Improving torsional behaviour of reinforced concrete beams strengthened with carbon fibre reinforced polymer composite

Three groups of rectangular beams, comprising of sixteen specimens with various amounts of torsional steel reinforcement were tested under pure torsion. Evaluating the effects of various steel torsional reinforcement ratios on the torsional behaviour of the strengthened beams was the objective of this investigation. Carbon fibre reinforced polymer (CFRP) sheet wrappings consisted of different configurations including anchored U-wrapping and full and strip wrappings. The total steel ratios, including longitudinal and transverse reinforcements considered in this study were 1.56%, 2.13%, and 3.03%, respectively. This study indicates that the CFRP contributions to the torsional strength of the strengthened beams, having identical volumetric ratio of CFRP reinforcement are quite dependent on the total amount of torsional reinforcements. The above mentioned CFRP contributions will increase as the steel torsional reinforcement is increased. Experimental results show that increasing the steel reinforcement by 37% and 94%, increases the CFRP contribution to torsional strength by up to 54% and 91% for strengthened beams with one CFRP ply; and by up to 60% and 111% for strengthened beams with two CFRP plies, respectively. In this experimental work, the effect of the number of CFRP plies is also investigated. It is found that the increase in CFRP contribution to torsional strength concerning the beams strengthened by one ply and two plies of CFRP sheets is close for various steel reinforcement ratios, when compared to increasing the total amount of steel reinforcement