Axial compressive behavior and strengthening of BFRP-reinforced concrete circular columns after elevated temperature exposure

Replacing steel reinforcement in concrete columns with basalt fiber-reinforced polymer (BFRP) bars can prevent durability damage caused by steel corrosion, thereby significantly extending the service life of concrete structures. The potential fire-related performance of BFRP-RC columns remains to be clarified because the resin matrix of BFRP bars may degrade significantly at elevated temperatures. This research investigates the mechanical properties of BFRP reinforcement within concrete and BFRP-reinforced concrete cylinders after high-temperature exposure. It further evaluates the effectiveness of CFRP wrapping as an external strengthening technique for BFRP columns. Results showed that the degradation of BFRP reinforcement in concrete was slightly slower than that under exposed conditions, with rapid deterioration occurring between 250°C and 500°C. High-temperature exposure mainly reduced the tensile and compressive strengths of BFRP bars, while the elastic modulus remained relatively stable. For unstrengthened specimens, increasing damage temperature significantly reduced mechanical performance, with ultimate load capacity decreasing by more than 40%, stiffness reducing by 19–89%, and strain ductility continuously declining. The confinement effect provided by stirrups became negligible when the damage temperature exceeded 500°C. CFRP wrapping effectively restored the load-bearing capacity of fire-damaged columns, and two-layer CFRP strengthening increased the ultimate load capacity to 172.95%–292.51% of the corresponding unheated specimens, although stiffness recovery remained limited. Furthermore, simplified prediction models considering thermal damage and CFRP confinement were proposed and demonstrated good predictive accuracy.