Shear behavior of reinforced concrete T-beams strengthened with carbon fiber reinforced polymer (CFRP) sheets and CFRP anchors
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The objective of this research is the evaluation of shear behavior of full-scale reinforced concrete T-beams strengthened with carbon fiber reinforced polymer (CFRP) sheets and CFRP anchors. Although the CRFP material has high tensile strength, premature failure due to debonding CFRP sheets prevents utilizing that strength. The use of CFRP anchors prevents this failure, so the CFRP sheets are able to reach ultimate strain. The current shear design is based on plasticity, which assumes that all steel (ductile material) stirrups, across the critical section yield at ultimate. However the strain in the CFRP (brittle material), is essential to estimate the shear contribution of CFRP. To evaluate the validity of CFRP strengthening for shear, 24 tests were conducted with several parameters including shear-span-to-depth ratio, depth of beams, different transverse reinforcement ratios, and the layout of CFRP strips. In addition, a simple shear behavior model was developed to explain the differences between ductile and brittle material. From test observation, the use of CFRP anchors resulted in U-wrap application to perform like continuous wrapping which implies that a CFRP strip reached rupture strain because the anchors prevented debonding failure. However, all FRP strips did not rupture simultaneously because the strain distribution across a critical crack was not uniform. The average strain across the critical crack was about 0.005. Therefore a conservative value of effective strain (0.004) was selected for design purposes. In addition, when a beam is strengthened with CFRP, interactions between the contributions of the CFRP, steel or concrete must be taken into account. Factors ka, ks, and kf were introduced in the proposed shear design equations. Factor ka reflects the change in the material contributions as the shear span to depth ratio (a/d ratio) changes in deep beams. Factors ks and kf account for the change in steel or CFRP shear contribution due to the change in the critical crack angle as well as the interactions between the steel and FRP transverse reinforcement. As the amount of either steel or FRP material increase, the efficiency of the other material decreases.