Fracture Mechanisms of 3D-Printed Ultra-High-Performance Concrete (2026-03)¶

3D-printed ultra-high-performance concrete (3DP-UHPC) is promising for structural engineering, but its flexural anisotropy remains poorly understood due to the lack of integrated analysis of pores, fibres, and interface. These are key factors often studied in isolation in existing literature. To address this gap, this study innovatively establishes a multiscale, synergistic framework that combines X-ray computed tomography, digital image correlation, and four-point bending tests to decode the fracture mechanisms of 3DP-UHPC. Unlike prior works that only report fibre orientation or pore characteristics separately, this study firstly quantifies three distinct pore types: structural voids, rheological-defect pores, and entrapped-air pores, while identifying structural voids as primary crack initiation sites. Secondly, the study reveals the coupled influence of fibre alignment (69.6% within 15° of printing direction) and content on crack propagation and failure modes. Fibres parallel to tensile stress induce tortuous, energy-dissipating crack paths while misaligned ones cause linear propagation, with 0.5 vol% fibres enhancing toughness and ≥ 1.0 vol% activating the “pull-out dissipation–crack restraint” mechanism to suppress brittle failure. Finally, the results indicate that printing–loading orientation plays a decisive role. The P-Y-Z and P-Z-Y configurations achieve up to 140% improvement in flexural strength at fibre volume fractions ≤ 2.0%, indicating these as preferable and print-feasible strategies. These findings provide quantitative guidance for optimising fibre content, alignment, and printing orientation to enhance the flexural performance of 3DP-UHPC.