Damage-rehydration synergy in strength evolution of ultra-high performance seawater sea sand concrete under marine tidal zones

Abstract

Ultra-high performance seawater sea sand concrete (UHPSSC) is an advanced cementitious composite material with significant potential for coastal and marine infrastructure applications. Marine tidal zones represent one of the most aggressive environments for concrete structures due to coupled chemical-physical degradation mechanisms. This study aims to elucidate the long-term strength evolution mechanisms of UHPSSC under marine tidal zone exposure. Specifically, the mass loss, mechanical properties, and chemically bound water content of UHPSSC and ultra-high performance concrete (UHPC) are evaluated after exposure to seawater/freshwater freeze-thaw cycles and seawater/freshwater dry-wet cycles. Microstructural characterization was employed to elucidate the underlying strength evolution mechanisms. The results demonstrate that the coupling of damage and rehydration governs the strength evolution of UHPSSC and UHPC under tidal zone conditions. Following 1000 freeze-thaw cycles and 360 dry-wet cycles, the mechanical properties and the chemically bound water content of UHPSSC and UHPC increased slightly, but the flexural strength under seawater dry-wet cycling decreased. In particular, after 360 freshwater-dry-wet cycles, the compressive strength, flexural strength, and chemically bound water content of UHPSSC increased by 18.7%, 14.2%, and 21%, respectively. For UHPC, the corresponding increases were 16.2%, 30.6%, and 20%, respectively. The effect of tidal zone erosion on the mass was negligible, with a maximum of no more than 0.9%. Remarkably, even after 1000 freeze-thaw cycles and 360 dry-wet cycles, the steel fibers within the UHPSSC matrix were uncorroded. Both UHPSSC and UHPC exhibited exceptional erosion resistance in marine tidal environments.