Ultra-high-performance concrete (UHPC) exhibits superior mechanical performance and durability compared to normal concrete; thus, it is suitable for strengthening reinforced concrete (RC) beams. However, the existing literature mainly focuses on flexural and shear strengthening, and relevant research on torsional strengthening is insufficient. To investigate the feasibility of strengthening RC beams using UHPC, pure-torsion tests were conducted on nine specimens, including one unstrengthened RC beam (the control beam) and eight UHPC-strengthened RC beams. The variables considered for the investigation included the wrapping form of the UHPC layer, thickness of the UHPC layer, shape of the steel fibers added to the UHPC layer, UHPC-RC interfacial treatment, and steel reinforcement ratio of the UHPC layer. Based on the torsion tests, the torsional failure modes, torque-twist curves, torque-strain curves, distribution of cracks, and UHPC-RC interfacial slippage were acquired, and the effects of the considered variables on the torsional performance were systematically investigated. The results showed that the cracking and ultimate torques of all strengthened beams were higher than those of the control beam, with maximum increases of 488. 5% and 593. 2%, respectively. The full wrapping form could resist the external torque more effectively; thus, the beam strengthened using this form achieved the best strengthening efficiency. Therefore, this wrapping scheme is preferred for strengthening RC beams. By contrast, the beam strengthened using a 2-sided wrapping form exhibited the lowest strengthening efficiency. In addition, the two UHPC layers peeled almost entirely from the RC beam when the specimen was damaged. Therefore, this wrapping scheme should be avoided during torsional strengthening. The strengthening efficiency of the U-shaped form was between those of 2-sided and full wrapping forms, and no obvious slippage was observed at the UHPC-RC interface during the entire loading process. When the original beam could not be fully wrapped, a 3-sided wrapping form was acceptable. Determining the thickness of the UHPC layer should comprehensively consider the strengthening effect, cost of strengthening, and specific requirements for the RC beam dimensions. The cracking and ultimate torques of the strengthened beam with end-hooked steel fibers were higher than those of the beams with straight fibers. The RC beam surfaces should be fully roughened before casting the UHPC to ensure that the UHPC layers and the RC beam can act together to resist torsional loads. The torsional capacity of the strengthened beam with UHPC layers containing steel bars was 347. 2% higher than that of the beam with UHPC layers without steel bars, which is recommended for torsional strengthening. Finally, by fully considering the excellent tensile behavior of UHPC, a theoretical formula was proposed for calculating the torsional capacity of UHPC-strcngthened RC rectangular beams. A comparison between the experimental and theoretical results for six UHPC-strengthened beams in this study demonstrated the feasibility of the proposed formula. © 2023 Xi'an Highway University. All rights reserved.
