Macro-mechanical properties and fine scale damage mechanism of carbon fiber-modified recycled coarse aggregate concrete under sodium sulfate-dry-wet and freeze-thaw cycling coupling effects

To investigate the combined impact of sodium sulfate dry-wet cycles and freeze-thaw cycles on the mechanical properties of carbon fiber modified recycled coarse aggregate concrete (CFRAC), uniaxial compression tests were conducted. The peak stress (σp), peak strain (εp), and compressive stress-strain curve of CFRAC under various sulfate dry-wet-freeze-thaw cycles (SDFCs) were determined. Microscopic testing methods such as acoustic emission (AE), scanning electron microscope (SEM), X-ray energy dispersive spectroscopy (EDS), and nuclear magnetic resonance (NMR) were employed to analyze the microcrack propagation process, micro-morphology, corrosion product composition, and pore structure changes of the specimens. The results indicated that the σp and elastic modulus (E) of RAC specimens decreased, while the εp increased with an increase in SDFCs. The σp of concrete with carbon fiber (CF) was 10.4%, 15.2%, 25.0%, and 29.7% higher than that of recycled coarse aggregate concrete (RAC) under different SDFCs. The study revealed the coupling effect of sulfate, dry-wet cycles, and freeze-thaw cycles, as well as the influence of CF on the meso-damage mechanism of RAC, and explained the phenomenon of the peak signal of AE lagging behind the σp from the perspective of effective stress using a statistical damage constitutive model. Furthermore, the concept of the "effective stress extreme value" theory was introduced, providing insights into the intricate relationship between meso-damage evolution mechanism of RAC under challenging circumstances. This study holds significant importance for the life prediction and durability evaluation of concrete structures in Northwestern China.