Effect of steel-polypropylene hybrid fiber on the flexural behavior of RC beams under cryogenic freeze-thaw cycles and repeated loading

Reinforced concrete (RC) structures in cold regions face challenges from cryogenic freeze-thaw cycles and mechanical stresses, which render conventional concrete materials inadequate. This study investigates the flexural behavior and ductility of RC beams modified with steelpolypropylene hybrid fibers (HF) exposed to cryogenic freeze-thaw cycles and repeated loading. The results indicate that the inclusion of hybrid fibers improves the cracking load, ultimate load, stiffness, and residual deformation of RC beams after cryogenic freeze-thaw cycles. After three freeze-thaw cycles (from ambient to - 80 degrees C), the cracking load and peak load of specimens with 1% steel fiber (SF) and 0.1% polypropylene fiber (PF) increased by 39.8% and 8.4%, respectively, compared to plain RC beams. Although hybrid fibers improved the overall performance of the beams, the addition of steel fibers reduced the displacement ductility coefficient by limiting the ultimate deformation capacity. Therefore, based on the curvature ductility coefficient and effective moment of inertia, methods were proposed to calculate the ductility coefficient and deflection of HFRC beams after cryogenic freeze-thaw cycles, evaluating the influence of various factors on their bending deformation capacity.