Multi-scale damage evolution and fracture precursors of coral powder-modified UHPC under Brazilian splitting

The sustainable utilisation of waste coral reefs has drawn significant attention in marine infrastructure development. Ultra-high-performance concrete was fabricated with 0%–25% cement replaced by coral powder (CP) at 5% intervals. Damage evolution during Brazilian splitting tests was tracked using acoustic emission (AE) and computed tomography (CT). Key precursors of progressive failure were identified, and the precursor response coefficient and precursor response stress ratio were introduced for comprehensive evaluation. Results showed that the mechanical performance strongly depended on CP content. A 5%–10% CP content shortened the compaction stage, steepened the linear-stage slope and delayed the pre-peak nonlinearity onset, whereas ≥15% CP produced opposite effects. Tensile strength peaked at 5% CP, with a 6.24% increase; the subsequent decline at higher CP contents may result from dilution and weakened cementation. Under the present CT conditions, 5% CP showed the lowest detectable defect level. With increasing CP content, defects increased, with limited meso-pore growth at 10%–15% and pronounced macropore expansion above 15%. CP content modulated the damage evolution pattern. During the unstable fracture stage, the CG5 group exhibited the smallest damage increment and the strongest crack-arrest capacity, whereas ≥15% of content intensified local deterioration. Multi-parameter AE analysis revealed that ρ and S provided relatively early warning signals yet exhibited substantial data dispersion, whereas tensile crack ratio and b value, although slightly delayed, demonstrated superior stability. A multi-indicator strategy was recommended to balance timeliness and robustness in the identification of failure precursors.