FRP Reinforcing Bars in Reinforced Concrete Members

ACI MATERIALS JOURNAL

TECHNICAL PAPER

Title no. 90-MS

FRP Reinforcing Bars in Reinforced Concrete Members

by Vicki L. Brown and Charles L. Bartholomew

This paper presents a preliminary experimental study on the behavior of concrete members reinforced with FRP (fiberglass reinforced plastic) reinforcing bars, in which both the bending capacity and bond strength of FRPreinforced concrete specimens were tested. Flexural behavior was studied by testing six simply supponed FRP-reinforced beams to failure under thirdpoint loading, while bond strength was investigated 1ry conducting twentyfour pullout tests. Experimental results for flexural strength indicated that the FRP-reinforced beams in many aspects behaved in the same manner as would be expected in beams reinforced with steel bars. In addition, strength design methods for steel-reinforced beams adequately predict the ultimate moment capacity of FRP-reinforced beams. However, the FRP-reinforced beams exhibited much larger deflections than would be expected in steel-reinforced beams, due to their lower elastic modulus. Finally, pullout test results appear to suggest that the FRP reinforcing baNo-concrete bond is approximately two-thirds that of steel reinforcing bar-to-concrete strength.

Keywords: beams (supports); bond (concrete to reinforcement); corrosion; reinforced concrete; reinforcing materials.

The construction industry has, for more than 100 years, used steel bars as a reinforcing medium for structural concrete members. Steel has, in general, performed quite well in such applications except where members have been exposed to aggressive environments, in which case deterioration of the structure due to corrosion of the steel can be quite rapid and dramatic. As a result, in the last 15 years there has been an increase in the use of alternative reinforcing materials for concrete in harsh environments. In the United States, plastics (particularly fiberglass reinforced plastics, or FRP' s) have been substituted for steel. Recently, plastic fibers have begun to replace steel wire segments in many fiber reinforced concrete applications, where the reinforcement is spread relatively uniformly throughout the member. Less common is the substitution of FRP reinforcing bars for steel bars, perhaps in part due to the fact that there is not a large enough body of published analytical and/or experimental data upon which to base routine design procedures. Originally manufactured for use in aggressive environments, the corrosion resistance of FRP reinforcing bars has resulted in commercial applications in chemical and waste-

34

water treatment plants, sea walls, floating docks, and under water structures. In addition, excellent dielectric properties have led to uses in structures where electric and magnetic fields would be affected adversely by steel reinforcing, such as substation reactor bases, airport runways, hospitals, and laboratories. FRP reinforcing bars, however, would not be as suitable for applications in which the possibility of a hightemperature fire existed. Although most modem plastics have excellent temperature stability in the range of -90 to +225 F (-68 to +107 C), flexural strength for the FRP reinforcing bars decreases significantly at temperatures in excess of 400 F (204 C). Temperature effects would therefore require special consideration for applications in buildings and bridges; but uses such as in footings, earth retaining walls, drilled piers, floor slabs on grade, pavements, and sidewalks are quite viable. In fact, FRP reinforcing bars would appear to have a distinct advantage over steel in such applications, because their superior corrosion resistance should result in a significant decrease in the required concrete cover for bars in concrete which is cast against earth. In addition, these types of structures (footings, retaining walls, slabs on grade, etc.) do not experience large bending deflections, and thus the lower flexural stiffness of FRP reinforcing steel would not be as critical a factor as in other members. There are currently several American companies that manufacture FRP reinforcing bars by pultrusion. In this process, strands of slightly twisted glass fiber are drawn through a catalyzed vinyl ester resin bath, then carefully aligned and pulled through a heated steel die which strips away excess resin and produces the desired rod diameter, with final rod composition approximately 30 percent thermosetting resin and 70 percent glass fiber. A band of glass fibers is wound around the rod in a spiral, creating the final indented surface that provides much of the reinforcing bars-to-concrete bond strength. Although ACI Materials Journal, V. 90, No. 1, January-February 1993. Received June 13, 1991, and reviewed under Institute publication policies. Copy: right © 1993, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion will be published in the November-December 1993 ACI Materials Journal if received by Aug. 1, 1993. ·

