Effect of geopolymer concrete cover on improving tensile and transverse shear behaviors of BFRP bars after exposure to high temperature

The integration of high-temperature resistant geopolymer concrete, reinforced with basalt fiber reinforced polymer (BFRP), addresses several challenges encountered by conventional reinforced concrete structures, such as elevated energy consumption, substantial carbon emissions, and susceptibility to steel corrosion. To investigate the impact of geopolymer concrete cover on the mechanical performance of BFRP bars after high temperature treatment, this study conducted mechanical tests on a total 406 tensile specimens and 406 transverse shear specimens, spanning a temperature range from ambient temperature to 700 °C. The BFRP bars with diameters of 6 mm, 12 mm, and 16 mm underwent heat treatment durations of 0 h, 1 h, 2 h, and 3 h. Subsequently, they were examined for mass loss rat, residual tensile properties, and residual shear strength following natural cooling. Notably, this study also compared the impact of different geopolymer concrete cover thickness (30 mm, 40 mm, and 50 mm) on these performance parameters. The test results revealed that with an increase in heating temperature, the resin of BFRP experienced decomposition, leading to alterations in surface morphology, as well as changes in tensile and shear failure modes. In the temperature range of 400 °C to 700 °C, bare BFRP bars exhibited a significant decrease in both tensile and transverse shear strengths, with strength loss rates ranging from 50% to 90%. Moreover, the degradation in shear strength was comparatively slower than that in tensile strength. The presence of geopolymer concrete cover effectively mitigated the strength degradation of BFRP bars following high temperature treatment, and this protective effect improved with an increase in the concrete cover thickness. Additionally, scanning electron microscope (SEM) analysis was employed to elucidate the damage mechanisms for both bare and geopolymer concrete-covered BFRP bars. SEM results indicated that the deterioration induced by high temperature treatment gradually propagated radially from the edge to the core area of the bar. Finally, a simplified calculation model was adopted to evaluate the residual tensile strengths of BFRP bars wrapped in geopolymer concrete after high temperature treatment.