, load capacity, and postfailure concrete containment.

This study addressed not only the possible replacement ofsteel reinforcement with composite grids, but also investigated

enhancement ofthe composite application through load-sharing with steel reinforcement in a complementary fashion.

Various manufacturing improvements also were explored, including the novel use of disposable toolings.

14. SUBJECTTERMS

CPAR

composite materials

fiber-reinforced plastic (FRP)

concrete

structural engineering

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ABSTRACT

SAR

Standard Form 298 (Rev. 2-89)

Prescribed by ANSI Std 239-18

298-102
jv USACERL TR 98/81

Foreword

This study was conducted for Headquarters, U.S. Army Corps of Engineers

(HQUSACE) under the Construction Productivity Advancement Research

(CPAR) Work Unit LX4, "Composite Grid/Frame Reinforcement for Concrete

Structures." The U.S. Army Construction Engineering Research Laboratories

(USACERL) was the Corps Partner and the U.S. Army Cold Regions Research

and Engineering Laboratory (USACRREL) was a Corps Partner participant.

The technical monitors were M.K. Lee (CECW-EG), D. Chen (CEMP-ET), and T.

Wilford (CEMP-CE).

This work was executed through a CPAR Cooperative Research and

Development Agreement (CPAR-CRDA) between the Materials Science and

Technologies Division (FL-M), USACERL, and the Composites Design Center,

Stanford University (SU). The USACERL Principal Investigator was David M.

Bailey, CECER-FL-M, and the co-Principal Investigator was Dr. Piyush Dutta,

USACRREL. The SU Principal Investigator was Dr. Stephen W. Tsai. The

following industry firms and other organizations participating with the CPAR

Industry Partner are acknowledged for their technical contributions and in-kind

support for this project:

Brigham Young University—David Jensen, testing and analyses

RL Industries, Fairfield, OH—Steven Linnemann (President), filament winding

Fiberflex, Houston, TX—Steve Morris (Vice President), pultruded FRProds

TPI, Warren, RI—Everett Pearson (President), vacuum infiltration processing

Fibergrate, Richardson, TX—composite grating

University ofZaragoza, Spain—Prof. Antonio Miravete, composites applications

Kanazawa Institute ofTechnology, Japan—Prof. Yasushi Miyano, life prediction modeling

Hong Kong University of Science and Technology—Prof. Ricky Lee, grid design

Hanyang University, Seoul, Korea—Prof. Sung-Kyu Ha, analytical modeling

SAMTECH, Liege, Belgium—Yves Lombard, finite element analysis

Dr. Ilker R. Adiguzel is Acting Chief, CECER-FL-M, and L. Michael Golish is

Acting Operations Chief, CECER-FL-M. The USACERL technical editor was

Gordon L. Cohen, Technical Information Team.

COL James A. Walter is the Commander of USACERL, and Dr. Michael J.

O'Connor is Director.
USACERLTR-98/81

Contents

SF298 iii

Foreword iv

List of Tables and Figures ix

1 Introduction 1

Background 1

Objective 2

Approach 2

Units of Weight and Measure 2

2 Technology Overview 4

Steel-Reinforced Concrete 5

Durability of Steel Reinforcement 6

Fiber Reinforced Plastic (FRP) Composites 7

FRP Reinforcement for Concrete 8

FRPRebar g

Bonded FRP Plates 11

Durability of FRP 12

GeneralAging 13

Moisture 13

Freeze-ThawEffects 14

Ultraviolet Radiation 15

Alkaline Environment. 75

Creep Behavior. 15

Fatigue 17

Fire Hazards and Flammability. 18

Economics of FRP Reinforcement 18

3 Grid Reinforcement Concept 20

Review of Composite Grid Reinforcement Research 21

Grid Geometry and Design 23

Grid Types and Terminology 24

Grids as Structures 25

Composite Material Grids 26
vi USACERL TR 98/81

Previous Composite Grids 27

Properties and Behavior of Composite Grids as Concrete Reinforcement 28

Ductility 28

Strength and Stiffness 28

FRP Reinforcement Bond Characteristics 29

Flexural Behavior. 30

Analysis Of Grid Reinforcement 33

Geometric Parameters 34

Mechanical Properties of Grids 35

Stiffness of Quasi-lsotropic Laminates 36

Stiffness ofIsogrids 37

Composite Grid Reinforcement Design Approach 38

GENLAM Modeling for Concrete-Filled Grids 39

Manufacturing the Composite Grids 42

BYU Direct Crossover Interlaced Joints 43

Lay-up Procedure 43

Cure Procedure 44

Strain Gage Placement 44

Stanford Pin Enhanced Geometry (PEG) Process 45

PEG Assembly. 46

Manufacture ofLarge Orthogrid (n/2) Structures 48

Manufacture ofLarge Isogrid (n/3) Structures 48

Stanford Tooling-Reinforced Interlaced Grid (TRIG) Process 49

TRIG Manufacturing Methodology 49

Advantages ofthe TRIG Method. 50

Disadvantages ofthe TRIG Process 51

Integration of PEG Techniques into the TRIG System 51

Load Testing of TRIG Grids 51

Material Properties 52

Panel Properties 52

Concept Development for Full-Scale TRIG Grid Production 54

Grid-Reinforced Concrete (GRC) Panels 56

GRC Testing at BYU 56

Description of Test Specimen Geometry. 57

The Test Matrix 57

Manufacturing the Concrete Beams 58

Test Fixture Setup 59

Flexural Specimen Testing 59

Test Results 60
USACERLTR-98/81 vii

Results from Steel Rebar Reinforced Beams 61

Results from Commercial Carbon Grid Reinforced Beams 62

Results from Commercial GFRP Grid Reinforced Beams 62

Results from BYU Plain Carbon Grid Reinforced Beams 62

Results from BYU Deformed Carbon Grid Reinforced Beams. '. 63

Results from Hybrid Carbon/Steel Grid Reinforced Beams 63

Results from Steel Grid ReinforcedBeams 63

Overall Comparison 63

GRC Investigations at Stanford 65

Testing 65

Model Via GENLAM '. 66

Demonstration and Result. 68

GRC Panel Testing at Stanford 69

Grid Behavior. 70

GRC Behavior 70

GRC Toughness 70

GRC Concrete Containment 71

Summary 71

7 Column-Reinforcing Grids 72

StackedJoint Grids 72

StackedJoint Grid Specimen Manufacturing 73

Test Matrix 74

Manufacturing the Concrete Columns 74

Curing 75

Compression Samples 75

Experimental Procedure 76

First-Generation Columns, 2.0 in. Spacing 77

First-Generation Columns, 4.0 in. Spacing 77

Second-Generation StackedJoint Grids 78

Discussion of Results 79

Failure Modes 79

Stress-Strain Behavior. 79

Load Transfer Effects. 80

8 Conclusions, Recommendations, and Commercialization 81

Conclusions 81

Design Issues 82

Manufacturing Issues ;....83

Recommendations 84

Technology Transfer and Commercialization 85
viii USACERL TR 98/81

References 86

Tables and Figures Cited in Report 93

Distribution
USACERLTR-98/81 ix

List of Tables and Figures

Tables

Table 1. Tensile strength and bending requirements of concrete reinforcing steel

(billet steel bars) 95

Table 2. Comparison of mechanical properties of steel and FRP rebar (Faza

1995) 96

Table 3. Fire tests of building materials 96

Table 4. Comparisons of stiffness of isotropic laminates and isogrids for three

materials on absolute and specific bases 97

Table 5. Reinforcement areas and theoretical mechanical properties 97

Table 6. Comparison of ultimate load, effective stiffness, deflection, and toughness

results for all beams 98

Table 7. GENLAM materials 98

Table 8. 13 mm beam lay-ups 98

Table 9. 38 mm beam lay-ups 99

Table 10. Grid-only load/deflection slopes 99

Table 11. Average load-deflection slopes of GRC panels and frames 99

Table 12. Concrete column specimens included in test matrix 100

Table 13. Comparisons of column specimens A, B, C, and K 100

Table 14. Comparisons of column specimens D, E, F, and K 101

Table 15. Comparisons of column specimens G, H, I, M, and P. 101

Figures

Figure 1. Deformed steel reinforcing bars 102

Figure 2. FRP tensile coupon test specimens 102

Figure 3. Commercially available FRP reinforcing rods 103

Figure 4. Moisture absorption of polymer composites 103

Figure 5. Schematic showing isometric view of isogrid structure 104

Figure 6. Schematic showing isometric view of orthogrid structure 104

Figure 7. Basic grid structure and terminology. 105

Figure 8. Example of isogrid configuration 105

Figure 9. Plot of specific strength and specific stiffness for FRP and metals 106
USACERL TR 98/81

Figure 10. Interlace of fiber rovings through anode 106

Figure 11. Typical steel and FRP stress-strain characteristics 107

Figure 12. Results of bending tests of steel, cfrp and hfrp specimens 107

Figure 13. Configurations for square grids, angle grids, and isogrids 108

Figure 14. Rib areal fractions of square grids and isogrids 108

Figure 15. Definition of area fraction (f) of square and isogrids 108

Figure 16. Isogrid portrayed as a regrouped laminate 109

Figure 17. Beam diagram for modeling 109

Figure 18. Composite grids with slotted joints 110

Figure 19. Typical relations for orthogrid 111

Figure 20. Isogrid with offset nodes 111

Figure 21. Interlaced nodes 112

Figure 22. Geometry of the plain (a) and deformed (b) orthogrids 113

Figure 23. Molds for plain (top) and deformed (bottom) orthogrids 113

Figure 24. Schematic drawing of winding process 114

Figure 25. Placement of strain gages and fiber optic sensors 114

Figure 26. Rib flaws 114

Figure 27. Central node pin 115

Figure 28. Central node pin with 1.3-degree lead-in angle 115

Figure 29. Two lay-up methods 116

Figure 30. First PEG specimen 116

Figure 31. PEG mold design steps 117

Figure 32. Sample PEG winding pattern 118

Figure 33. PEG Isogrid mold and specimen 118

Figure34. 7X7TRIGpanel 119

Figure 35. TRIG panel mold 119

Figure 36. Conventional processing cycle 120

Figure 37. TRIG processing cycle 120

Figure 38. Test specimens — commercial, reinforced commercial, and TRIG grids. ...121

Figure 39. Tensile modulus of test specimens 121

Figure 40. Panel bending test 122

Figure 41. Panel twisting test 122

Figure 42. Results from panel bending test 122

Figure 43. Results from panel twisting test 123

Figure 44. Tooling for grid block 123

Figure 45. Grid mold 123

Figure 46. Sliced grid block 124
USACERLTR-98/81 xi

Figure 47. Cross-section of reinforced concrete beams 124

Figure 48. Test configuration for reinforced concrete beams 125

Figure 49. Uneven crack distribution in grid-reinforced concrete 126

Figure 50. Four-point load test fixture 126

Figure 51. Deflection lever arm 127

Figure 52. Load-deflection curve showing calculation of effective stiffness 127

Figure 53. Load vs deflection for steel rebar reinforced beam 128

Figure 54. Expanded load vs deflection for steel rebar reinforced beam 128

Figure 55. Load vs deflection for commercial carbon grid reinforced beam 129

Figure 56. Expanded load vs deflection for commercial carbon grid reinforced

beam 129

Figure 57. Load vs deflection for fiberglass grid reinforced beams 130

Figure 58. Expanded load vs deflection for fiberglass grid reinforced beams 130

Figure 59. Load vs deflection for in-house plain carbon grid reinforced beams 131

Figure 60. Expanded load vs deflection for in-house plain carbon grid reinforced

beams 131

Figure 61. Load vs deflection for in-house deformed carbon grid reinforced

beams 132

Figure 62. Expanded load vs deflection for in-house deformed carbon grid

reinforced beams 132

Figure 63. Load vs deflection for in-house hybrid carbon grid reinforced beams 133

Figure 64. Expanded load vs deflection for in-house hybrid carbon/steel grid

reinforce beams 133

Figure 65. Load vs deflection for steel grid reinforced beams 134

Figure 66. Expanded load vs deflection for steel grid reinforced beams 134

Figure 67. Overall load vs deflection comparison 135

Figure 68. Expanded load vs deflection comparison 135

Figure 69. Identification of load and deflection values for crack propagation

photographs 136

Figure 70. Crack propagation in in-house carbon grid reinforced beam at 6000 lb 136

Figure 71. Crack propagation in in-house carbon grid reinforced beam at 13,000

lb. 137

Figure 72. Crack propagation in in-house carbon grid reinforced beam at 17,600

lb (failure) 137

Figure 73. Effective precracking stiffness of the different beams tested (each

symbol represents results from a single test) 138

Figure 74. Concrete compressive strength for each beam type 138

Figure 75. Post-cracking effective stiffness of the different beams tested 139

Figure 76. Axial rigidity (EA) of reinforcement 139
xii USACERL TR 98/81

Figure 77. Linear relationship between post-cracking stiffness and reinforcement

rigidity. 140

Figure 78. Ultimate load vs toughness for the different beam types tested 140

Figure 79. Grid reinforced concrete slab (concrete contained within a bi-grid

frame) 141

Figure 80. Bi-grid frame construction 141

Figure 81. Thick GRC slab 142

Figure 82. GRC testing methods (the upper two test compressive and tensile

properties; beams test stiffness models; panels test GRC structural behavior) 142

Figure 83. GRC beam cross-sections 143

Figure 84. 13 mm GRC beam data 143

Figure 85. GRC compression test specimen 143

Figure 86. Basic laminate and expected stress distribution 144

Figure 87. Modified material stacking sequences 144

Figure 88. 13 mm beam stress distribution 144

Figure 89. 13 mm beam predicted results 145

Figure 90. 38 mm beam stress distribution 145

Figure 91. 38 mm beam predicted vs. experimental results 145

Figure 92. Summary of final beam lay-ups 146

Figure 93. Panel bending and twisting tests 146

Figure 94. Testing data for grid structures 146

Figure 95. Grid vs GRC load/deflection slopes 147

Figure 96. Testing data for grid structures 147

Figure 97. Failure curve of a 38 mm dual-grid GRC panel (toughness is

demonstrated through the post-yield behavior) 148

Figure 98. Failure toughness of GRC panels (demonstrating repeatability of the

GRC toughness in multiple panel thicknesses and loading conditions) 148

Figure 99. Concrete containment through first yield in a 13 mm panel 149

Figure 100. Concrete containment through failure in 13 mm panel 149

Figure 101. Concrete containment through failure in a 38 mm panel 149

Figure 102. Stacked joint grid 150

Figure 103. Stacked joint grids of various geometric configurations 151

Figure 104. End failure for of a first-generation column 152

Figure 105. Load vs time graph for 2.0 in. spacing of fiber-reinforced columns 152

Figure 106. Stress vs longi strain for 2.0 in. spacing of fiber-reinforced columns 153

Figure 107. Stress vs circ strain graph for 2.0 in. spacing of fiber-reinforced

columns - 153

Figure 108. Stress vs longi strain graph for 4.0 in. spacing of fiber-reinforced

columns 154
USACERLTR-98/81 xiü

Figure 109. Stress vs circ strain graph for 4.0 in. spacing of fiber-reinforced

columns 154

Figure 110. Load vs time for second-generation fiber-reinforced columns 155

Figure 111. Stress vs circ strain for second-generation composite fiber-reinforced

column 155

Figure 112. Stress vs longi strain for second generation composite fiber reinforced

column 156

Figure 113. Ultimate load vs number of longis for fiber-reinforced concrete

columns 156

Figure 114. Comparison of stacked grid composite grid structures in concrete

columns 157

Figure 115. Separation of external concrete from composite 157
USACERLTR-98/81

1 Introduction

Background

Traditional steel reinforcements of concrete, and their currently anticipated

composite replacements, are circular rods. The rod surfaces must be tied

together mechanically and their ends must be bent to provide additional surface

for good anchorage. Grid structures are receiving increasing attention as generic

structural forms for a variety of applications. These structures may offer many

advantages when used as reinforcement. This mechanism of reinforcement is

fundamentally different from reinforcement rods (or "rebars") used in

conventional concrete. With reinforcing rods, load transfer is one-dimensional,

achieved by shear lag or the interfacial friction between concrete and

reinforcements. With a reinforcing grid, load transfer is two- or threedimensional. The ribs transverse to the applied load provide the transfer ofload

between concrete and reinforcements. The latter transfer mechanism is more

efficient, more evenly divided throughout the entire structure, and can resist

bidirectional or tridirectional stresses resulting from freezing (i.e., microcracking). Steel reinforcement of concrete is susceptible to corrosion and fatigue

problems. Concrete reinforced with composite materials can solve these

problems. Productivity is automatically enhanced because composite reinforcements weigh less than steel reinforcements. Transportation, storage, and

erection can all be accomplished at lower costs. Additional cost reduction is

possible for using prefabricated grid reinforcements because a very large

prefabricated section can be easily lifted and positioned in place. Field assembly

of grid reinforcements is also cheaper and faster than conventional reinforcements made by adhesively bonded joints.

Developing an entirely new class of concrete reinforcement that is corrosionresistant and less expensive has the potential to save the U.S. Army Corps of

Engineers millions of dollars annually in construction and eliminated corrosion

related maintenance costs. Potential savings for the U.S. construction industry

could amount to many times that total.

Under the Corps of Engineers Construction Productivity Advancement Research

(CPAR) program, a Cooperative Research and Development Agreement (CPARCRDA) was initiated to investigate and demonstrate the application of composite
USACERL TR 98/81

grid reinforcement in a full-scale reinforced concrete structure. The Corps

partner laboratories were the U.S. Army Construction Engineering Research

Laboratories (USACERL) and the Cold Regions Research and Engineering

Laboratory (USACRREL). The industry partner was the Composites Design

Center, at Stanford University, CA.

Objective

The objective of this research was to develop and demonstrate the use of

grid/frame1structures made of unidirectional composite materials as reinforcements for concrete.

Approach

The project was divided into the following tasks

• Survey and evaluate existing methods of concrete reinforcement and identify

physical performance requirements for composite grid frames.

• Develop preliminary designs for family of grid/frame systems.

• Perform lab-scale mechanical performance and environmental exposure tests

on grid/frame specimens.

• Develop software package for designing composite grid frames for structural

concrete applications.

• Design and construct full-scale demonstration of a composite grid/frame

reinforced concrete structure.

• Monitor and evaluate performance.

• Complete final technical report.

Units of Weight and Measure

U.S. standard units of measure are used throughout this report. A table of

conversion factors for Standard International (SI) units is provided below.

' In this report, "grid/frames" are referred to as "grids" for brevity.
USACERLTR-98/81

SI conversion factors

1 in. = 25.4 cm

1ft = 0.305 m

1 sq in. = 6.452 cm2

1 sqft = 0.093 m

2

11b = 0.453 kg

1 kip = 453 kg

1 psi = 6.89 kPa

°F = (°Cx1.8) + 32
USACERL TR 98/81

2 Technology Overview

FRP grid structures are made with interlaced unidirectional fiber composites.

Work at the U.S. Air Force Phillips Laboratory has proved that grid construction

with continuous graphite fibers gives exceptional stiffness and lighter weight to

flat or curved panels, and such panels have found ready applications in space

structures (Koury and Kim 1991; Koury et al. 1993). Following recent serious

concerns in the civil engineering community about the deterioration of

thousands of bridge decks, application of continuous-fiber composite grids as

concrete reinforcement for bridges in the United States was first proposed by

Koury and Dutta (1993). It was proposed that despite possible higher material

cost, fiber-composite grids can offer a competing advantage of higher speed of

construction, and can present a viable alternative to steel reinforcement of

concrete. Similar grid-type reinforcement has been under experimental

investigation in Japan, and some successes have been reported by Sugita (1993).

In the United States, field tests of the concept using the Japanese grids were

performed by Schmeckpeper and Goodspeed (1994) with encouraging results.

The first serious attempt to develop the technology in the United States, which

this report describes in detail, was discussed by Dutta and Bailey in 1995.

Grids made of composite materials offer low mass with high stiffness and

strength that are competitive with traditional composite laminates. Commonly

available manufacturing processes such as filament winding, pultrusion, and

tubes made from female molds are used to produce composite grids. Costeffective grids can then be made in large sizes and quantities. Grids derive their

global stiffness and strength from their ribs. The models for stiffness and failure

modes can be viewed as simple extensions of laminated plate theory. Simple

formulas for calculating grid stiffness will be shown in this report. It is foreseen

that grids will emerge as one of the common composite structural forms along

with solid, stiffened, and sandwich panels.

The most immediate potential payofffor FRP grid reinforcement is likely to be in

structural applications that involve environments that are detrimental for steel

use and that do not have restrictive deflection requirements. Applications in

areas of harsh chemical exposure (e.g., salt) or restrictive electromagnetic

requirements offer high potential payback for FRP grid use.
USACERLTR-98/81

In order for the FRP grid technology to gain widespread acceptance it will be

necessary to develop viable grid manufacturing processes. Such processes must

lower the direct cost of manufacture as well as the cost of field usage. To

facilitate field use, the manufacturing process must be sufficiently flexible to

permit grid size and fiber volume to be varied in a manner that supports the

structural design process.

This chapter presents a brief overview of why reinforcement of concrete is

needed, how the reinforcing steel gets corroded, how corrosion affects the

structure, and why the alternative reinforcing material (FRP composite) would

be more suitable. Finally, the anticipated problems and durability issues of the

FRP composites are discussed.

Steel-Reinforced Concrete

The ancient Egyptians realized the importance ofreinforcing brittle cementitious

materials with stronger tensile reinforcement. Large clayey blocks, reinforced

with straw, were used to build some ofthe largest and most enduring structures

in the world. Today, concrete is one of the most widely used materials in

construction. Its high compressive strength, ability to be molded, and the

abundance of its raw constituent materials make concrete a very versatile

structural and architectural component.

Due to its small tensile capacity, concrete must be reinforced with materials

strong in tension. Traditionally, concrete has been reinforced with slender,

cylindrical steel reinforcing bars (or rebars). These bars are placed in the

structural members at locations where bending, torsional, or axial forces will

produce internal tensile stresses in the member. To transfer the forces from the

concrete to the steel reinforcement, a good shear bond must be developed

between the concrete and steel. Steel reinforcing bars are manufactured with

small surface deformations to enhance shear transfer (see Figure 1). Even with

these deformations the steel rebar must be of adequate length to develop the full

capacity of the bar. This is often difficult or impossible due to design

considerations, making it necessary to use bends or hooks to provide adequate

anchoring ofthe bars.

The combination of concrete reinforced with steel provides a very durable

building material that is well suited for the construction industry and is

probably the most versatile material available for building structures. It can be

formed on-site to almost any shape or size and can be designed to the strength

necessary to withstand predicted loads and stresses. The basic advantage of
USACERL TR 98/81

reinforcing concrete with steel is to compensate for its weak tensile strength.

Another important advantage includes reduction in the likelihood of catastrophic

failure due to the ductile properties of steel and the consequent reduction and

control of cracking in the concrete.

Durability of Steel Reinforcement

In ideal terms, the combination of steel and concrete are complementary. The

properties of reinforcing steel are given in Table 1. The steel provides the

strength and the alkaline nature of the concrete provides the passivation to

protect against oxidation (rusting) of the reinforcing steel. However, in order for

this complementary system to properly function, deteriorative environments

within the concrete surrounding the steel must be prevented. Deteriorative

conditions created by factors inherent in the concrete and by imposed

environmental conditions subsequently lead to corrosion of the reinforcing steel.

The effects ofsteel corrosion not only compromise the integrity of the reinforcing

steel but also, in turn, further aggravate and accelerate deterioration within the

concrete.

Steel corrosion is the result of iron oxide scale (rust) formation on the surface of

the steel that normally develops when steel is exposed to air. During the

hydration of concrete, alkaline compounds are liberated from the cement

component, and these form a protective film around the reinforcing steel. Within

this matrix, approximate pH values between 12.5 and 13.6 allow the steel to

remain passive. The cement matrix also provides a barrier against the intrusion

of moisture and oxygen, thus providing physical and chemical protection of the

steel. However, aggressive deteriorative agents that attack and destroy the

cement matrix (e.g., acids, chlorides, sulfates, etc.) cause reduction in pH values

and subsequent depassivation of the steel. Therefore, it is important that the

integrity of the cement matrix (and thus the state of passivity) remains intact to

avoid initiation of steel corrosion. The prevention of corrosion is highly

dependent on the properties of the surrounding concrete. Such properties

include concrete cover, water/cement ratio, permeability, and consolidation.

Common factors that promote aggressive attacks include improper mix design,

poor placement techniques, and contaminated materials.

Significant problems involving corrosion of reinforcing steel began to emerge in

the 1960s. Solutions to the problem have included galvanization of the

reinforcing steel, polymer concrete, epoxy-coated rebar, low-permeability

concrete, and FRP rebar. The galvanized reinforcing steel has been found to lose

its corrosion protection due to an electrolytic reaction between the steel and the
USACERLTR-98/81

zinc-based coatings. Polymer concrete has been found not to be compatible with

steel reinforcement because of the large differences in thermal properties.

Epoxy-coated rebar is used widely today despite concerns about damage from

nicks and scratches that could lead to corrosion concentrations and doubtful

bond (development length) properties. The cost of low-permeability concrete is

almost twice that of standard concrete, but it is still a common remedy for

corrosion problems. Composite reinforcements are currently making their way

into the spotlight.

Fiber Reinforced Plastic (FRP) Composites

The class of materials designated as FRP composites has been defined by the

American Concrete Institute (ACI 440R-3, 1996) as "a polymer matrix, whether

thermosetting (e.g., polyester, vinylester, epoxy, phenolic) or thermoplastic (e.g.,

nylon, PET), which is reinforced by fibers (e.g., aramid, carbon, glass)." Glassfiber reinforced plastics are usually referred as GFRP and carbon-fiber

reinforced plastics are called CFRP. The performance of any FRP composite

depends on the materials ofwhich the composite is made, the arrangement ofthe

fibers, and the interaction ofthe fibers with the matrix.

The most common types of FRP used in civil and structural applications today

use glass fibers within a vinyl ester resin matrix. Glass fibers are used because

oftheir low price and high strength. The tradeoff between glass and other fibers

is price for stiffness. Carbon fibers provide significantly higher stiffness, but at a

much higher price. As new applications evolve and as new processing methods

are developed, the price of carbon fibers is expected to come down. When this

happens carbon will become the fiber of choice. "Vinyl ester resins are slightly

more expensive than other resins, such as polyesters. The high strength and

durability ofvinyl ester resins, however, warrant the extra costs.

Composite structures can be fabricated by many methods. In civil engineering

applications it is essential that the most cost-effective method is followed. The

current methods of producing composite structures include pultrusion, filament

winding, resin transfer molding (RTM), fiber placement and other lay up

processes. In all these processes the aim is to achieve a balanced stiffness and

strength in multiple directions. For this purpose it is essential that

unidirectional fibers be placed in multiple directions. Such homogeneous

structure inhibits delamination failure. Construction applications of FRP

composites have been recently surveyed by Kant et al. (1997).
USACERL TR 98/81

The mechanisms that contribute to the composites failure are well known. They

include (1) chemical or physical changes, or microcracks of polymer matrix, (2)

loss of adhesion (debonding) at the fiber/matrix interface, and (3) reduction of

fiber strength and modulus. These processes contribute to the growth of flaws or

cracks that can result in eventual failure. For unidirectional composites, the

compressive failure mechanisms include microbuckling of fibers (Rosen 1965;

Greszczuk 1975; Davis 1975; Hahn and Williams 1984), kink banding (Berg and

Salama 1973; Hahn et al. 1986), matrix yielding (Piggott 1981), and shear

through both the fiber and matrix (Hancox 1975). The tensile failure is

accompanied by brittle failure of the matrix, fiber pullout, interface matrix shear

failure, and constituent debonding (Chamis 1974). However, in most cases of

multidirectionally reinforced composites, the initial failure takes the form of

cracking of constituent plies; that is, cracks appear parallel to fibers long before

the final failure of the laminate. The initial failure is very often referred as the

first-ply failure (FPF). The FRP tensile coupon test specimen on the left in

Figure 2 shows an examples of the first ply failure, and the coupon on the right

shows the complete failure.