ACI Materials Journal . I January-February 1993

ACI member Vicki L. Brown is an associate professor of civil engineering at Widener University, Chester, Pa. She received her BSfrom the University of Pittsburgh in 1976 and her PhD from the University of Delaware in 1988 and is a licensed professional engineer in Pennsylvania. Her research interests include FRP reinforcing bars in reinforced concrete structures and the stability of gusseted connections in steel-framed structures. ACI member Charles L. Bartholomew is Professor and Chairman of the Department of Civil Engineering at Widener University. He received his BS degree in civil engineering from the University of Kansas in 1959 prior to working for two years in materials research for the Kansas Highway Department. He received his MS from the University of Illinois in 1962 and his PhD in civil engineering in 1974from the University of Illinois. Before his present position, he served as Chi~f Soils Engineer for the Illinois Department of Transportation, was vice-president of a consulting engineering firm, and taught at Bradley University and the University of Colorado. He operated his own consulting firm from 1968 until /984. His research interests are geotechnical analysis and innovative construction materials.

LOAD FROM TESTING MACHINE

JrdPOINT COMPRESSION LOADING TOOL

CONCRETE BEAM

1------ JO''--'-----1

DEFLECTOMETER AT MIDSPAN

TESTING MACHINE BOTTOM PLATEN

Fig. 1 -Beam test schematic

bar deformations do not conform to ASTM A 615, the geometry was selected to maximize bond strength. There is currently no ASTM standard for FRP reinforcing bar deformations. Being somewhat temperature dependent, the material is sufficiently variable that standardized stress-strain diagrams are not available from the reinforcing bar manufacturers. However, a typical tensile stress-strain diagram is linear to almost the point of failure. Tensile strength of the bars is on the order of 100 to 160 ksi (690 to 1100 MPa), which is higher than that of Grade 60 steel reinforcing bars; while the tensile modulus of elasticity ranges from 6 to 10 million psi (40 to 70 GPa), which is significantly lower than that of steel. Shear strength is only about 8500 psi (58.5 MPa), while the coefficient of thermal expansion is similar to that of normal weight concrete. Although FRP reinforcing bars have ultimate strengths greater than that of commonly used Grade 60 steel reinforcing bars, they cannot replace steel bars on an equal area basis because their tensile modulus is only about one-fourth that of the steel bars. The lower modulus of elasticity affects deflection and crack width parameters in addition to strength. Proper anchorage of the higher strength FRP bars in the concrete is also a major consideration.

RESEARCH SIGNIFICANCE If FRP reinforcing steel bars are to be used to full advantage, appropriate design procedures must be derived from the governing theory and confirmed by test data. Several investigators have studied the feasibility of using FRP bars for reinforcing concrete. An initial study by the Army Corps ofEngineers1 concluded that design formulas for steel-reinforced concrete could be applied to FRP bars. They also found that deflections in FRP-reinforced beams were two to three times as large as those in steel-reinforced beams, and that poor bond strength was a major problem. Other investigators 2-4 also reported that the behavior of FRP-reinforced beams was in many aspects comparable to steel-reinforced beams, although deflections were greater and cracking was more extensive. Although section capacity could be predicted accurately from ultimate strength theory, use of the ACI equation for effective moment of inertia resulted in calculated deflections that were lower than observed values.

ACI Materials Journal I January-February 1993

A study conducted at Widener University on the behavior of FRP-reinforced concrete beams provides additional experimental data which largely confirms behavior demonstrated elsewhere. The experimental work was carried out in two phases, the first of which involved destructive testing of concrete beams to determine ultimate flexural behavior and strength, as well as deflection characteristics. In the study's second phase, pullout tests were performed to determine the strength of the FRP reinforcing bar-to-concrete bond.

BEAM TESTS Six FRP-reinforced concrete beams were cast in standard 6 x 6 x 30 in.-(152 x 152 x 762-mm) metal forms. The concrete was made from Type I portland cement, river sand, and %-in. (19-mm) crushed stone. The mix proportions were 0.49:1.0:2.2:3.0 (water:cement:fine aggregate: coarse aggregate) by weight. Concrete test cylinders gave the 28-day compressive strength.fc' as 5200 psi (35.9 MPa). Each beam was cast with one 3/8-in. (9.5-mm) diameter FRP bar, which was supplied by two of the American manufacturers who market their products specifically for reinforcing concrete. One inch (25.4 mm) of bottom cover was provided for the flexural reinforcement, with bars centered laterally in the forms. All beams reinforced for flexure were also reinforced for shear, through the use of steel stirrups. After casting, beams were cured in 100 percent humidity at laboratory temperatures for 28 days prior to testing. A schematic of the test setup, giving relevant dimensions, is shown in Fig. 1. All beams spanned 26 in. (660 mm) between simple supports, with symmetric load points located 8 in. (200 mm) from each support, thus approximating thirdpoint loading. The beams were tested to failure in a 180,000lb (800-kN) capacity universal testing machine. Results from the six beam tests are given in Table 1. Behavior of the FRP-reinforced beams was in many respects similar to what would be expected from conventional steelreinforced beams. Tension cracks initially formed on the underside of a beam in the center portion of the span. As load was increased gradually, the cracks widened progressively and propagated upward through the cross sections. The moment causing failure in these beams averaged 2.37 times the cracking moment (Table 1), as determined by testing unreinforced control beams from the same concrete mix. As ex-

