Bond of NSM FRP strengthened concrete: Round Robin Test initiative

Despite the extensive research on the debonding behavior of fiber-reinforced polymer (FRP) strengthening systems, no standard methodology has been established on its experimental characterization. To assess the performance and reliability of small-scale testing on near-surface mounted (NSM) FRP strengthening systems, an experimental program was carried out on a series of nine NSM FRP strengthening systems (bars and strips), in the framework of an international round robin testing (RRT). Eleven laboratories and seven manufacturers and suppliers participated in this extensive international exercise, which regarded both NSM and EBR FRP strengthening systems. The experimental results are discussed and compared in this paper in order to investigate (1) the bond behavior of NSM strips and bars, (2) the influence of alternative single-bond or double-bond shear pulling setups adopted by the participating laboratories, and (3) the influence in concrete strength due to different constituent materials between countries in applying a prescribed mix. The results show no significant influence of the concrete strength on both failure modes and loads. On the other hand, axial stiffness and surface treatment of the FRP NSM systems strongly affect both the bond strength and the parameters identifying the bond laws. Differences of bond strength experimentally recorded for the same FRP NSM systems tested according to different setups range between 10 and 25%. Therefore, for standard bond test protocol, the single shear bond test setup is more reliable, due to its higher feasibility compared with the double shear test setup.