The effect of concrete strength on the performance of the external concrete beam-to-column joint reinforced by fiber-reinforced-polymers (FRP) using the near-surface-mounted method (NSM)

Beam-to-column joints in reinforced concrete structures play an essential role in the overall behavior of the structure. Therefore, it seems necessary to strengthen joints that have been damaged over time under various loads or designed only based on gravity loads. Nowadays, shear retrofitting of joints with fiber-reinforced-polymers (FRP) has attracted the attention of researchers. Since the shear force applied to the joint is transferred by the truss mechanism formed by horizontal and vertical bars and also the compressive strength of the concrete, so the concrete of the joint plays the main role in the shear capacity before retrofitting by FRP materials. In this study, 7 reinforced concrete external joint specimens were designed and constructed on a scale of 2/3 and in two concrete compressive strength groups of 30 and 42 MPa. Seismic criteria were not considered in these joints. Two specimens were damaged up to 1.5% drift and the other two specimens up to 3% drift, and then retrofitted with FRP materials by Near Surface Mounted (NSM) method. Of the other three specimens, one with and two without considering seismic criteria as control specimens were subjected to cyclic loading at two levels of concrete strength. The purpose of considering two different levels of concrete strength was to investigate the effect of increasing concrete strength within the normal range of engineering on the performance of the NSM-retrofitted joint. As the results show, damaged retrofitted specimens with concrete compressive strength of 42 MPa were able to increase the bearing capacity of the joint by 12% compared to the seismic control specimen with concrete compressive strength of 30 MPa. Also, the increase in concrete strength by 12 MPa caused a 20, 40, and 65 percent increase in the bearing capacity, hardness, and energy dissipation ability of the NSM retrofitted specimens, respectively.