Compressive Stress-Strain Behavior of Deteriorated Concrete after Sulfate Attack, Wetting-Drying, and Freezing-Thawing Cycles under Repeated Loading

This study investigates the repeated compressive behavior of concrete subjected to sulfate wet-dry cycles and combined sulfate-wet-dry-freeze-thaw cycles. Stress-strain curves were systematically obtained for deteriorated concrete across varying numbers of cycles, complemented by microstructural characterization using white light interferometry (WLI) and backscattered electron (BSE) imaging. Key findings reveal a dual-phase response during sulfate wet-dry cycles: an initial 8.1% increase in peak stress after 15 cycles due to crystallization-induced compaction, followed by an 18.7% decrease after 30 cycles resulting from crack development given expansive crystallization pressure. In contrast, combined sulfate-wet-dry-freeze-thaw cycles led to progressive strength loss, with a 35.2% reduction in peak stress after 15 wet-dry and 75 freeze-thaw cycles, attributable to synergistic damage from both crystallization and ice formation. Microstructural analysis confirmed that combined cycles produce deterioration exceeding individual effects. Moreover, damage accumulation resulted in increased concavity of unloading curves—indicating accelerated stress decay and reduced strain recovery—along with pronounced reloading curve degradation. Finally, constitutive models integrating these damage mechanisms were developed and validated.