Salt frost damage evolution and transport properties of recycled aggregate concrete under sustained compressive...

The frost damage behavior of recycled aggregates concrete (RAC) in a cold region is inherently more complex due to the incorporation of recycled coarse aggregate (RCA). In real-world service environments, the combined effects of mechanical loading and environmental conditions further make RAC's damage mechanism more intricate. This study explores the impact of uniaxial compressive loading (at 0.1f_c, 0.3f_c, and 0.5f_c, respectively), freeze-thaw cycles, and chloride penetration on the relative dynamic elastic modulus (RDEM), mass transport properties, and microstructure of RAC with varying RCA replacement ratios. The results indicate that specimens loaded at 0.3f_c exhibit enhanced frost resistance, with reduced water absorption and chloride ion content. Additionally, a damage model is developed to quantify the effects of mechanical loading, freeze-thaw cycles, and chloride penetration on RDEM degradation. The investigation using X-ray computed tomography (X-CT), mercury intrusion porosimetry (MIP), and scanning electron microscopy (SEM) techniques reveals that as compressive stress levels increase, the microstructural density and porosity of RAC initially decrease before increasing. Moreover, the RDEM of RAC decreases with decreasing pore sphericity. Compared to the R100-S55 samples, the pore sphericity of R100-S55-0.5f_c samples increased by 60.4 % in the range of 0.4-0.5, resulting in a decrease of approximately 17.72 % in the RDEM. Furthermore, the initial sorptivity of frost-damaged RAC exhibits a significant linear relationship with porosity. Overall, this study elucidates the evolving trends of mass transport properties and microstructure in RAC under loading and freeze-thaw conditions, laying a theoretical groundwork for the widespread application of RCA.