Experimental and Analytical Evaluation of Strengthening Techniques for Ultra-High-Performance Concrete Circular Columns

This study presents an experimental investigation to evaluate the effectiveness of four schemes for strengthening circular reinforced concrete (RC) columns. The columns were cast using ultra-high-performance concrete (UHPC). Ten RC columns, categorized into five groups including four strengthening schemes, were tested under concentric axial compression. The first group (G1) included the unstrengthened specimens (control specimens). Group G2 included specimens strengthened using two carbon fiber-reinforced polymer (CFRP) layers and 4Ø12 near-surface-mounted (NSM) steel rebars. Group G3 consisted of specimens strengthened using three CFRP layers. In group G4, a 3 mm thick steel tube was used for column jacketing. The specimens of group G5 were strengthened using UHPC jacketing reinforced with three welded wire mesh (WWM) layers and 4Ø8 longitudinal steel rebars. The fibers of CFRP layers in all schemes were oriented in the circumferential direction. The experimental results indicated that all strengthening schemes significantly increased ultimate load by 30% to 69% and initial stiffness by up to 130%. The G5 strengthening scheme (UHPC jacketing reinforced with WWM and 4Ø8 steel rebars) was most effective in enhancing load capacity and stiffness, while CFRP wrapping (G3) offered a superior balance of strength and ductility without increasing column dimensions, preserving post-peak integrity.