FRP Reinforcement for Concrete

While there have been numerous papers written on the topic of composite

reinforcement, the concept is still relatively new and unexplored. ACI

Committee 440 has recently reported on the general aspects of using FRP for

concrete reinforcement (ACI 440R-96). This committee's report summarizes the

findings of research that has been done to date on all types of composite

reinforcement for concrete structures. While most ofthe previous work has been

done on FRP reinforcing bars, which are modeled after deformed steel

reinforcing bars, many of the conclusions are applicable to other types of

composite reinforcement as well.

Advantages of composite reinforcement for concrete include corrosion resistance,

high specific strength, high specific stiffness, electromagnetic permeability, and

impact resistance. In fact, the largest market today for composite reinforcing is

in the medical fields. The use of composite reinforcement around MRI (magnetic

resonance imaging) medical equipment eliminates the magnetic interference

found with steel reinforcement. However, the primary motivation for research in

this field has been the durability or corrosion problems associated with steel

rebar.

In 1986, the first FRP reinforced concrete bridge was built in Germany (ACI

440R-96; Taerue 1993). At the end of 1993 there were nine companies actively
USACERLTR-98/81

marketing FRP reinforcing bars in North America. As the demand increases for

composite reinforcement the market will respond with high-quality, readily

available products as well as high-tech, cost effective manufacturing techniques.

The ACI report concludes that the basic engineering principles generally applied

to the design of steel-reinforced concrete can effectively be used to design FRPreinforced concrete. These principles include (1) stress equilibrium on the crosssection, (2) strain compatibility between the reinforcement and the concrete, (3)

the Kirchoff hypothesis (plane sections remain plane) and, for the concrete, (4)

the Whitney rectangular stress block to approximate the concrete stress

distribution. Designers are warned of the differences between steel and FRP

reinforcement, the main difference being the lack of ductility in advanced

composites. The ACI report suggests that adequate reduction factors or possibly

limiting ultimate strains might compensate for the lack of warning prior to

failure. Lack of energy absorption due to elastic response also needs to be

accounted for in seismically excited structures. In any case, designers should be

familiar with the engineering characteristics of the materials in their designs

and apply appropriate factors of safety. The following sections summarize some

ofthe research taking place in the field ofFRP reinforcement of concrete.

FRP Rebar

Perhaps the most common type of composite reinforcement currently being

implemented is composite reinforcing bars (Bakis 1993). These commercially

available FRP reinforcing bars, one type of which is shown in Figure 3,

generally mimic steel rebar in their shape and size. The bars are made by

pultruding fibers and resin through a round die to produce slender, cylindrical

rods of unidirectional fibers. An additional bundle of fibers wrapped helically

around the rods produces a spiral deformation to provide a good bond between

reinforcement and concrete. Research conducted by Larralde et al. (1989)

investigated the similarities between composite and steel rebar reinforced

beams. Their results showed that theoretical values for ultimate load and

deflections did not correlate with actual values. The difference in ultimate loads

is attributed to the failure mode being controlled by diagonal tension rather than

tensile failure of the reinforcement. Research by Chaalal and Benmokrane

(1996) showed a much better correlation between both ultimate load and ACI

deflection predictions. The ACI code, however, overestimates the effective

moment of inertia and does not account for a significantly larger number of

cracks in the FRP reinforced beams.

Wu et al. (1990) have reported that E-glass reinforced composite bars may have

tensile strength in excess of 100 ksi and longitudinal elastic modulus of about
10 USACERL TR 98/81

7500 ksi. In tensile tests the bars fail without any significant yield (brittle

failure). Table 2 gives a comparison of mechanical properties of the steel rebars

and the FRP rebars.

Currently there are several FRP rebar companies actively marketing their

products in the United States. Most FRP rebars contain by volume about 55

percent E-glass fiber and about 45 percent thermoset resin. Faza (1995) has

reported a number of successful applications of rebars in the United States.

These include applications in seawalls, medical MRI facilities, reactor pads,

compass calibration pads, mill roofs, laser test facilities, highway barriers,

residential foundations, and bridge decks.

One of the most critical developments necessary for successful large-scale

applications of FRP rebars is improvement of FRP bond strength with concrete.

As stated before, some designs provide a helically convex surface made with a

strand spirally-wound and cured on the surface. Other designs use sand or grit

coating on the rebars. A recent design includes a pultruded, ribbed surface. The

authors are not aware of comparative survey data on the bond quality of these

various surface modifications.

There are several major barriers to widespread FRP rebar applications,

including lack of sufficient durability or performance data under extreme

environments. Creep, fatigue, and corrosion from concrete's alkaline

environment need to be investigated. Kumar et al. (1995) have studied the

fatigue performance of concrete slabs with FRP rebars and concluded that the

loss of stiffness was significant. Other impediments to the widespread use of

FRP rebars include high material costs, lack of design guidelines, and lack of

product quality and industry standardization. Recently, standardizing

committees have been formed by ACI, ASTM2, and ASCE3to address the issues

of developing materials standards, test methods, design specifications, and

design guidelines. Some researchers (Faza 1995; Kumar et al. 1995) have

recently presented equations for evaluating FRP rebars and designing with

them.

2 ASTM: American Society for Testing and Materials.

3 ASCE: American Society of Civil Engineers.
USACERLTR-98/81 11

Bonded FRP Plates

A vast majority of aerospace applications employ composite materials in a

laminated form. Thin sheets of unidirectional fibers preimpregnated with resin

are stacked on top of one another and bonded together through curing to form a

two-dimensional composite member.

Similar to the bonding of steel plates for repairing concrete structures, FRP

laminates have also been considered for use in such repairs. The associated

advantages as described before are obvious in terms of lighter weight and

corrosion resistance. In Switzerland very promising results have been obtained

by using CFRP for concrete repair (Meier and Kaiser 1991). Because such

repairs involve only small amount of FRP composites, the high-cost carbon fiber

composites are the most preferred system. Prestressing of the bonded

strengthening elements improves the performance. In the United States, similar

work has been reported by Triantafillou and Plevris (1991). They have shown

that strengthening or repair by FRP bonding to the concrete structure can

improve the load-carrying capacity as well as the stiffness. Concerns in this

technique revolve around the behavior under sustained loading, fatigue, thermal

cycling, and humidity cycling. Recently Shahawy (1995) reported results for load

cycling of a few FRP-repaired beams which clearly show a substantial increase in

both the stiffness and the ultimate strength capacity ofthe repaired beams.

Research conducted by the Florida Department of Transportation Structures

Research Center and the Florida Atlantic University Department of Ocean

Engineering (Shahawy et al. 1996) and at the University of Delaware (Kaliakin

1996) shows the effectiveness ofthis type ofstrengthening. Both research efforts

effectively predicted the behavior of composite-strengthened beams using nonlinear finite element analysis. The research done at the Florida Department of

Transportation and Florida Atlantic University focused on tension reinforcement

and strengthening by applying composite laminates to the bottom surface of

rectangular beams. The University of Delaware, on the other hand, wrapped

composite plies around the web ofreinforced concrete tee beams resulting in both

tensile and shear reinforcement. Both experiments resulted in significantly

increased stiffness and strength. Recently, research under a separate CPAR task

group produced a technical report documenting concrete repair and external

concrete reinforcement (Marshall 1998).

A widely used technique for retrofit of concrete columns is the placement of

cylindrical steel jackets around the existing columns (Priestley and Seible 1991).

To create a constraining effect, epoxy or concrete is pumped into the gap between

the existing column and the jacket. A major disadvantage of this method is that
12 USACERL TR 98/81

it is very time-consuming and difficult. Furthermore, the steel jackets ultimately

corrode. However, the confining action is effective in increasing both

compressive strength and ductility of concrete structure. The use of FRP

composite wraps as confining layers for concrete columns has thus become an

increasingly popular idea in recent years (Harmon and Slattery 1992; Priestly et

al. 1992; Hasegawa et al. 1992; Katsumata et al. 1988; Karbhari et al. 1993; Ma

1993).

Various FRP materials for column wrappings have been tried in the recent past.

These include glass, aramid, and CFRP. The efficiency ofthe process depends on

the final compaction of the fabric on the column. Most commonly used methods

of wrapping include a variation of hand lay-up and wet lay-up/winding. An

automated lay-up/winding system has also been developed. To improve the

compaction of the FRP on the concrete column, Karbhari et al. (1993) have

recently reported the resin-infusion technique. In this process the resin is

injected under a vacuum film using a resin distribution medium. Since the

vacuum is maintained until the resin is cured, the compaction level is generally

high.

Durability of FRP

The durability issue of FRP composites has remained an open question to

composite structural designers. Durability of FRP composites has recently been

reviewed by several researchers (GangaRao et al. 1995; Dutta 1995; Dutta and

Hui 1996; GangaRao and Dutta 1996). Many factors play roles in reducing the

life of an engineering component.

Characteristics of most materials change by interaction with their surroundings

over time. For example, oxidation of steel leads to rusting; rubber ages and

becomes brittle under the effect of ozone; and loss of water leads wood to split.

Therefore, before using composites, the designer must know what changes are

likely to occur to FRP composites under given service conditions.

It is also important to define the effective service life of the materials. Polymeric

composite materials may not fail or separate under a certain service loading

condition, but can continue to deform or deflect beyond an acceptable limit.

Thus, the effective service life ceases when a predetermined material

deformation or deflection limit is exceeded.

Environment plays a crucial role in changing the properties of polymer matrix

composites. The environment includes both the ambient environment and the
USACERLTR-98/81 13

mechanical loading environment, because both can affect the durability of

composites. Considering the ambient environment, both matrix and fibers may

be affected by moisture, temperature, solar ultraviolet (UV) radiation, and

presence of degrading chemicals like salts or alkalis. Repeated excursions to

very high and low temperatures (freeze-thaw cycling) may introduce some

changes. Under a mechanical loading environment, as for steel, repeated

loading may introduce fatigue in composites. A sustained load over time may

cause the material to creep. The following discussions briefly summarize the

effects ofvarious service factors that affect the durability ofFRP composites.

General Aging

Durability of FRP composites must be considered in the context of the role ofthe

polymer matrix. It is normal for all polymers to undergo an extremely slow

change of chemical (molecular) structure. The ambient environment—mainly

the temperature and humidity—controls this change, which is known as the

process of aging. When a polymer is cooled below its glass transition

temperature, the material does not achieve instantaneous thermodynamic

equilibrium. Instead, its free volume equilibrium evolves over time, and during

this time the mechanical properties can change (Monaghan and Brinson 1994).

As a result of physical aging, some polymers can become suffer and brittle,

which can influence the matrix-dominated properties. However, the effects on

the fiber-dominated properties will be minimal.

Moisture

Polymer matrix composites absorb moisture by instantaneous surface absorption

and diffusion. Initially the absorption rate is high, then the rate slows down as

the material approaches the saturation point (equüibrium). The time to reach

the saturation point depends on material thickness and the ambient

temperature. Drying can reverse the process but may not result in complete

reattainment ofthe original properties.

The uptake of water by polymer composites in general follows the generalized

Fick's Law of Diffusion. Figure 4a and 4b, extracted from a recent publication

by Dutta and Hui (1997b), shows the typical Fickian behavior of two different

FRP's. Figure 4a shows the moisture absorption rate of a 0.25 in. thick gelcoated FRP laminate and Figure 4b gives the moisture absorption rate of a

commercially available FRP rebar. In reality, however, the exact rate ofmoisture

uptake depends on several factors, including void content, fiber type, resin type,

temperature, applied stress level, presence ofmicrocracks, and thermal spikes.
14 USACERL TR 98/81

Absorption of water by resin in some instances may change the resin properties,

including the glass transition temperature. Moisture also causes the resin to

swell. Hahn and Kim (1978) reported that the ingress of water through the voids

or interfaces can cause plastification of the resin and blisters. Glass fibers are

generally resistant to damage by water, but aramid fibers can absorb

considerable quantities of water, resulting in swelling. Moisture does not have

any known degrading effects on carbon fibers (Mallick 1988).

Temperature influences distribution of the water, both the quantity and the rate

at which it is absorbed. As the temperature increases the amount and rate

increase rapidly, as shown by Dewimille and Burnsell (1983). They have shown

that damages induced by immersion in boiling water for only a few hours

produced debonding and cracking of the same degree as at 50 °C (122 °F) over a

period of 200 days. At room temperature, the same composite specimens showed

no indication of damage. These observations spurred development of a technique

for accelerated aging tests of composites. It also proves that in warmer water,

polymeric composites should be used with caution.

Freeze-Thaw Effects

Unless a composite contains a significant percentage ofinterconnected voids that

are filled with water, the freeze-thaw effects on the strength within the normal

range of temperature (+30 °C to -20 °C [+86 °F to -4 °F]) is insignificant.

Commercially available GFRP usually contains about 0.4 percent voids, which

does not allow any appreciable frozen moisture to cause serious damage.

Low-temperature thermal cycling, however, has other effects on composites.

Residual stresses occur in composite materials due to differences in coefficients

of thermal expansion of constituent elements in the material microstructure.

Under extreme low-temperature conditions these stresses can form microcracks

in the resin matrix or at the resin-fiber interface. The chances of microcrack

growth under a normal range of service temperatures, as noted above, is usually

remote or marginal. Under severe thermal cycling conditions, for example

between +60 °C and -60 °C, (+140 °F and -76 °F) microcracks can grow and

coalesce to form matrix cracks, and these may propagate in the resin matrix or

at the matrix-fiber interfaces (Lord and Dutta 1988). Under prolonged thermal

cycling they continue to grow in density, intensify cracking, and can result in

stiffness degradation and degradation of other matrix-dominated properties

(Dutta and Hui 1996).

FRP thermal expansion characteristics compare favorably with those of steel for

use in concrete. The coefficient of thermal expansion for GFRP is 9.9 x 10"6/ °C
USACERLTR-98/81 15

(5.5 x 10"6/ °F). For CFRP the coefficient is -0.3 x 10s/ °C (-0.2 x 10-6 / °F),

parallel to the fiber, and 28.1 x 10"6/ °C (15.6 x 10"6/ °F) perpendicular to the

fiber. With the longitudinal fiber orientation found in reinforcement, CFRP has

virtually no expansion or contraction response to temperature change. The

coefficient ofthermal expansion for steel is 11.7 x 10"6/ °C (6.5 x 10'6/ °F). While

concrete properties depend on the type of aggregate used, an average coefficient

for concrete is 9.9 x 10"6/ °C (5.5 x 10"6/ °F).

Ultraviolet Radiation

The effect of ultraviolet (UV) light on polymeric compounds is well known. On

prolonged exposure to sunlight the matrix may harden and discolor. The

problem is generally overcome using UV resistant coating to the composites. Of

major concern is the degradation of reinforcing polymeric fiber such as aramid.

An example strength loss of 50 percent is reported for aramid fabric of light

weight 75 g/m2(0.25 oz/ft2) after 5 weeks exposure in Florida sunlight (Larsson

1988). However, the effect is a self-screening type, which means only the skin of

the composite structure is affected. So, in thicker composites, the degradation

effect is minimal on structural properties.

Alkaline Environment

In using GFRP composites in an alkaline environment it is essential to ensure

that high-alkali-resistant glass is used. This is because the alkaline solution

reacts with glass fibers to form expansive silica gels. This precaution is

especially important for application of GFRP composite material as reinforcing

bars in concrete. During the hydration period of the concrete, a highly alkaline

(i.e., pH>12) pore water solution is created. This highly alkaline solution can

affect the glass fiber and reduce durability ofthe bars. Relatively inexpensive Eglass fibers are considered not to have much resistance against alkali attack.

High-alkali-resistant glass can improve the durability of such cementitious

applications. There is ongoing research aimed at reducing the long-term

negative effects of alkaline environment on glass fibers embedded in polymer

matrix. Results of these studies may help to improve the FRP composite's

durability in alkaline environment.

Creep Behavior

Creep refers to a slow deformation over time under a constant stress that is

lower than the material's yield stress. When a constant load is applied to a

viscoelastic material (except for a short initial duration when the strain may

increase quite rapidly), the strain increases steadily. This increase in strain is
16 USACERL TR 98/81

the creep. If the creep increases beyond a certain limit the effective stress

increases due to the decrease in cross-section area. The increased stress

produces further deformation, which in turn increases the stress even more.

Thus, the creep deformations suddenly accelerate leading to the failure of the

material.

At the microstructural level, creep occurs due to the presence of mobile defects

such as dislocations that move (enlarge) primarily at increased stress and

temperatures. Thus the general mathematical formulation of creep rate takes

the form

ds/dt = F(a,T) [Eq 1]

where e is the strain, t is time variable, and F(a,T) is the function of the stress a,

and temperature T.

Creep in polymeric composites has been the subject of investigation for a long

time (Glaster et al. 1983, 1984; Budelman & Rostasy 1993). Tunik and

Tomashevskii (1974) discussed creep and long-term strength of glass FRP in

interlaminar shear. Weidmann and Ogorkiewicz (1974) studied tensile creep of a

unidirectional glass fiber epoxy laminate. Creep strength of discontinuous fiber

composite has also been studied by Bocker-Pedersen (1974). The power-law

approach to modeling the creep behavior of plastics and FRP primarily owes to

the original work by Findley (1960), which he subsequently updated in 1987.

Numerous other studies about creep behavior of FRP have also been reported in

composites literature. These studies include the work by Holmes and Rahman

(1980) on creep in FRP beams. Brinson et al. (1980), Hiel and Brinson (1983),

and Dillard and Brinson (1983) used numerical methods of predicting creep and

delayed failures. Eggleston (1994) studied transverse creep and tensile behavior

of composite laminates, whereas Huang and Gibson (1990) performed both

theoretical and experimental studies on sandwich beams with linear viscoelastic

cores. Creep behavior of Kevlar/epoxy composites was studied by Beckwith

(1984), who concluded that the creep behavior in the laminate composites is

primarily "fiber-dominated" and independent of resin modulus. Krishnaswamy

(1991) presented the results of finite element model of ductile behavior of

polymers. Chen and Lottman (1991), Ueng (1991), and Vinogradov (1989)

studied the creep effects in composite columns. Slattery (1994) developed a

procedure for predicting the accelerated failure rate due to creep by

extrapolating short-term data and taking into consideration the progression of

the fundamental damage mechanism. Recently, Mossalam and Bank (1991) and

Mossalam and Chambers (1995) presented a simplified and efficient design

procedure to predict deflection of pultruded composites under sustained load and
USACERLTR-98/81 17

a laboratory procedure for determining the creep coefficients. Dutta and Hui

(1997a) recently studied the creep behavior of a commercial FRP rebar over a

temperature range from -10 °C (-23 °F) to 49 °C (120 °F), and observed

insignificant creep.

Fatigue

The fatigue properties of a structural material represent its response to cyclic

loading. Composites are generally well known for their excellent fatigue

behavior, but they exhibit a gradual softening with increasing cycling. Thus,

tests are sometimes done not to a failure represented by the separation of the

specimen, but to a limit of specimen stiffness or residual strength that is

predetermined.

The fatigue behavior of a material is usually characterized by an S-N diagram

representing the relationship between the stress amplitude (S) and the number

of cycles (N) to failure on a semilogarithmic scale. In general, the cycle to failure

increases continually as the stress level is reduced.

The S-N curve for fiber composites can be represented by a straight-line

relationship given by

S = u[mlogN+b] [Eq2]

where S = maximum fatigue stress

N = number of cycles to failure

u = static strength

m, b = constants.

Values of m and b for E-glass/epoxy 0° fiber orientation are -0.1573 and 1.3743,

respectively (Lorenzo and Hahn 1986).

It has generally been observed that in tension-tension fatigue tests the

unidirectional carbon and aramid composites exhibit exceptionally good fatigue

strength. However, when the fibers are oriented in off-axis directions the fatigue

strengths often depend on the proportion of fibers aligned with the loading axis,

the stacking sequence, and the parameters of cycling. It should be noted that a

tension-compression cycling may produce a steeper S-N plot than the tension-
18 USACERL TR 98/81

tension cycling. The fatigue performance of both E- and S- glass fiber reinforced

composites are poorer than those of carbon or aramid composites (Mallick 1988).

Fire Hazards and Flammability

FRP Composites used for any structural construction must comply with local

construction code requirements, including fire safety and other life-safety

specifications. Where fire hazards exist the fire hazard characteristics—

including the intended use of the structure to be designed; potential ignition

sources; potential mode of flame and smoke spread; and means for detection,

suppression, and extinguishment—must be identified and the proper building

code and other fire code requirements determined. Heger (1981) has

summarized the specific standards for plastics in a model building code. The

following is a typical example:

The approved plastic materials shall be those that have self ignition

temperature of 650 °F or greater when tested in accordance with the

Uniform Building Code (U.B.C.) Standard 52-3 (ASTM D1929) and a

smoke density rating not greater then 450 (ASTM E84). Approved plastic

shall be classified in accordance with U.B.C. Standard 52-4 (ASTM E84).

Fire tests for FRP composites are to be based on end use, quantity, location, and

special requirements by the owner. After identifying the fire hazard, a suitable

test method must be either selected from standard tests, or developed for the

specific need. These include ignition tests, flame spread tests, extinguishment,

smoke evaluation, tests for toxicity, and fire endurance. Table 3 summarizes

some of the standard fire tests. A review of industry literature on FRP

composites shows that flammability properties are usually specified by the

manufacturers.

Economics of FRP Reinforcement

In selecting an FRP reinforcement technology one must note that while cost of

materials is an important criterion, it does not constitute the total cost of the

project. Besides material cost, the designer should also consider the related costs

of installation, maintenance over time, replacement of debilitated materials and

worker safety. In many instances it has been shown that when life-cycle costs

are considered, the FRP composite costs come out to be lower, especially in laborintensive operations (Fibergrate 1993).
USACERLTR-98/81 19

In relative terms, the conventional steel-reinforced concrete structure is not

expensive. Given the limitations of FRP materials that are outlined herein,

there will not be many immediate applications where FRP use will be more

economical than steel, except where the local environment creates conditions

under which steel is not economical Economics will most readily favor FRP grid

use in environments that create steel corrosion problems. The most obvious

examples are marine environments and highway bridge decks. In highway

bridge deck applications, where underlying girders may provide sufficient

support to minimize deck deflection problems, FRP grid may prove to be effective

in regions where road salt is heavily used. There will, for example, be some

direct construction cost savings associated with the need for less reinforcing bar

placement labor.

The critical issue here is the viability of manufacturing FRP grid on a large

scale. Current manufacturing processes for composite materials and structures

are based on weaving, braiding, pultrusion, and/or lamination. They require

expensive facilities, manufacturing equipment, and processing. As a result, total

processing costs amount to many times the material cost. However, this report

will illustrate that the cost of manufacturing composite grids can be reduced to

the level ofmaterial cost. In such a case, composite structures can then compete

against more conventional materials.
20 USACERL TR 98/81

3 Grid Reinforcement Concept

Unidirectional composite grid-reinforced concrete structures provide a wide

range of potential applications. These include concrete decks and slabs, curtain

walls, highway bridge decks, and support and lining of underground tunnels or

oil storage rock cavities. The advantages include the suppression of

delamination and built-in redundancy. Corrosion and fatigue resistances of

composite materials can be claimed for the grids. In general, the grid

structure—being lightweight and formed into either curved or flat plates in

factories—drastically reduces assembly work at the construction site. Offsite

construction will allow for better quality control of the products. Because of its

corrosion resistance it is a promising technology for reinforcing concrete in ocean

coastal regions, and in cold regions of the world where corrosive salts are used

for deicing roads.

In 1964 the National Aeronautics and Space Administration (NASA) began to

search for an efficient geometry to be used in aerospace structural applications.

The result was "a lattice of intersecting ribs forming an array of equilateral

triangles" termed the "Isogrid" (see Figure 5). This proved to be a lightweight,

economical, and efficient load carrying geometry (McDonnell Douglas

Astronautics 1975; Kouri and Dutta 1993; Kouri et al. 1993). Researchers in the

area of composite reinforcement for concrete would see this two-dimensional grid

structure as a possible alternative to conventional one-dimensional steel

reinforcement. A two-dimensional orthogrid structure (see Figure 6) makes it

possible to take advantage of the unidirectional strength characteristics of

advanced composite materials. The longitudinal ribs provide high strength and

stiffness in two perpendicular directions. This makes the grid very suitable for

applications such as bridge decks, flat slabs, walls, shear walls, and dams. The

two-dimensional nature also allows for structures to be designed as twodimensional elements rather than one-dimensional elements in two directions.

While the orthogrid efficiently resists normal forces in two orthogonal directions,

the isogrid may provide improved shear resistance for shear wall applications.

It is also possible that FRP grids could be employed in seismically active areas to

provide confinement for reinforced concrete frame elements, particularly bridge

and building columns. Recent earthquake experience (Northridge, CA and Kobe

Japan) shows that confining frame elements—particularly columns—can greatly

enhance their ductility during earthquake-induced motions. FRP wraps are
USACERLTR-98/81 21

already being employed as retrofits to older structures. FRP grids may provide

good confinement mechanisms in both new construction and retrofits if the

proper combinations ofstrength and stiffness can be developed.

The use of composites in aerospace applications came about because oftheir high

strength and stiffness per unit weight. In the case ofreinforced concrete, a more

lightweight reinforcement does not significantly affect the overall weight of the

structure. While the specific strength and stiffness do not in themselves warrant

the use of composites, they are added benefits to a noncorrosive concrete

reinforcement. The inherent geometry of an orthogrid structure also provides a

more reliable load transfer mechanism than do steel reinforcing bars. The

internal loads in the beam can be transferred through bearing on the orthogrid

cross-members rather than through shear development along the length of the

bars. Furthermore, composite manufacturing usually employs a "build-up"

approach where materials are combined and added rather than cut or machined.

This allows for geometric alterations, such as surface deformations, that can

enhance the bond characteristics between the concrete and the composite grid.

Finally, composite grid reinforcement has the potential to significantly reduce

labor costs. The tying of steel rebar is a tedious and labor-intensive process.

Premanufactured grids can simply be cut and placed as a continuous sheet of

reinforcement. Transportation costs would also be reduced significantly because

of weight reduction in the reinforcement. Also, because the grid reinforcement

will not corrode, the life of the structure can dramatically be increased and the

maintenance costs significantly reduced. The high composite material costs can

be offset by lower overall costs for the structure. The noncorrosive nature, high

specific strength, high specific stiffness, and moldability make composites a

natural choice for critical applications where corrosion is a problem.

Review of Composite Grid Reinforcement Research

Early research in the field of composite grid reinforcement of concrete was

reported by Sugita et al. (1992) ofJapan, who work with a New Fiber Composite

Material for Reinforced Concrete (NEFMAC) grid made of either carbon fibers or

a hybrid combination of carbon and glass fibers in a polymeric matrix. Its

primary use is to reinforce concrete. The applications to date include reinforcement for tunnel lining, shotcrete reinforcement, LPG tanks, fender plates,

and precast curtain walls (none of which are primary structural components).

