Shear performance evaluation of damaged RC beams strengthened with cast-in-place U-shaped UHPFRC shell

The shear behavior of reinforced concrete (RC) beams may degrade during service, necessitating effective strengthening interventions for these shear-degraded RC members. This study aims to investigate the shear performance of damaged RC beams strengthened by a cast-in-place U-shaped ultra-high performance fiber reinforced concrete (UHPFRC) shell. Four-point flexural tests were performed on one un-strengthened and four UHPFRC shell-strengthened damaged RC beams. In this study, different pre-damage levels for the RC beams (55%, 70%, and 85%) and stirrup ratios within the UHPFRC shell (0.7% and 1.34%) were considered. The failure mode, ultimate capacity, and ductility of these tested RC beams were assessed. The results indicate that the shear capacity of the strengthened beams can be greatly improved by a U-shaped UHPFRC shell, reaching 2.7-3.0 times that of the un-strengthened beams. As the stirrup ratio increased from 0.7% to 1.34%, the failure mode of the strengthened beams changed from shear compression failure to flexure failure, and the ductility increased from 1.56 to 1.96. However, the shear capacity of strengthened beams is slightly influenced by the pre-damage level of the RC beams. Moreover, planting steel rebars at the UHPFRC-RC interfaces effectively improved the interfacial bonding performance. Finally, the shear capacity of the strengthened beams was evaluated by a truss arch model, and the calculated results of the truss arch model are safe as the lower limit of shear capacity.