Experimental study on the initial damage and mechanical property evolution of recycled coarse aggregates under freeze–thaw cycles and repeated loads

The natural aggregate fraction in recycled coarse aggregate (RCA) can yield initial damage during service and repeated crushing, which might affect the performance of recycled aggregate concrete (RAC). Therefore, in this study, freeze–thaw cycles and repeated loads of limestone were used to simulate the freeze–thaw cycles and load-induced damage of natural aggregates in RCA during service and repeated crushing. The properties of natural aggregates with initial damage were investigated to explore the mechanism of initial damage evolution and property degradation for natural aggregates in RCA. The results revealed that the compressive strength and tensile strength of RCA both decreased with the increasing number of freeze–thaw cycles or repeated loads. Furthermore, the interior damage for limestone under freeze–thaw cyclic loading was dominated by frost cracking, and the exterior damage was dominated by macroscopic cracking. However, when under repeated loads, the internal damage of limestone was dominated by macropores and fine pores, and the external damage pattern was dominated by interlayer cracks and intergranular tensile cracks. Additionally, both tensile strengths and elastic moduli decreased linearly with the increasing number of freeze–thaw cycles, while the loss of compressive strength was quadratically related to the number of freeze–thaw cycles.