Other types of commercial FRP grids include IMCO (molded grating),

DURADEK (pultruded grating), SAFE-T-GRATE, KORDEK (rectangular

grating), KORLOK (pultruded grating), and custom-manufactured grids.
22 USACERL TR 98/81

The design of reinforced concrete structure requires that flexural behavior be

understood. The flexural behavior of a reinforced concrete beam can be

characterized by its ultimate strength, failure mode, stiffness (or amount of

deflection), and predictability. The following paragraphs summarize the results

ofresearch related to composite grid reinforcement.

Composite materials generally have a higher ultimate strength than steel, which

allows for higher ultimate loads in composite-reinforced concrete. Bank, Xi, and

Mosallam (1991) tested a wide range of pultruded and molded gratings

embedded in concrete beams. All but one of the grid-reinforced beams exceeded

the ultimate strength of the steel reinforced beam. Schmeckpeper and

Goodspeed (1994) compared beams reinforced with NEFMAC composite grids

with beams reinforced with an equivalent amount ofsteel based on axial rigidity

of the reinforcement. Their results showed significantly higher ultimate

strengths with the composite-grid-reinforced beams over the steel-reinforced

beams. In both research efforts the higher composite tensile strength resulted in

shear failure modes rather than tensile failure ofthe reinforcement.

Composite materials have the unique characteristic of a modulus of elasticity

that can vary over a wide range depending on the amount and type of fibers used

as well as the manufacturing process employed. Typical carbon fibers have a

tensile modulus, similar to steel, around 29 x 106pounds per square inch (psi).

Once the fibers are embedded in a polymer matrix, the effective stiffness of the

composite is lower than that of steel. This means that generally, in order to

obtain a composite reinforcement with an equivalent stiffness to steel, a larger

cross-sectional area must be employed. As stated earlier, Shmeckpeper and

Goodspeed (1994) were able to compare composite grid reinforcement and steel

reinforcement having equal stiffnesses. The results showed that the beams had

the same load/deflection behavior up to the point where the steel yielded. The

results from Bank, Xi, and Mosallam (1991) showed the effects of varying

reinforcement stiffnesses. Some of the beams reinforced with the commercially

available grids approached the stiffness of the steel-reinforced beams. Larralde

and Zervai (1991) took a different approach by comparing the flexural behavior

of FRP grating materials alone and embedded in concrete. The purpose of the

tests was to show that FRP structural grating which was designed to carry load

independently can be enhanced by adding concrete. The authors concluded that

concrete can be used to enhance the stiffness ofthe FRP grids and that using the

gratings for concrete reinforcement in corrosive applications is feasible.

As stated previously, the general principles and theories currently applied to the

design of reinforced concrete structures can be effectively applied to composite

reinforcement as well. Banthia(1995) tested the flexural response of reinforced
USACERLTR-98/81 23

concrete slabs and found that current code equations can effectively and

accurately be applied to FRP-reinforced concrete. Shmeckpeper and Goodspeed

(1994) investigated beams reinforced with NEFMAC grids and also concluded

that the flexural behavior can be predicted with current design procedures. In

another article, however, Goodspeed et al. (1991) found that the actual

deflections were slightly greater than predicted.

Sugita (1993) and Sugita et al. (1992) indicate that the Japanese have also

explored the use of FRP-grid reinforcement for shotcrete applications. The

prefabricated nature ofthe grid lowers construction effort. The flexible nature of

the grid that results from its low stiffness permits easier placement on nonplanar surfaces such as those found in tunnels. These researchers have also

found that the higher flexibility of the FRP grid results in fewer voids in the

shotcrete matrix that later require filling, further reducing construction costs.

This may indicate a viable use for FRP reinforcement in constructing concrete

elements with curved surfaces (e.g., domes, etc.).

The variability in the types of composite grids available has created some

problems for research in the area of composite-grid-reinforced concrete. As can

be inferred from the work that has been done, there is not a well established

basis for comparison. For the most part researchers have used what is

commercially available. This includes pultruded sections with mechanically

attached crossmembers, molded gratings, different fibers, different volume

fractions, and different spacings. Despite the difficulties, the past research has

been fairly successful as a preliminary investigation. Researchers have shown

that the fundamental principles used in design of reinforced concrete structures

are directly applicable to composite reinforcement in concrete.

Grid Geometry and Design

Grid design is not a new structural concept (Meyer 1973). As noted,

astronautical engineers use such lattices to stiffen spacecraft solar cell arrays.

Aeronautical engineers stiffen fuselage and engine components with them.

Architectural engineers reinforce vertical structures and towers with frame

elements. Facility design engineers employ them as flooring and catwalks. Civil

engineers use them in numerous applications, ranging from sewer grates to

bridges. The grid lattice structures are generally created by bonding together

isotropic material beams. Unfortunately, isotropic materials are not optimal for

use in grid structures because multidirectional strength and stiffness properties

are wasted in beams sustaining mostly unidirectional load. Moreover, the

secondary processing required to bond the beams together at each crossover
24 USACERL TR 98/81

point is highly labor-intensive. It appears that successful exploitation of grid

structures leads to better material utilization and more efficient manufacturing

methods.

Advanced fiber-matrix composite materials are uniquely suitable for structures

subjected primarily to unidirectional loading. The fibers, such as E-glass and

graphite, are unidirectional in strength and stiffness. This implies that a grid

structure, with unidirectional loads in the beams, would make full use of the

material's capabilities. With such materials available, the challenge is to develop

manufacturing processes that efficiently utilize the materials to produce lattice

structures with uniformly high properties at all points, provide controlled final

part geometry without post-processing, and reduce or eliminate manufacturing

waste material to reduce costs.

Grid Types and Terminology

The basic grid is a latticework of rigid, interconnecting beams in two, three, or

four groups and directions. Figure 7 demonstrates the most basic grid structure

and the terminology used to describe it. Nodes, ribs, beams, and cells are the

grid structure elements. Nodes are the crossover points, ribs are the linear

segments that span adjacent nodes, and beams are a collection of aligned ribs

and nodes. Cells are the spaces enclosed between ribs. Structurally related

terms are center-to-center, in-plane, and out-of-plane. Center-to-center indicates

the distance between the centers of adjacent parallel beams. In-plane actions

take place within the plane of the grid. Out-of-plane actions occur orthogonal to

the plane of the grid. Element-level terms describe the rib cross-sectional

dimensions where width is an in-plane measurement while depth (thickness) is

out-of-plane.

The displayed grid segment comprises of many beams placed in a bidirectional

pattern, giving rise to the reference term of bi-grid. A special case of the bi-grid

is one in which the beams intersect orthogonally with equal spacing. In this

configuration, there are two identical mechanical directions, and the terms

orthogrid and %I2 structure are applied.

Tri-grids are the next level of complexity, with three beam groups and directions.

A special case ofthis tri-grid is known as an isogrid or TC/3 structure. The isogrid

has three identical mechanical directions from the uniform distribution of beams

at 0 degrees / ± 60 degrees to form equilateral pockets. Figure 8 shows an

example ofthis grid type.
USACERLTR-98/81 25

A final grid configuration, the quadri-grid, uses four beam groups. When equally

distributed at 0/±45/90 degrees, this grid has four equivalent directions and is

labeled as a TC/4 structure. The main benefit of this structure over the isogrid is

not in the mechanical performance, but rather in the usefulness of the geometry.

A rectangular plate cut from an isogrid has only two sides along which beams

span the entire distance, which makes joining to surrounding structures

difficult. The same size plate cut from a TC/4 grid can have beams spanning the

entire perimeter, allowing for simple joining to surrounding structures.

Grids as Structures

Grids are generally added to solid panels and shells to increase out-of-plane

flexural stiffness with minimal added weight. A simpler method is to use a grid

only, designing the beams to support both flexural and in-plane loads. Such a

grid would be an independent structural element with unassisted load-bearing

capability in all directions. The expected benefits of this configuration may be

seen by comparing grids to sandwich structures, in which the cells are filled.

Problems with sandwich structures include core moisture entrapment and bonds

between the skin and core, which cannot be inspected. Conversely, grid

structures have open cells that facilitate inspection and moisture escape. Grids

also solve such vexing applications problems such as providing for passage of

wires, pipes, and fluids through a structure.

Making a structurally independent grid requires proper design of both the beam

and grid geometries. The design of the beam geometry must provide significant

moments of inertia to resist flexural loads and nominal cross-sectional area to

handle axial loads. The grid geometry must have a beam pattern that generates

balanced reinforcement against all possible in-plane loads. The isogrid is the

best configuration for this purpose. The McDonnell Douglas Special Space

Programs (Kouri and Dutta 1993) found "the isogrid lattice to be a complete

structure by itself; that is, it can effectively resist tension, compression, shear,

and bending loads" while also being "torsionally stiff" (McDonnell Douglas 1981).

Chen (1995) investigated the relative mechanical competitiveness ofsolid panels,

sandwich panels, and isogrid panels—all configurations in both isotropic

materials and composites. He discovered that isogrids made from unidirectional

fiber composites have (1) the highest in-plane strength with stiffness only

slightly reduced from other configurations, (2) flexural strength greater than

solid panels and similar to sandwiches, and (3) minimal hygrothermal response.

Grids, which are structurally complete, have the possibility of being used in

many applications from high-tech aircraft primary or secondary structures and
26 USACERL TR 98/81

jet engine components to low-tech cargo containers and concrete reinforcement.

However, cost generally limits their use. Isotropie grids are usually made from

separate beams that are clamped and bonded together at crossover points.

Straight beams may be easy to manufacture, but the clamp and bond procedure

is time-intensive and strewn with opportunities for error. More complex and

costly beams are necessary if constant thickness is desired, for example, when

the beams are shaped to pass through each other in key-and-hole fashion. In

higher quality no-bond methods, a solid panel is machined down to a grid,

removing all pocket material. This takes a considerable amount of time and the

vast majority of original material becomes scrap. Isotropie materials are not

efficient for application to grid structures.

Composite Material Grids

Laminated structures are a standard design when using advanced composite

(fiber-matrix) materials. Sheets (plies) of aligned fibers are stacked, each

aligned in a specified direction to optimize structural strength and stiffness in

multidirectional load cases. This method of generating composite structures is

well established but not without its difficulties. Panel design is often driven by

transverse properties since the lamination is subject to damage at the interface

of cross-aligned plies. This occurs because the brittle matrix is the only bonding

agent in these interfaces. Conversely, plies with parallel fibers tend to bond both

adhesively and mechanically. Fibers in these unidirectional plies tend to cross

and reinforce the interface, as if the plies were sewn together. These aligned

plies are rarely susceptible to delamination damage. Different from laminates,

grid structures take advantage of both the fiber-aligned damage resilience

through unidirectional beams, and fiber-unaligned multidirectional strength

through multidirectional beam configurations. Unidirectional fiber ribs also

allow for failure strains to be determined mostly by the fiber rather than the

matrix properties.

As noted above, structurally independent grids need to be manufactured from

beams with axial properties sufficient to resist in-plane tension, compression,

shear, and torsion. Isotropie materials best resist these in-plane loads as solid

plates rather than as grids. Advanced composite materials have many

characteristics that make the grid configuration reasonable. Grid beam

elements inherently bear loads in a single direction, as do composites fibers.

Metal, however, is a tridirectional material, which means that the grid

configuration uses only one-third of the metal's load-carrying capacity. In the

single direction of interest, the specific strength (strength/weight) and stiffness

(modulus/weight) of unidirectional composites are many times greater than
USACERLTR-98/81 27

standard metals. Even after reducing the properties to account for multidirection beam alignments, the ideal composite grid structures perform better

than metal grids, as seen in Figure 9. The concern for isotropic behavior is also

resolved by composites in the grid configuration because the isogrid pattern

displays isotropic panel behavior, as would a solid metal panel. Thus, combining

the material benefits of unidirectional composite beams with the geometric

benefits of the isogrid pattern, advanced composite materials are ideally suited

for making structurally independent grids.

Manufacturing is also a major consideration driving the exploration of composite

grids. Currently, metal grids are manufactured either by bonding together

individual metal beams or by machining a grid from a solid metal panel. Both

methods have problems with respect to cost, time, implementation effort, and

opportunities for error. Composite grid manufacturing can provide net shape

parts directly from the mold without secondary processing steps. Filament

winding and tow placement methods may be used to provide continuous fiber

deposition for low cost at a high production rate. Composite grids can be

manufactured in patterns providing the desired unidirectional beams, interlaced

at the nodes at each layer, as seen in Figure 10. These interlaced nodes are

susceptible to damage similar to laminated panels, but nodal delamination

failures are generally non-propagating because the unidirectional ribs are not

susceptible to delamination.

Previous Composite Grids

Various commercial and research attempts have been made to manufacture

composites grid structures. Most have restricted themselves to a simple

orthogrid structure, and have achieved only limited success. The methods have

spanned a wide variety of processes, from filament winding to fiber draping, and

from using molds of rigid metal to soft silicon rubber. Several companies have

succeeded in producing orthogrid structures for commercial sale. However, they

produce only flat orthogrid structures that do not utilize the fibers to their best

advantage. The issues of distributed fiber crossovers and achieving high-density

interlacing still remains. Typical specimens from these companies have fiber

volumes of Vf node = 45 percent and Vf rib = 25 percent—much lower than the

desired quantity ofVf total = 55 percent.

One research group with moderate success is the U.S. Air Force (USAF) Phillips

Laboratories. Work there has centered on flat panel isogrids for use as solar

panel stiffeners. The method developed in this lab uses an inverted master mold

of steel to generate silicon rubber molds. These silicon molds make up to six
28 USACERL TR 98/81

panels before they are terminally damaged and discarded. Because the silicon is

soft, tight control of the various grid dimensions is impossible; and the rib width

is exceedingly thin. The width limits the grids to use as stiffening agents for

solid panels rather than as complete structures with in-plane strength and

stiffness properties able to resist axial loads without excessive deformation.

Finally, the USAF Phillips isogrids have offset beam groups designed so that all

three do not cross through a single node center (Koury and Dutta 1993). This

approach generates two different cell shapes, making multigrid joining and

modular repair difficult.

Properties and Behavior of Composite Grids as Concrete

Reinforcement

In general, reinforcement in concrete structural elements is used to sustain

tensile forces while the concrete sustains compressive forces. Concrete is

significantly stronger in compression than it is in tension; conventional

structural design procedures usually ignore its tensile strength. Whether the

reinforcement used in an element is placed for flexural tension, shear (diagonal

tension), or confinement, it is there to provide tensile strength. Conventional

steel reinforcement also provides deformation ductility to structural elements

through its ability to sustain large strains after its initial yield. Some of the key

properties to consider in evaluating concrete reinforcement materials include

ductility, tensile strength and stiffness, the mechanism for force transfer

between the reinforcement and surrounding concrete (i.e., bond), and flexure.

Ductility

FRP composites are typically elastic in their tensile stress-strain characteristics

up to the point of fiber rupture. Each individual fiber fails completely, with no

ductility. The literature generally suggests that, in aggregate, bundles of plastic

fibers will fail sequentially, so that behavior somewhat resembling ductility

occurs. This "pseudo-ductility" is random and depends heavily upon the

fabrication process. This phenomenon is not as pronounced or as predictable as

the yielding found in steel. Therefore, FRP reinforcement cannot be relied upon

to provide the hysteretic behavior found with steel reinforcement.

Strength and Stiffness

A report by the American Concrete Institute (ACI-440R 1995) indicates that

individual E-glass fibers in FRP rebars, which may be considered as the baseline

reference material, have tensile strengths of approximately 3.5 GPa (500 ksi)
USACERLTR-98/81 _^ 29

and a tensile modulus of 73 GPa (10.5 x 103ksi). Strain at tensile failure is

approximately 5.0 percent. Individual carbon fibers have tensile strengths

similar to that of E-glass, but are suffer, with tensile moduli of approximately

230 GPa (33 x 103ksi). Strain at failure is approximately 1.5 percent to 2.0

percent. More important than individual fiber properties, however, are the

properties of the composites that result from using these materials. GFRP bars

with tensile strengths of 500 - 1200 MPa (75 - 175 ksi) have corresponding

tensile moduli of 40 - 55 GPa (6 - 8 x 103ksi); similar CFRP bars have tensile

strengths of 160 - 2400 MPa (240 - 350 ksi) and tensile moduli of 150 -165 GPa

(22 - 24 x 103ksi). Corresponding failure strains were 3.5 percent to 5.0 percent

for GFRP bars and 1.0 percent to 1.5 percent for CFRP bars. The strength and

stiffness characteristics are affected directly by the volume offibers contained in

the composite matrix. The resin in the composite matrix contributes only

minimally to the overall stiffness and can practically be ignored. The above

characteristics were obtained in bars with 45 percent to 70 percent fiber volumes

in the total matrix.

One should also note that the tensile characteristics of both the GFRP and the

CFRP composites contrast with those of steel—the traditional means of

reinforcing concrete—in two key ways. First, steel is significantly stiffer than

the composites. The tensile modulus of steel is 200 GPa (29 x 103ksi). For a

given load that is resisted by the reinforcing elements, both the deflections ofthe

structural element and the cracking in the surrounding concrete matrix will be

larger with the FRP composites than with steel (assuming that concrete crosssection is the same in both cases and the reinforcement is sized for strength).

Additionally, the FRP composites exhibit no range of yielding beyond a failure,

(i.e., ultimate strength). They behave elastically up to the point of tensile

failure, and then failure, or rupture, is sudden. Steel behaves in a ductile

manner, with a defined yield strength followed by a relatively large yielding

envelope. Steel strains at tensile failure are typically on the order of 10 percent,

following an initial yield strain of significantly less than 1 percent. Figure 11

illustrates typical steel and FRP stress-strain characteristics. In the figure, "CFRP" represents CFRP, while "H-FRP" represents a hybrid carbon and glass

FRP

FRP Reinforcement Bond Characteristics

Effective force transfer between the reinforcement and surrounding concrete

matrix is essential. In conventional steel-reinforced concrete elements, the

individual reinforcing bars are fabricated with deformations that are either

raised above the nominal bar circumference or indented into the circumference.

These deformations facilitate load transfer and prevent the bars from slipping
30 USACERL TR 98/81

within the concrete matrix. This load transfer mechanism is commonly known

as bond.

While individual FRP reinforcing bars that have been fabricated usually

incorporate some type of surface deformations that facilitate load transfer, the

surface finish on the FRP grid ribs used in this study are smooth. Essentially no

load transfer in the traditional sense of bar bond is possible. Instead, the ribs

that are orthogonal to the primary reinforcing ribs provide direct bearing

surfaces for the concrete matrix to transfer force to the reinforcement. As

opposed to the distributed force transfer that is provided by conventional

deformed reinforcement, force transfer in a composite grid is concentrated at

these intersecting grid lines. There are several implications from these

differences. First, it will be necessary to ensure that the grid joints (where the

orthogonal ribs intersect) are structurally capable of transferring the force

without failure. Second, the concentration of force transfer in the concrete

matrix may lead to larger individual crack widths that are concentrated at the

grid points (Schmeckpeper and Goodspeed 1994). A third, less-explored issue is

the possible formation of shear failure planes in the concrete matrix along these

orthogonal grid lines that could accompany the concentrated load transfer

mechanisms.

Flexural Behavior

Schmeckpeper and Goodspeed (1994) have reported the results of beam bending

tests at the University of New Hampshire. Researchers there cast reinforced

concrete beams in three configurations: steel-reinforced, CFRP-reinforced, and

hybrid CFRP/GFRP-reinforced (referred to here as HFRP). Both FRP types were

grids. The steel-reinforced beams were reference specimens designed in

accordance with ACI requirements for flexural design that ensure ductile

behavior. The researchers sought to match the stiffness of the reference beam

with both types of FRP-reinforced beams. This was accomplished by matching

the reinforcement axial rigidity, which is defined as the area of longitudinal

reinforcement in the beam cross-section multiplied by its tensile modulus. The

researchers were able to match the reference stiffness in the case of the CFRPreinforced beams; the required CFRP reinforcement area was 2.4 times that of

the steel reference. The lower tensile modulus of the glass fiber in the HFRP

specimens required that a large volume of the hybrid reinforcement be used to

match the steel reinforcement axial rigidity. The required GFRP reinforcement

area would have been 4.8 times that of the steel reference, but concrete

placement prohibited using this high volume of reinforcement. Therefore, the

stiffness of the hybrid-reinforced beams was lower than the reference stiffness.

The FRP composite-reinforced specimens failed in a brittle manner instead ofthe
USACERLTR-98/81 31

ductile manner of steel reinforcement, largely due to concrete crushing. The

lower-stiffness hybrid composite specimens had significantly higher deflections

at service load than either the CFRP specimens or the steel specimens. Figure

12 presents the results of some of these bending tests, comparing the steel,

CFRP, and hybrid fiber-reinforced (HFRP) specimens. There were several

additional significant observations:

1. The ultimate capacities ofboth FRP specimens were significantly higher (more

than 3 times) than the steel specimen (noting that reinforcement volumes were

higher, too).

2. Energy dissipation, as measured by the area under the load/deflection curve, was

quite good, comparable to steel for both types of composite specimens. The

nature ofthe energy dissipation was quite different, however. With steel

reinforcement, energy dissipation occurs through large inelastic strains at

relatively low stress levels. With FRP reinforcement, energy dissipation occurs

through elastic strains with corresponding higher stresses.

3. In examining service load/deflection ofthe beam, the CFRP-reinforced beam

specimens had the same load capacity as the steel-reinforced reference. TheACI

Building Code (ACI 318-95) limits live load/deflection ofnoncritical structural

elements to 1/180 ofthe beam length. At this midspan deflection, the CFRP and

steel specimens resisted the same load. The hybrid-reinforced specimen resisted

60 percent ofthis load at the same midspan deflection.

Fujisaki et al. (1993), Sugita (1993), and Sugita et al. (1992) reported the results

of a test series in Japan. They performed research on various FRP gridreinforced concrete elements. In addition to the same types of beam tests as

those reported in Schmeckpeper and Goodspeed (1994)—which yielded

essentially the same results—the Japanese authors tested slab specimens in

bending and wall sections in shear. In the slab bending tests, they made many of

the same observations about the behavior of FRP-reinforced elements as those

made from the beam tests. The Japanese authors also examined prestressing

the FRP reinforcement in some of the slab tests. The relatively high ultimate

strength and low creep susceptibility of the FRP reinforcement can lead to

efficiency in the prestressing application, but problems associated with brittle

failure remain.

The Japanese tests of shear wall models included both steel-reinforced reference

specimens and two configurations of HFRP specimens. The shear walls were

loaded cyclically in-plane through a number of reversed cycles of increasing

displacement magnitude to failure. Prior to reinforcement yield, the FRP

specimens performed very similarly to the steel reference specimen; hysteretic
32 USACERL TR 98/81

behavior for the different specimens is comparable. After reinforcement yield,

the FRP specimens behaved in a less ductile manner than the steel specimen.

The Japanese papers also indicate that the researchers have developed a

mechanized means of producing the FRP grid, but few details are presented.
USACERLTR-98/81 33

4 Analysis Of Grid Reinforcement

Grids are like the skeleton of a human body or the frame of old airplanes made of

wood and cloth cover. The grid carries the primary load, where as, skins or

covers are there for another function. The FRP grids are formed by a network of

ribs made of unidirectional composites, which are many times stronger and

lighter than metallic materials. The key is to exploit the unidirectional

properties. While concrete and metallic grids have been made, their

performance is limited because the ribs are isotropic. Only when ribs are

unidirectional can the true potential of grids be realized. By modeling, it will be

shown how to capitalize on this principle and combine it with low-cost

manufacturing.

A grid analysis model analogous to classical laminated plate theory has been

developed at Stanford University (Chen and Tsai 1996). Like laminated plate

theory, in this model also the applied in-plane stress {N} and bending moment

{M} are user-defined. The model performs a point-stress analysis of a grid

having ribs oriented in 0, 90, and +/- degrees. Thus, any of the grid systems

shown in Figure 13—i.e., isogrid, orthogrid, or angle grid—could be used for

analysis. Grid analysis is performed using a spreadsheet-based program (Chen

and Tsai 1996) that yields stiffness and compliance components of the structure.

It also identifies the strength at the initial failure and the particular mode of

failure. Failure modes include rib strength, rib buckling, skin ply-by-ply

strength, and skin buckling.

The structural optimization of the design is performed in two steps. First, the

grid-analysis program is used to select an optimal grid at highly stressed points

within a structure. The resulting effective stiffness of the grid is used in a finite

element analysis of the structure. From a new set of internal loads at critical

locations the grid-analysis program is used again to generate a new grid design.

Then, another finite element analysis is performed. This iterative process is

continued until an optimal design is achieved.

The grids can be formed into a flat, curved, or three-dimensional grid shape.

They can also employ sandwich construction in which two sheets of orthogrids

structurally interconnected through offset blocks may be the facing material. In

concrete slab reinforcement the placement will be nonsymmetric, with only one

layer of grid located on the tensile side. The models for such symmetric and
34 USACERL TR 98/81

nonsymmetric configurations have been developed at Stanford University

following the well established models of laminated construction commonly used

for composite structures. On the grid layer level, a contact model between the

grid and the concrete filler can predict the effective stiffness of the filled grid

layer.

Geometric Parameters

The principal geometric parameters of grids are the length L, width b, and

height h ofthe ribs. Because ribs are treated as beams, it is assumed that length

L is considerably larger than b and h. The definition of L for square grid is

different from that for isogrids, as shown in Figure 13. A useful dimensionless

measure is the rib area fraction f within a unit cell. This fraction is related to

the length and width of the ribs and their orientations in the end. Three values

of f are shown in Figure 14 for isogrids (on the top row) and square ends (the

bottom row). These pictures are representations of grids that are sparse on the

left and increase in density as they move to the right. A dense square or

rectangular grid is also called a waffle plate, characterized by heavy ribs that

will not buckle.

The value of area fraction f is the same as the rib volume fraction if the grid

pattern remains constant throughout the grid height. The rib fraction is

analogous to the fiber volume fraction of a composite material. But fiber fraction

in composite plies is not a common design variable because material suppliers

often predetermine the fraction. For grids, however, rib fraction is an important

design variable and must be deliberately selected for a given design. As will be

illustrated in later text, the fractions shown in Figure 14 depict practical ranges

from the considerations of global stiffness, strength and mass, the assembly

methods, and the design ofthe rib intersections.

Rib height h is also a critical design parameter that determines the flexural

rigidity of the grid. A height-to-width ratio or hub less than unity is a shallow

grid; a ratio greater than unity is a tall grid. Height affects the assembly

method of the grid. Grids with stacked joints are suitable for shallow grids,

while those with slotted and interlace joints are suitable for tall grids. Ribs may

fail when their strengths are exceeded, or they may buckle under compressive or

torsional load. If Euler buckling occurs the ribs move in the lateral direction.

For shallow grids where h/b < 1, Euler buckling is governed by length-to-height

ratio L/h; for tall grids where h/b > 1, Euler buckling is governed by the lengthto-width or slenderness ratio L/b. Such failure mode must be compared with
USACERLTR-98/81 35

failure by compressive strength and buckling by shear. Whichever value is the

lowest is considered the controlling failure mode.

The relation defining the rib length L or L/b and the area fraction f of a grid are

functions of the grid configuration. Figure 15 illustrates definitions of L and f

for iso- and square grids. A visual presentation of an isogrid compared with

square grids is featured. All grids have the same rib width. The smaller square

grid on the left has the same area fraction f but different L as the isogrid in the

middle. The larger square grid on the right has the same L but different f as the

isogrid. For the smaller square grid, the length L is reduced by V3; for the larger

square grid, the area fraction fis reduced by V3.

