Experimental Investigation of the Eccentric Performance of Columns Reinforced by Steel and GFRP Bars and Strengthened with GFRP NSM Bars and Mat Techniques

An experimental program was conducted to study the eccentric behavior of columns reinforced with glass fiber-reinforced polymer (GFRP), steel bars, or a hybrid of both. Two strengthening methods were employed: GFRP mats and near surface-mounted (NSM) GFRP bars. Eighteen specimens with different reinforcement configurations, all having slenderness ratios 19, were tested under axial compression loads with eccentricities of 0.0 mm, 30 mm, and 60 mm. Results showed that using GFRP bars as longitudinal reinforcement significantly decreased the axial bearing capacity as eccentricity increased, with reductions of about 43% at 30 mm and 71% at 60 mm without additional strengthening. Specimens strengthened with three layers of GFRP mats exhibited reductions of approximately 35% and 56% at the same eccentricities, while a combination of GFRP mats and NSM bars showed reductions of 19% and 49%. At zero eccentricity, NSM GFRP rebars combined with GFRP wrap increased the loading capacity by about 10% and enhanced ductility by approximately 54%. Combining NSM GFRP rebars and GFRP wrap can significantly increase the loading capacity and improve displacement ductility. The obtained results show that these specimens outperform others. For example, the axial load is increased by 10% at e = 0, by 56% at e = 30, and by 93% at e = 60 compared to the control specimens. The elastic stiffness was comparable for columns reinforced with GFRP and steel. Using GFRP for longitudinal reinforcement and steel for stirrups resulted in a 16% reduction in axial load and a 22% reduction in axial displacement. The strengthening techniques proved more effective as the loading eccentricity approached the P–M curve’s equilibrium point. © The Author(s) 2025.