X-ray micro-CT quantification of 3D steel fiber distributions in carbonated high-toughness recycled cementitious composites

This study employed X-ray micro-CT to quantitatively characterize the three-dimensional distribution of steel fibers in carbonated high-toughness recycled cementitious composites (HTRCC). Two distribution-effect coefficients— η ₁ (fiber orientation) and η ₂ (azimuth uniformity)—were established to evaluate the spatial uniformity of fiber dispersion and were further correlated with multiple mechanical indices. The results indicate that improved fiber distribution uniformity significantly enhances macro-scale mechanical performance: peak stress, peak strain, and elastic modulus increased substantially, while the post-peak modulus decreased to approximately 0.35, reflecting a delayed softening behavior. Regression analysis revealed reliable functional relationships between η ₁ , η ₂ , and the mechanical indices, demonstrating the predictive capability of the proposed distribution coefficients. Moreover, carbonation contributed to performance enhancement through CaCO₃ precipitation and densification of the Interfacial Transition Zone (ITZ), further synergizing with uniform fiber distribution. Based on these findings, this study develops a systematic framework linking 3D fiber distribution, carbonation-induced densification, and multi-index mechanical performance, providing a model-driven foundation for the prediction and design of recycled cementitious composites. • Quantified 3D steel fiber distributions in carbonated composites via micro-CT. • Proposed η₁–η₂ coefficients to predict strength, stiffness, and toughness indices. • Revealed carbonation–fiber synergy densifying ITZ and improving ductility.