Modeling the behavior of externally bonded reinforcement using a rigid-trilinear cohesive material law

Externally bonded reinforcement (EBR) represents a valid solution for strengthening and retrofitting existing structures. Among possible reinforcements, fiber-reinforced composites gained large popularity in the construction industry mainly due to their high strength-to-weight ratio and durability. Due to their high strength, failure of externally bonded fiber-reinforced composites is usually due to debonding, which makes understanding the bond behavior of these materials of paramount importance for the effectiveness of the strengthening application. Within this framework, bond tests are generally employed to obtain information on the stress-transfer mechanism between the composite and the substrate. The results of these bond tests can be employed to derive the interface cohesive material law (CML), which describes the relationship between the shear stress and corresponding slip at the interface where debonding occurs. In this paper, a rigid-trilinear CML is proposed to describe the stress-transfer mechanism of externally bonded composites that show the presence of frictional stresses at the debonding interface. The analytical solution of the full range behavior of composite-substrate joints with long and short bonded lengths and with free and fixed far end is provided using the proposed CML. Then, the results of thirty-five single-lap direct shear tests of PBO fiber-reinforced cementitious matrix- (FRCM-) concrete joints are presented and employed to calibrate the CML. Good agreement is found between the analytical and corresponding experimental direct shear test load responses.