Research on the freeze resistance and mesoscopic damage mechanism of modified recycled concrete

In response to the national strategic goals of "carbon peak and carbon neutrality," this experiment utilized recycled coarse aggregate (RCA) as the aggregate and metakaolin (MK) as a mineral admixture to replace a portion of cement. Compression analysis was carried out on recycled aggregate concrete (RAC) under various numbers of freeze-thaw cycles. Additionally, the mechanism of freeze-thaw deterioration and the effect of MK on the microstructure of RAC were investigated by electron scanning (SEM) and other microscopic instruments. Finally, a constitutive relationship for RAC subjected to freeze-thaw cycles was formulated utilizing the principles of statistical damage model. The results indicated that the physical expansion due to freeze-thaw cycles leads to continuous coarsening of RAC pores, leading to an ongoing reduction in its strength and elastic modulus, along with an escalation in the peak strain. The secondary hydration reaction and filling effect of MK significantly mitigated the effect of freeze-thaw cycles, enhancing RAC freeze resistance and strength. Taking into account the reinforcing effect of optimizing and adjusting the force-bearing skeleton at the mesoscale, the complete process of deformation and rupture was understood through the lens of effective stress, shedding light on the mesoscale reasons for the delay of the acoustic emission peak stage relative to the peak of the stress-strain curve, and distinguishing between the peak state and the critical state. The evolutionary laws between freeze-thaw damage parameters were quantitatively analyzed, verifying the rationality of the statistical damage model for use in freeze-thaw environments.