Indirect Identification Method of Bilinear Interface Laws for FRP Bonded on a Concrete Substrate

The mechanical response of Reinforced Concrete (RC) beams strengthened by an Externally Bonded Reinforcement (EBR) made out of Fiber-Reinforced Polymers (FRPs) is deeply influenced by the interaction between the concrete substrate and the FRP system, either cured-in-place (sheets) or preformed (plates). In particular, the strength of FRP-EBR RC beams is often controlled by debonding phenomena to develop at the adhesive-to-concrete interface. The most recent theoretical formulations and some experimental results obtained in the last years pointed out the differences that characterize the debonding strength of FRP sheets and plates. According to the findings of those studies, the fracture energy is a fundamental parameter governing the debonding phenomenon. However, determining its value is not sufficient for simulating the behavior of the FRP-to-concrete interface and modeling relevant problems such as intermediate debonding in RC beams externally strengthened by FRP. Consequently, formulating and calibrating local bond-slip models, which take into account the different behavior of sheets and plates, is of fundamental importance for modeling FRP-strengthened RC members. This paper is aimed at identifying bond-laws for sheets and plates through an Indirect Identification Method (IndIM), recently implemented and validated by the authors. A wide collection of experimental results obtained by pull-out tests on FRP sheets and plates is first reported and then employed for identifying the previously noted bond-slip laws. Finally, the results of the identification procedure demonstrate that the debonding phenomenon, described as a fracture process in mode II, should be modeled by assuming different bond-slip relationships for FRP plates and sheets.