One control and three strengthened Reinforced Concrete (RC) beams with box-section were subjected to pure torsion. The proposed strengthening schemes were U-jacketing or U-jacketing with longitudinal strips of fiber-reinforced polymer (CFRP). The U-jacketing strips exhibited a reasonable improvement to the ultimate torque from the experimental investigation. The CFRP strengthening schemes, failure modes, cracking angles, and strain in fibers were investigated and recorded. Besides, three models used for computing the torque of RC members and two for FRP torsional contribution were carefully reviewed and discussed. Based on these models, six combinations were formed by the two-part models addition for predicting the torsional capacity of the strengthened members. The database of 25 strengthened beams was collected from the literature for evaluating prediction of the ultimate torque of the specimens by these six combinational models. In comparison with the laboratorial numbers of the ultimate torque of specimens, the combination of Comb 2 (ACI and fib Bulletin 14 models) and Comb 6 (Chinese and Ghobarah models) presented rational prediction in the ultimate torque of retrofitted beams. However, Comb 6 is a more suitable combination for calculating the ultimate torsional capacity of strengthened beams with rectangular or box-section by FRP entirely wrapping or Uwrap.
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