Concrete structures in cold marine regions are exposed to chloride attack， freeze-thaw action， and alternating wetting and drying. The combined effects of these processes promote internal damage accumulation and compromise long-term structural safety. Steel fibers can bridge cracks and restrict their propagation， thereby improving concrete resistance to environmental attack. However， the durability and flexural toughness evolution of steel fiber-reinforced concrete （SFRC） under the coupled effects of chloride attack， freeze-thaw cycling， and dry-wet cycling remain insufficiently understood. To address this gap， this study adopted a staged coupling regime reflecting the seasonal conditions of cold marine environments and investigated the macroscopic deterioration and microscopic damage mechanisms of SFRC.
Laboratory accelerated tests were conducted on C40 ordinary concrete （NC） and SFRC containing approximately 2% （volume fraction） corrugated steel fibers. A 3.5% （mass fraction） NaCl solution was used to simulate the chloride environment of seawater. Two exposure regimes were designed： salt freeze-thaw cycling alone and a coupled regime that combined salt freeze-thaw with dry-wet cycling. In the coupled regime， 50 salt freeze-thaw cycles were first applied to simulate cumulative winter freeze-thaw damage， followed by seven dry-wet cycles to represent alternating wetting and drying in tidal or splash zones during the non-freezing season. This sequence constituted one coupled cycle. The mass loss rate， relative dynamic modulus of elasticity， flexural strength， load-displacement response， and flexural toughness were evaluated. Scanning electron microscopy （SEM） and energy-dispersive spectroscopy （EDS） were used to characterize deterioration of the cementitious matrix and changes in the interfacial transition zone between the steel fibers and cementitious matrix.
The results show that coupled salt freeze-thaw and dry-wet cycling markedly accelerate concrete deterioration. After four coupled cycles， the relative dynamic modulus of elasticity of NC falls below 60%， while the flexural strengths of NC and SFRC decrease by 33.3% and 29.5%， respectively. The smaller reduction in SFRC demonstrates that steel fibers enhance concrete durability under the coupled environment. Nevertheless， the flexural toughness of SFRC progressively decreases with cycling. After four coupled cycles， its peak load decreases by 27.6%， and the characteristic double-peak response tends to disappear， indicating a continuing loss of fiber-bridging and energy-dissipation capacity.
Microstructural observations indicate that frost-induced expansion and drying shrinkage jointly promote crack propagation and increase pore connectivity. As a result， Cl- ingress is accelerated. Chloride-bearing reaction products， including Friedel’s salt， accumulate in pores and cracks； their expansion generates additional stresses and induces further microcracking. Chloride-induced corrosion roughens the steel-fiber surface， damages the interfacial transition zone， weakens fiber-matrix bonding and consequently reduces frictional energy dissipation during fiber pullout. These processes create a self-reinforcing sequence of crack propagation， ion diffusion， chemical attack， accumulation of expansive products， and renewed crack growth， which is identified as the primary mechanism driving the rapid degradation of the physical and mechanical properties of concrete. By linking macroscopic performance loss with microstructural evidence at the matrix and interface scales， this study clarifies the coupled deterioration mechanism of SFRC and demonstrates that flexural toughness is particularly sensitive to environmental damage. The findings provide a scientific basis for durability design， in-service condition assessment， and service-life prediction of SFRC structures in cold marine environments.
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