Torsional strengthening of T-shaped RC members with FRP composites using EBROG method: Experimental investigation and analysis

Despite extensive research on reinforced concrete (RC) members strengthened with fiber-reinforced polymer (FRP) sheets, limited knowledge exists regarding their torsional behavior. This study aims to contribute to this research field by conducting experimental tests on novel strengthening configurations to achieve more efficient strengthening systems under torsion. Two different surface preparation methods of externally bonded reinforcement (EBR) and externally bonded reinforcement on grooves (EBROG) were evaluated and compared in torsional FRP-strengthening. The combined utilization of longitudinal and transverse FRP strips and the impact of different parameters such as spacing between transverse FRP strips as well as an anchoring system of FRP fans, were explored in torsional strengthening of RC members. The strengthening patterns were evaluated by comparing torque-twist curves, cracks documenting, and strain field analysis using digital image correlation (DIC). Results showed that the EBROG technique significantly enhances the transfer of shear stresses to deeper concrete layers and ensures uniform strain distribution across transverse FRP strips, resulting in a significant increase in cracking and ultimate torque, up to 72 % and 63 %, respectively. The results also showed that the deformability index improved by 3.0 and 5.6 times, respectively, in the strengthening systems applied through the EBR and EBROG methods. A comparison of the tested specimens proved that implementation of the EBROG technique and FRP fans can increase the torsional strengthening efficiency by more than 6 times.