While there is a one-to-one relation between fraction f and L/b, each serves its

own purpose in the design of composite grids. Area fraction f can be treated as a

material property that governs both in-plane and flexural stiffnesses in a

consistent manner. Slenderness ratio L/b is useful in its direct relation to Euler

buckling of the ribs. The area fraction fis preferred here because it reflects the

weight and amount ofmaterial used in a grid.

With regard to the hub parameter, grids that have the same rib configuration

along their height have the same homogeneous characteristics as solid panels.

Their flexural rigidities increase with the cube of the height. Sandwich panels,

on the other hand, have flexural rigidities that depend on both the heights ofthe

core and the laminated face sheets. They have the most efficient flexural

rigidities because the load-carrying face sheets are located in the outermost

positions. Grids can also have one or two face sheets in order to improve their

flexural rigidities. Even without face sheets, grids can be used effectively for

flexural applications when the height exceeds the limit of conventionally

available sandwich core.

Mechanical Properties of Grids

The following sections describe the stiffness and strength of grids and compare

them with comparable properties of laminates. It is useful to compare the

stiffness of laminates and equivalent grids. The simplest comparison is that

between isotropic laminates and isogrids.
36 USACERL TR 98/81

Stiffness of Quasi-lsotropic Laminates

There is a close relation between composite laminate and composite grid. Grids

can be viewed simply as a special case of laminates. This perspective will be

used in deriving the stiffness and strengths of grids.

Laminates become quasi-isotropic with equally spaced ply orientations of n/3,

id A, 7c/5 and so on. Similarly isotropy of the grid is assured when, say, the three

ribs are spaced at 60 degrees or TI/3 apart.

There are closed-form solutions of the plane stress stiffness components (Tsai

1992). The quasi-isotropic invariants (the terms that are not affected by

orientations, and thus are most useful in designing with composites) are linear

combinations of the ply stiffness components shown below (see Table 3.1 in Tsai

1992):

Ux = 3/8 (Q„ + Q„) + 1/4 Q^ + 1/2 QM

U4 = l/8(QIJI + Qyy) + 3/4Qv-l/2QM

U5 = l/8(Q„ + Qyy)-l/4Qxy+l/2QM [Eq3]

where Q denotes the "reduced stiffness" and U represents constants that give the

effective measure of the lamina stiffness (Jones 1975). The subscripts of U as

1,2,3... refer to contracted numerical notation, and subscripts of Q refer to the

corresponding regular letter notations.

The quasi-isotropic Young's modulus, Poisson's ratio, and shear modulus of the

laminates are functions ofthe invariants:

E^ = D/U,, ViM = U/U;, G^ = U5; [Eq 4]

where D = V1'-TJ*

This simplifies when the degree of anisotropy of a composite ply increases to the

upper limit (unidirectional composite); the only dominant stiffness component is

the longitudinal Young's modulus E,. The matrix-related components become

vanishingly small. Then the invariants can be represented by the following

expressions:

U, = 3/8 Ex, U4 = 1/8 E„ U5 = 1/8 Ex [Eq 5]
USACERLTR-98/81 37

The resulting engineering constants ofthis limiting quasi-isotropic laminate are:

Eiso =1/3 Ex, Viso =1/3, G^ =1/8 Ex [Eq 6]

Stiffness oflsogrids

A composite isogrid is portrayed in Figure 16 as a regrouped laminate where the

matrix stiffness is approaching zero. The same 1/3 factor in Equation 6 can he

applied for the contribution of rib stiffness to the isogrid stiffness. This global

stiffness however must be weighted by area fraction f. This factor is proportional

to the ratio of width b and spacing L of each rib. The Poisson's ratio can also be

shown to have the same value of 1/3 as a laminate without matrix. A

straightforward but rigorous derivation of the Poisson's ratio of an isogrid as a

truss will produce this special value. The following global Young's modulus and

Poisson's ratio of an isogrid are easy to use and to remember:

V=

m E

* =

2

^

3

(

L/b

»

E

*; ^= 1/3 [Eq 7]

where b = rib width and L = rib length as per the relationships shown in Figure

15. For square ends, the relation between the global stiffness and area fraction f

or slenderness ratio L/b will be different.

The relations here apply to interlaced isogrids where all ribs are in the same

plane. For stacked joint grids, ribs of different orientations run in different

planes and yield an effective stiffness lower than that of interlaced grids having

the same overall rib geometry. The global thickness of the grid is the sum of the

rib thicknesses. The effective stiffness of such grids would be lower than that of

interlaced grids having the same rib geometry.

The grid stiffness depends on the rib stiffness and geometry. It can be shown

that the detail of the joint at rib intersections, which can be either pinned or

fixed, does not affect the grid stiffness if the slenderness ratio is high. This is

the case when the area fraction is small, but will not be the case for a grid that

looks like a waffle plate.

When ply anisotropy in a composite is moderate, like that of E-glass/epoxy

composite, the matrix-related components are no longer negligible. The quasiisotropic laminate stiffness will obey the relations expressed in Equation 4. A

comparison of laminate and grid stiffness for different materials is listed in

Table 4.
38 USACERL TR 98/81

For comparison among different grid materials, the area fraction correction is

the same. Its effect is included in Table 4; i.e., for a given fraction f, the global

stiffness is this factor multiplied by the Young's modulus shown in the table.

Relative efficiencies between the grid and laminate are shown. Highly

anisotropic material like CFRP is most efficient; the isotropic material is the

least efficient. Solid laminates are still more efficient than grids but using

highly anisotropic ribs in grids provides performance within 13 percent of the

laminates.

Composite Grid Reinforcement Design Approach

Analyses that have been performed in conjunction with FRP-reinforced concrete

research indicate that flexural capacities in sections with the reinforcement in

tension may be calculated using the same force equilibrium and strain

compatibility relationships that are used in conventional reinforced concrete

design. The significantly lower compressive strength and stiffness characteristics of FRP composites, relative to their tensile strength and stiffness, would

dictate that the conventional procedures be modified to differentiate between

tensile and compressive behavior. Analysis of shear and diagonal tension

capacities of flexural sections is currently poorly defined. The higher degree of

concrete cracking that accompanies the use of FRP composites will reduce

section shear/diagonal tension capacity, but there has not been sufficient

research performed to determine these relationships.

While the analytical approach to flexural analysis is similar in principle to

current practice, actual design procedures are not well defined, due to significant

differences in tensile stiffness and failure mechanism between steel and the FRP

composites. The less stiff the composite (i.e., the lower its tensile modulus), the

more severe the design discrepancy will be. ACI-440R (1996) provides an

excellent overview of the current state of this dilemma. A summary is presented

here, but the reader is referred to the original document for added detail.

To present this clearly, a brief summary of the conventional steel-reinforced

concrete design approach is helpful. At any given instant in the loading history

of a flexural member, the compressive forces (usually borne by the concrete) and

the tensile forces (borne by the steel) are in equilibrium within the member

cross-section. A "balanced" flexural section is one in which, at the instant the

tensile steel yields, the outer fibers of the concrete that is in compression will

crush. The ACI Building Code ensures that the steel will always yield before the

concrete crushes by limiting the amount of tensile steel to 3/4 of the amount that

results in the balanced condition. Since steel will yield to very large strains after
USACERLTR-98/81 39

it reaches its yield stress but before it reaches its ultimate stress, the structural

member will behave in a ductile manner and absorb large amounts of energy

before complete collapse. Figure 12 illustrates the differences in energy

dissipation capacity and ductility. Analysis of section capacity is also

straightforward at this condition; the conventional design equilibrium equations

easily accommodate the use of the steel tensile yield strength and the concrete

compression strength.

The FRP composites behave elastically up to their yield stress, but they have no

postyield ductility in a conventional sense. Ductile behavior, in the traditional

sense, is very difficult to attain in FRP composites. A condition where the FRP

reinforcement is near the balanced condition may be analyzed in essentially the

same manner as that now used in conventional design, with the understanding

that failure will not be ductile. Ifthe FRP reinforcement volume is significantly

above that required for balanced conditions, concrete compression will occur

before reinforcement yield. If the FRP reinforcement volume is significantly

below that required for balanced conditions, failure will occur by rupture of the

reinforcement. In both cases, failure will be brittle. ACI-440R (1996) mentions

two possible approaches to addressing this problem from a design perspective.

The first is to apply more stringent capacity-reduction factors to the design

equations than are used for conventional design. Conventional design with steel

reinforcement applies a flexural capacity-reduction factor of 0.9. ACI-440R

(1996) suggests the factor might be 0.85 for FRP reinforcement volumes below

the balanced condition and 0.70 for FRP reinforcement volumes above the

balanced condition. Because of the need to control deflections, there will likely

be a tendency to use the higher FRP reinforcement volumes.

The second approach is to examine the use of a new definition in ACI 318-95,

wherein a "tension-controlled section" is defined as one in which the net tensile

strain at nominal strength is 0.005 or greater. Both of these approaches reduce

the efficiency of the materials that are being used, especially the concrete.

Research described in ACI-440R (1996) indicates that concrete of high

compressive strength (fe of 69 MPa or 10 ksi) should be used to improve

efficiency.

GENLAM Modeling for Concrete-Filled Grids

The basic method developed to model reinforced concrete structures treats them

as laminated panels. The computer code GENLAM (Tsai 1992), based on

composite lamination theory, is used to stack layers of varying material
40 USACERL TR 98/81

properties to produce a composite structure. The lay-up pattern is varied to

produce a structure that smoothly distributes stresses internally. With proper

modeling refinement, accurate predictions of beam behavior may be obtained.

This method is explored below, and the model predictions are compared with

experimental results in Chapter 5 to demonstrate the model's ability to

accurately predict beam stiffness.

Supplied to the GENLAM code are material on-axis engineering constants, a

material stacking sequence, and an applied load. GENLAM then calculates the

total laminate stiffness and compliance properties in the form of [A] [B] [D] and

[a] [ß] [5] matrices, respectively. For beams in three-point testing, the most

important performance parameters for out-of-plane deformation behavior are

found in the bending stiffness and bending compliance matrices, [D] and [5]].

These relate the applied moment vector, {M}, and out-of-plane curvature vector,

{K}:

Figure 17 diagrams the beam test. Simple equations can convert GENLAM

compliance information into beam load/deflection and load-strain behavior. The

derivation of the load/deflection relationship begins with the definition of

curvature:

d

2

w re «,

ax

The matrix formulation and known loading conditions are:

K, =8,1MI+812M2+813M3 M2=M3=0 [Eq 10]

M1( or moment per unit width, may be expressed as a function oflocation, fl(x):

M L{a-x)

1 2b

Combined, this produces the differential beam equation:

d

2

w _8uL(a-x)

dx22b

[Eql2]
USACERLTR-98/81 41

Integrating and applying boundary conditions results in the following

load/deflection relationship when evaluated at the beam end relative to the beam

center:

^U=0 w(*)L=0 [Eql3]

ax

w(a)=^L [Eql4]

60

Rearranging yields the following load/deflection relationship:

L 48o

w(a) 8n/

3

[Eql5]

A similar approach is taken to obtain the load-strain relationship. Given axial

strain as a function of axial curvature:

s XX (Z)=ZK, [Eql6]

and substituting with Equations 9 and 12 yields:

e»L..... lb

With the strain gage mounted on the surface at

z=

l

- [Eql8]

the load-strain relationship is found to be

L %b

*xx thn(l-2x)

[Eq 19]

In each of these equations, the following constants apply to all beams in this

study:

b = 55.9mm I = 355.6mm x = 50.Smm [Eq20]
42 USACERL TR 98/81

5 Manufacturing the Composite Grids

The initial effort to manufacture composite grids at Stanford University was

based on a traditional slotted joint manufacturing system that is most frequently

used in carpentry (Figure 18a). An isogrid made with slotted wood ribs is

shown in Figure 18b. The composite grids (Figure 18c) were manufactured

using pultruded thin unidirectional sections as the rib material. The slots were

machined with a generous tolerance over the rib thickness. This facilitated

assembly of the grid. The process was intended for low-cost manufacturing of

large and small flat panels. For an orthogrid the typical relations between the

rib stiffness and that of the grid panel are shown in Figure 19. The

disadvantages ofthis design include:

1. cost ofmachining slots

2. difficult assembly ofmany ribs having multiple slots

3. low rib strength introduced by machined slots and notches

4. low stiffness and strength from imperfect fit at slotted joints

5. limited to square or rectangular grid configuration.

Following the lessons learned from the slotted grid configurations it became

clear that a more appropriate FRP grid manufacturing process must be dictated

by three major considerations:

1. All ribs must be unidirectional.

2. Rib cross-section must be well defined and remain constant.

3. Nodes must be interlaced with continuous composite layers.

The volume fraction of fiber in the node will dictate the volume fraction of fibers

in the rib. The upper limit of the fiber volume fractions in most available

composite gratings is 60 percent at the nodes and 30 percent at the ribs.

However, Koury and Dutta (1993) have explained a methodology of designing

isogrid in which a higher fiber fraction in the rib could be incorporated. In their

methodology they designed the isogrid with offset nodes as shown in Figure 20.

It will be seen that one triple-layered node in this case has been spread over

three double layered nodes. The proposed grid has two triangles of different

sizes. The smaller triangle is the offset triple node. At Stanford University,

Colwell (1996) has developed an innovative approach to accommodate larger

volume fraction of fibers. In their design they have provided for wider nodes
USACERLTR-98/81 43

(Figure 21) that reduce the thickness of the interlacing layers. In the limit, the

fiber volume fraction of the nodes will be the same as that of the ribs. For

double-layered nodes, the width would be twice that of the rib; for triple-layered

nodes, the width would be three times the rib. This method of filament

placement, in conjunction with a vacuum-infiltration resin transfer process,

offers an economical solution to the manufacturing problem and produces grids

ofsuperior structural performance.

The techniques of manufacturing interlaced joints were developed both at

Stanford and at Brigham Young University (BYU), Provo, UT.

BYU Direct Crossover Interlaced Joints

In order to manufacture FRP grids for actual placement in concrete specimens

for laboratory testing, two master orthogrids were cut from 5/8 in. thick steel

plates using an abrasive water jet cutter. These grids were used as blanks to

make a mold for fabricating composite grids (Smart 1996). One grid was plain,

with straight sections, and the other had deformed surfaces with ribbed sections

(see Figure 22).

The steel grids were then placed in a form made from wood and lined with

plastic. The grids and form were released using a release agent. A silicone

rubber compound was mixed and placed in the form. The silicone was allowed to

cure overnight. The next day the mold was removed from the form and the steel

was carefully removed from the silicone mold. The mold was then postcured in

the oven at 250 °F overnight. Figure 23 shows a picture ofthe finished molds.

A winding platform was prepared using a % in. thick piece of plywood, some

lumber 2 by 4s (nominal dimensions, inches), and some sharply pointed steel

winding pins. The 2 by 4s were attached to the bottom side of the plywood to

form a flat, stable surface. Nails were then driven through the plywood in line

with the grid intersections. The nails served as winding pins in the fabrication

process.

Lay-up Procedure

Before each grid fabrication, the silicone mold was cleaned and a releasing agent

was applied. Once the mold was prepared, approximately 700-800 grams ofvinyl

ester resin was mixed with 1 percent by weight of methyl ethyl ketone peroxide

(MEKP). This ratio was chosen to allow approximately 1 hour of working time

before gelation ofthe resin. The resin mixture was then poured into a resin bath
44 USACERL TR 98/81

used for wet winding. The resin bath was equipped with two rollers that served

to induce proper resin saturation into the fiber bundle. One tow of Akzo Nobel

Fortafil 3(C) 50k (50,000 fibers/tow) X95 14 fiber was then pulled through the

resin bath and wound through the silicone mold and around the winding pins

(see Figure 24). The fiber was tensioned by hand with approximately 10-16 lb

oftension as the winding proceeded.

The fibers at the joints were consolidated periodically by hand to maximize

compaction and fiber volume. The process was repeated until the proper number

of tows had been wound onto the grid. The winding procedure was repeated 11

times for the carbon grids, producing 11 tows in each direction. For a set of the

hybrid grids, a 0.146 in. diameter steel wire was placed down the length of the

longitudinal ribs after the fifth repetition. Four additional carbon tows were

wound on top of the steel wire for a total of nine repetitions. When the proper

number of tows had been wound into the mold, a peel ply release cloth was

placed over the top of the mold. The fibers were then cut between the nails and

the silicone mold and the excess fibers were removed. The wet grid and mold

were placed in the oven with an aluminum plate placed on top, along with

additional weights, to obtain a flat surface finish on the grid.

Cure Procedure

The grids were oven-cured in the silicone mold for approximately 2 hours at 250

°F. The relatively large positive coefficient of thermal expansion (CTE) of the

silicone molds provided good compaction and consolidation during curing. After

2 hours the molds were removed from the oven and the grids were carefully

removed from the molds. The grids were trimmed to the proper size using a

band saw.

Strain Gage Placement

A pair of electrical resistance strain gages was mounted on each of the

reinforcements to monitor the internal strain during loading. The strain gages

were placed on the reinforcement (rebar or grid) at the midspan of the beam and

under one of the loading points (see Figure 25). The mounting of the strain

gages followed the manufacturer's suggested procedure. The surface was

prepared through degreasing, surface abrasion, and conditioning. The strain

gage was mounted with a small amount of 5-minute epoxy. Pressure was applied

to the gage as the epoxy cured to ensure a good bond. Lead wires were then

soldered to the tabs and the gages were checked with a strain indicator box to

ensure that they were functioning properly. The completed gage was covered

with epoxy to protect it while the concrete was placed and cured.
USACERLTR-98/81 . 45

Stanford Pin Enhanced Geometry (PEG) Process

Manufacturing high-quality grid structures requires the implementation ofthree

concepts: (1) high nodal interlace density, (2) high fiber linearity, and (3)

distributed fiber crossovers. When the Direct Crossover Interlaced Joints similar

to the BYU method was tried at Stanford (mostly from BT250-1, a lowtemperature cure thermoset resin), the molds were not satisfactory (Colwell

1996). They showed highly visible flaws (voids). As shown in Figure 26, the

fibers could not properly compensate for the gap created between plies at the

edges ofthe laminated nodes. Resin pools, fiber warping, and wrinkles appeared

within each rib. Similarly visible from the exterior were the nonlinear paths of

each ply. Because each ply in the rib was distinct from adjacent plies, goal of

true unidirectional beams—no defined plies or ply interfaces—was not achieved

by this method.

Figure 27 shows the PEG pattern with a central node pin without any lead

angle in the mold, and Figure 28 shows a pattern with 1.3 degree lead-in angle

in the mold to smoothen the rib-node transition. However, alternating

placement of each roving to opposite sides of the pin did not reduce the node

thickness to rib thickness. As a solution, a two-layer laminated lay-up pattern

(shown in Figure 29) was used. In doing this, it was soon realized that while

the expanded node volume and lead-in angle helped improve the Vf ratio between

ribs and node, the single pin in the middle disturbs the fibers and initiates dry

zones.

A follow-up modification was to replace the 6.4 mm central pin with four 1.5 mm

pins at the node. Figure 30 shows the first mold and resultant specimen with

the four-pin concept. In this test mold, the rib width was sized to the wetted

diameter of the roving—2.5 mm. The pins were located on the center axis of the

ribs and set back from the node center such that four fiber paths had

unobstructed 2.5 mm channels through the node. The wall geometry was

designed to provide a minimum 2.5 mm gap between any wall and pin. The pins

were intended to remain in the specimen following cure. A bolt at the end of

each beam channel guided the roving and provided a tensioning point. This

mold also used a new concept in part removal. Instead of machining channels

into a solid metal plate, individual blocks were bolted to a base plate to form the

channel pattern. After part lay-up and cure, the blocks were unbolted and lifted

from the base plate with the specimen. The blocks were then knocked out from

the specimen individually. This minimized the wall friction to overcome at any

one time during removal and eliminated the need for release draft angles.
46 USACERL TR 98/81

Based on visual inspection, the specimens from this mold were successful. The

fibers were straight throughout the specimen. The rib ply layers were

indistinguishable and the vertical fiber density of the rib and node appeared

equivalent. Measurements of the lateral fiber spread confirmed a nodal fiber

width of approximately twice the rib width (ratio of 1.9). Most specimens had

geometry variations of less than 0.25 mm over the absolute design value. This

variation was the result of the combined errors of mold machining and mold

assembly, both of which could easily be reduced in better settings.

In later attempts to force the fibers to conform properly, excessive pressure was

applied during the cure cycle. Resin pools, fiber undulations, and wrinkles

remained. In addition, resin-poor regions in the nodes also appeared. The nodal

double-points supported the entire applied load, and the high nodal pressure

forced most resin to flow into the low-pressure zones of the ribs. Beyond

producing poor specimens, this method was found to damage the aluminum of

the mold due to the concentrated load at the nodes.

Following these experiences a process was developed that integrates methods of

specified fiber placement, innovations in nodal geometry, and fiber tension. The

greatest effort involves adding fiber guides to the nodes and modifying the shape

ofthe nodes. The new process is termed Pin-Enhanced Geometry, or PEG.

PEG Assembly

Figure 31 demonstrates the four major steps (A, B, C, D) to define PEG node

geometry. Specifically shown is an orthogrid node, but the steps are similar for

the isogrid node.

In Step A, the beam of width w is determined from structural design criteria.

Guide pins (1.59 mm diameter) are placed in the node region on the centerline of

their respective beam, away from the center to provide an edge-to-edge gap of

one beam width. With this gap, the roving placement accuracy remains constant

throughout the mold rather than requiring an increase in placement accuracy

through the node region.

Step B illustrates designing the node lead-in angle to account for the pins and

the expected side-to-side fiber placement. At the node center, a double-width

beam is simulated with a line, shown as 2w. Then, transverse to the associated

pin, a half-width line (0.5w) is drawn. These lines are connected by a line

extending into the original beam, and they define the flare of the beam in the

rib-node transition zone. The reasons for these particular measurements are

that after lay-up and pin removal, the fibers are expected to fill across the 2w
USACERLTR-98/81 47

line to produce a double-width spread. At the pin, the half-width between pin

and mold wall will enforce a uniform thickness during lay-up because only half of

the rib fibers will be on either side ofthe pin.

In Step C, the rib flare is replicated in the transverse rib and a corner radius is

applied. The corner radius ofthis design happens to be numerically equal to half

of the rib width. Optimally, the corner would be sharp in order to constrain the

fibers exactly to the desired path, but that would create a point of high stress

concentration. Additionally, manufacturing a male mold with sharp corners or

very small corner radii is extremely difficult.

Finally, in Step D, the rib-node transition zone is replicated around the entire

node. It should be noted that the design guidelines provided here are not fully

optimized. However, the guidelines did produce uniform molds and removed

concerns over pin placement and rib-node flare for each new situation. As

demonstrated later, even this imperfect design methodology produced

exceptional results in terms of grid strength and stiffness.

The implementation of this node in a mold is rather simple. The node design is

replicated four times and connected to define the "square" mold block for the

female mold. This block design is cut from 1 in. aluminum plate and holes are

also placed to bolt the blocks to a base plate. The base plate is merely a flat

plate with bolt holes positioned to align the blocks, pin holes to place the guide

pins, and end-of-beam bolts for fiber tension.

Figure 32 demonstrates the winding pattern used for the specimens. The

winding pattern is intended to create the smoothest transition between nodes.

The roving first winds around an end pin and then proceeds down the length of

the beam, traversing the ribs at a very slight angle to shift from one side of the

node pins to the other in successive nodes. At the end of the beam, the roving

bends around the end pin and is pulled with slight tension to straighten the

fibers down the length of the beam. The roving is then placed back down the

same beam going to the alternate sides of the node guide pins, thus creating a

complete single layer in the nodes, and a double layer in the ribs. The roving is

taken to the next beam, and the pattern is repeated around the entire mold until

the mold is filled. It may appear that the fiber angle in the ribs will cause

distinct layers, but the angle is very slight. And, after the male mold is placed

over the fibers, pressed down, and the pins removed (either through the male or

female plates), the slight fiber tension causes the rovings to collapse towards the

center of nodes. The side-by-side positioning is retained, but the rib fiber angle

is reduced to an even smaller amount. The end result is a beam that behaves as

if it were perfectly unidirectional and does not delaminate. If the rovings are
48 USACERL TR 98/81

combined into large bundles, the resin layer will be too thick and there will be

distinct layers that will delaminate.

Basically, the PEG concept ofmanufacturing achieved the following:

• rigid male and female tooling to promote dimensional control

• hard and smooth surfaces to promote parts release

• high-density interlace to promote high fiber density in the ribs

• a method of parts removal which allows large scale structures

• fiber pretension to promote linearity

• node pins to guide the fibers

• lay-up patterns to promote node thickness reduction

• new node geometry to promote smooth rib-node transitions

• wet lay-up with room condition cure to eliminate extraneous material waste.

Manufacture of Large Orthogrid (n/2) Structures

PEG processing demonstrated the ability to manufacture grids with high

material properties utilizing a combination of proper node geometry, fiber

manipulation, and fiber tension. However, the question remained as to whether

this method could be generalized to larger structures.

A 50-cm x 50-cm square orthogrid panel structure was fabricated to discover

large-scale problems. As in the test mold, 1.5 mm pins were used to spread the

fibers. Also, because the rib width was 2.5 times the fiber roving width, each

layer required bundle placement of three rovings. The difficulty with this mold

involved the manual labor effort for setup, lay-up, and parts removal. The steps

are repetitive and simple, but time-consuming when done by hand. Automation

would greatly simplify the process. Assuming that basic automation could be

applied to this process, this mold and grid demonstrated that this PEG process is

feasible on a large scale.

Manufacture of Large Isogrid (n/3) Structures

The final application of the PEG method was an attempt to produce a highquality isogrid specimen. With traditional direct crossovers, this triple crossing

is impossible to manufacture with even a semblance of quality. As noted

previously (Kouri and Dutta 1993), the USAF Phillips labs developed a method

to convert this triple point into three double points by offsetting one rib in each

node, but still failed to resolve the double-point problem. The Phillips method

involves soft silicon tooling and is limited to very thin rib widths. The Phillips

isogrid would be useful for stiffening structures against out-of-plane loads, but
USACERLTR-98/81 49

not for resisting in-plane loads. More desirable would be a structurally

independent isogrid.

To that end, the PEG process was generalized to this triple crossing. A pin is

placed in each rib-node transition, making for a 6 pin node. Also utilized, are

lead-in angles, end bolts for tension, and postlay-up pin removal. The tool and

specimen shown in Figure 33 are the results of this design. The produced

isogrids had a node/rib fiber width ratio of 2:1, as expected from splitting each

beam into two side-by-side bundles. The ideal solution for isogrids calls for

splitting each beam into three bundles. So, some points of excessive fiber density

are expected, and inspection of the specimen confirmed this. Because the ideal

node fiber manipulation did not occur, small resin pools accumulated in the ribnode transition zone.

Stanford Tooling-Reinforced Interlaced Grid (TRIG) Process

The PEG process was derived from studying grid micro-mechanics with the

intent to improve the nodes and ribs. The process produces high quality grids,

but the liabilities include: high effort, high cost, high investment in automated

tooling, and a limitation in design to flat panels. The Tooling-Reinforced

Interlaced Grid (TRIG) process is derived from studying ways to improve grids at

the structural level with the intent of addressing the needs for extensive

configuration capabilities and low-tech processing. These are accomplished by

inverting the conventional concepts surrounding production tooling.

Traditionally, the tooling has belonged to the manufacturer. TRIG changes this

to incorporate the tooling as an integral part of the finished structure, thus

removing any design limitations incurred by a desire to separate the part from

the tool. The term tooling in association with the TRIG process denotes the tube

segment blocks used to construct the TRIG mold.

