Experimental and Theoretical Analysis of the Torsional Behavior of Flanged RC Members Strengthened with Bond-Critical FRP Systems

This study aims to enhance the understanding of the torsional behavior of RC members strengthened with fiber-reinforced polymer (FRP) sheets, a topic that remains underexplored. It advances the field through experimental tests and proposes a design model for side-bonded strengthening of flanged RC beams. Two surface preparation methods are also compared: externally bonded reinforcement and externally bonded reinforcement on grooves (EBROG). The effect of FRP fan anchors on shifting the bond-critical mechanism to contact-critical behavior is also examined. Finally, it introduces an effective method that transforms bond-critical systems into contact-critical, significantly enhancing the efficiency of the strengthening systems. The design model is also developed to account for the effects of FRP surface preparation methods and FRP anchors on system response. It calculates the optimal FRP percentage in both transverse and longitudinal directions. The results highlight that the EBROG technique transfers shear stresses to deeper concrete layers, which boosts torsional resistance by up to 68%. It demonstrates that combining the EBROG method with FRP fans can quadruple the efficiency of torsional strengthening.