Experimental Investigation of GFRP-RC Slender Circular Columns under Axial Loads with Various End Eccentricities

This study investigates the structural performance of full-scale slender circular concrete columns reinforced with glass fiber–reinforced polymer (GFRP) bars and spirals under various eccentric loading scenarios. Seven full-scale specimens with a slenderness ratio of approximately 45 were tested under concentric loading and varying end moment conditions, including single and double curvature. The study aimed at evaluating the influence of end moment ratio and curvature type on strength, stiffness, and second-order effects. The results revealed that reinforced concrete (RC) columns with unequal end moments exhibited increased load-carrying capacity and lateral stiffness, primarily due to the reduction of second-order effects. In contrast, columns with equal end moments showed more pronounced lateral displacements and higher secondary-to-primary moment ratios, leading to stability-related failure. All specimens ultimately exhibited concrete crushing at the critical compression region; however, the eccentricity configuration influenced the governing member-level failure mechanism, with most specimens showing material-controlled response and a subset exhibiting stability-controlled behavior prior to crushing. The results highlighted that double-curvature columns exhibited improved stability and more controlled load–deformation responses, whereas single-curvature columns experienced intensified second-order effects and were more susceptible to instability-driven failure. A comparison between experimental results and analytical predictions based on American code and Canadian standards for fiber-reinforced polymer (FRP)-RC structures demonstrated the overly conservative prediction of axial capacity and flexural stiffness by current design provisions, primarily due to neglecting or underestimating the compressive contribution of GFRP bars. Flexural stiffness derived from experimental moment–curvature relationships exceeded code-based estimates for most specimens. Experimental peak strengths exceeded the corresponding code-based predictions by 20–30%, confirming the structural adequacy of GFRP reinforcement in slender columns within the investigated range of slenderness and eccentricity.