Flexural behaviour of concrete-filled pultruded GFRP box beams after exposure to elevated temperatures

Concrete-filled pultruded glass fibre-reinforced polymer (GFRP) box beams are innovative structural members with superior corrosion resistance and flexural capacity. However, GFRP profiles are vulnerable to thermal degradation at elevated temperatures, potentially compromising structural integrity. This study experimentally investigates the flexural behaviour of concrete-filled pultruded GFRP box beams after exposure to elevated temperatures to evaluate the fire resistance effects of the infilled concrete. Residual material properties of GFRP profiles were first studied by 88 groups of coupons subjected to temperatures from 50 to 350 °C (25 °C increments) for 15–60 min (15 min increments). A logistic-function-based theoretical model was then proposed to predict residual tensile and compressive strength. Subsequently, four-point bending tests were performed on hollow and concrete-filled GFRP box beams after exposure to temperatures of 250, 300 and 350 °C for various durations. The experimental results revealed that the flexural behaviour of the hollow GFRP box beams deteriorated significantly after being exposed to temperatures exceeding T g , with the flexural capacity decreasing by 65% at 350 °C after 60 min exposure. In contrast, the infilled concrete remarkably enhances the fire resistance of the GFRP box beams due to the isolation effect from direct air exposure, resulting in only minor reductions in flexural capacity and stiffness after heating. Finally, a residual flexural capacity prediction model of the beams was proposed and showed good agreement with experimental results. • A comprehensive experimental program examined GFRP material degradation at 50–350 °C. • A logistic-model-based approach was proposed to predict residual tensile and compressive strengths. • Ten hollow and concrete-filled GFRP box beams were tested in four-point bending after heating. • Concrete infill provided strong thermal protection, retaining flexural capacity even at 350 °C. • A practical flexural capacity prediction model was developed for fire-damaged GFRP box beams