35

Table 1 -

where

Beam test results

Specimen Ultimate load, Ultimate moment, No. kips in.-kips

MJM"

MuiMn

Uu P

I

15.58

62.32

2.38

1.12

2

14.70

58.80

2.25

1.06

3*

11.37*

45.48*

1.74*

0.82*

4

15.99

63.96

2.44

1.15

5

16.27

65.08

2.49

1.17

6

14.93

59.72

2.28

1.08

AVG.

14.81

59.23

2.37

1.12

e=

Note: I kip= 4.45 kN; I in.-kip = 113 N-m. *Results from Specimen No. 3 were not included in averages.

pected, the FRP-reinforced beams had more extensive cracking and larger deflections than would be anticipated in steel-reinforced beams. Midspan deflections, as measured by deflectometer, were approximately three to four times larger than calculated values for similar beams with steel reinforcement. In addition, the FRP reinforcing bars did not yield in the manner of steel reinforcing bars. Some stretching occurred as the bars bent, but this was accompanied by fraying and in some cases by actual severing of the fiberglass strands that formed the ribbing on the outside surface of the reinforcing bars. The lacerations on the face of the reinforcing steel may have been the result of slippage between the concrete and the reinforcement, even though total pullout of the bars did not occur. Some splitting of the concrete surrounding the reinforcing bars did take place, but there was no conclusive evidence that the beams failed prematurely due to loss of the bond between the reinforcing bars and concrete. Although the beam tests were conducted to observe ultimate load and deflection characteristics, beam behavior was such that the possibility of bond failure was considered in interpreting test results.

PULLOUT TESTS In designing reinforced concrete members, it is assumed that no slippage will occur between the reinforcement and the concrete when load is applied. If the bond between the two is not strong enough to support the resulting stresses, the concrete surrounding the reinforcement may crush or split. The strength of the reinforcing bar-to-concrete bond is dependent upon a number of factors, with concrete compressive strength, reinforcing bar diameter and spacing, and embedment length being most significant. Nominal bond strength is determined experimentally by the pullout test, which basically involves measuring the force needed to produce excessive slippage or to pull out a bar embedded in concrete. There are various measures of bond strength. Perhaps the simplest is the nominal bond force per unit embedment length, which can be calculated as the pullout force divided by the reinforcing bar embedment length, or p

Uu =£.

36

= nominal bond force = pullout force

(1)

