Study on the bonding properties of hybrid fiber ordinary concrete and recycled concrete with BFRP bars under freeze–thaw cycles

The promotion of recycled concrete is a sustainable development trend in the construction industry, but its durability issues, especially in cold regions, may become an obstacle. Studying the difference between the effects of freeze–thaw (FT) on the bonding behavior of recycled concrete and ordinary concrete will help evaluate its reliability in harsh environments. This study used central pullout tests to investigate the bonding behavior of BFRP bars in hybrid fiber normal concrete (HFNC) and hybrid fiber recycled aggregate concrete (HFRC) under FT conditions. The specimens mixed with 0% and 50% recycled aggregate were subjected to 0, 25, 50, 75, and 100 FT cycles, respectively. The results showed that FT cycles significantly reduced the bond strength of both concretes, with the reduction being more pronounced in HFRC. Specifically, the ultimate bond strength of HFNC without FT cycles was 24.3 MPa, while that of HFRC was 23.3 MPa. After 100 FT cycles, the bond strengths of HFNC and HFRC decreased by 36.21% and 45.49%, respectively. In addition, the peak slip value first increased and then decreased with the increase in the number of FT cycles, and the peak slip value of HFRC was 1.12–1.15 times that of HFNC. Finally, this study successfully established a bond–slip constituent model under FT conditions, which provides a valuable basis for understanding and predicting the durability and performance degradation of concrete structures under FT environments.