Rate-dependent damage–constitutive coupling and CT-quantified fibre–matrix interaction in micro-fibre-reinforced recycled cementitious composites

The rate-dependent damage and constitutive behaviour of micro-fibre-reinforced recycled cementitious composites (MF-RCC) were systematically investigated under high strain rates (40–380 s−1) using Split Hopkinson Pressure Bar (SHPB) tests combined with in-situ X-ray computed tomography (CT). A secant-stiffness-based damage variable was defined, and a dynamic damage – constitutive model incorporating a Dynamic Fibre Influence Factor (DFIF) was developed to characterise the coupled effect of strain rate and fibre content. The model reproduced the nonlinear responses across pre- and post-peak stages with high accuracy. CT quantification revealed that increasing fibre content effectively reduced pore volume fraction and crack connectivity, confirming the fibre-induced suppression of damage evolution. The results show that 2.0% fibre content at 377.6 s−1 enhanced peak stress and ultimate strain by 11.0% and 65.0%, respectively, whereas excessive fibres (2.5%) led to mild deterioration due to non-uniform dispersion. The integrated testing–modelling-CT framework provides a mechanism-based approach to predict the dynamic response and microstructural damage of MF-RCC, offering insights for fracture-resistant design of sustainable cementitious materials.