Degradation properties and meso-damage mechanism of carbon fiber modified recycled concrete under freeze-thaw cycles

Recycled aggregate concrete (RAC) materials not only realize the resourceful reuse of construction waste but also effectively relieve the pressure of municipal solid waste disposal. This study examined the impact of varying how many of freeze-thaw cycles (N = 0, 25, 50, 75, 100, 125) and the carbon fiber (CF) content (0 % and 0.3 %) on the mechanical properties, microstructure, and damage processes for RAC. The compression analysis of carbon fiber-reinforced recycled aggregate concrete (CFRAC) under different numbers of freeze-thaw cycles was carried out by uniaxial compression test. In addition, two kinds of microscopic tests, scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR), were used to analyze the microscopic mechanism of CFRAC under different conditions. The results showed that the mechanical properties of the specimens decreased continuously with the increase of the number of freeze-thaw cycles, and the peak stress and elastic modulus showed a nearly linear decreasing trend; the freezing resistance of the specimens was significantly improved with the addition of 0.3 % CF. At N = 125 times, the peak stress and elastic modulus increased by 80.3 % and 45.7 %, respectively, compared to the RAC without CF. Combined with the statistical damage constitutive model, the effects of the number of N and CF content on the mesoscopic damage evolution and yield behavior of CFRAC were quantitatively revealed, and the correlation between its macroscopic mechanical properties and the mesoscopic damage mechanisms was established.