Postheating Compressive Behavior of Carbon Fiber–Wrapped, Geopolymer Concrete–Filled GFRP Composite Tubes after Elevated-Temperature Exposure

Glass fiber polymer–reinforced (GFRP) composite profiles offer advantages such as corrosion resistance and a favorable strength-to-weight ratio, but their limited ductility and poor fire resistance hinder broader structural use. This study examines the thermal and compressive behavior of carbon fiber–reinforced polymer (CFRP)–confined, geopolymer concrete–filled pultruded GFRP square tubes under elevated temperatures. A total of 90 specimens were prepared using geopolymer concrete with three different compressive strengths (average strengths of 59.8, 68.3, and 89.6 MPa), controlled by sodium hydroxide molarity: 4 M (geopolymer concrete with 4 M sodium hydroxide, denoted as GC4), 8 M (GC8), and 12 M (GC12). Specimens were externally wrapped with CFRP wraps and exposed to temperatures ranging from 25°C to 350°C. Results show that CFRP confinement significantly enhanced compressive capacity, particularly in lower-strength cores, with average strength gains of 87%, 63%, and 27% for GC4, GC8, and GC12 specimens, respectively. Interestingly, elevated temperatures improved strength further, with peak load increases of up to 25% at 350°C. Average ductility indices decreased with increasing concrete strength, ranging from 1.42 (GC4) to 1.30 (GC12). One-way ANOVA revealed that temperature accounted for 82%, 79%, and 63% of variance in compressive capacity for GC4, GC8, and GC12 groups, respectively. These findings highlight the effectiveness of CFRP confinement and the potential of sustainable geopolymer-filled GFRP systems in fire-prone structural applications.