Frost Resistance and Damage Mechanism of Recycled Aggregate Concrete

This study systematically evaluates the influence of the recycled coarse aggregate (RCA) replacement rate and the number of freeze–thaw cycles (FTCs) on the frost damage of recycled aggregate concrete (RAC) through rapid freeze–thaw tests, and delves into the underlying damage mechanisms. The findings demonstrate that the incorporation of recycled aggregates deteriorates the frost resistance of concrete to a certain extent, primarily manifested by increased apparent damage with rising FTC numbers and RCA content. Specimens with an RCA replacement rate exceeding 50% exhibited extensive mortar spalling and aggregate exposure after 50 FTCs. The mass loss rate initially decreased in the early freezing-thawing stage, then began to increase after approximately 20 cycles, reaching a maximum of 5.09%. The relative dynamic elasticity modulus (RDEM) decreased with an increase in both FTCs and RCA content, dropping to a minimum of 71.99%. Furthermore, based on the relative dynamic elastic modulus, this study developed a GM(1, 1) freeze–thaw damage prediction model applicable to a full replacement range of 0%–100% RCA with a precision level of Grade I. Microstructural analysis revealed that microcracks and pores within the interfacial transition zones (ITZs) and the surrounding matrix of both NCA and RCA are critical for the initiation and propagation of freeze–thaw damage, thereby elucidating the damage mechanism in RAC.