Dry–Wet Cycle Fracture Performance of Recycled Concrete Sulfate Based on Computerized Tomographic Images

To investigate the damage degradation of recycled concrete under mesoscale morphology and the critical expansion force of concrete cracking following sulfate wet–dry cycles, an experimental sulfate wet–dry cycle was designed. In situ scanning of recycled concrete was conducted using X-ray computed tomography (CT). Analysis of the CT images revealed the relationship between the gray scale changes and the sulfate salt wet–dry cycle, along with pore alterations and crack propagation in recycled concrete. A CT image analysis method based on grayscale inversion for crack propagation was developed. By integrating sulfate attack with fracture mechanics, this study explored the phenomenon of pore expansion in recycled concrete subjected to dry–wet cycling tests. The concrete fracture criterion provided the basis for determining the critical expansion force of recycled concrete after the wet–dry cycles. Results indicated that as the duration of sulfate wet–dry cycles increased, the gray scale first increased and then decreased. On the 40th day of the cycling test, the average grayscale value increased by 10.4%. The number of pores in recycled concrete continuously decreased, pore size diminished, and cracks appeared on the specimen’s weak surface. The use of gray scale changes to reveal the degradation of recycled concrete after sulfate wet–dry cycles proved to be both feasible and effective. As the length of the interface crack increased, the stress intensity factor at the crack tip also increased, while the critical expansion force decreased. Additionally, as the pore diameter increased, the stress intensity factor at the crack tip rose. The critical expansion force of a symmetric crack at the edge of a pore was 53 times greater than that of a single crack.