rebar embedment length

In this study, the nominal bond force Uu is used as the measure of bond strength. All references to "bond strength" imply the nominal bond force per unit embedment length. In any case, it should be noted that bond strength can be affected by a large number of variables, with the result that experimental values may predict unrealistically high nominal bond strengths, especially in situations where values are based on test beams in which favorable conditions for bond exist. Consequently, extensive testing under all manner of conditions is required to establish an expression that predicts nominal bond strength with a reasonable degree of confidence. The classic pullout specimens used in these tests surround the bars with concrete in compression (in contrast to the tension field that occurs in actual members), and thus are recognized generally as providing somewhat tainted results. At any rate, the experimental results reported herein represent preliminary work in this area and are not intended to be regarded as either definitive or complete. Fig. 2 gives the basic pullout specimen geometry. A single %-in. (9.5-mm) diameter reinforcement rod was embedded a specified length into a standard 6 x 12-in. (152 x 304-mm) concrete test cylinder. Testing was limited to one size reinforcing bar to eliminate the need for multiple sets of custommade grips. A total of 24 pullout tests were performed with reinforcing bars from the same companies that supplied the FRP bars for the beam tests. Test parameters included: (a) reinforcing bar embedment length, with half of the tests at 4 in. (102 mm) and half at 6 in. (152 mm) of reinforcing bar embedment; and (b) concrete compressive strength, which varied from 1160 psi (8.0 MPa) to 4200 psi (28.9 MPa). Specimens were again cured in 100 percent humidity for 28 days prior to testing. Pullout tests were performed on the same universal testing machine as was used in the beam tests. The specimens were loaded in the machine (Fig. 3) with the top surface of the concrete test cylinder bearing against the bottom surface of the testing machine's center cross head. A bearing plate was placed over the top of the concrete cylinder to provide uniform distribution of stress. The end of the reinforcing bar protruding from the cylinder was grasped by a custom-made chuck, which was designed specifically for testing the FRP reinforcing bar. The chuck was, in tum, bolted to the testing machine's upper crosshead. Load was applied at a constant rate until the bond between the reinforcing bar and the concrete was broken. In all cases, the failure loads were less than the calculated "yield" load of 12,100 lb (42.8 kN) for the FRP bars. In 11 of the 12 tests conducted with the lower strength concrete (the reinforcing bar itself failed in the twelfth test), bond failure resulted when slippage of the reinforcing bar occurred, with little or no visible surface cracking of the surrounding concrete. Testing was terminated prior to the bar's completely pulling out of the concrete specimens, to avoid damaging the testing machine. On the other hand, in those specimens tested


I

I 3/8"D~~I

FRP REBAR

LOAD APPLIED BY TESTING MACHINE

TESTING MACHINE UPPER PLATEN

TESTING MACHINE STATIONARY CROSSHEAD

3/8" FRP ROD

I I

I

36

I

I I

•

4" OR 6" EMBEDMENT

72"

'

L

I Ill I I

~lj

I I

I I

I

I

[\__ 11 60 TO 4260 PSI NCRETE cOMPRESSIVE sr,RENGTH

HARDENED STEEL BEARING PLATE WITH RUBBER GASKET

co

6" X 72" CONCRETE TEST CYLINDER

Fig. 2 -Pullout test specimen Fig. 3 -Pullout test schematic with the higher strength concrete, little or no slippage of reinforcing bars occurred, while specimens did exhibit significant cracking and splitting of the surrounding concrete. Averaged results from the pullout tests are shown in Table 2, along with corresponding nominal bond strengths. Those results were reasonably consistent, given the poor quality of several of the concrete mixes. As expected, the specimens from the stronger concrete had greater bond strengths. A comparison of the results from Mixes A and B, which had similar concrete strengths, shows that the specimens with the 6in. (152-mm) embedment lengths had somewhat (approximately 20 percent) higher nominal bond strengths than those with the 4-in. (102-mm) embedment lengths. A similar comparison cannot be made directly with the data from Mixes C and D, as the concrete compressive strengths were not the same. To overcome the difficulty of varying concrete compressive strengths, Table 2 also reports K values, which are the nominal bond strengths Uu normalized with respect to the square root of the concrete compressive stress. K = .!:!.!.___ = ____!_____

{1:

f{l:

(2)

Table 2 indicates that specimens with similar embedment lengths had similar K factors, regardless of the concrete compressive strength. In addition, the largest variation in K factors was the 23 percent difference between the 18.02 (38.1) value for Mix B and the 22.17 (46.8) value for Mix C. The overall average K value from all tests was 20.37 (43.0) with a 2.25 (4.75) standard deviation, indicating fairly good repeatability of results. Because the K factor normalizes data with respect to reinforcing bar size, embedment length, and concrete strength, it provides a means of comparing results ACI Materials Journal I January-February 1993

Table 2 -

Pullout test results .f/,

Bond strength u•. lb/in.

psi

Average P, lb

A

6

6

4200

8455

1409

Mix

No. of Length, Tests in.

