Study on the mechanical properties and mesoscopic damage mechanism of recycled aggregate concrete under different dynamic strain rates after freeze-thaw cycles

In order to study the comprehensive influence of freeze-thaw cycles and dynamic strain rates on the mechanical properties of recycled aggregate concrete (RAC), uniaxial compression tests on RAC were carried out. The complete stress-strain curves under different numbers of freeze-thaw cycles (N = 0, 50, 100) and different strain rates ($\dot{\epsilon }$ = 10⁻⁵/s, 10⁻⁴/s, 10⁻³/s, 10⁻²/s) were obtained. Methods such as scanning electron microscopy (SEM) and acoustic emission (AE) were adopted to analyze the microstructure of the specimens and the development process of microcracks. The results showed that the specimens had poor frost resistance and significant strain rate sensitivity after undergoing freeze-thaw cycles, the strength and elastic modulus of the specimens decreased with the increase of the number of freeze-thaw cycles (N), and the strength and elastic modulus increased with the increase of the strain rate ($\dot{\epsilon }$) under the same N. Combined with the statistical damage constitutive model, through the analysis of five parameters (E₀, ${\epsilon }_a$, ${\epsilon }_h$, ${\epsilon }_b$, and H), the influence laws of freeze-thaw cycles and strain rates on the meso-damage evolution mechanism of RAC were revealed.