Compressive behavior of square glass-fiber-reinforced polymer geopolymer concrete columns exposed to elevated temperatures

This study presents an experimental investigation into the compressive performance of square glass fiber-reinforced polymer (GFRP) pultruded profiles filled with geopolymer concrete (GPC) under elevated temperatures. Ninety concrete-filled GFRP (CFGP) specimens were prepared using GPC mixes of three different compressive strengths (59.85 MPa, 68.31 MPa, and 89.57 MPa), achieved by varying NaOH molarity (4M, 8M, 12M). The specimens were exposed to temperatures ranging from 25°C to 350°C to evaluate post-heating compressive behavior. Results showed that confinement by the GFRP profile improved performance by restraining lateral dilation of the concrete core, thereby enhancing effective compressive strength and stiffness, particularly in low-strength cores where larger lateral expansion activated confinement earlier. However, experiencing elevated temperatures degraded the resin matrix and fiber–resin interface, reducing hoop strength and stress transfer, while microcracking and vapour pressure effects in the core further contributed to capacity loss. At ambient conditions, high-strength cores displayed the greatest peak loads, but they suffered the steepest degradation under heating, retaining only 74.37% of their original strength at 350°C, compared to 82.5% retention in low-strength cores. Two-way ANOVA confirmed that temperature and concrete strength contributed 58.85% and 27.90% of the variation in compressive strength retention, respectively, while their interaction accounted for an additional 8.05%, underscoring that thermal degradation is strongly dependent on concrete grade. These findings demonstrate that while higher-strength cores improve initial capacity, they are more vulnerable to thermal deterioration, and that optimal design of CFGPs must consider both material pairing and exposure conditions.