K

= U. I ..,fl:

21.75 (45.89)

B

6

4

4200

4672

1168

18.02 (38.02)

c

5

6

1930

5843

974

22.17 (46.78)

D

6

4

1160

2705

676

19.85 (41.88)

Note: I in.= 25.4 mm; I psi= 6.88 kPa; I lb = 4.45 N; I lb/in. = 0.175 N/mm.

from different mixes and of contrasting the behaviors of different materials. For example, No. 11 and smaller deformed steel reinforcing bars have an average bond stress Uu, as reported by Leet, 1o of approximately 9.5 JlZ /db (20 Jlj!db). Nominal bond strength Uu is found by multiplying the average bond stress by bar circumference

(3) Thus the K value for steel is 30 (63.3), so that the FRP reinforcing bars in this study had a bond strength approximately two-thirds that of typical steel bars. INTERPRETATION OF RESULTS The moment capacity of the FRP-reinforced beams was computed analytically, using the Whitney equivalent stress block concept for calculating the ultimate moment capacity. The yield.stress for the FRP reinforcing bars was taken as 110 ksi (760 MPa) in the computations. This was the. yield stress specified by one of the FRP reinforcing bar manufacturers

37

Table 3 - Comparison of midspan deflections for steel-reinforced and FRP-reinforced beams Reinforcing Bar/Type FRP Steel

Exp. Deflection, in. 0.060

/, Deflection, in. O.OZ5

0.019

Ia Deflection, in. 0.173 0.044

Notes: I in. = 25.4 mm, Deflections shown occur at 8400 lb (37 .4 kN) total load. Experimental value is averaged from several tests.

and represented 70 percent of the ultimate strength reported by the other manufacturer (no value for yield stress was available for those bars). Calculations yielded a nominal moment strength Mn of 55.5 in.-kip (6.27 kN-m). Agreement between the theoretical prediction and the experimental results was quite good (Table 1), with average experimental moment capacity 12 percent higher than the theoretical value. Beam specimen 3 was not included in the average values, as it failed at a significantly lower load than did the other five beams, due possibly to either a flaw in the beam or a bond failure. Using a nominal bond strength of 20 .[1), which the pullout tests indicated was reasonable, the length of bar needed to develop the yield stress can be found by equating the pullout force to bar capacity fd

= Ahfy = 0.11(110,000) = 8.4 in. (213 mm) Uu 20..,)5200

(4)

As the actual distance from the ends of the reinforcing bar to the points of maximum moment was 9 in. (229 mm), the bars could theoretically develop full strength. However, the factor of safety in these instances was only 1.07, so loss of bond could certainly be a possible cause for the premature failure in Specimen 3. Deflection computations also produced interesting results. Average midspan deflections for the beams tested were compared to calculated values at 8400 lb (37.4 kN) of total load, a value not only significantly larger than that which would cause first cracking but also reasonably smaller than the failure loads, thus modeling a typical service load for purposes of comparison. Concrete deflections are computed typically using an effective moment of inertia fe as given in Paragraph 9.5.2.3 of the ACI 318-89 Specification.5 For situations in which concrete cracking is expected to be extensive, fe is replaced with the moment of inertia of the cracked transformed section fer. Table 3 compm;es experimental deflections to deflections computed using both fe (as specified in the ACI Building Code, Section 9.5.2.3) and fer· To provide a frame of reference on relative size of deflections in the FRP-reinforced beams, computed deflections for similar beams reinforced with steel bars are also listed. As the table indicates, the FRP-reinforced beams had midspan deflections that were three to four times larger than would be expected in a steelreinforced beam. Furthermore, the actual (experimental) deflections were two to three times larger than the deflections calculated with fe for the FRP-reinforced beams. Averaged experimental results for the FRP-reinforced beams fell between the values computed with fe and fer· Such behavior would appe,.ar logical since the ACI equation for effective moment of inertia is empirical in nature, being based upon observed deflections in

38

steel-reinforced beams. Clearly, a separate equation for effective moment of inertia must be developed for FRP-reinforced beams, as these beams are subject to more extensive cracking than are similar beams with steel reinforcement. The current ACI equation bases the effective moment of inertia upon the third power of the cracking moment-to-service moment ratio, using it to interpolate between the moments of inertia of the cracked and uncracked cross sections. The FRPreinforced beam deflections in this study correlated more closely with an effective moment of inertia based upon the fifth power of the cracking moment-to-service moment ratio; additional testing, however, is needed to confirm these results as the number of tests performed was limited.

