Bond Model of NSM-FRP Strips in the Context of the Shear Strengthening of RC Beams

Abstract

Gluing of fiber reinforced polymer (FRP) strips by a structural adhesive into thin slits cut in the cover of reinforced concrete (RC) beams’ lateral faces is a shear strengthening technique that is gaining increasing attention of the FRP community. Nonetheless, certain aspects related to the failure modes affecting the behavior at ultimate of such near surface mounted (NSM) strips need to be clarified. Recent findings have spotlighted that besides debonding and tensile rupture of the strips, another possible failure mode should be considered, which involves the fracture of concrete in the shape of semicones along the strips. Thus, a comprehensive analytical model accounting for all of those failure modes was recently developed for predicting the NSM contribution to RC beams shear capacity. Despite its consistency with experimental recordings, that model still needs improvement since some aspects related to the interaction between the force transferred by bond to the surrounding concrete and its tensile fracture still need to be clarified. The need to improve that analytical model led us to further address the issues regarding the debonding failure mode affecting the behavior, at ultimate, of a NSM FRP strip. A new local bond stress-slip relationship is proposed and closed-form equations to be implemented in that analytical model are derived and appraised on the basis of some of the most recent experimental results available in the literature.