Durability and Long-Term Performance of FRP-to-Concrete Joints under Environmental Conditioning: Experimental and Analytical Study

The external bonding of fiber-reinforced polymer (FRP) composite plates and sheets to the tensile surface of reinforced concrete structural elements is a viable retrofitting technique. The performance of such strengthened members is strongly influenced by the capacity of the FRP-to-concrete bonded interface. However, over time, harsh environmental conditions can compromise the integrity of the interface. This paper reports an experimental investigation on 135 single lap shear tests that sought to quantify the change in bond strength and local bond stress-slip properties due to thermal cycling loading, as well as wet-dry cycling in both potable water and saltwater conditions. Two types of FRP strengthening materials are tested, namely carbon FRP (CFRP) plates formed by pultrusion and wet lay-up, in addition to three types of epoxy adhesives with varying degrees of viscosity. The results show that the failure modes of specimens under all the considered environmental conditions transitioned from cohesive failure to interfacial failure, regardless of the conditioning regime. For cyclic thermal conditioning, the transition from cohesive to interfacial failure is due to the reduction of the adhesive bonding strength, whereas for cyclic wet-dry conditioning the change in failure mode is likely due to the combination of lowered adhesive bonding strength and enhanced concrete strength arising from post-curing in the presence of moisture. Finally, a bond stress versus slip model that incorporates degradation is used to analyze the changes in the key bond parameters that define the trilinear relationship adopted in a partial-interaction modelling approach.