A Study of the Properties of Fiber Composite Bars for Use in Highway Bridge Deck Reinforcement

A study and evaluation of the basic properties of fiber reinforced polymer (FRP) rods for use as bridge deck reinforcement was completed in April, 1996. Two reinforcing fibers were studied, E-glass and carbon. The glass fiber (GFRP) bars tested were straight production line bars, using vinylester resin as the polymer matrix, manufactured by PolyStructures, Inc. of Little Rock, Arkansas. The data for GFRP bars using a polyester resin matrix were available for comparison from other tests done for the manufacturer under a separate contract. The carbon fiber (CFRP) bars were hand laid-up by Marshall-Vega Corporation of Marshall, Arkansas, using an epoxy matrix. The study was conducted in two parts, tension tests and bond pull-out tests. The tension tests were done at the University of Arkansas. These tests were controlled at a constant stroke rate of 0.07 inches (0.178 cm)/minute in a 110 kip (439 kN) MTS test frame to failure. A secant modulus of elasticity between the points of 5% and 50% of ultimate strength was calculated for each bar. Also, fatigue tests were done by subjecting bars to repeated loads at one Hz at various percentages of the bar failure strength ranging between 10% and 50% for the GFRP bars and between 50% and 90% for the CFRP bars. These latter tests indicated a definite need for a reduction in applied stress to be able to achieve a larger number of repeated loadings. The GFRP bars were consistent in their strength, averaging 70 ksi (483 MPa) ultimate stress for the #4 and #5 bars, and 95 ksi (655 MPa) for the #6 bars. The #4, #5, and #6 bars (#'s 13, 16, 19 bar sizes by new metric designation) showed average moduli of elasticity of 4.9, 5.3, and 6.0 million psi (33.8, 36.5, and 41.4 GPa), respectively. The results for the CFRP bars were more scattered because of the hand manufacturing process. The carbon bars showed an average ultimate tensile strength of 122.5, 125.7, and 107.2 ksi (844.6, 866.7, and 739.1 MPa) for the #4, #5, and #6 bars, respectively. The #4, #5, and #6 bars showed average moduli of elasticity of 11.3, 12.2, and 10.9 million psi (77.9, 84.1, and 75.2 GPa), respectively. The second part of the study was completed at Auburn University. Bond pull-out tests were made of each size of GFRP bar to determine the bond strength coefficient. The coefficient, k, varied with size of bar, having an average value of 19.88, 26.79, and 32.75 for glass reinforced #4, #5, and #6 bars, respectively. This indicates the need for development lengths some 75%, 31 %, and 7% greater than the current ACI Code formulation for individual steel rebars of the same respective "size." This tendency of increased bond capacity with increasing bar size is consistent with earlier tests done at the University of Arkansas on GFRP bars from a different manufacturer. Because of the distortion and inconsistency of the outer shape of the carbon reinforced FRP bars there was excessive scatter in the bond results. It was decided to not continue with the bond strength tests of CFRP bars. However, it was felt that good bond strength consistent with that of GFRP bars will be available in the future from CFRP bars when they are machine made.