TRIG Manufacturing Methodology

The TRIG manufacturing process for the flat panel orthogrid shown in Figure

34 involved several laboratory steps.

Step 1 generates the mold blocks, similar to those used for the PEG molds. For

the TRIG process, these are typically filament-wound composite tube segments

with outer cross-sectional geometry becoming the grid channel geometry.

Step 2 constructs the mold by fixing the tooling blocks to a wooden base plate

with lines to guide the tooling block placement. The base plate is covered with a
50 USACERL TR 98/81

clear vacuum bag film to allow easy separation of the part from the base.

Finally, the blocks are aligned to the guidelines and glued to the release film.

For fiber guidance and tension, nails are positioned at the ends of each channel.

Figure 35 demonstrates the mold at this stage.

In Step 3, the wetted fiber rovings are wound into the channels between tooling

blocks. Each layer is thin, using only the minimum number of rovings necessary

to place a single wall-to-wall layer of fiber in the channels. This generates a

highly interlaced grid, as discussed before.

The mold is set aside so the resin can cure. Once cured, the grid is lifted from

the base plate and trimmed to size. In contrast to typical composites processing,

the only disposables from this process are the release film, the trimmed material,

and perhaps the wooden base plate.

Advantages of the TRIG Method

TRIG simplifies various steps in the manufacturing process. A simplified

manufacturing cycle using conventional mold technology is shown in Figure 36.

In mold preparation, the tool is assembled and prepared with a release agent. In

lay-up and cure, the fiber rovings are draped into the mold as rapidly as possible

with minimal interlace density, and the mold is set aside to let the resin cure.

During parts removal, pins in the mold base are used to press the grid out of the

mold, and the mold is submitted for cleaning. Depending on a number of factors,

residual resin may bond to the mold corners and pin holes. It must be removed

without damaging the mold, a cleaning effort that is often difficult.

The TRIG process improves this cycle by increasing interlace density and output

quantity, and eliminating mold cleanup. Figure 37 demonstrates the TRIG

cycle. The grid quality is improved by placing many thin layers to produce a

high interlace density. The extra time required to accomplish this may be

regained both by improved efficiency in the use of factory resources and by the

elimination of mold cleanup. A better utilization of factory resources is achieved

by eliminating the waiting time for the mold tools to become available.

Availability is dictated by the number of molds in the factory and by the period

for resin cure. For conventional manufacturing, each solid metal mold can be an

very expensive investment, and during cure it is unavailable for new production.

In TRIG processing, a new panel is made on demand rather than waiting for the

processing cycle to return a mold to the lay-up step. Additionally, because the

tooling blocks are made on demand by pultrusion, extrusion, or winding, each

panel may have unique tube dimensions that are tailored to the customer's need.
USACERLTR-98/81 51

This combination of improvements makes TRIG an attractive means for

commercial processing of grid structures.

Disadvantages of the TRIG Process

The TRIG process has many beneficial characteristics as discussed above, but

some of the benefits involve hidden costs. Typically, TRIG mold setup time and

setup tool requirements are increased over conventional methods; TRIG involves

direct crossover in the nodes; the open inspectability of molded grids is lost; and

the integrated tooling increases grid weight.

Integration of PEG Techniques into the TRIG System

The PEG study demonstrated that structural improvements might be achieved

through the use of tension, nodal geometry, nodal pins, and a male mold.

Without using these methods, the grids exhibit problems associated with nodal

buildup, low rib Vp and warped fibers (typical of commercial grids). The TRIG

process can easily incorporate two of the four PEG techniques: node geometry

and fiber tension. Tubes with a specifically designed nodal lead-in angle can be

produced as easily as tubes without this angle. Also easily implemented is the

placement of nails for tension at the ends of each beam. However,

implementation of pin-guided fiber spread and male molds would appear to add

significant complexity to TRIG.

Load Testing of TRIG Grids

Load tests were conducted to evaluate the TRIG-processed grid characteristics

and performance. The geometries and tooling types represented only the

orthogrid type of TRIG grids. Other configurations may have different

characteristics. However, these results demonstrate what can be achieved using

nonoptimal tooling made from GFRP tubing with thick walls and pultruded fiber

alignment.

The results were compared with two orthogrid panels that were commercially

available. The TRIG panels were each fabricated by different people with very

different skills and degrees of experience. It was found that the TRIG method is

simple and robust enough that both panels exhibited similar behavior (within 15

percent). These result were obtained even though the panel was manufactured

by someone with minimal experience and supervision, and with minimal process

control offiber quantity, resin mixture, and fiber tension.
52 USACERL TR 98/81

Figure 38 shows the panels that were compared. At left is a commercial grid; in

the center is a commercial grid with tube segments bonded into the cells to

approximate the TRIG reinforcement; and at right is a full TRIG panel. For all

three, the unidirectional core cross-section of the rib is 0.51 cm wide and 1.4 cm

deep. Total grid geometry is based on a 7x7-cell configuration with a spacing of

5.08 cm between node centers. This made the grid outer dimensions

approximately 40.6 cm square.

All three grid panels used E-glass fibers in the core. The resins were different,

with the commercial grids using a polyester resin while the TRIG panel used an

epoxy resin. Therefore, fiber-dominated tensile and bending tests would provide

the most meaningful comparison. All reinforcement tube segments were cut

from the same pultruded GFRP tube (6.44 mm wall thickness). The reinforced

commercial grid had the reinforcement bonded to the grid with a high-strength

epoxy, while bonding in the TRIG panel occurred due to the core epoxy resin.

During full panel testing, no epoxy bonds appeared damaged, eliminating

concern that the structural load/deflection behavior might be controlled by the

reinforcement bond strength rather than by the grid and reinforcement

interaction.

Material Properties

It was expected that the TRIG beams would have greater stiffness

characteristics than beams from the commercial grid, but significantly less than

the PEG beams. As the chart in Figure 39 demonstrates, this is true. The

TRIG modulus in this chart is from the unidirectional fiber core only; the

reinforcement tooling was cut away before the test. Even though all of the TRIG

panels tested have direct crossover nodes, the TRIG axial modulus is 65 percent

greater than the direct crossover commercial grid. This may be attributed to the

increase in fiber linearity and quantity provided by fiber tension. In manufacturing the TRIG panels, the nails at the ends of each beam channel were set

at steep angles, with the nail heads pointing away from the grid. By pulling the

rovings tight around these nails, the rovings slide down the nail toward the mold

and base plate in order to release the tensile stress. This downward motion at

the nail carries the entire roving slightly deeper into the mold, compacting the

fibers slightly and increasing the fiber density.

Panel Properties

Two main structural comparisons are made using the panels in Figure 38.

First, the basic commercial grid panel (A) is compared to the reinforced

commercial grid panel (B) to isolate the effect of reinforcement on out-of-plane
USACERLTR-98/81 53

load/deflection behavior, and to compare weight-normalized behavior. The

second comparison is between the reinforced commercial grid panel (B) to the

TRIG panel (C) to determine the effect of interlace density and fiber tensioning.

Panel bending tests and panel twisting tests were performed on these grids as

described below.

Panel Bending Test. Panel bending is the most basic out-of-plane panel

test and resembles a 3-point beam bending test, see Figure 40. It is a sideto-side test with supports at the outer edges on opposing sides, and at a

midpanel loading point. On a 7x7 panel (i.e., 7 cells by 7 cells), the supports

are only under the one central rib, and adjacent nodes, on each side. Mostly,

this test challenges the axial stiffness of the two central beams. However, it

is a slightly more complex test than the pure beam-bending tests. Without

loading and supporting across the entire panel width, the panel is free to

deform in an anticlastic manner that slightly challenges the grid shear

modulus. This behavior helped verify the accuracy of two grid modeling

programs developed concurrently with this manufacturing program.

Panel Twisting Test. Panel twist (shear) testing is a complex out-of-plane

panel test very similar to four-corner tests. This is a corner-to-corner test

that mainly tests the grid shear stiffness. Two opposing corners are

supported across the corner cell and the two associated nodes. Likewise, the

load is applied across the midpanel cell and associated nodes, as seen in

Figure 41. With no linear path between the load bar and supports, the load

path must turn at each node. The deformed panel displays a saddle shape

compound curvature, as would be expected in four-corner loading. Two

opposing corners displace above the panel midpoint and the other two

displace equidistant below it. The out-of-plane displacement magnitude of

the corners is highly influenced by the panel shear stiffness.

The following four plots illustrate the behavior of the panels in three-point

bending and three-point twisting. Figure 42 shows the results from the bending

test, and Figure 43 shows results from the twisting test. The left-side plot in

each figure is the actual average slope for each specimen type. Normalizing each

slope by the weight of the grid produces the graph on the right. These

normalized values allow a more equitable comparison between the TRIG and

commercial panels.

The first comparison is between the commercial and reinforced commercial grid

panels. Since both panels have exactly the same unidirectional core, all

differences are due to the reinforcement. The reinforced commercial grid

load/deflection slope is 228 percent greater in the bending test, and 487 percent
54 USACERL TR 98/81

greater in the twisting test. The difference between these numbers is due to the

on-axis versus off-axis behavior ofthe grids. Load paths not aligned with a beam

direction require load transfer between orthogonal beams and challenge the grid

shear strength. Since the commercial grids are very weak in shear, the

reinforcement is able to make a much larger impact in the panel twisting

stiffness than on the panel bending stiffness that is mostly dependent on the

unidirectional fiber stiffness.

The graphs also show that stiffness increases with improved interlace density

and fiber pretension. The contribution from these process steps can be seen

when comparing the load/deflection slopes from the reinforced commercial grid

panel and the TRIG grid panel. These two grids have the same type and amount

of reinforcement and the same core geometry. Their main difference is in the

core fiber lay-up, as discussed previously. The TRIG panel slope is 49 percent

greater in the bend test, and 35 percent greater in the twist test. It is expected

that the bending test slope is the one most affected because this test is highly

dependent on the unidirectional core properties whereas the twisting test

depends more on the shear strength.

Of the three panel types, the TRIG panels have the greatest stiffness and also

the greatest weight. It is reasonable to question the load/deflection benefits

relative to weight. Displayed on the right side of Figures 42 and 43 are the

weight-normalized stiffness behaviors. The normalization weights are as

follows: basic commercial grid = 0.9 kg; reinforced commercial grid = 2.6 kg; and

TRIG = 2.8 kg. Comparing the basic and reinforced commercial grid normalized

responses, reinforcement does not benefit bending behavior, but adds 93 percent

to the normalized twisting behavior. Similarly, between the reinforced

commercial grid panel and TRIG panel, TRIG panels are significantly more stiff

in both bending (37 percent) and twisting (24 percent). This means that the

weight-based comparison is positive for TRIG, even when using the highly nonoptimal tooling of this study. But, even with suboptimal tooling, these results

indicate that as long as the total allowed structural weight is not exceeded, the

TRIG panel has the greatest specific stiffness and is ideal for use in applications

need low costs combined with good performance per weight.

Concept Development for Full-Scale TRIG Grid Production

The laboratory-scale TRIG method of grid production was successful but it was

necessary to translate this development to a commercial scale production process

in which the cost of production must be minimized by reducing the production

steps. To address this problem Dutta and Bailey (1996) conceived the novel idea
USACERLTR-98/81 ^ 55

of producing the TRIG grid panels as in Stanford from TRIG blocks. A TRIG

block is a TRIG grid several feet that is later sliced by industrial slicing machine

to produce the grid panels.

The block tooling conceived for this process is shown in Figure 44. It consists of

rows of FRP tubings of outside dimensions compatible with the rib widths and

lengths as discussed in the TRIG process. The major difference from the TRIG

process is that while the TRIG process allowed the tooling (tubing) height to be

limited to the grid height, this process will allow a grid to be made several feet

high. Thus, instead of a flat grid panel the product will be a grid block, as shown

in Figure 45. Figure 46 shows a pictorial view of four slices made from one

block. This approach would accelerate the grid manufacturing process and

would be expected to bring the cost down. In order to practically demonstrate

the process a trial block tooling was manufactured at USACRREL using

polyvinyl chloride (PVC) square tubing mounted on a wooden block. However,

further effort toward developing the tooling was suspended as the project

direction was changed from developing a demonstration model for flat grid

panels to a demonstration ofbeams and columns.
56 USACERL TR 98/81

6 Grid-Reinforced Concrete (GRC)

Panels

This chapter discusses the testing of the previously described composite grids as

reinforcement in concrete structures.

The conventional method of reinforcing concrete is to place the tensioning

element (rebar or grid) on the tension side of the beam or panel. But under

many loading conditions panels may require reinforcement on both sides. Thus

parallel investigations were conducted for both configurations. At BYU, the

composite grid reinforcement in concrete was tested as concrete beams and

panels with one set of grids as reinforcement. At Stanford University tests were

performed with flat panels of concrete reinforced with single grid and bi-grid

systems. The results ofthese efforts are discussed in sequence.

GRC Testing at BYU

Testing at BYU involved the use of FRP grids as primary reinforcement in

concrete beams as structures. Results of 26 beams tested in flexure under a

four-point loading system will be discussed to analyze the behavior of the FRP

grid reinforcement. The grids were of varying strength and stiffness. Control

beams were reinforced with steel to allow for comparison with grid reinforced

beams.

Due to the significant geometric and material differences between grid

reinforcement and traditional steel rebar reinforcement, issues such as load

transfer, stiffness, and toughness were investigated. The test matrix was

developed to enable identification ofthe effects ofmultiple variables. Grids were

made from carbon and glass fiber composites and steel. The transfer of load

between the concrete and the reinforcement was investigated by varying the

geometry of the grids. Commercially available grids were not adequate to

investigate all ofthese parameters; therefore, the manufacturing of grids became

an important consideration. The techniques developed to manufacture

composite orthogrids were described in Chapter 5.
USACERLTR-98/81 57

Description of Test Specimen Geometry

As shown in Figure 47, 30 in. long beam specimens having a 6 in. square crosssection were chosen for convenience because beam molds already were available

for this size specimen. The control beams were reinforced with the mini'miim

amount ofsteel according to ACI (1992) Building Code and Commentary, 318-89,

Section 10.5. This corresponds to one #3, grade 60 reinforcing bar. The beams

reinforced with steel rebar provided concrete cover of % in. for corrosion

protection and development ofbond strength. The grids, on the other hand, were

placed flush with the surface ofthe beam.

Two types of grids were used in this experiment: commercially manufactured

grids and in-house fabricated grids. The commercial grids were composed ofribs

approximately 0.2 in. by 0.6 in. spaced at 2 in. on center. The fabricated grids

were composed ofribs approximately 0.17 in. by 0.63 in. spaced at 4 in. on center.

Table 5 shows the area of reinforcement as well as the theoretical mechanical

properties ofthe reinforcement.

The Test Matrix

The test matrix is summarized in Figure 48. Initial tests were performed using

commercially available grids fabricated by a commercial manufacturer. The

manufacturer produces GFRP composite grating material (i.e., grids) used for

flooring in chemical plants. Because ofthe low stiffness ofthese gratings, carbon

was chosen as the preferred better grid material. Carbon fiber tow supplied by

Akzo Nobel Fortafil Fibers, Inc., was sent to the grid fabricator for production of

carbon grids. The manufacturing procedure employed at this point did not

achieve a sufficiently high fiber volume fraction. This produced carbon grids

with undesirably low stiffness. Three concrete beams were reinforced with these

grids and the load/deflection curves showed deflections much larger than

originally anticipated.

The remaining test configurations were based on an equal rigidity approach.

New grids were fabricated with higher volume fractions, yielding much higher

stiffnesses. Rigidity was chosen as a baseline for comparison. The grids were

designed such that the modulus of the grid times the cross-sectional area of the

longitudinal ribs of the grid would be approximately equal to the modulus of

steel times the area of one #3 reinforcing bar. This would result in test beams

with the same stiffness or load/deflection behavior.

Load transfer mechanisms were another issue that arose from the tests

performed with the commercial grids. The grid-reinforced beams did not show
58 USACERL TR 98/81

an evenly distributed crack propagation. Cracks and deflections were concentrated primarily in one spot, possibly indicative of poor bonding or load

transfer (see Figure 49). To identify the effects of the different load transfer

mechanisms, grids were manufactured with ribs to simulate the ribs found on

steel rebar. The behavior of these deformed (ribbed) grids is compared to grids

without deformations.

A major concern associated with composite-reinforced concrete is the lack of

ductility and tendency toward brittle failure. The possibility of a high-strength,

high-stiffness, noncorrosive reinforcement that demonstrates some amount of

ductility would be extremely desirable. To investigate this possibility, three

hybrid grids were fabricated using a combination of carbon fibers and steel wire.

Steel wire embedded in the composite grid might provide additional ductility

after failure ofthe composite fibers.

Manufacturing the Concrete Beams

A concrete mix design corresponding to a compressive strength of 5000 psi was

chosen for the beam specimens. This strength is common for reinforced concrete

structural members. A Type III portland cement was used to expedite the

research. The high-early strength cement allows 14-day tests to be done in place

of 28-day tests while still maintaining a relatively high-strength concrete. The

beams were formed using prefabricated steel molds. Prior to pouring the

concrete, the rectangular steel molds were assembled and coated with a release

oil. Small holes were drilled in the end plates of the molds to allow the strain

gage wires to exit the beam molds. The reinforcement was placed in the bottom

ofthe molds and the strain gage wires and/or optical fibers (as appropriate) were

threaded through the holes in the end plates. The concrete was mixed in a large

portable mixer. The steel molds were filled with concrete in three lifts, each

approximately 2 in. in height. The beams were poured in sets ofthree or four.

Three 6 in. diameter, 12 in. long compression cylinders were poured with each

set of beams. Like the beams, the cylindrical molds were filled with the same

material in three lifts of equal height. These samples were tested at the same

time as the beams in order to determine the concrete strength at the time of

testing.

The completed rectangular concrete beams and cylindrical compression

specimens were left in the molds overnight. The next day the beams and

cylinders were removed from the molds and placed in a fog room where the

temperature was maintained at approximately 75 °F and the humidity was
USACERLTR-98/81 59

maintained at 90-95 percent. The beams and cylinders were cured in the fog

room for 14 days.

Test Fixture Setup

The test fixture used in the beam tests was a standard 4-point loading test

fixture. The fixture (Figure 50) consists of a channel section approximately 8

in. wide and 32 in. long with threaded holes at varied increments. The supports

are made of % in. diameter cylindrical steel rod fixed to steel blocks that can be

fastened with bolts into the channel in different locations to produce a wide

variety ofspans. One ofthe supports is allowed to swivel. The compression load

head is equipped with two load points very similar to the supports on the

channel, which can also be set at varying positions to change the loading span.

The entire load head is allowed to swivel in order to ensure equal load to each

loading point.

The span of the supports was set at 27 in. and the distance between load points

was set at 9 in. This resulted in three even spans of 9 in. each.

In order to measure large deflections with a high degree of accuracy, a linear

voltage differential transducer (LVDT) with a 1 in. range was attached to a lever

arm with a 10:1 ratio, allowing digital deflection readings up to 10 in. Using the

stand and the bearings for an existing deflection gage, a longer lever arm and an

LVDT holder were manufactured from aluminum (Figure 51).

Flexural Specimen Testing

After removal from the fog room, the concrete beams were allowed to dry until

their surfaces were free of excess moisture. Alignment marks were placed on the

beams and the beams were placed on the 4-point loading fixture. The strain

gage and LVDT lead wires were attached to the computer lead wires and the

LVDT deflection arm was positioned beneath the center point ofthe beam.

For each test, the load, midspan deflection, and electrical resistance strain gage

data at multiple locations were collected, along with photographs at various load

stages.

For each test, the data acquisition was started and the load crosshead was

lowered until contact was made with the beam. The range on the Baldwin 300-

kip test system was set to the low range (0-16 kips) and the load dial was zeroed.

The hydraulics were then started and load was applied to the beam at an

approximately constant rate. The loading rate varied somewhat between
60 USACERL TR 98/81

specimens (between 20 and 50 lb per second) and was only approximately

constant due to limitations of the manual controls on the Baldwin 300-kip test

system. In each case, the load was increased until a significant drop in load

indicated failure.

As the testing began the load increased steadily until initial cracking of the

beam occurred. This often happened when the load reached approximately 5.0 to

5.5 kips. A slight drop in load indicated that initial cracking had occurred. The

load continued to climb steadily as the flexural cracks continued to elongate and

widen. The next drops in load corresponded to the formation of shear cracks.

These cracks propagated from the beam supports and extended toward the load

points. These cracks eventually led to failure. As the ultimate load was reached,

a large shear crack formed and a loud noise was emitted simultaneously with a

dramatic drop in load, indicating failure.

Test Results

The data stored automatically by the AutoNet operating system was exported to

the DOS-accessible drive as a Lotus 1-2-3 file and then read into Microsoft Excel

spreadsheets. All of the data reduction was performed using Excel spreadsheets

and the Microsoft Excel Visual Basic macro language.

Initial data reduction involved removing unnecessary data points that were

logged during the space oftime between the initiation of data acquisition and the

actual commencement ofloading. The excess data at the end ofthe file were also

removed. The calibration ofthe LVDT deflection arm did not allow for re-zeroing

with every test. Zeroing the deflection data with respect to the initial readings

compensated for slight variations in the deflection starting point.

Load/deflection graphs were generated in order to compare the behavior of the

different beam types.

The load/deflection curves were analyzed to determine the effective stiffness

(both pre and postcracking) and the toughness for each beam. Stiffness relates to

the amount of deflection caused by a unit load, or the slope of the load/deflection

curve. The effective stiffness was found by determining the best-fit linear

approximation of the pre and postcracking portions of the load/deflection curve.

The Excel graphing functions were used to accomplish this. Figure 52 shows

the portions of a typical graph used to calculate the effective stiffness.

The toughness was calculated by finding the total area under the load/deflection

curve using a program written in the Excel Visual Basic language. The program
USACERLTR-98/81 61

used the trapezoidal rule of numerical integration. The toughness value

indicates the amount of energy absorbed by the beam up to failure.

Theoretical load/deflection values were obtained using an effective moment of

inertia (Je) according to ACI 318-89 (Revised 1992), Equation 9-7:

L =

rMcr

Ma

•/* + 1-

(Mcri

Ma

[Eq21]

where

Mcr = [Eq 22]

and

Ma = maximum moment at section where Ie is being calculated

Ig = gross moment ofinertia, neglecting area ofreinforcement

ICr = cracked moment ofinertia

fr- Modulus ofRupture, ( 7.5 x V fc')

yt = distance from neutral axis to tensile face of concrete, for uncracked

section.

The experimental modulus of elasticity as measured from the compression tests

was considerably lower than the modulus as predicted by ACI 318-89, Section

8.5.1 (1992). As a consequence, the initial theoretical deflections were significantly lower than the actual deflections observed during testing. Therefore, the

modulus obtained from the compression tests was used instead of the ACIpredicted modulus.

Results from Steel Rebar Reinforced Beams

Figure 53 shows the load/deflection behavior of the beams reinforced with one

#3 deformed steel reinforcing bar. Notice the large deflections that the beam

undergoes after the steel reinforcement begins to yield. The large drop in load at

about 1 in. of deflection corresponds to failure ofthe beam by compression failure

in the concrete. The load recovers slightly before a tensile failure of the steel
K2 USACERL TR 98/81

reinforcement. Figure 54 shows the pre and postcracking stages prior to

yielding of the steel. Notice the correlation with the ACI-predicted deflections.

Table 6 presents the average values for ultimate load, pre and postcracking

stiffness, maximum deflection, and toughness results, as compared with the

results from the other reinforced beams. The large deflections and toughness are

indicative ofthe ductility ofthe steel-reinforced beams.

Results from Commercial Carbon Grid Reinforced Beams

Figure 55 shows the overall load/deflection curve for the commercial carbon grid

reinforced beams. The low modulus of the reinforcement is evident from the

large deflections following the initial cracking ofthe beam. Figure 56 shows the

initial portion of the load/deflection curve. The theoretical load/deflection curve

does not predict the sawtooth behavior caused by crack propagation in the beam.

The failure mode on these and the other composite grid reinforced beams was a

diagonal tension failure in the concrete. Again, the low modulus of the

commercial carbon grid (Table 6) results in a lower postcracking stiffness (37

kips/in.), compared to the baseline steel rebar reinforced beam (81 kips/in). The

commercial carbon grid also has a lower toughness and maximum deflection.

Results from Commercial GFRP Grid Reinforced Beams

Figure 57 shows the load/deflection behavior of the beams reinforced with

commercial GFRP grids. Figure 58 shows an expanded portion of the

load/deflection curve. The nonlinearities in the range of 0 to 2 kips can be

attributed to localized crushing of the concrete and settling of the supports.

These curves demonstrate that the theoretical deflections for the postcracking

segment are fairly nonconservative. Once again, the low modulus grid (Table 6)

results in very little stiffness (18 kips/in.) after the beam has cracked. The

postcracking stiffness is the lowest for the GFRP grid reinforced beam.

Results from BYU Plain Carbon Grid Reinforced Beams

Figure 59 shows the overall deflections for the BYU plain carbon grids. One of

the beams showed significantly different behavior than the other two prior to

initial cracking of the beam. This difference can be attributed to differences in

concrete strengths. Figure 60 shows the initial region of the load/deflection

curve. Once again, the differences in behavior can be attributed to differences in

the concrete. The nonlinear effects of settling and localized crushing were

greater in two of the beam specimens. The postcracking stiffness (Table 6) is

slightly larger (88 kips/in.) than that of the steel rebar reinforced beams (81

kips/in.).
USACERLTR-98/81 63

Results from BYU Deformed Carbon Grid Reinforced Beams

Figure 61 and Figure 62 show the overall and expanded load/deflection curves

for the BYU deformed carbon grid reinforced beams. The three beams showed

very similar behavior to the plain BYU carbon grid reinforced beams. The

postcracking effective stiffness, toughness, and maximum deflection, as shown in

Table 6, are almost equivalent to the values for the plain BYU carbon grid

reinforced beams.

Results from Hybrid Carbon/Steel Grid Reinforced Beams

The effects of the variations in beam strength are again seen in Figure 63 and

Figure 64. The beams reinforced with grids composed of carbon fibers and steel

wire showed behavior similar to that ofthe all-carbon grids. As seen in Table 6,

the toughness is unexpectedly lower (2.8 kips/in.) in the hybrid grid reinforced

beams than in the all-carbon grid reinforced beams (3.8 kips/in.).

Results from Steel Grid Reinforced Beams

The load/deflection behavior of the beams reinforced with steel grids is shown in

Figure 65 and Figure 66. The cutting of the steel grids resulted in some

nonuniformities in cross-section that can account for the differences between the

deformed steel grid and the plain steel grid.

Overall Comparison

Figure 67 shows typical load/deflection curves for some ofthe beam types. The

BYU composite grids showed similar load/deflection behaviors with significantly

higher ultimate loads over the steel rebar reinforced beams and the commercial

composite grid reinforced beams. The steel rebar reinforced beams

demonstrated increased toughness due to the large deflections undergone during

the yielding of the reinforcement. Figure 68 shows the initial portion of the

load/deflection curves. If the nonlinearities associated with the BYU composite

grid reinforced beams were neglected, the postcracking behavior would be

similar to that of the steel rebar reinforced beams. Notice the large toughness

value (Table 6) for the steel rebar reinforced beam and the higher ultimate loads

for the composite grid reinforced beams.

