Mechanism Analysis of Freeze–Thaw Cycles and Chloride Erosion on Damage to Recycled Aggregate Concrete

This study analyzed the impact of freeze–thaw cycles on a 3% NaCl solution and water in recycled concrete, utilizing a rapid freezing method and a water–cement ratio of 0.45. The weight, relative dynamic modulus of elasticity, compressive strength loss, and changes in compressive strength of damaged concrete samples containing recycled aggregates were analyzed with substitution ratios of 0%, 25%, 50%, and 100% for recycled aggregates. Scanning electron microscopy was employed to investigate the composite damage mechanisms of recycled concrete subjected to chloride and freeze–thaw cycles. Test results revealed that the relative dynamic elastic modulus of recycled concrete experiences three distinct stages: a slow decrease, a rapid decrease, and an accelerated decrease. Compressive strength gradually declines, with loss accelerating as the number of freeze–thaw cycles increases. The impairment of compressive strength correlates with the expansion of the damage layer, which heightens the overall degree of damage. When exposed to a 3% NaCl solution, the degradation observed during the initial freeze–thaw cycle is less severe than that in water. However, after 200 cycles, degradation in the saline solution becomes more pronounced. As the cycle count increases and interactions with chloride ions occur, Friedel salts and gypsum form, contributing to material degradation. The Weibull model effectively characterizes damage changes, aiding in the estimation of service life. To comply with the durability criteria specified in Chinese standards, it is recommended that the content of recycled aggregates remains below 30%.