Flexural and shear strengthening of lightly reinforced concrete beams using CFRP bars and UHPC overlays

Existing studies predominantly focus on enhancing either the flexural or shear performance of reinforced concrete (RC) members, with limited research on simultaneous improvements. There is a strong motivation to develop a holistic approach that addresses both flexural and shear deficiencies in lightly reinforced concrete beams. This study addresses this critical gap by providing a dual-strengthening approach that integrates ultra-high performance concrete (UHPC) overlays with near surface mounted (NSM) carbon fiber reinforced polymer (CFRP) bars to achieve a synergistic effect, improving both flexural and shear capacities simultaneously. An experimental program was conducted on five RC beam specimens, with key variables including the integration of NSM-CFRP bars and the type of UHPC overlay: disconnected prefabricated panels, lap-spliced panels, and cast-in-place monolithic overlays. The performance of the beams was evaluated via damage modes, force-deflection responses, shear deformation, strain demand of NSM-CFRP bars, stiffness, and ductility. Experimental findings revealed that simply retrofitting with NSM-CFRP bars adversely affected strength and drift capacity due to early shear crack localization. The combination of CFRP bars and UHPC overlays effectively enhanced both flexural and shear performance, increasing peak strength by 16–24 % and developing higher strains in the CFRP bars. Among the retrofitted beams, the one with disconnected UHPC panels exhibited the highest structural ductility because the dislocation of the panels alleviated shear crack localization. In addition to the experimental study, suitable strength models were proposed to predict the flexural strength and dominant behavior of the retrofitted beams.