To illustrate the failure mechanisms, a series of photographs was taken during

the testing of one of the BYU plain carbon grid reinforced concrete beams. The

load/deflection curve of this test is shown in Figure 69, identifying the loading

points at which the photographs were taken. The photographs shown in
64 USACERL TR 98/81

Figures 70, 71, and 72 illustrate the propagation of cracks in the beam as the

load increases.

Figure 73 shows that there was a large variation in the precracking stiffnesses

ofthe beams. There does not, however, appear to be any correlation between the

type of reinforcement and the precracking effective stiffness. This would

indicate that the type of reinforcement does not significantly influence the

amount of deflection until after cracking occurs. This was to be expected because

the type of reinforcement does not significantly affect the gross moment of

inertia. The differences in precracking stiffness can instead be attributed to the

large variations in material properties often associated with concrete. The

correlation between the beam type and the compressive strength can be seen in

Figure 74. It is interesting to note the similarities with Figure 73 indicating

that the precracking stiffness is primarily controlled by the compressive

properties of the concrete. The postcracking stiffness, on the other hand, shows

a very good correlation between the axial rigidity of the reinforcement and the

postcracking. Figure 75 shows the postcracking stiffnesses for the various beam

types. Figure 76 shows the axial rigidity ofthe different types ofreinforcement.

Figure 77 shows the linear relationship between the postcracking stiffness and

the axial rigidity ofthe reinforcement.

There was very little difference between the load/deflection curve for the beams

reinforced with plain grids and those reinforced with deformed (ribbed) grids.

This would indicate that the transfer of internal loads through bearing on the

crossmembers of the grid structure is just as effective as the transfer of load

through shear between the concrete and the reinforcement. The crack

distribution for the plain and deformed grid reinforced beams was also very

similar. For the grid-reinforced beams, cracking occurred at the locations of

transverse grid ribs. This led to a uniform crack distribution with cracks

forming at discrete intervals.

The results from the beams tested show that perhaps the least desirable

characteristic of composite grid reinforcement is the inherent lack of toughness.

Figure 78 shows the comparison oftoughness and ultimate load for the different

configurations. The large amounts of deflection attained during the yielding

phase leads to large amounts of inelastic energy absorption and greater warning

prior to failure. While the ultimate load of the composite grid reinforced beams

is significantly higher than for the steel rebar reinforced beams, the brittle

failure of the composite reinforcement does not allow for the same energy

absorption and ductility. This challenge needs to be overcome.
USACERLTR-98/81 65

GRC Investigations at Stanford

GRC investigation at Stanford focused primarily on using concrete as a filler

material and the composite grid as the major load-sharing structure in

compression, tension, and shear. These results were finally used to validate the

GENLAM model as discussed in Chapter 4 for prediction of grid-reinforced

concrete behavior. Thus, the Stanford design approach for test specimens was

different than the one used at BYU.

Figure 79 demonstrates the schematic of a bi-grid reinforced concrete slab with

the grids on the outer surfaces. Figure 80 shows the steps for its construction.

Using tube segments as spacers (external dimensions equal the orthogrid cell

dimensions and fit snugly into the grid cells), five spacers were placed in each

7x7 orthogrid panel. The spacers were placed in panel center cell and four

perimeter cells at the testing load points, as seen previously in Figures 40 and

41. The spacers were bonded with epoxy into the first grid. The second grid was

aligned to the first via the spacers and was bonded to the spacers to create the

frame. Figure 81 shows one ofthe completed GRC panels.

To make the reinforced concrete slabs, a fine aggregate concrete was used. Using

Type I/II Portland cement, #30 grit sand, and water in a mass ratio of

100:250:60, respectively, a mortar-like concrete mixture was formed. This ratio

allowed the cement to easily flow through the cells and completely fill the grid.

The grid frames were placed in containment boxes and the concrete was poured

to the level of the upper grid surface. Because they have a very low specific

gravity, the grids tend to float upwards in the concrete unless restrained during

the air removal vibration process. The concrete was allowed to cure for a

complete 28 days, after which the GRC structure was ready for testing.

Testing

Two material tests and three structural tests were performed on the GRC, as

illustrated in Figure 82. Material tests determine beam-aligned tensile and

compressive moduli, and were used in models to predict beam bending behavior.

The compressive test was also used to demonstrate the benefits ofthe grid—high

compressive strength and concrete containment after fracture.

The structural tests were out-of-plane tests: 3-point bending of beam, 3-point

bending of panels, and 3-point twisting of panels. Panel bending and twisting

tests are fundamentally different due to the demands each test places on the

grids. The bending test was basically aligned to one of the grid beam directions

and tested the grid axial stiffness. The twisting test was cross-aligned to both
66 USACERL TR 98/81

beam directions and tested the grid shear stiffness much like a four-point test

that loads all four corners. By observing the actual deformation of the panels in

the twist test, a saddle shape may be seen, exactly as would occur with the fourpoint test. These panel tests check the overall behavior of larger structures,

especially in terms offailure toughness and concrete containment.

The three-point bending tests were conducted on three beams of each crosssectional type shown in Figure 83. All cells were filled with concrete.

Displacement data were measured at midbeam where the load is applied, and

the strain was measured on one of the two axial beams at a point 2 in. from the

beam midspan. The experimental load/deflection and load-strain data are

presented in Figure 84.

Model Via GENLAM

The application of GENLAM to beam bending was discussed in Chapter 4. The

goal of this model is to prepare ply material properties that appear within the

GRC beams, determine a rule of lamination for combining these plies for

accurate beam modeling, and then use the GENLAM-generated 8U value to

calculate beam load/deflection behavior. Finally, the model predictions are

checked against the experimental values in order to evaluate the model validity.

GRC Plies

Six distinct ply materials, listed in Table 7, were designed for GENLAM

analysis. These are divided into groups of pure concrete and grid/concrete

materials, with each group having a material that reflects the compressive,

tensile, and average behavior. The axial properties are determined as described

before, using compressive and tensile tests as represented in Figure 85.

Assuming isotropic behavior in the concrete, the shear modulus is obtained from

the standard relationship between the axial modulus, shear modulus, and

Poisson ratio:

G

«=U^) [Eq23]

The Poisson ratio varies slightly for each state ofstress within the concrete. For

uni- and biaxial tension, the ratio is 0.18. For uniaxial compression it is 0.20. In

biaxial states with mixed tension and compression, the ratio is 0.19. The GRC

shear moduli are copied directly from the concrete materials on the assumption
USACERLTR-98/81 67

that the concrete contribution towards GRC shear stiffness is much greater than

the grid contribution.

Lamination Methods

Among other results, GENLAM calculates the compliance matrix and stresses in

laminates. For these GRC beams, each grid layer is divided into four plies of

3.175 mm depth. This provides the grid with multiple stiffness zones to allow

placement of appropriate materials at appropriate locations. For example, the

upper halfis expected to be in compression and should require the application of

GRC compressive properties while the lower halfis expected to be in tension and

should require use of GRC tensile properties. Multiple plies within the grid

allow modeling ofthese changing properties.

There are two ways to evaluate the effectiveness of the model. The first and

most obvious way is to compare with the experimental data. However, the goal

is to develop a predictive method independent of experimental verification.

Therefore, a second method uses tools within GENLAM. By applying a moment

to the laminate, Mt = 1MN, GENLAM will calculate the stresses within each ply.

For the purpose of this study, it is assumed that the most accurate lay-up model

is a laminate in which the stress distribution varies as smoothly as possible

across the beam thickness, as drawn in Figure 86. A smooth stress distribution

is not generally the rule in laminated composites, but in this situation it is found

to be an effective method for evaluating the accuracy of any given lay-up.

The basic lay-up for modeling the beams starts by assuming that the beam

neutral axis (N/A) exists at the beam midplane. In the tensile half, plies with

tensile properties are applied; in the compressive half, plies with compressive

properties are used.

After evaluating the basic lay-up and examining the resultant stress

distribution, refinement may be necessary. The two basic refining steps are

presented in Figure 87. First, the exact N/A location and hence the exact stress

state near the N/A, is assumed to be unknown. To avoid using the incorrect

material, an average material is applied to the plies surrounding the N/A. The

second refinement is to examine the stress-resultant calculations and allow a

shift in the assumed location of the N/A away from midplane. The compressive

materials in this model are significantly suffer than the tensile materials. Thus,

equally strained compressive and tensile plies will support much different loads.

To balance the beam's internal forces, the N/A shifts upward to make a smaller

compressive region and larger tensile region. To model this shift, tensile materials are applied up to the assumed N/A location. This location may be confirmed
68 USACERL TR 98/81

in an iterative manner by using GENLAM to display the ply stresses. The

tensile, compressive, and average stiffness plies are manipulated until the

calculated and assumed locations agree.

Demonstration and Result

The 13 mm beam is the first one to be modeled and verified. The beam is divided

into four plies, each taking one of the three GRC material properties (Gt, Gc, Ga).

A unit moment of M1 = 1MN is applied in GENLAM and the calculated ply

stresses are examined. As described in the previous section, the most basic

initial lay-up assumes the N/A at the midplane, with tensile materials in the

lower half and compressive materials in the upper half. Lay-up A in Table 8 and

Figure 88 represents this case. Upon examination of the stress plot, it appears

that the assumed N/A location is incorrect, and it is actually in the upper half,

causing tensile loading of compressive materials. A sharp jump in stress levels

also occurs at the tensile/compressive materials interface.

Following the refinement guidelines, the N/A is allowed to move and the average

material property is applied to the layer that contains it. This leads to Lay-up B,

which does smooth the stress distribution—but not completely. In the low-stress

states, intuition would expect the beam to behave in a uniform manner with a

midplane N/A. To satisfy curiosity, Lay-up C applies the average material to the

ply below the N/A. The resulting stress distribution looks coarse, but it starts to

satisfy intuition by shifting the N/A back toward midplane. Extending this trend

leads to Lay-up D. Lay-up D is the result from using only the average material

properties. The N/A returns to midplane, surrounded by average material

properties, and the resultant stress distribution is entirely smooth. Figure 89

demonstrates the accuracy of each of these lay-ups by overlaying them on the

experimental data. As may be seen, Lay-up D is a good first success for this

modeling process because it provides the most accurate predictions, within the

experimental scatter, on both the load/deflection and load/strain graphs.

Like the 13 mm beam, the 38 mm beam uses 3.175 mm plies (12 total). Unlike

the 13 mm beam, all six materials are used in order to model the three distinct

layers (i.e., lower grid panel, concrete core, and upper grid panel). Table 9 and

Figure 90 summarize the progression of lay-ups used in this model. Instead of

starting with the basic lay-up of tensile and compressive materials, Lay-up A

utilizes all average materials in an attempt to learn from the final lay-up for the

13 mm beam. Unfortunately, the stress distribution is extremely jagged. In an

attempt to remove the stress discontinuity around the N/A, tensile and

compressive concrete properties are applied to the core in Lay-up B. This
USACERLTR-98/81 69

reduces the tensile side discontinuity but aggravates the problem on the

compressive side.

Lay-up C returns to the basic lay-up with one refinement; placing an average ply

above the N/A. The resulting stresses smooth out slightly and the N/A shifts up

into the second layer of the compressive half. Lay-ups D and E are intended to

account for the N/A shift. Lay-up D uses tensile and compressive properties

around the new N/A and Lay-up E adds an average material layer at the N/A

location. Both generate the same numerical results but Lay-up E has the better

stress distribution; however, even Lay-up E has a small stress discontinuity in

the concrete core. A final lay-up, F, uses only tensile properties in the core and

one average with three compressive GRC plies in the upper grid panel. As can be

seen, this method produces the smoothest stress graph.

Again, it is reasonable to question the accuracy of the Lay-up F results. The

stress distribution is smooth, but the actual predictions may be completely in

error. Figure 91 overlays the predictions from these lay-ups onto the

experimental data. Notably, each refinement relative to the stress distribution

also improved the accuracy of the prediction. Furthermore, the final lay-up falls

on the edges and within the actual experimental data.

Considering the success of this modeling method as applied to GRC beams with

both the mono-grid panel and the more complex dual-grid frame, it appears that

this method may be successful at modeling GRC beams of arbitrary thickness.

Figure 92 summarizes the final lay-up models for both the 13 mm and 38 mm

beams. As stated above, these lay-ups are found by starting with a basic

distribution of plies and then refining the material stacking sequence until the

stress distributions vary smoothly between lower to upper surfaces. Further

research can extend this method to include strength predictions and application

to larger, more generalized structures.

GRC Panel Testing at Stanford

For these tests, 7x7 grid panel structures (each measuring 36 x 36 cm) of various

thicknesses were used, from 13 mm mono-grid panels to 38 mm and 64 mm dualgrid frames. These were tested both as grid-only and GRC structures. They

were all constructed as described in Chapter 5. The perimeter struts used to

make the grid frame were positioned at the center of the sides for the panels

used in the bending tests, and at the corners for the panels used in the twisting

tests. These locations are designed to place the struts at the loading and support

locations so frames without concrete can transfer the loads between the upper

and lower grid panels. As a part of a GRC structure these struts lose their
70 USACERL TR 98/81

importance because the concrete takes over the role of bearing the loads between

surfaces. Figure 93 shows the mono-grid panel specimen types and test setups.

Tests were conducted as described previously, with the load/deflection slope

results summarized below.

Grid Behavior

The first question to address was the baseline response to be expected from the

grids before placement in concrete structures. A graphical summary of the test

results is presented in Figure 94, and numerically tabulated in Table 10.

GRC Behavior

With concrete fill, the panel and frame structures become considerably more

stiff. To provide a comparison that highlights this increase in grid stiffness, the

charts in Figure 95 compare the mono-grid panel and dual-grid frame behavior

to the equivalent GRC behavior. For the 13 mm panel case, bending and

twisting stiffness each increase by approximately 150 percent. For the 64 mm

frame case, the stiffness increase jumps to 1500 percent in the bending test and

a staggering 5000 percent in the twisting test. Figure 96 summarizes the GRC

test results that are averaged and tabulated in Table 11.

GRC Toughness

Toughness is one of the design requirements of steel-reinforced concrete

structures. Iftoo much reinforcement is used, the SRC panel will fail in a brittle

manner due to the inability of the tensile reinforcement to yield at a level

consistent with the compressive strength of the concrete (MacGregor 1991).

With too little reinforcement, the panel will yield excessively and support far less

than the necessary maximum load. The ideal design will support loads up to a

desired maximum before yielding, at which point the structure will continue to

support that yield load through a significant range of deformation prior to final

failure collapse. Such postyield behavior is what constitutes toughness, and it

has been observed in all GRC panels, both in the bending and twisting tests.

Recalling that the bending test is essentially a test of the grid on-axis stiffness

properties and that the twisting test significantly challenges the grid shear

stiffness, it is impressive that the GRC panels display failure toughness in both

stress states.

Figure 97 explains the various regions of the load/deflection behavior through a

failure curve displaying the desired toughness. This is actual data from a 38 mm

dual-grid GRC panel. In the first region, the panel deforms elastically up to the
USACERLTR-98/81 71

point of initial yielding at 17.5 kN. At this point, the panel has absorbed

approximately 85 joules of energy, calculated as the integrated product of load

and deflection (or the area under the load/deflection curve). After initial

yielding, the panel begins to deflect significantly, supporting roughly the yield

load. It progresses to slightly more than twice the yield deflection before

arriving at final failure, with a total absorbed energy of approximately 200

joules. The yield region loading accounts for 58 percent of the total absorbed

energy of deformation. This large percentage of total energy absorbed in the

yield phase is what defines these structures as tough in failure. Figures 98a

and 98b illustrate this behavior in other representative panels.

GRC Concrete Containment

The containment of the fractured concrete likely plays a significant role in the

degree of panel toughness demonstrated in the previous section. The structure

enters the post-yield phase as the grid and concrete begin to fracture, but by

containing all fractured concrete in position, the structure can continue to

support the yield load. With the concrete in position, internal forces are still

distributed through normal contact between fractured segments such that each

continues to support portions ofthe load. If cement portions were to break away,

the internal forces would rebalance by shifting to already overburdened

reinforcement elements, which would in turn rapidly induce total structural

failure. Photographs in Figures 99 through 101 demonstrate the containment

capabilities of GRC panels. These photographs illustrate panels in the post-yield

and postfailure stages. Significantly, even the 13 mm mono-grid panels contain

the concrete without having any concrete plugs "pop out" from the cells.

Summary

Through various tests of the grid-reinforced concrete structures, it is demonstrated that this reinforcement concept has excellent potential. The grid

reinforcement has shown complete concrete containment that results in a high

structural toughness, and the ability to avoid the corrosion and construction

difficulties found with steel reinforcement. The modeling capabilities can be

expanded beyond simple beam stiffness to benefit GRC structures and provide a

significant tool for future construction.
72 USACERL TR 98/81

7 Column-Reinforcing Grids

During this research, continuing attention was paid to developing a grid design

simple enough to be industrially produced and commercially applied. Through

continuing dialogue with the composites industrial partners during this study it

became apparent that the manufacturing processes so far developed in the

laboratory would not scale up to be economical in an industrial production

environment to be cost-competitive with conventional steel reinforcement. Not

only must the ultimate value in performance and durability be attractive, but its

first cost also must be attractive. Industry feedback gave a strong incentive to

develop a grid system that would be economical to manufacture but still retain

all the benefits of the grid structure developed in the study. It was also

considered that rather than flat grid panel type prototypes, longitudinal

structures such as columns would be easier to demonstrate as a new concept to

the traditionally conservative civil engineering community.

The proposed reinforcement system, called stacked joint grids, would use grid

structures composed of composite members that are placed circumferentially as

well as longitudinally. This method of reinforcement would allow concrete

columns to be subjected to greater compressive loads before induced shear

failure occurs. Concrete columns were tested in compression to explore alternative geometric configurations of the composite grid structures and optimize

their strength.

Stacked Joint Grids

Stacked joint grids were developed in a number of configurations to meet the

need for economical production. The circular reinforcement is composed of two

carbon fiber rings separated by extruded tubes. These tubes are bonded to the

carbon fiber rings by an epoxy fill to make circumferential reinforcement

elements called circs. The circs are connected via pultruded GFRP rods. These

longitudinal rods, called longis, resist flexure while the carbon fiber reinforced

circs contain the concrete under axial compression (Figure 102). This

containment of the concrete prevents the column from expanding radially,

delaying the primary mode of failure. Preliminary research on sample

prototypes successfully demonstrated the feasibility of this concept. This

research was continued to explore the role of longitudinal and circumferential
USACERLTR-98/81 73

members to develop suitable design criteria for the stacked joint reinforcement

system.

Stacked Joint Grid Specimen Manufacturing

The grid specimens to be manufactured were sized to reinforce concrete column

specimens 18 in. long with an 8 in. circular cross-section. This provided concrete

columns having a ratio of column diameter to column length that was

appropriate for axial compression tests. The concrete columns would be

reinforced with manufactured stacked joint grids ofvarious configurations.

The circs used in the stacked grids are V2 in. thick, composed oftwo hoop-wound

carbon fiber reinforced rings separated by interior guide rings made from

extruded polymers. The guide rings were bound to the inner and outer carbon

fiber rings by vacuum-infiltrated epoxy. The finished first-generation circs

contained 15 guide rings and the finished second-generation circs contained 21

guide rings, providing holes for the longis. The longis, GFRP rods pultruded

with a high fiber volume fraction, serve as longitudinal reinforcement while

providing spacing between the circs.

The first-generation composite grid structures contained circs having a 7 in.

outer diameter with a fully packed column of 15 longis; the second-generation

structures contained circs having an 8.125 in. outer diameter with a fully packed

configuration of 21 longis. Different geometric configurations of the composite

reinforcement were built by varying the number of longis and circs to optimize

the strength ofthe columns (Figure 103).

The grid structure was constructed by threading the longis through the holes

provided by the guide rings and spacing the circs accordingly. After two or three

longis connected the desired number of circs, more longis could be added to the

configuration. For the first-generation columns, a small amount of 5-minute

epoxy was used to bond each of the circs to the longis at 90-degree angles.

However, for the second-generation longis, only the end circs were bonded with

epoxy. The middle circs were spaced with sections of cylindrical cardboard molds

for concrete, and these also served as the mold for the concrete cylinder. The

edge ofthe cardboard tubing was bonded to the circs using caulk; the tubing and

caulk were later removed after the concrete had set.
74 USACERL TR 98/81

Test Matrix

The test matrix used for the laboratory investigations comprised oftwenty 18 x 8

in. concrete columns. Thirteen of these columns were constructed using firstgeneration circs, while four columns were constructed using second-generation

circs. The number of columns was limited by the amount ofmaterials available;

therefore, the test matrix was organized so that the compression tests would

cover a spectrum of circ/longi combinations. This approach allowed for a

comparison between different composite grid structures to optimize the

reinforcement.

Two sets of six different geometric configurations of the grid structures were

constructed using 2.0 and 4.0 in. spacing between first-generation circs, while

varying the number oflongis (5, 10, and 15). Also, three different configurations

were constructed using 2.5, 3.0, and 3.5 in. spacing between second-generation

circs while using 21 longis in each column (Table 12). To examine the influence

of containment, two fully packed columns of each column generation (15 and 21

longis) were tested with a circ on each end, but no circumferential reinforcement

in the middle ofthe column.

Manufacturing the Concrete Columns

An 8 in. length of cylindrical cardboard tubing was used as the mold for these

columns in an effort to make these tests conform with industry standards as

much as possible. The concrete columns made with the first-generation grid

structures had approximately 1/2 in. concrete covering the outer part of the

composite grid structure. However, due to the larger diameter of the secondgeneration circs, the outer surfaces were exposed. As discussed previously, the

cardboard tube was cut into sections (varying in width) and used to space the

circumferential reinforcement while remaining flush with the circs.

For the purpose of determining the critical locations of stress and strain, a

preliminary compression test was performed on a single composite grid structure

without any concrete using 12 electrical resistance strain gages mounted on the

circs and longis. For the columns included in this experiment at least one pair of

electrical resistance strain gages was mounted in each column (one on the longi

and one on a circ) before the concrete was poured. The strain gages were placed

on the longitudinal reinforcement at the mid span ofthe longi and on the outside

of the middle circ. As many as six gages were used on the second-generation

columns. The mounting of the strain gages followed the manufacturer's

suggested procedure. The surface was prepared through degreasing, surface

abrasion, and conditioning. The strain gage was mounted with a small amount
USACERLTR-98/81 75

of 5-minute epoxy. Pressure was applied to the gage as the epoxy cured to

ensure a good bond. Lead wires were then soldered to the tabs and the gages

were checked with a strain indicator box to ensure that they were functioning

properly. The completed gage was covered with epoxy to protect it while the

concrete was placed and cured.

A mix design corresponding to a compressive strength of 3000 psi was chosen for

the column specimens. Although an even higher strength is common for

reinforced concrete structural members, this mix design was used because of the

high strength of the composite reinforcement and also to ensure that the

maximum load of the compression machine (300 kips) would not be exceeded. A

Type I portland cement was used and the columns were allowed to cure for 28

days.

The columns were formed using concrete sono-tubes. Before pouring the

concrete, the cylindrical cardboard molds were cut and sanded to a length of 20

in. and a steel base plate was coated with a release oil. Small holes were drilled

in the molds to allow the strain gage wires to exit the column molds. The

composite grid structure was centered in the middle of the sono-tube while

standing upright. The concrete was mixed in a large portable mixer and the

molds were filled with concrete in five lifts, each approximately 4 in. in height.

Each lift was rodded 50 times with a blunt rounded rod and the sides ofthe mold

were struck sharply five times with a mallet. An aggregate of 1/4 in. diameter

was used with the first generation columns and an aggregate of 3/8 in. diameter

was used with the second-generation columns to place the concrete between the

longitudinal reinforcement.

Curing

The completed fiber-reinforced concrete columns and plain concrete column

specimens were left in the molds overnight. The next day the molds were

removed from the columns and the concrete specimens were placed in a fog room

where the temperature was maintained at approximately 75 °F and the

humidity was maintained at 90 to 95 percent. The concrete columns were cured

in the fog room for 28 days.

Compression Samples

For baseline concrete data an 8 in. diameter, 18 in. long compression cylinder

was poured with each set of columns (total of 3). Similar to the other columns,

these compression cylinders were filled in five lifts of equal height and each lift

was rodded 50 times. These samples were tested at the same time as the
76 USACERL TR 98/81

composite grid reinforced concrete columns to determine the concrete strength at

the time oftesting.

Experimental Procedure

The preliminary compression test of the single composite grid structure (with

and without concrete) was performed on a 300-kip Baldwin compression

machine. The entire load head was allowed to swivel to ensure an evenly

distributed load. Because the load required to fail this column (with

reinforcement of 6 circs and 5 longis) almost exceeded the load capacity of the

300-kip machine, the first- and second-generation columns were tested with a 4-

million pound Baldwin test machine.

After the columns had been allowed to cure they were capped using a sulfur

compound. A sulfur cap ensures that the load will be distributed over the entire

end of the column. This was important considering the configuration of composite reinforcement.

After removal from the fog room, the concrete columns were allowed to dry until

the surface was free of excess moisture. Strain gages were again checked with

the strain indicator box and marked with colored tape to identify longitudinal

strain and circumferential strain.

A data acquisition setup similar to that used for the reinforced concrete beam

testing performed by BYU was used. To begin a test the data acquisition system

was started and the swivel load head was lowered until contact was made with

the top ofthe column. The range on the Baldwin 4-million lb test system was set

to the low range (0-1000 kips) and the load dial was zeroed. The hydraulics were

then started and load was applied to the column at an approximately constant

rate of 200 to 300 lb per second. The loading rate varied somewhat between

specimens because of the limitations of the manual controls on the Baldwin 4-

million lb system. In each case, the load was increased until a significant drop in

load indicated failure.

As the testing began the load increased steadily until initial cracking of the

column occurred. For the first-generation columns, initial cracking occurred

when the load reached approximately 250 to 300 kips. A slight drop in load

indicated that the initial cracking had occurred, and concrete outside of the

reinforcement began to separate from the composite grid structure inside the

column. This external concrete layer displayed very dynamic behavior, shooting

the concrete outward at high velocity. After this initial failure the load continued
USACERLTR-98/81 77

to climb steadily until a sudden failure initiated at one end of the column. The

concrete crumbled locally, the end circ slid down the longis and the ends of the

pultruded longis exhibited brooming (Figure 104). Once this happened, the end

circ failed dramatically and again sent the crumbled concrete flying away from

the column. The load on the column would drop by one-third to one-half and

then slowly build back up to approximately the same load level until another circ

would fail in a similar manner. After two or three circs had failed the load would

no longer increase and the test was stopped. The second-generation columns

failed in a similar manner, but at much higher loads.

The plain concrete columns were tested on the same day as the fiber-reinforced

concrete columns. Load/deflection readings were taken with the load cell to

determine the concrete's modulus of elasticity. The ultimate load was recorded

for all ofthe concrete specimens (Table 12).

First-Generation Columns, 2.0 in. Spacing

Columns A, B, and C consisted of 10 circs spaced 2.0 in. apart (from center to

center of each circ), with the number of longis being varied (5, 10, and 15,

respectively). The plain concrete failed at 120 kips or 2.1 ksi; the 2.0 in. spaced

fiber reinforced columns failed around 300 kips or 5.9 ksi. This represents an

improvement in strength by a factor of approximately 2.8 (Table 13).

Because the load is controlled by the stroke ofthe compression machine, the load

vs time graph cannot be used to directly compare the columns. However, a

comparison ofload vs time shows how the column was affected as each circ failed

(Figure 105).

