Torsional Moment Capacity and Failure Mode Mechanisms of Concrete Beams Reinforced with Carbon FRP Bars and Stirrups

Fiber-reinforced-polymer (FRP) bars and stirrups have emerged as internal flexural and shear reinforcement for reinforced-concrete (RC) members in different applications. Nonetheless, the torsional behavior of FRP RC members has not yet been defined. This paper presents the results of an investigation of the torsional strength and behavior of full-scale concrete beams reinforced with carbon-FRP (CFRP) bars and stirrups. The beams measured 4,000 mm long, 250 mm wide, and 600 mm deep and were tested under pure torsion loading. The test specimens included four beams reinforced with CFRP bars and stirrups and one control beam reinforced with conventional steel reinforcement. The test variables were the type of reinforcement and CFRP stirrup spacing. The test results indicated that the CFRP RC beam exhibited similar strength, cracking behavior, and post-peak torsional stiffness compared with the counterpart steel RC beam. The hollow-tube, space-truss analogy with the 45 degrees inclination of diagonal compressive stresses was in good agreement with the observed diagonal torsion failure. In addition, the paper examines the validity of the new design provisions in CAN/CSA S806-12 in predicting the nominal torsional strength of CFRP RC beams. (C) 2014 American Society of Civil Engineers.