Effective strain of CFRP in RC beams strengthened in shear with NSM reinforcements

Abstract This paper investigates the structural behavior of reinforced concrete (RC) beams strengthened in shear by near surface mounted (NSM) method through experimental and numerical studies. Furthermore, the effect of applying different concrete strengths and steel shear bars is studied through the case studies. Four reinforced concrete beam specimens with dimensions of 2000 × 300 × 200 mm were cast in two series (with and without stirrups). The beams were tested under four-point flexural loading and their load-deflection curves as well as their failure mechanisms were compared. Experimental results showed that bonding carbon-fiber-reinforced polymer (CFRP) reinforcements into the grooves enhanced the shear capacity up to 42% and 68% in the beams with and without stirrups, respectively; and the reinforcing strips failed due to CFRP rupture. Furthermore, in NSM strengthened beams, ductility index decreased for both strengthened beams with and without stirrups compared to the reference beams. In addition, the overall stiffness of the strengthened specimen with stirrup was slightly more than that of the strengthened beam without stirrup. Finite element parametric studies through modeling bond behavior for the CFRP strips showed that as the concrete strength decreases, the debonding possibility may decrease, and as the shear reinforcement amount increases, the ultimate strain in CFRP strips decreases. Based on the results of the parametric study, a new relationship was proposed for determining the effective strain in NSM-CFRP strips.