Although the strength ofthe individual columns was not affected a great deal by

the number of longis, the stiffness was affected, as shown by the load/deflection

curves. In particular, the initial slope of the stress-strain curve is affected by

properties of the GFRP which is much suffer than concrete (Figure 106). This

affect on the stress-strain curves in longitudinal reinforcement is also seen in the

stress-strain curves ofthe circumferential reinforcement (Figure 107).

First-Generation Columns, 4.0 in. Spacing

Columns D, E, and F consisted of 10 circs spaced 4.0 in. apart (from center to

center of each doubled circ) with the number oflongis being varied (5,10, and 15,

respectively). These composite structures were built in this configuration to see

the effect of doubling the density ofthe circs by placing two circs at each location,

4.0 in. apart (see Table 14).
78 USACERL TR 98/81

Again, the plain concrete, which corresponds to the concrete used in these

columns, failed at 120 kip (2.1 ksi) while the 4.0 in. spaced fiber reinforced

columns failed around 250 kip (5.0 ksi). This represents an improvement in

strength by a factor of approximately 2.0. The number of longis made more of a

difference in the ultimate strength of the 4.0 in. spaced fiber-reinforced columns

than it did in the 2.0 in. spaced fiber-reinforced columns (Figure 108). An

additional 5 longis increased the axial compressive strength of the columns by

approximately 43,800 lb, or 17.5 percent.

These tests proved that doubling the density of the circs by spacing them closer

together (2.0 in. vs 4.0 in. apart) was more effective than doubling the density of

the circs by spacing two circs at each location, 4.0 in. apart. These results

suggest that total containment of the column is the optimal way to reach an

ultimate compressive strength.

The number of longitudinal reinforcements in the 4.0 in. spaced columns had a

significant impact on the stress-strain curves. Most ofthe gages inside the firstgeneration columns were ripped off the composite reinforcement due to shear

after the first circ failure. However, Column F in Figure 109 shows the load

transfer on a middle circ as several end circs failed in succession.

Second-Generation Stacked Joint Grids

The second-generation circs were by far superior to the first-generation ones.

This was due to the higher quality of the carbon fiber hoop windings and not the

additional number oflongis allowed for in the configuration (Table 15).

These columns were averaging over 700 kips, or 14.1 ksi, while the concrete

samples from which they were made had an ultimate axial compression strength

of 128 kips, or 2.5 ksi. This shows an increase of strength by a factor of

approximately 5.5. Although the second-generation columns failed at three

times the load of the first-generation columns, their behavior was very similar.

Column H is a good example of these columns and their ability to repeatedly

release stress through failure of an end circ and then build up to even a higher

load (Figure 110).

Figure 111 shows the load transfer to the other circs in the column as each end

circ failed. These axial compression tests on the second-generation columns

confirm that the number of longis is correlated to the initial slope of the stressstrain curve. Columns G, H, and I all had 21 longis, and the initial slope for

each column is almost identical (Figure 112).
USACERLTR-98/81 79

Discussion of Results

Figure 113 shows that the ultimate load is directly proportional to the number

of longis, which is expected because of the predominant influence of the longis

over concrete strength. Figure 114 shows groups of data depicting a dramatic

increase in the compressive strength ofthe second-generation grid structure.

Failure Modes

Plain concrete columns typically fail due to shear, which is evident in a 45- to 60-

degree angle crack after failure. In these advanced composite grid structures,

longitudinal rods resist flexure while the carbon fiber rings contain the concrete

under axial compression. This containment of the concrete prevents the column

from expanding radially, delaying the primary mode offailure. Failure generally

initiated at one end of the columns. The concrete crumbled locally, the end circ

slid down the longis, and the ends of the pultruded longis exhibited brooming.

Soon after this, the end circ would fail, which would further broom out the longis

and increase the area over which the load was being applied. One possible way

to overcome this local failure and realize an increase in strength would be to use

steel endcaps.

The data collected from these tests for the first- and second-generation columns

suggest that the only major factor determining failure of the column for a given

number of longis is strength of the circumferential reinforcement. A fullcontainment jacket made of the same high-quality carbon fiber hoop windings

would produce incredibly high strength.

Stress-Strain Behavior

Although the strength ofthe individual columns was not affected a great deal by

the number of longis, the load/deflection curves were. This is evident in the

initial slope of the stress-strain curves for circs in columns with the same

number of circs but varying number of longis. This is due to the properties of

GFRP, which is much suffer than concrete. The number of longis resisting

flexure also affected the initial slope of the stress-strain curve for the

circumferential reinforcement ofthe column. The stress-strain diagram for firstgeneration columns was interesting in this respect: as the first circ failed, the

stress dropped dramatically and the strain increased for the remaining circs in

the columns. Typically, one would expect the strain to decrease as the stress

dropped. This occurrence may be due to the stroke control of the compression

machine and the column's effort to expand radially. Between failures of end

circs, the stress-strain relationship is linear and repeats at the same slope.
80 USACERL TR 98/81

Load Transfer Effects

As shown in the overall comparison of ultimate loads for the various

configurations of columns, there is a huge difference between the plain concrete

columns and those with even minimal composite reinforcement. Preliminary

tests performed on plain grids with no concrete showed them to have a low

ultimate load strength. The high ultimate compressive strength of the

reinforced columns suggests that the load is being carried synergistically

between the concrete and advanced composite reinforcement. From the stressstrain diagrams, it appears that the concrete initially takes the load and at its

yield point the composite grid begins to carry the load. There is a linear

relationship here that is based on the stiffness of the GFRP longitudinal

reinforcement. With an increase in load the longis have the tendency to buckle.

However, buckling is prevented by the circs, which brace the longis while

confining the concrete that is attempting to expand radially.

Apparently the concrete outside of the composite grid structure is not an integral

part of the load transfer between the concrete and the composite. At the load

where plain concrete columns fail, the reinforced columns showed separation of

the external concrete with the grid structure (Figure 115).
USACERLTR-98/81 81

8 Conclusions, Recommendations, and

Commercialization

Conclusions

This research introduced a new concept of concrete reinforcement and grid

manufacture. Instead of simply replacing steel reinforcement bars with

composite versions of the same form, the proposed reinforcement method was

designed especially to make use of the grid's unique properties. In this concept,

grid panels were placed in the outermost layers of the structure, creating a

grid/concrete sandwich. The concept was proven to be economically feasible and

mechanically sound.

It must be noted that the work was brought to a conclusion before a

demonstration model was produced, as had originally been proposed in the scope

of work. Before building any demonstration model it is essential that full-scale

testing is done in the laboratory. This testing and model demonstration would

have required additional resources and time that were not available before

conclusion of the project. However, the work serves as a proof-of-concept for

using composite grid systems for reinforcing concrete structures.

The concept of composite grid reinforced concrete has been shown to be both

predictable and reliable. Based on experimental data, load/deflection behavior of

grid-reinforced concrete (GRC) is strictly a function of the mechanical properties

of the reinforcement and the concrete. The load transfer mechanism involved

with GRC is adequate to transfer internal stresses from the concrete to the

reinforcement, and is possibly more reliable than relying on a shear transfer.

There are encouraging results from the experimental tests that tend to validate

the initial model developed at Stanford University. Based on an examination of

manufacturing process alternatives, innovations will be required in material

selection, processing, fabrication, and placement techniques. It is clear that a

completed system as proposed by this investigation could produce a concretereinforcing methodology that would offer simple design procedures and cost

savings in field assembly (place and pour) while providing durability and

damage tolerance.
82 USACERL TR 98/81

Design Issues

Traditional methods for designing steel-reinforced concrete elements are

intended to force ductile failure. Such failure provides warning, dissipates

energy through inelastic action, and often avoids complete structural collapse.

This characteristic is assured by forcing designs to provide "under-reinforced"

elements, meaning that reinforcing quantities are limited to amounts that force

steel to yield before concrete crushes. Referring to the grid-reinforced concrete

beam test results given in Figures 67 and 68, it seems clear that a similar

ductile characteristic could be induced with the FRP grids. However, the

mechanism of failure for FRP grids is totally different from the ductile nature of

steel. The grids fabricated at BYU had almost the double the ultimate strength

and toughness of the commercially fabricated grids but, as shown in Figure 78,

the toughness was only about 30 percent for that of steel bar. It is interesting to

note that the ultimate load capacity of the FRP grids was about 30 percent

higher than for the steel reinforcement. The FRP provides a greater margin of

safety, but provides less warning.

Theoretically, significantly larger volumes of FRP reinforcement could achieve

the same deflection limitations provided in a steel-reinforced concrete element

subjected to the same loads. However, the factor by which FRP reinforcement

volume could be increased to lower deflections without failure of the concrete by

crushing or shear was a concern, and was explored at Stanford University with

analysis and grid-reinforced slab testing. The possibility of using concrete as a

filler material with composite grid frame as the major load-sharing structure in

compression, tension, and shear was investigated. Results from the tests of

dual-grid GRC panels shown in Figures 97 and 98 illustrate that after yielding,

the panels deflect significantly and support roughly the yield load. The yield

region accounts for roughly 60 percent of the total absorbed energy of

deformation. More significantly, as the structure enters the post-yield phase and

the grid and concrete begin to fracture, all internal fractured concrete is

contained in position, and the structure continues to support the yield load.

With the concrete crushed, the internal forces rebalance by shifting the forces on

the grids until inducing the total failure as shown in Figure 101. GENLAM

provided an analytical tool for this design.

The application of composite grid frame as the major load-sharing structure in

compression, tension, and shear was adapted in a stacked joint grid

configuration as discussed in Chapter 7. Concrete was used as a partial

compression load bearing structure. Here the stacked grid reinforcement was

used in columns that were subjected to compressive loading. The FRP

longitudinal rods (longis) shared the major load in this configuration, as can be
USACERLTR-98/81 83

seen in Figure 113. This figure shows that the ultimate load is directly

proportional to the number of longis. Figure 114 shows that under this configuration the ultimate load capacity would be 750 percent of the plain concrete

compressive load capacity. The toughness of these columns is controlled by the

number of longis and the number of circular reinforcements (circs). The innovative stacked joint grids with circs and longis over-performed when compared

with the baseline steel-reinforced concrete.

The failure mechanisms of the orthogrid reinforcements of concrete were also

different from the failure mechanisms ofsteel-reinforced concrete. As opposed to

the distributed force transfer provided by conventional deformed steel rebars,

force transfer is concentrated at the intersecting FRP grid lines. Several

implications of these differences were of concern. First, it was necessary to

ensure that the grid "joints" (where the orthogonal ribs intersect) are

structurally capable of transferring the force without failure. Second, the

concentration of force transfer in the concrete matrix may lead to larger

individual crack widths that are concentrated at the grid points. Third is the

possibility offormation ofshear failure planes in the concrete matrix along these

orthogonal grid lines that could accompany the concentrated load transfer

mechanisms. The experiments performed with grids and discussed in Chapter

6 addressed these issues and clearly showed that the transfer of internal loads

through bearing on the cross rib members of the grid structures is just as

effective as the transfer of load through shear between the concrete and

reinforcement. Figures 70 through 72 clearly demonstrate that the crack

distribution for the grid-reinforced beams was uniform, and cracks formed at

discrete intervals.

To summarize, in both the FRP grid-reinforced concrete slab design and the

stacked joint grid-reinforced concrete column design, the composites were

intended to share major loads, and consequently the load transfer mechanisms

are significantly different from those for conventional steel-reinforced concrete.

In the grid-reinforced slabs, the concrete was crushed in individual cells under

compression and shear loads, but the slabs did not fail until the composite ribs

failed. Similarly, in the stacked joint grids, the concrete crushed before the circs

failed or the longis buckled.

Manufacturing Issues

The manufacture and use of composite grid structures have been explored and

refined through this research project. A demonstration of current technology

revealed that composite grids generally suffer from manufacturing problems of

low density interlacing, no fiber tension, and excessively thick nodes with direct
84 USACERL TR 98/81

roving crossovers. Simple steps were taken to resolve the first two of these

problems, and the PEG process was then developed by Stanford University to

properly distribute the fibers through the nodes in such a way as to generate

grids with uniform fiber volume fractions and thicknesses throughout the

structure. It was shown that through the use of even a single nodal guide pin,

this process significantly increased the grid material properties. The method

was even able to be generalized from a basic orthogrid panel to the

unprecedented isogrid panel.

The TRIG architecture returns to the basic and simple direct crossover through

the nodes, but uses composites tubing in the grid cells to reinforce the structure

and provide a uniform thickness dimension. This also allowed implementation of

a unique tooling concept that integrates into the reinforcement during the

manufacturing process—the reinforcement tubes are used to make the mold for

each panel. This disposable tooling concept eliminates the significant

investment in conventional reusable molds. The concept was further developed

to produce large numbers of grids by slicing tall grid blocks manufactured with a

modified TRIG technique. This process has the potential for tremendous cost

reduction in grid manufacture.

Recommendations

As indicated above, the composite grid system in its current phase of development is not yet ready for field demonstration or commercialization. The recommendations specified below address technical requirements and issues that

should be carefully considered during any future commercialization phase.

1. Optimize resin and fiber combinations to improve strength, stiffness, ductility, and

durability. The key areas ofinvestigation in this project included fiber types and

volume in the resin matrix. Both stiffness and ductility need improvement. With

higher stiffness and strength FRP grids may possibly overcome the problems

associated with low postyield ductility, but such procedures will require industry

acceptance.

2. Optimize tfie stackedjoint grid geometric configuration. The strength ofthe

partially contained columns is highly dependent on the strength of

circumferential reinforcement. This research suggests that the more area

confined the greater the strength ofthe columns. Optimization is needed toward

either full containment or partial containment using high-strength

circumferential reinforcement spaced at regular intervals. The greater the

circumferential reinforcement, the higher the ultimate strength, but the cost-to-
USACERL TR-98/81 85

benefit ratio may prove that partial containment using high-strength

circumferential reinforcement is more cost-effective.

3. Conduct furtherstudies ofcreep and fatigue properties. Studies ofthese

properties in the civil engineering area have been relatively limited to date.

More data are needed to ensure that these properties are well understood, not for

the FRP grids alone, but for the reinforced system itself.

4. Conduct long-term studies ofenvironmental effects on structuralproperties. This

recommendation is made especially for GFRP reinforcement. Areas for further

study include acid and alkaline exposure, salt exposure, ultraviolet radiation

exposure, and fire exposure. Each ofthese factors will affect the required depth

ofconcrete cover over the FRP grid.

5. Collect data on the use ofFRPgrids as confining elements in reinforced concrete

frame construction, particularly in seismically active regions. The research

community has become particularly aware ofthe benefits of confining column

elements as a means of enhancing their ductility in bridges and buildings.

Whether FRP grids may be used effectively will depend first on the proper

combinations ofstrength and stiffness, which currently favors CFRP over GFRP.

Beyond these requirements, the results of environmental effects studies and

improved manufacturing flexibility will be influential.

6. Develop large-scale manufacturing techniques using the improved fiber and resin

combinations. This is critical in making the technology cost-competitive. The

techniques must also be flexible in permitting significant variations in grid sizes

and configurations to accommodate designers' need for flexibility.

7. Commercialize the technology with recognition oflong-establishedACIstandard

practice. It is important for potential commercializers ofthis technology to

recognize that widespread acceptance oftheACI Building Code has come after

more than 75 years ofresearch and application experience with steel

reinforcement. The commercialisation ofgrid-reinforced concrete technology will

proceed most effectively when a larger experimental database is available for

consideration by the targeted mainstream end user. Another essential aspect for

successful commercialization will be the development ofan acceptable design

philosophy that recognizes the differences between plastics behavior and steel

behavior.

Technology Transfer and Commercialization

At the time this CPAR project concluded, the industry Partner Participants were

ready to fabricate full-scale components for testing. The CPAR Partner, Stanford

University and their partners, had planned to continue testing, development,

and commercialization with the resources available to them.
86 USACERL TR 98/81

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Canadian Society for Civil Engineering, 1992.

Sugita, M., "NEFMAC grid type reinforcement," Alternative Materials for Concrete Reinforcement,

Blackie Academic, NY, 1993, pp 55-69.

Sugita, M., (1993). NEFMAC - Grid Type Reinforcement, Concrete Structures: Properties and

Application, ed. A Nanni, Elsevier Publishers, B.V., pp 355-381.

Taerue, L.R., (1993). "FRP Developments and Applications in Europe," Fiber-Reinforced Plastic

(FRP) Reinforcementfor Concrete Structures: Properties and Applications, Elsevier Science

Publishers B.V., pp 99-113.
92 USACERL TR 98/81

Triantafillou, T.C. and Plevris, N. (1991). Post strengthening ofR/C Beams with Epoxy-bonded

Fiber Composite Materials, Advanced Composites Materials in Civil Engineering Structures,

ed. S. L.Iyer, and R.Sen, ASCE, pp 245-256.

Tsai, S. W., Theory ofComposites Design, Think Composites, Palo Alto, CA, 1992.

Tsai, S.W. (1992). Theory ofComposites Design, Think Composites, Dayton, OH, 1992.

Tsai, S.W. and Chen, H.J., (1995). "Composite Grid/Frame Structures," Proceedings ofthe Tenth

International Conference on Composite Materials, Whistler, British Columbia, Canada, August

13-18.

Tunik, AL. and Tomashevskii, V.T. (1974). Mekhanika Polimerov, Vol 7, p 893.

Ueng, C.S. (1991). "The Elastic Stability ofthe Laminated Composite Columns," Proceedings of

ASCE Mechanics, Computing in 1960's and Beyond, ed. H.Adeli, and R.L.Sierakowski, pp 971-

974.

Vinogradov, AM. (1989). "Long-term Buckling of Composite Columns," Proceedings ofASCE

Structures Congress, San Francisco, pp 536-545.

Weidmann, G.W. and Ogorkiewicz, R.M. (1974). Composites, Vol 5, p 117.

Wu, W.P., GangaRao, H., and Prucz, J.C. (1990). "Mechanical Properties of Fiber Reinforced

Plastic Bars," Constructed Facilities Center, College of Engineering, West Virginia University.
USACERLTR-98/81 93

Tables and Figures Cited in Report
USACERLTR-98/81 95

Table 1. Tensile strength and bending requirements of concrete reinforcing steel (billet steel

bars).

Code: A 615 (ASTM)

Testing Code: A 370 (ASTM)

Tensile Requirements

Grade 40 Grade 60 Grade 75

Tensile Strength (psi) 70,000 90,000 100,000

Yield Strength (psi) 40,000 60,000 75,000

Elongation in 8@, %

Bar No.

3 11 9 —

4,5 12 9 —

6 12 9 7

7,8 — 8 7

9, 10, 11 — 7 6

14, 18 — 7 6

Bend Test Requirements

Bar No. Grade 40 Grade 60 Grade 75

3,4,5 3.5d 3.5d —

6 5d 5d 5d

7,8 ~ 5d 5d

9, 10, 11 — 7d 7d

14, 18(90deg.) - 9d 9d

Test bends 180 deg. unless otherwise noted; d = nominal diameter of specimen.

Low-Alloy Steel Deformed Bars for Concrete Reinforcement (common)

Code: A 706 (ASTM)

Tensile Requirements Bend Test Requirements (180 deg.)

Tensile (psi) 80 000 Bar No. Dia. Pin

Yield min (psi) 60 000 3,4,5 3d

Yield max (psi) 78 000 6,7,8 4d

Elongation (%) 9,10,11 6d

Bar No. 14,18 8d

3,4, 5, 6 14

7, 8, 9, 10, 11 12

14, 18 10
96 USACERL TR 98/81

Table 2. Comparison of mechanical properties of steel and FRP rebar (Faza 1995).

Steel rebar FRP rebar

Specific gravity 7.9

Tensile strength (MPa) 483 -690

Yield strength (MPa) 276 -414

Compr. strength (MPa) 276 - 414

Tensile modulus (GPa) 200

Thermal Expansion Coeff (1(fPC) 11.7

1.5- 2.0

517 -1207

310 -482

41 - 55

9.9

Table 3. Fire tests of building materials.

Tests Test Methods Object

1. Ignition Tests ASTMD-1929

ASTM E-136

Measures flash-ignition

temperature.

Material is classified as

combustible or not

2. Flame Spread Tests ASTM E-84

(Tunnel Test)

ASTM D635

UL-94

Measures surface flame

spread, smoke generation, and

total heat release

Measures horizontal burning

rate

Measures inflammability in

vertical bar specimens

3. Tests for Smoke

Evaluation

NFPA258 Measures maximum optical

smoke density

4. Tests for Fire

Endurance

ASTM E-119 Determines flame penetration

on unexposed face and

structural collapse
USACERLTR-98/81 97

Table 4. Comparisons of stiffness of isotropic laminates and isogrids for three materials on

absolute and specific bases.

Properties Units Source 1300/5208 Glass/epoxy Aluminum

E. GPa Ply data [4] 181.00 38.60 70.00

E. GPa Ply data [4] 10.30 8.27 70.00

v. Ply data [4] 0.28 0.26 0.90

E- GPa Ply data [4] 7.17 4.14 26.90

A pM

Ply data [4] 1.60 1.80 2.60

B Drsogd] fpM 1.60f 1.80f 2.60f

C

ppso] GPa Equation[4] 69.70 19.00 70.00

D

pPsoodl GPa FE73 60.30f 12.90f 23.33f

E

pM; -M GPa C/A 43.56 10.55 26.92

F

pösostf]/ -psoadl GPa D/B 37.68 7.17 8.97

G Efficiency1

*

300"1

/

1

*

01 F/E 0.87 0.68 0.33

Table 5. Reinforcement areas and theoretical mechanical properties.

Type of Reinforcement Area Ultimate Strength

(in2) (ksi)

Modulus of

Elasticity

(x10spsi)

Axial Rigidity

Area x Modulus

(kips)

Steel Rebar 0.11 60 30 3300

Commercial GFRP Grid 0.32 - 2 600

Commercial Carbon Grid 0.32 - 5 1600

ln-house Plain Carbon Grid 0.22 190 14.5 3200

ln-house Deformed Carbon Grid 0.22 190 14.5 3200

ln-house Hybrid Carbon/Steel 0.22 170 16.6 3600

Grid

Plain Steel Grid 0.27 60 30 8100

Deformed Steel Grid 0.24 60 30 7200
98 USACERL TR 98/81

Table 6. Comparison of ultimate load, effective stiffness, deflection, and toughness results for

all beams.

Ultimate

Load (kips)

Pre-cracking

Stiffness

(kips/in)

Postcracking

Stiffness

(kips/in)

Max

deflection

(in)

Toughness

(kips-in)

Glass FRP (GFRP)

Grid

7.5 400 18 0.22 1.2

Commercial carbon

Grid

9.6 350 37 0.17 1.1

BYU Plain Carbon

Grid

16.6 280 88 0.33 3.9

BYU Deformed

Carbon Grid

16.2 280 87 0.30 3.6

BYU Hybrid Carbon

Grid

15 280 83 0.26 2.8

Steel Rebar 12.1 420 81 0.92 9.5

Average 12.8 335 66 0.37 3.7

Std. Dev 3.7 64 30 0.28 3.1

Table 7. GENLAM materials.

E. GPa G„GPa V

Compressive concrete 22.43 9.35 0.20

Average concrete 12.94 5.44 0.19

Tensile concrete 3.45 1.46 0.18

Compressive GRC 7.63 9.35 0.15

Average GRC 5.47 5.44 0.15

Tensile GRC 3.31 1.46 0.15

Table 8. 13 mm beam lay-ups.

Lay-up Layup [bottom...top] 5„ (Nm)-1 L/w, kN/m L/e,kN

A [G,/G,/G„/GJ 1.21 E-3 49.3 100.5

B [G./G./G./GJ 1.22 E-3 48.9 99.8

C [G./G./G./GJ 1.15 E-3 51.9 105.7

D [G./G./G./GJ 1.07 E-3 55.7 113.6
USACERLTR-98/81 99

Table 9. 38 mm beam lay-ups.

Lay-up Lay-up[bottom...top] 5„ (Nm)"1 L/w, kN/m L/s, kN

A [GJGJGJGJCJCJCJCJGJGJGJGJ 3.78 E-5 1578 1806

B [GyGyGyGyc/c/cycyG,/G./G,/Gj 4.85 E-5 1549 1067

C [G/G/G/G/c/cycycyGyGyGVGj 4.45 E-5 1340 923

D [G/G/G/G/C/C/C/CyGyGyGyGJ 4.49 E-5 1328 915

E [G/G/G/G/C/C/C/CyGyGyGyGJ 4.49 E-5 1328 915

F [G/G/G/G/C/C/C/C/GyGyGyGJ 4.53 E-5 1317 906

Table 10. Grid-only load/deflection slopes.

Structural thickness Bending load/deflection [KN/m] Twisting load/deflection [KN/m]

13 mm mono-gridpanel 84.1 21.0

38 mm dual-grid frame 401.0 57.8

64 mm dual-grid frame 478.8 115.6

Table 11. Average load-deflection slopes of GRC panels and frames.

Structural Thickness Bending load-deflection [MN/m] Twisting load-deflection [MN/m]

13 mm mono-gridpanel 0.217 0.045

38 mm dual-grid frame 2.874 2.567

64 mm dual-grid frame 7.543 5.896
100 USACERL TR 98/81

Table 12. Concrete column specimens included in test matrix.

Generation Column # of Circs Spacing (in) # of Longis Aggregate

size

Ultimate

Load (lb)

1* A1 10 2 5 VA" 267844

r A2 10 2 5 VA" 283438

f B1 10 2 10 w 286594

r B2 10 2 10 vs 310469

f C1 10 2 15 VA" 281594

f C2 10 2 15 VA" 339563

f D1 10 4 5 VA" 220406

r D2 10 4 5 VA" 173313

t E1 10 4 10 VA" 244156

f E2 10 4 10 VA" 244156

1" F.1 10 4 15 VA" 308844

r F2 10 4 15 VA" 260094

2" G 6 2.5 21 3/8" 666656

2" H 7 3 21 3/8" 717250

2* I 8 3.5 21 3/8" 752313

1" 0 2 18 15 VA" 217125

2" J 2 18 21 3/8" 274125

Plain K - - - VA" 105281

Plain M - - - 3/8" 141844

Plain P - - - 3/8" 114688

Table 13. Comparisons of column specimens A, B, C, and K.

Column Column Description Ultimate Load (lbs) Average load (lbs)

A A1 2.0" spacing, 5 longis 267844 275641

A2 2.0" spacing, 5 longis 283438

B B1 2.0" spacing, 10 longis 286594 298532

B2 2.0" spacing, 10 longis 310469

C C1 2.0" spacing, 15 longis 281594 310579

C2 2.0" spacing, 15 longis 339563

k K Plain concrete (1/4") 105281 105281
USACERLTR-98/81 101

Table 14. Comparisons of column specimens D, E, F, and K.

Column Column Description Ultimate Load (lbs) Average Load (lbs)

D D1 4.0" spacing, 5 longis 220406 196860

D2 4.0" spacing, 5 longis 173313

E E1 4.0" spacing, 10 longis 244156 244156

E2 4.0" spacing, 10 longis 244156

F F1 4.0" spacing, 15 longis 308844 284469

F2 4.0" spacing, 15 longis 260094

K K Plain concrete (1/4") 105281 105281

Table 15. Comparisons of column specimens G, H, I, M, and P.

Column Description Ultimate Load (lb)

G 6 circs, 3.5" spacing 666656

H 7 circs, 3.0" spacing 717250

1 8 circs, 2.5" spacing 752313

M&P Plain concrete (3/8") 128266
102 USACERL TR 98/81

äs*

/•^^MPii

Figure 1. Deformed steel reinforcing bars.

