Performance of reinforced concrete columns with reduced steel ratios strengthened by hybrid FRP/steel and high-strength concrete systems

Reinforced concrete (RC) columns with insufficient steel reinforcement present a significant structural concern. This issue is most commonly found in aging buildings, poorly constructed members, or structures designed before the adoption of modern codes. This study investigates the effectiveness of external hybrid strengthening techniques in enhancing the axial load-bearing capacities of such deficient columns. A series of rectangular RC column specimens with intentionally reduced longitudinal reinforcement ratios were tested under axial compression in the laboratory and simulated using ABAQUS software to evaluate structural performance. The specimens were strengthened using a combination of near-surface-mounted (NSM) steel or glass fiber-reinforced polymer bars with externally bonded glass fiber textile mesh covered with different types of high-strength concrete to form a hybrid strengthening solution. The influence of different strengthening configurations on the columns' ultimate load, energy absorption, and failure modes was evaluated. The results show that the proposed hybrid systems improve the structural behavior of deficient RC columns. They also demonstrate that these systems significantly enhance the structural performance of deficient RC columns, achieving increases in the axial load-bearing capacity ranging from 11% to 78% compared with the deficient specimens. Additionally, the energy absorption capacity improved by up to 382%, highlighting the effectiveness of the combined NSM and external confinement techniques. Furthermore, these systems provide a practical solution for field engineers by helping them strengthen existing structures with inadequate internal reinforcement.