Interaction forces in RC beams strengthened with NSM FRP round bars

The near-surface mounted (NSM) strengthening method has attracted an increasing worldwide attention in the last decade. Although the bond efficiency between FRP and concrete in the NSM method is much improved compared with the externally bonded (EB) FRP strengthening method, debonding failures have also been often observed in reinforced concrete (RC) beams strengthened with NSM FRP bars. In such FRP-strengthened RC beams, debonding may initiate at either of the two ends of the NSM bar (i.e. end debonding), due to the existence of large localized interaction forces between the NSM bar and concrete near the bar ends. This paper presents an analytical solution to the interaction forces in RC beams strengthened with NSM FRP round bars, which are one of the most popular types of FRP bars used for NSM strengthening. The key elements of the proposed analytical solution are the two interfacial stiffness parameters (i.e. tangential interfacial stiffness and normal interfacial stiffness) and the eccentricity of the tangential interaction force to the centroid of the NSM bar. The accuracy of the analytical solution is verified with predictions from a sophisticated 3D finite-element (FE) model of a RC beam strengthened with a NSM round bar.