Degradation behavior and damage mechanism of pre-cracked concrete subjected to freeze-thaw cycles

Concrete structures in service often develop cracks due to various factors, significantly compromising their frost resistance in cold regions. This study investigates the degradation behavior and damage mechanisms of pre-cracked concrete subjected to freeze-thaw (F-T) cycles. Rapid F-T tests were conducted on concrete specimens with different crack widths and numbers of cracks, both with and without the incorporation of basalt fibers (BF) and calcium sulfate whiskers (CSW). Macroscopic properties including compressive strength, mass loss, porosity, and crack morphology, along with microstructural characteristics, were analyzed. The results indicate that after 200 F-T cycles, the compressive strength loss rates were 10.3% for crack-free specimens, 17.9% for those with a single 0.5-mm crack, 11.9% for a single 1-mm crack, and 30.6% for two 1-mm cracks, highlighting the significant influence of crack geometry. The combined addition of BF and CSW reduced the strength loss rate by 45.2% compared to the unreinforced cracked group, an improvement attributed to the hybrid mechanisms of fiber-induced crack bridging and whisker-enhanced matrix densification. Microstructural analysis revealed a vicious cycle involving water penetration, frost heave, pore expansion, and crack propagation. The findings offer valuable insights into enhancing the durability of cracked concrete structures in cold environments. • Freeze-thaw damage in cracked concrete accelerates with increasing cycles, characterized by C-S-H gel disintegration, CH crystal fracture, and progressive pore structure deterioration. • Wider and more numerous cracks lead to lower initial strength and more severe freeze-thaw damage, underscoring the critical importance of controlling crack geometry. • Basalt fibers form a three-dimensional network that inhibits crack propagation, while calcium sulfate whiskers refine pores and strengthen interfacial bonding, collectively enhancing frost resistance. • The damage mechanism follows a self-reinforcing chain in which cracks channel water and accelerate frost heave stress, thereby aggravating microstructural and macro-property degradation.