Experimental studies on torsional behavior of reinforced concrete beams strengthened using hybrid carbon and basalt FRP sheets

Torsional strengthening of reinforced concrete (RC) beams using fiber-reinforced polymer (FRP) composites faces challenges such as brittle failure modes that reduce strength and limited ductility, notably in carbon FRP (CFRP) applications. This study addresses these issues by experimentally investigating a hybrid CFRP-basalt FRP (BFRP) system to enhance the torsional behavior of RC beams. The hybrid system leverages the high strength and modulus of CFRP and the superior ultimate strain of BFRP, bonded with epoxy. Eight RC beams are subjected to pure torsion. Seven beams are strengthened with various wrapping configurations and numbers of layers of CFRP and BFRP sheets. The beams strengthened by hybrid CFRP-BFRP systems are compared with the control beam and beams strengthened with only one type of FRP in terms of torsional strength, crack patterns, failure modes, and torque-twist curves. Generally, the hybrid-strengthened beams were more effective in increasing the torsional strength than those strengthened with one type of CFRP or BFRP sheets. The strength of the hybrid beams increased by 68.1%-112.6% compared to the original specimen. In addition, the ultimate twist indices of the hybrid specimens exhibited an enhancement of up to 15.2% relative to those strengthened only with CFRP sheets. Finally, the accuracy of torsional strength predictions from National Cooperative Highway Research Program Report 655 and fib Bulletin 90 was evaluated, revealing conservative estimates, while the modified softened truss model for torsion with FRP accurately predicted cracking and ultimate strengths.