Torsional behaviour of RC box beams strengthened by CFRP under combined shear and torsion

A preliminary experimental investigation of box-section reinforced concrete (RC) beams strengthened with carbon-fibre-reinforced polymer (CFRP) under shear and torsion was carried out. Based on different torque-to-shear ratios (TSRs) (0.958, 1.597, and 2.556), 12 box-section specimens were divided into three groups. In each group, one beam was a non-strengthened control while the other three beams were strengthened with CFRP U-jackets or CFRP U-jackets and longitudinal strips. Tests on the beams showed that the average cracking angle at the ultimate state was inversely proportional to the TSR. Two failure patterns were observed: debonding in the beams strengthened with U-jackets and fracture in the beams strengthened with both U-jackets with longitudinal strips. At the ultimate state, the best improvement in torsional capacity was achieved in the specimens retrofitted with longitudinal strips bonded on the U-wrapping strips. Equations using a modified effective strain expression were developed to compute the ultimate torque of all the specimens, the results of which were in good agreement to existing experimental data.