Behavior of GFRP-Reinforced Concrete Members under Combined Bending Moment and Low Axial Load

Abstract

This study examined nine concrete members reinforced with glass fiber–reinforced polymer (GFRP) bars, exploring the impact of combined bending moment and low axial loads. Three reinforcement ratios (1.7%, 2.5%, and 3.3%) were considered under various axial loads (0, 125, and 250 kN). In the absence of a standardized test method for determining the compressive properties of rebars, a simple approach was adopted. GFRP bars demonstrated a compressive modulus of elasticity that was roughly equivalent to their tensile modulus, along with a compressive strength reaching approximately 70% of their tensile strength. The main tests on members showed that increasing the reinforcement ratio to 3.3% resulted in a 4% reduction in bending resistance with a 2% axial load, while a ratio of 2.5% led to a 7% decrease; conversely, a lower ratio of 1.7% showed a 2% increase in bending resistance under the same load. An analytical model incorporating GFRP bar compression contributions was developed for cross-sectional analysis. It was verified against experimental and literature data, to conduct parametric studies on the impact of reinforcement ratio, concrete strength, GFRP modulus, and strength on the interaction diagram shape under low axial loads. The results demonstrated that there are two major cases of interaction diagram slope in the proximity of the pure moment axis. At higher reinforcement ratios, the moment resistance diminishes at low axial loads when compared with pure moment conditions. However, more research is needed to verify the repeatability of the test results and draw conclusive empirical evidence.