Cross-Scale Mechanisms of Anisotropy in 3D-Printed Ultra-High-Performance Concrete (2025-11)¶

Integrating ultra-high-performance concrete (UHPC) into 3D printing (3DP-UHPC) offers a rebar-free reinforcement solution, yet the effect of fibre fraction on micro/meso-structural and mechanical properties remains poorly understood. This study bridges this gap through a multi-scale framework combining DIC, X-ray CT, and in-situ micro-loading to elucidate the interrelations among processing, structure, and performance. Results show that fibre volume fraction significantly affects fracture anisotropy through its impact on interfacial quality, pore morphology, and fibre alignment. Elongated pores are mainly distributed along interlayer and interstrip interfaces, but extrusion-induced “de-airing” enhances matrix densification, reducing porosity below that of cast UHPC. Steel fibres align along the print path, constrained by layer height, with dominant orientations between 60-90°. A novel compression-interlock strip-stacking strategy further improves interlayer compactness by eliminating macro-pores. These improvements lead to superior compressive strength compared to prior reports, attributed to enhanced fibre orientation and matrix density. The study provides a quantitative basis for understanding mechanical anisotropy in 3DP-UHPC and proposes design strategies to optimise fibre distribution, pore architecture, and layer integration, delivering valuable cross-scale insights for tailored material design and enhanced printing control.