FRP-to-concrete debonding - global and local bond behaviour

FRP (fiber reinforced polymer) strips are used to flexurally strengthen reinforced concrete (RC) beams. The global bond strength namely the maximum transferable bond stress due to the increase of FRP tensile stress in a cracked concrete element (subsequently named as global bond shear stress) is a relevant aspect of flexural strengthening design. The differences between the bond strength in an uncracked concrete (end anchorage) and in a cracked concrete element have been highlighted in the existing literature; however, despite a large number of research works on the simple lap-shear test (local bond shear stress), less effort has been dedicated to investigate and model debonding on a global view. The current version of Swiss code on externally bonded reinforcement (SIA 166-2004) limits the local and global bond strength by means of the shear stresses, τf,max=τl0 and τl,lim, which are both assumed to be only functions of the concrete tensile strength. Nevertheless, it has been demonstrated in the literature that the maximum interfacial shear stress between two adjacent flexural (or flexural-shear) cracks depends also on the stress level in the FRP (σo). In the current paper, the difference between the local and global bond behavior and its debonding processes is explained and discussed with analytical and numerical models. In analogy to the existing approach developed to study the bond behavior of prestressed carbon fiber reinforced polymer (CFRP) strips during force release, a fracture energy-based model is proposed in the current study to determine the maximum global bond developed in a so-called intermediate crack element (ICE). The proposed model is a function of the FRP tensile stress (σo) and assumes a constant shear stress law. It is here demonstrated that the proposed model provides similar results to the more complicated models available in the literature. The main findings are here discussed with the aim to evaluate the feasibility of this new model for a future enhancement of the Swiss code.