Abstract: In this paper, the torsional behavior of CFRP reinforced concrete box beams under different torque-to-shear ratios were investigated in the experimental study. According to different torque-to-shear ratios (ie, 0.958, 1.597, and 2.556), 12 reinforced concrete box beams were divided into 3 groups, and 3 beams in each group were strengthened by CFRP with different configurations, such as U-shaped strips and U-shaped strips with longitudinal strips. The test results showed that at the ultimate load the cracking angles of all beams were proportional to the torque-to-shear ratios. But, when the torque-to-shear ratio was 0.958, the angle of cracks was close to the 45° angle of the crack angle under pure shear failure. Besides, two failure modes appeared in the experiment: U-shaped strips peeled off and the longitudinal strips ruptured, which shows the longitudinal strips compressed the U-shaped stripes to contribute the torsional strength for the "CBSL" series strengthened beams. Moreover, the improvements of torque of the beams presenting 76.2% for 3CBSL2, 55.2% for 1CBSL2, and 46.9% for 2CBSL which shows increase in torsional resistance was inversely proportional to the torque-to-shear ratio. For beams strengthened with only U-shaped strips, however, the torsional capacity was increased moderately, due to the U-shaped strips which have not significantly enhanced the reinforcement effect of the box girder. Therefore, the torsional behavior and strengthening effectiveness of the strengthened beams is affected by the CFRP configuration and the torque-to-shear ratio.

Key words: reinforced concrete box beam, carbon fiber reinforced polymer, strengthening pattern, torque-to-shear ratio, torsional behavior, composites