Axial behavior of hollow reinforced concrete columns confined by circular concrete jackets reinforced with dual reinforcements (steel/ GFRP )

Abstract Hollow reinforced concrete (RC) columns offer significant advantages in terms of structural efficiency and reduced self‐weight. However, their limited confinement and premature concrete crushing, particularly in square sections, adversely affect their load‐carrying capacity and ductility. Despite the proven effectiveness of circular concrete jacketing and fiber‐reinforced polymer reinforcement, their combined application to hollow square RC columns remains largely unexplored. This study experimentally evaluates a structural strengthening method that transforms square hollow RC columns into circular cross‐sections through RC jacketing. Ten specimens, each with an outer dimension of 150 mm × 150 mm and a central 75 mm‐diameter void, were constructed. Specimens were cast in two lengths—1350 and 2250 mm—resulting in length‐to‐depth ratios of 9 and 15, respectively. Eight columns were strengthened with 250 mm‐diameter circular RC jackets incorporating various reinforcement configurations combining glass fiber‐reinforced polymer (GFRP) and steel. Two unjacketed columns served as control specimens. Reinforcement layouts in the jacket included four variations: full steel (longitudinal and transverse), full GFRP, hybrid reinforcement with GFRP longitudinal bars and steel ties, and steel longitudinal bars with GFRP hoops. All specimens tested under concentric axial compression loading. Test results demonstrated that circular RC jacketing substantially enhanced the structural performance of hollow RC columns by increasing strength, stiffness, and ductility through combined concrete confinement and additional longitudinal reinforcement. Steel RC jackets yielded the greatest improvements in load capacity and stiffness, whereas fully GFRP RC jackets exhibited superior deformation capability. The benefits of GFRP were more pronounced in shorter columns, while steel reinforcement proved more effective in slender configurations due to its higher axial stiffness. Furthermore, experimental outcomes closely matched predictions from current design codes, confirming the applicability of existing slenderness and strength models for strengthened hollow RC columns.