Torsional behaviour of RC beams strengthened by NSM GFRP bars

Abstract

Near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement is an innovative, serviceable, and beneficial method for strengthening reinforced concrete (RC) structures. Previous studies have proven that the epoxy-resin bonded NSM FRP method effectively achieved shear and flexural strengthening. However, only a few researches have employed this technique for improving torsion but, no research has been conducted on the use of NSM with GFRP bars and epoxy adhesives. This study investigates the behavior of RC beams subjected to combined torsion when they have been enhanced with NSM GFRP U-shaped and closed stirrups with different overlap sites. Four RC beams with dimensions of (150×250×2000) mm (width X depth X length) were considered in the experimental tests. One RC beam is not strengthened and used as a control beam (CB), while three beams are enhanced with NSM GFRP bars in different shapes and overlap configurations. The findings of the tests in terms of twist angle_applied torque curves, the torque of initial cracking, the ultimate torsional carrying capacity, and the ultimate angle of twist were all recorded and evaluated. It has also been shown that a considerable improvement in the performance of RC beams in twisting can be achieved by using NSM GFRP stirrups. From the different NSM strengthening configurations used in the tests, results have demonstrated that beams strengthened by closed GFRP stirrups at the opposite overlap with 100 cm spacing outperformed other strengthened beams of about 181.5% compared to the CB. In contrast, the beam, strengthened by U-shaped GFRP stirrups with spacing at 200 cm, had the lowest torsional strength of about 48.1% relative to the CB.