Chloride-induced corrosion characteristics of reinforcing steel in steel-polypropylene hybrid fibre reinforced concrete

Chloride-induced corrosion of reinforcing steel is a major durability concern for marine concrete structures. To clarify the corrosion behaviour of reinforcing steel in steel–polypropylene hybrid fibre reinforced concrete (SPHFRC), this article combines experimental investigation with simplified multi-mode modelling, aimed at developing a morphology-informed corrosion model. Accelerated corrosion tests were performed on four SPHFRC specimens with varying fibre volume contents and target mass loss ratios. Corrosion morphology was examined using field-emission scanning electron microscopy and quantitatively analyzed using image-processing techniques. A key finding is that corrosion in SPHFRC is driven by the coupled development of non-uniform corrosion and wide–shallow pitting, with width-to-depth ratios ranging from 1 to 7. When the mass loss ratio is below 5%, the steel surface exhibits only mild non-uniformity and retains a nearly regular cross-sectional contour. Based on the observed pit evolution, a two-stage multi-mode corrosion model integrating pitting and non-uniform corrosion is developed and characterised by a critical relative corrosion depth threshold. Validation against independent experimental data demonstrates that the proposed model achieves improved prediction accuracy for residual steel area, with an average experimental-to-predicted ratio of 0.97. Overall, this work advances the mechanistic understanding of chloride-induced corrosion in SPHFRC and provides a computationally efficient and experimentally grounded framework for durability assessment and service-life prediction of HFRC structures in marine environments, with direct relevance to practical engineering evaluation.