CONCLUSIONS AND RECOMMENDATIONS In this series of laboratory tests, FRP-reinforced beams exhibited ductile failure modes and demonstrated approximately the same behavior pattern as would be expected in steel-reinforced beams. In addition, ultimate moment capacities were predicted adequately by the strength design method as developed for steel-reinforced beams. On the other hand, pullout tests showed that the bond between the FRP reinforcing bar and the concrete was only approximately two-thirds of that which would ordinarily be expected between steel reinforcing bars and concrete. Thus, proper anchorage of bars should be a major consideration when designing with the higher strength FRP reinforcement. Another major design consideration must be the deflection levels in FRP-reinforced concrete beams; as seen in this study, deflections at service loads may be in the neighborhood of four times those in steel-reinforced beams. Although FRP reinforcing bars, have ultimate strengths greater than those of commonly used Grade 60 ~teel reinforcing bars, their tensile modulus is only about one-fourth that of the steel bars. It is this lower modulus of elasticity that results both in increased cracking and in larger deflections than would be present in a steel-reinforced beam. Larger crack widths in themselves will not necessarily present a problem, since FRP bars do not require the same protection from corrosive environments as do steel bars. However, the increased cracking does have a significant effect on deflection computations, necessitating the development of a separate expression for effective moment of inertia. Based upon the generally favorable overall results obtained through laboratory testing, it can be concluded that FRP reinforcement can provide an attractive alternative to steel for structural applications in aggressive environments. The ultimate goal of further experimental work in this area should be the development of criteria for the design of FRP-reinforced concrete members. ·


ACKNOWLEDGMENTS The authors wish to acknowledge the contributions of two groups of senior civil engineering students at Widener University, who conducted the testing as their senior projects. 6•7 The successful completion of this work is in large part due to their enthusiasm and professionalism.

NOTATION The following symbols are used in this paper: Ab = cross-sectional area of reinforcing bar db = reinforcing bar diameter fc' = 28-day concrete compressive strength Ia = moment of inertia of cracked transformed section I, = effective moment of inertia K = bond strength coefficient £ = reinforcing bar embedment length fd = development length Mer = cracking moment Mn = nominal (theoretical) moment capacity of the cross section M. = ultimate (actual) moment capacity of the cross section P = pullout force u. = average bond stress u. = nominal bond strength

REFERENCES I. Wines, J. C., and Hoff, G. C., "Laboratory Investigation of Plastic-Glass Fiber Reinforcement for Reinforced and Prestressed Concrete," Reports No. I & 2, U.S. Army Engineers Waterways Experiment Station, Vicksburg, 1966.


2. Goodspeed, C.; Schmeckpeper, E.; Henry, R.; Yost, J.; and Gross, T., "Fiber Reinforced Plastic Grids for the Structural Reinforcement of Concrete," Serviceability and Durability of Construction Materials, V. 2, Denver, 1990, pp. 593-604. 3. Nawy, E. G.; Neuwerth, G. E.; and Phillips, C. J., "Behavior of Fiberglass Reinforced Concrete Beams," Journal of the Structural Division, ASCE, V. 97, No. ST9, Sept. 1971, pp. 2203-2215. 4. Nawy, E. G., and Neuwerth, G. E., "Fiberglass Reinforced Concrete Slabs and Beams," Journal of the Structural Division, ASCE, V. 103 No. ST2, Feb. 1977, pp. 421-440. 5. ACI Committee 318, "Building Code Requirements for Reinforced Concrete and Commentary (ACI 318-89/ACI 318R-89)," American Concrete Institute, Detroit, 1989, 353 pp. 6. Bertoni, J.; Jesseman, S.; Maffei, J.; Read, H. F.; and Zaun, C., "PlasticBased Reinforcing Bars in Concrete," Senior Project Group 2 Report, School of Engineering, Widener University, Chester, PA, 1988. 7. Boehning, S.; Cahill, B.; Saloma, S.; Taylor, J.; and Wiser, L., "Fiberglass Reinforced Plastic Rebar in Concrete," Senior Project Group 3 Report, School of Engineering, Widener University, Chester, PA, 1989. 8. Daniali, S., "Bond Strength of Fiber Reinforced Plastic Bars in Concrete," Serviceability and Durability of Construction Materials, V. 2, Denver, 1990, pp. 1182-1191. 9. Kodiak Fiberglass-Reinforced Plastic Rebar, International Grating Inc., Houston, 1988. 10. Leet, Kenneth, Reinforced Concrete Design, McGraw-Hill, Inc., New York, 1990. 11. Pleimann, L. G., "Tension and Bond Pull-Out Tests of Deformed Fiberglass Rods" Final Report for Marshall-Vega Corporation, Department of Civil Engineering, University of Arkansas, Fayetteville, 1987.

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