Figure 2. FRP tensile coupon test specimens.
USACERLTR-98/81 103

Figure 3. Commercially available FRP reinforcing rods.

o

5 1,

£ -a o&- * ^"

1 -S 0.6 •

• .

- 8 0.4 -

§•§ 02-

a n i

"

0) - " > i i i

0. 0 2000 4000 6000 8000

Number of hours immersed in

water

Moisture absorption of a glass/polyester composite pultruded sheet

"O

0)

§ « 04 T .a Bi aaa

» 9 0.3 -

.n o

ö S 0.2 J

2 a- 0.1 {

i

= o> _ " % ° °«

o 0 2000 4000

No. of hrs immersed

Moisture absorbed in a glass/polyester pultruded bar.

Figure 4. Moisture absorption of polymer composites.
104 USACERL TR 98/81

Figure 5. Schematic showing isometric view of isogrid structure.

Figure 6. Schematic showing isometric view of orthogrid structure.
USACERLTR-98/81 105

Bad*

eeaf«r*toHWB*er

Figure 7. Basic grid structure and terminology.

Figure 8. Example of isogrid configuration.
106 USACERL TR 98/81

Fiber Reinforced Plastics vs. Metals

100-

75--

E

Q.

D

£ 50-

M

e

O. 25-

glass fiber graphite fiber_ _=E

w

metals

unidirectional [0°

orthogrid |0°/90°1

faogrid |0°/±60°]

H h

3 4 5

Specific Stiffness, E/p (106 m)

Figure 9. Plot of specific strength and specific stiffness for FRP and metals.

Figure 10. Interlace of fiber rovings through a node.
USACERLTR-98/81 107

STRESS (MPa)

t Ann—, (ksi)

1 |TWW

1,200 RUPTURE ö

_

L

C-FRP

200

1,000 y

y

y H-FRP

150

800 ty

600

400

y

y

k ty

y _

f~ y

y

y

0*

RUPTURE

100

CONTINUOUS YIELDING

§ t*

t.« ** 50

200

n

/ y .**' I ty tt- • *

/

#

* * * *'

n

0.000 0.005 0.010

STRAIN

0.015 0.020

Figure 11. Typical steel and FRP stress-strain characteristics.

80s

60

40

APPLIED LOAD (kN)

STEEL

C-FRP

H-FRP

Kips

15.0

10.0

5.0

50 100 150

MIDSPAN DEFLECTION (mm)

200

Figure 12. Results of bending tests of steel, cfrp and hfrp specimens.
108 USACERL TR 98/81

B.W.WJ e=±6o £

*/y\ TT/3#lS0°

R,D I

width b YV j£^JLzlJj®L JL

»WWvVl

*| Length L ^-Length L-4f

Figure 13. Configurations for square grids, angle grids, and isogrids.

f == 12 percent f == 23 percent f =

: 39 percent

f= 13 percent f = 21 percent f = 38 percent

Figure 14. Rib areal fractions of square grids and isogrids.

SQUARE

GRID

ISOGRID

SQUARE GRID »yr-?-"—■■ "?' ' ' "'**

b.

2

.: >: ;.!-. : ;.= •-.; % ;-?.:.;.r,:

i EQUALL

'ISO

'sq L/b Iso- L/b f

sq~ L/b ~ 73

Figure 15. Definition of area fraction (f) of square and isogrids.
USACERLTR-98/81 109

\\AAAAA////

ITT/3]LAM ~if Regrouping

/ / AAAAAAA

tji *ji *./> M M, f^., M /> /i n

ISOi ISO6RID"

c

c

u

CB

Figure 16. Isogrid portrayed as a regrouped laminate.

Figure 17. Beam diagram for modeling.
110 USACERL TR 98/81

SLOTTED JOINT

fin carpentry)

Figure 18. Composite grids with slotted joints.
USACERLTR-98/81 111

SQUARE GRID

If""1^"1" 1 "^^^""'^! I^r^^^* I f '

-rift fEx ^VfEf E

iE2- {Jb- 2 2

E6= 612 = 0

V =0

P^fPrlD-fvfPf

where pr1b = vmpm + vfpf

Figure 19. Typical relations for orthogrid.

»sT^s^^ *

' h

Figure 20. isogrid with offset nodes.
112 USACERL TR 98/81

Figure 21. Interlaced nodes.
USACERLTR-98/81 113

O <>

s k. s k ~.irnr 1

/ \

r

/ N. / k «

V / \ y 'v

a

\\ b 8

Figure 22. Geometry of the plain (a) and deformed (b) orthogrids.

Figure 23. Molds for plain (top) and deformed (bottom) orthogrids.
114 USACERL TR 98/81

!•" iiQun. i'

J

- • M1 —-a»^«i

Figure 24. Schematic drawing of winding process.

P/2

L/3

P/2 i

I 73 ^SL

strain

gages

- L

1 fiber optic

strain sensor

4

Figure 25. Placement of strain gages and fiber optic sensors.

node rib node

resin pool wrinkles fiber warping distinct plies

Figure 26. Rib flaws.
USACERLTR-98/81 115

fiber bunching

against the pin

crossover point of

one halfthe fibers

moid wall

fibers

node pin

Figure 27. Central node pin.

lead-in

angles fiber bunching

against the pin

moid wall

fibers

node pin

crossover point of

one half the fibers

Figure 28. Central node pin with 1.3-degree lead-in angle.
116 USACERL TR 98/81

Initial pinned node mold. The pin is large relative to the ribs and affects the node poorly:

causing large, resin poor regions.

(A) woven layer (B) laminated layer

Figure 29. Two lay-up methods.

resin rich

zone

non-ideal

corner

pin

location

Figure 30. First PEG specimen.
USACERLTR-98/81 117

Figure 31. PEG mold design steps.
118 USACERL TR 98/81

Figure 32. Sample PEG winding pattern.

^SWflfyrafpSjgg^

Figure 33. PEG Isogrid mold and specimen.
USACERLTR-98/81 119

tooling tube segment

unidirectional fiber core

Figure 34. 7 X 7 TRIG panel.

base board with

vacuum bag layer

tooling

tube segment

fiber core

channel

guide nail

alignment line

Figure 35. TRIG panel mold.
120 USACERL TR 98/81

Figure 36. Conventional processing cycle.

Figure 37. TRIG processing cycle.
USACERLTR-98/81 121

T

j

_

(A) commercial

iKmMmmMm

i.^^t -JÄÄJ; -PK :;^^ eflHg JPP|: $P%

-*Ä JW**äM ;W| 4PM wRa JHI'

>^*K ;<^^^ ^^WE j^^Pe '^ÄjK* rflfla \

JÄPW*

-^» -<^^ -.MR ä* iB> Äs WS.

.on

■■■■■■■

(B) reinforced commercial (C) TRIG

Figure 38. Test specimens — commercial, reinforced commercial, and TRIG grids.

Tensile Stiffness

D commercial

TRIG core

4-pin PEG

Figure 39. Tensile modulus of test specimens.
122 USACERL TR 98/81

J&

SMS AW

mmmmmmm

J-Z.

Figure 40. Panel bending test.

///VSSSJ

>>vvvvvvf ,<r

w

Figure 41. Panel twisting test.

Load/Deflection 150 - (Load/Deflection)/Weight:

400 T

300-

200-r 5

100 T

100 -r it

s

50-

D commercial

commercial reinforced

TRIG

Figure 42. Results from panel bending test.
USACERLTR-98/81 123

Load/Deflection

160-

120-

80-

E

2

40- KMI

0- 21

604

(Load/DefIection)AVeigbt I

40* j»

s

20-^

D commercial

commercial reinforced

TRIG

Figure 43. Results from panel twisting test.

Figure 44. Tooling for grid block.

7U£LZL£Lä,

raZEZCZEil oizizrzizr

S^AILKZL 71 71 71 71 71

Figure 45. Grid mold.
124 USACERL TR 98/81

ZLZLAZL£L CäzczrzEZi ra^rzczra:

OrZCZEZEZI

Figure 46. Sliced grid block.

I

- 6 in.

SUSI

Figure 47. Cross-section of reinforced concrete beams.
USACERLTR-98/81 125

SSäS:

aas

15*

JE

SKfe

SÄT

HP

335 m

;SöS

05.:

M

ffld

in;: fei

w

ssBR!

••Id:

IÄTTI

JsS?

tSSs

-«• ÄS1Ü

*3e äp

Sä? s38S

US

lit ill

sis

Figure 48. Test configuration for reinforced concrete beams.
126 USACERL TR 98/81

Figure 49. Uneven crack distribution in grid-reinforced concrete.

Swiveling

Load Head

Swiveling

Support Adjustable

Load Points

Figure 50. Four-point load test fixture.
USACERLTR-98/81 127

Deflection

Point

Pivot

Point ^

LVDT

-' '-. ■■■■■■■''

CounterBase weight

Figure 51. Deflection lever arm.

- JOUUU '

-16000-

-14000

Post-Gacking ^r\ ^r~y^*S*^ X-*^"*^

Stiffiiess S)s^^^ **

Load (lbs)

y=-81138x-2136.1 yT *

-6000-

•4000-

rS' Pre-Cracking

/ Stiffiiess

-2000

0^

Av y=-237628X+1423

i 1 1 1 1 1 i

0.000 0.050 0.100 ai» 0.200

Deflection (in.)

0250 0.300 a350

Figure 52. Load-deflection curve showing calculation of effective stiffness.
128 USACERL TR 98/81

-+-

0.000 0.200 0.400 0.600 0.800 1.000

Deflection (la)

1.200 1.400 1.600 1.800

Figure 53. Load vs deflection for steel rebar reinforced beam.

•on

o

0.000 0.010 0.020 0.030 0.040 0.050 0.060 0.070 0.080 0.090 0.100

Deflection (in.)

Figure 54. Expanded load vs deflection for steel rebar reinforced beam.
USACERLTR-98/81 129

-10000 •

-8000-

JO

v -6000-

3

-4000-

-2O0OU -T

0.000 0.020 0.040 0.060 0.080 0.100 0.120

Deflection (in.)

0.140 0.160 0.180 0.200

Figure 55. Load vs deflection for commercial carbon grid reinforced beam.

CO

.Q

n

o

-ouuu -

-7000- theoretical -»^-^»""T^S

-6000-

-5000-

•4000-

IfkjSf^jYjnJ'Y^jr* f

-3000-

-2000-

-1000-

0^ Z. 1 i 1 1 1 1 i 1 i

0.000 0.010 0.020 0.030 0.040 0.050 0.050 0.070 0.080 0.090 0.100

Deflection (in.)

Figure 56. Expanded load vs deflection for commercial carbon grid reinforced beam.
130 USACERL TR 98/81

-8000

o

es

o

0.000 0.050 0.100

Deflection (in.)

0.150 0.200 0.250

Figure 57. Load vs deflection for fiberglass grid reinforced beams.

0.000 0.010 Q020 ao30 ao« ooso aoso aero ooeo ao9o QIOO

Deftedtonfm.)

Figure 58. Expanded load vs deflection for fiberglass grid reinforced beams.
USACERLTR-98/81 131

Jg -10000-

a

id

o

0.000 0.050 0.100 0.150 0.200

Deflection (In.)

0.250 0.300 0.350

Figure 59. Load vs deflection for in-house plain carbon grid reinforced beams.

H r0.000 0.010 0.020 0.030 0.040 0.050 0.060 0.070 0.080 0.090 0.100

Deflection (in.)

Figure 60. Expanded load vs deflection for in-house plain carbon grid reinforced beams.
132 USACERL TR 98/81

-1ÖUUU •

-16000- ^^

-14000

-12000-

1" -10000-

g -8000

-6000-

-4000

-2000-

1 u ^

O.OOO 0.050 0.100 0.150 0.200

Deflection (in.)

0.250 0.300 0.350

Figure 61. Load vs deflection for in-house deformed carbon grid reinforced beams.

•a

n

o

H h0.000 0.010 0.020 0.030 0.040 0.050 0.060 0.070 0.080 0.090 0.100

Deflection (In.)

Figure 62. Expanded load vs deflection for in-house deformed carbon grid reinforced beams.
USACERLTR-98/81 133

-lOUUU

-14000-

-12000-

-10000-

"n

ir -8000- Loa

^»000-

-2000-

0.000 0.050 0.100 0.150

Deflection (in.)

0500 0550 0.300

Figure 63. Load vs deflection for in-house hybrid carbon grid reinforced beams.

1 H +-

0.000 0.010 0.020 0.030 0.040 0.050 0.060 0.070 0.080 0.090 0.100

Deflection (in.)

Figure 64. Expanded load vs deflection for in-house hybrid carbon/steel grid reinforce beams.
134 USACERL TR 98/81

-ZUUUU 1

-18000-

DeforrrEd Steel Grid

-16000

-14000

Plan Steel Grid

^ -12000-

~-10000-

3 -8000-

-6000

-4000-

-2000

1 ;— V 1

0.000 0300 0.4a 0.600 0.800

Deflection fia)

1.000 1.200 1.400 1.60

Figure 65. Load vs deflection for steel grid reinforced beams.

-16000 -, — — '—~ I

-14000

-12000 -

-10000

n

J3

r

-

-8000

Loa

Deformed Steel Grid

-4000

Plain Steel Grid

-2000

_.-. 1 1 __ U f-—

0.000 0.010 0.020 0.030 0.040 0.050

Deflection (la)

0.060 0.070 0.080 0.090 0.100

Figure 66. Expanded load vs deflection for steel grid reinforced beams.
USACERLTR-98/81 135

Ä -12000

w

~-10000

<o

3 -8000 ..

0.000 0.200 0.400 0.600

Deflection (in.)

0.800 1.000

Figure 67. Overall load vs deflection comparison.

-10000

M

IB

o

. Steel Rebar

. In-House Deformed Carbon Grid

. In-House Rain Carbon Grid

. Commercial Carbon Grid

. Fiberglass Grid

. Hybrid Grid

0.050 0.100

Deflection (in.)

0.200

Figure 68. Expanded load vs deflection comparison.
136 USACERL TR 98/81

ioorv>

-16000 -

^^ E

-14000 . c M

-12000 .

/*V D

.Q -10000 . B J

^*

o

O

-8000 .

-6000 .

-4000.

-2000.

r> /

A

0.000 0.050 0.100 0.150 0.200

Deflection (In.)

0.250 0.300 0.350

Figure 69. Identification of load and deflection values for crack propagation photographs.

v. ,v *- - '^SSHHHS

•; U" "X»,'. VMS}

1 ÜrKux'''^i«^^tfHHfl

Figure 70. Crack propagation in in-house carbon grid reinforced beam at 6000 lb.
USACERLTR-98/81 137

Figure 71. Crack propagation in in-house carbon grid reinforced beam at 13,000 lb.

Figure 72. Crack propagation in in-house carbon grid reinforced beam at 17,600 lb (failure).
138 USACERL TR 98/81

500-,

450-

in)

o

o

* I

& 350- A

S

S 300- A

g 250

X

A

g

X

4

« 200-

t> 150-

£

S 100-

50-

0-

to

rcialGrid

c

o

CD

Xt

©

a •= a> o

§ c

2

a> ö

2>-c £ c

(0 "O CC o

c

<5 O E o <-> s 2 8 = 8 Xt E-e c» m

u. O CD •* * Q

«Ü»°"gCD

O O Q. CO Ü Ü

Figure 73. Effective precracking stiffness of the different beams tested (each symbol represents

results from a single test).

» -

w

& 7 • : s 2 1

o» 6 - t 1

c

2 5-

A A A

§ 4- 8 £ £

CO

» 3 -

ex _

E 2

o

w 1-

U H

to

to

J2 -o

"5 ?

c

o

XI

CO Xt

CD

•0

a -c

g O

E c

lie

Grid

o>-= 2 c

«J ,*o (T 0 5

«3 O

xt

E o

E € ^<5 "5 0 0

•8 -e

3 0

u. o co O ü

CO

0-

CO a « CO

O

Figure 74. Concrete compressive strength for each beam type.
USACERLTR-98/81 139

1ZU -

~ 100 -

c

'S" 80-

(0

o

| 60-

CO

X

X X

i

X

1 •

o

ectlve

O

i

m 20

0-

&Fiberglass Grid

Ü o

E s E -E O (0

ü Ü

c

o

.a

cä s

^§

(0

to

.Q

<D

m

55

T3

•a ~

g CD

Q 10

O

Ductile

Carbon Grid

Figure 75. Post-cracking effective stiffness of the different beams tested.

(O

2j 4UUU •

1 3500

E

g 3000

1 2500 -

4>

K 2000-

o

S 1500 -

a

«§ 1000 -

© 500 • >

s .

UJ

09

CO

.2 -D

2 §

e

o

CO

CD

T3

p> -c <» c <o S o: "S 5

<D CD

LL.

£ o

E -e

o eg

O Ü

<0

Q.

"3

«>

55

o o

'S -S

Q CO

O

ca Ü

Figure 76. Axial rigidity (EA) of reinforcement.
140 USACERL TR 98/81

^^

e ^^

(0

a

90 •

80 - R

2

= 0.9586 ^S^^*

70 ^x*1^

SJ ^^^

CO

a 60 • ^r

c ^^^

50 - ^^

E ^^^

9

40 - ^S%

o 30 • y^

Q. ^^^

> 20 - ^s00^ 9 ^^

a

10 -

!CUi _

(

) 500 1000 1500 2000 2500 3000 3500

Reinforcement Rigidity E*A (kips)

4000

Figure 77. Linear relationship between post-cracking stiffnessand reinforcement rigidity.

18 -

•

16 +

In-Hoiife-Grids

+x

14 •

•

12 Steel Rebar -|X

a.

5 8-

E

3

5

s

4

.Commercial Grids

•

X

+

Xaverage

•#1

2

0

c

+*2

-#3

2 4 6 8 10 1 2

Toughness (k*ln)

Figure 78. Ultimate load vs toughness for the different beam types tested.
USACERLTR-98/81 141

£\ concrete

gnd

."OOOOii

)%S

y^-

V

,.^f\\\

xii^HN^vsl >Oo^>^\N

^^^

\VvV\S'

Figure 79. Grid reinforced concrete slab (concrete contained within a bi-grid frame).

EiEEEE FEEEEE

::::::: EzEEEEE

Figure 80. Bi-grid frame construction.
142 USACERL TR 98/81

Figure 81. Thick GRC slab.

Figure 82. GRC testing methods (the upper two test compressive and tensile properties; beams

test stiffness models; panels test GRC structural behavior).
USACERLTR-98/81 143

56 mm

J

13mm il 38 mm

grid

N^ concrete

(A) It panel (B) 3t frame

Figure 83. GRC beam cross-sections.

L/w « 56 kN/m

5

200-

Z

A

5

100-

0

1

*#

- «#. „ „-. ;*

." -• is. .«* *

-•-. •**..-•-.

* • •• «I *w>« iS*—i

" * • it ' J^f* « *'» *• • •»

•VT*-. C*"V*.

% •««* :» *

0 12 3 4'

deflection, mm

Figure 84. 13 mm GRC beam data.

2 x 1 x single grid thickness

grid

fj\] concrete

Figure 85. GRC compression test specimen.
144 USACERL TR 98/81

CompressiveGRC

- Compressive Concrete

*N/A\^

^0\— Tensile Concrete

Tensile GRC

desired stress

distribution

Figure 86. Basic laminate and expected stress distribution.

Refinement #1 Refinement #2

Compressive GRC

_Compressive Concrete

Average Concrete

Tensile Concrete

Tensile GRC

Compressive GRC

Compressive Concrete

Tensile Concrete

Tensile GRC

Figure 87. Modified material stacking sequences.

compression, -a- -

~ "tension, +o

bottom surface (A) (B) (Q (D)

Figure 88. 13 mm beam stress distribution.
USACERLTR-98/81 145

300

200

predict» L/w » 56 kN/m

o

100-

300

predicts L/e -114 kN/mm/mm

200-

2 3

deflection, mm

Figure 89. 13 mm beam predicted results.

Figure 90. 38 mm beam stress distribution.

predicts L/w-1,317 kN/m

< 1-

predicts L/e = 906 kN/mm/mm

0.2 0.4 0.6 0.8

deflection, mm

1.0 0.2 0.4 0.6 0.8

micro-strain

1.0 1.2

Figure 91. 38 mm beam predicted vs. experimental results.
146 USACERL TR 98/81

Figure 92. Summary of final beam lay-ups.

V\\M_ Compressive GRC

^mw. , IXS^I Average GRC

l\N/Al

- Average GRC

^N/AN—

V\\V— Tensile Concrete

IVNSJZ Tensile GRC

bending test twisting test

Figure 93. Panel bending and twisting tests.

Bending Test

5 10

deflection, mm

15

Twisting Test

1000

5 10

deflection, mm

15

Notes: squares are for the 13 mm panels; dashes are for 38 mm frames; crosses are for 64 mm frames.

Figure 94. Testing data for grid structures.
USACERLTR-98/81 147

Bending Load/Deflection

6t

2 -

13mm 38 mm 64 mm

Twisting Load/Deflection

6-

2T

grid only

grid with concrete

13mm 38 mm 64 mm

Figure 95. Grid vs GRC load/deflection slopes.

Bending Test

1 2

deflection, mm

Twisting Test

i

lit

iK*-*»«»—«>*• •«••<••,j*~i. .*«*~%t*#»<

0 1 2

deflection, mm

Notes: squares are for the 13 mm panels; dashes are for 38 mm frames; crosses are for 64 mm frames.

Figure 96. Testing data for grid structures.
148 USACERL TR 98/81

38 mm GRC Panel in 3-Point Bending

ZU ~

16-

z

121 ^

///

£/

!

'

large

region

deformation

of "toughness":

without failure

n „ti / ! — „_ bl

et #7o /

>

8- / 1" yield final failure

4-

U -

1 r

Ii i

0 4 8 12 16

deflection, mm

Figure 97. Failure curve of a 38 mm dual-grid GRC panel (toughness is demonstrated through

the post-yield behavior).

13 mm Panel in 3-Point Bending

•a«

o

10 20 30

deflection, mm

64 mm Panel in 3-Point Twisting

5 10 15

deflection, mm

20

Figure 98. Failure toughness of GRC panels (demonstrating repeatability of the GRC toughness

in multiple panel thicknesses and loading conditions).
USACERLTR-98/81 149

Figure 99. Concrete containment through first yield in a 13 mm panel.

test-yield with, surface flaking,

but no Ices from within the CPPS.

UltditHte failure with cplTnlar fill brckai cut

due to pundi-tixjugh by the loading ram.

Figure 100. Concrete containment through failure in 13 mm panel.

Final failure:

grid separation

and concrete cracking

Figure 101. Concrete containment through failure in a 38 mm panel.
150 USACERL TR 98/81

Figure 102. Stacked joint grid.
USACERLTR-98/81 151

J-B:Ä*""tir"B3P**

ISJS9 &»fl

A) 2.0" SPACING, 5 LÖNGIS

D) 4.0" SPACING, 3 LONG15

B) 2.0" SPACING, IOLONGIS

E) 4.0" SPACING, IOLONGIS

Q 2.0" SPACING, 15 LONGIS

F) 4.0" SPACING, 15 LONGIS

G) 3.5" SPACING, 21 LONGIS H) 3.0" SPACING, 21 LONGIS I) 2.5" SPACING, 21 LONGIS

Figure 103. Stacked joint grids of various geometric configurations.
152 USACERL TR 98/81

Figure 104. End failure for of a first-generation column.

1st Generation 2.0" Spacing

Column A (5 Longis)

Column B (10 longis)

Column C (15 Longis)

Column K (plain concrete)

50 100 150 200 250

Time (sec)

300 350 400

Figure 105. Load vs time graph for 2.0 in. spacing of fiber-reinforced columns.
USACERLTR-98/81 153

6,000

5,000

ä 4,000

*35

B.

"ST 3,000

«3 2,000

1,000

1st Generation 2.0" Spacing

-2500 -2000 -1500 -1000

Longi Strain (in/in)

5 Longis

10 Longis

•15 Longis

-500

Figure 106. Stress vs longi strain for 2.0 in. spacing of fiber-reinforced columns.

8,000

7,000 -j1st Generation 2.0" Spacing

500 1000 1500

Circ Strain (in/in)

• 5Longis

10 Longis

-15 Longis

2000 2500

Figure 107. Stress vs circ strain graph for 2.0 in. spacing of fiber-reinforced columns.
154 USACERL TR 98/81

6,000

5,000 f

4,000

S 3,000 -

tfl

2,000

1,000 -

1st Generation 4.0" Spacing

-3500 -3000 -2500 -2000 -1500

Longi Strain (in/in)

-1000

- 5Longis

10 Longis

•15Longis

-500

Figure 108. Stress vs longi strain graph for 4.0 in. spacing of fiber-reinforced columns.

1st Generation 4.0" Spacing

1000 2000 3000 4000

Grc Strain (in/in)

5 Longis

10 Longis

15 Longis

5000 6000

Figure 109. Stress vs circ strain graph for 4.0 in. spacing of fiber-reinforced columns.
USACERLTR-98/81 155

800,000-

Column H (3.0" spacing, 21 longis)

700,000- s\

600,000 A >r /

Load (lb;

w *» at[/ f

200,000- f

100,000

c) 100 200 300 400

Time (sec)

500 600

Figure 110. Load vs time for second-generation fiber-reinforced columns.

Column H (3.0" spacing, 21 Longis)

1000 2000 3000 4000 5000 6000

Ore Strain (in/in)

7000 8000 9000

Figure 111. Stress vs circ strain for second-generation composite fiber-reinforced column.
156 USACERL TR 98/81

Second Generation Columns

Column G (6 Circs)

Column H (7 Circs) I

Column 1 (8 Circs) j

-7000 -6000 -5000 -4000 -3000 -2000

Longi Strain (in/in)

-1000

Figure 112. Stress vs longi strain for second generation composite fiber reinforced column.

800,000 T

700,000

600,000

*35

•S 500,000

•a«

e

•i 400,000

a

J 300,000 -

5

200,000

100,000

0

Comparison ofAxial Compression Tests

O A - 1st gen (2.0" spacing) ;

* B-1st gen (2.0" spacing) I

C -1 st gen (2.0" spacing)

o D - 1st gen (4.0* spacing)

• E- 1st gen (4.0" spacing)

• F-1st gen (4.0" spacing)

X G - 2nd gen (3.5" spacing)

B H - 2nd gen (3.0* spacing) i

B I - 2nd gen (2.5" spacing) \

B O-1st gen (18" spacing)

J - 2nd gen (18" spacing) j

A P- Rain Concrete (PC1)

10 15 20 25

No. ofLongitudinal Reinforcement Bars

30 35

Figure 113. Ultimate load vs number of longis for fiber-reinforced concrete columns.
USACERLTR-98/81 157

800,000

Comparison ofAxial Compression Teste

700,000 - a

X

^ A -1 st gen (2.0" spacing)

600,000 -

^ B - 1st gen (2.0" spacing)

+ C -1 st gen (2.0" spacing)

Sf 500,000 •

o D -1 st gen (4.0" spacing)

e E -1st gen (4.0" spacing)

03

O

^ 400,000-

4*

• F - 1st gen (4.0* spacing)

H G - 2nd gen (3.5" spacing)

Ultimi

COooooo

%

g H - 2nd gen (3.0" spacing)

B I- 2nd gen (2.5" spacing)

200,000 -

•

B

6

m O -1st gen (18" spacing)

J - 2nd gen (18" spacing)

100,000 -

A P - Plain Concrete (PCI)

0

Figure 114. Comparison of stacked grid composite grid structures in concrete columns.

Figure 115. Separation of external concrete from composite.
158 USACERL TR 98/81

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