Effect of Secondary Loading on Shear Performance of Damaged Thin-Walled Web RC Beams Strengthened with a UHPC Layer and Postinstalled Adhesive Bolts

Damaged RC structures are typically strengthened in the load-carrying state in practice, which limits the improvement of their mechanical performance due to stress lag in the strengthening components. In this study, the shear performance of damaged thin-walled web RC beams strengthened with an ultrahigh-performance concrete (UHPC) layer and postinstalled adhesive bolts under secondary loading was experimentally and numerically investigated. Direct shear tests were first conducted to study the interfacial shear behavior between UHPC and normal concrete with different interface treatments. Subsequently, the three-point bending tests were performed to assess the performance evolution of the strengthened beams under secondary loading, i.e., strengthening under sustained loading conditions. The results demonstrated that even under secondary loading, the UHPC layer could still effectively constrain crack propagation and improve the shear performance of the damaged RC beams. However, due to the strain lag of UHPC induced by secondary loading, the UHPC strength was not fully utilized, leading to 15.7% and 10.8% reductions in the stirrup yielding load and ultimate load, respectively, compared with the strengthened beam without secondary loading. Finite-element analysis further indicated that the detrimental effect of secondary loading intensified with increasing sustained load levels but was mitigated as the UHPC layer thickness and postinstalled bolt ratio increased. Based on the experimental and numerical results, a combined surface treatment of mechanical chiseling followed by high-pressure water jetting at 80 MPa is recommended for effective UHPC-based shear strengthening of thin-walled web RC beams.