Evaluation of FRP-concrete bond strength: Insights from slant shear and double shear tests

Fiber-reinforced polymer (FRP) composites are emerging as the most effective and promising materials for strengthening reinforced concrete (RC) structures. However, the transfer stresses are dependent on the bond between FRP and concrete, and any weakness in it may cause premature failure of RC structures. It is for this reason that the interfacial bond between concrete and epoxy resin was investigated under slant shear compression and tension tests for different values of slant angles (20°, 40°, 60°, and 70°) to achieve a wide range of shear-to-normal stress ratios. A key contribution of this research is the use of partial bond tests, as the full surface bond in slant shear tests was insufficient for accurately estimating the epoxy resin-bond characteristics, the partial bond was employed. The Mohr-Coulomb modeling, a crucial aspect in the design of FRP-concrete bonded structures, was used for this investigation to enhance the understanding of interfacial bond behavior and optimize the performance of concrete-epoxy resin systems. The FRP-concrete interfacial bond in externally bonded FRP systems commonly employed to strengthen existing RC structures has also been investigated through double shear bond tests performed for single and double layers of GFRP and CFRP sheets, giving the average bond strength for one layer of GFRP and CFRP as 1.28 and 1.79 MPa, respectively. The test results reveal the significance of the surface preparation and the adopted wet layup procedure for employing multiple layers of FRP sheets. A model is also proposed for estimating the upper bound for the FRP-concrete interfacial bond based on the Mohr-Coulomb model of the epoxy resin-concrete bond.