Based on X-CT and DEM Simulation (2026-03)¶

Based on a 3D concrete printing system, this study investigates the anisotropic mechanical properties and permeability of concrete specimens printed at different speeds, considering the influence of in situ stress and groundwater pressure in geological environments on 3D printed concrete shaft linings. X-ray computed tomography (X-CT) was employed to perform three-dimensional reconstruction of specimens extracted in different directions, enabling a systematic analysis of pore distribution and its influence on permeability. Furthermore, discrete element method (DEM) numerical simulations were conducted to explore crack propagation mechanisms and force chain distribution characteristics under loading in different directions. The results indicate that printing speed and sampling direction significantly affect density: density decreases with increasing printing speed, accompanied by a reduction in P-wave velocity, suggesting a higher porosity at higher speeds. The wave velocity and permeability coefficient were highest in the Z-direction (parallel to the printing path), where the concrete exhibits a continuous strip-like morphology with improved integrity. Compressive strength was relatively higher in the Z- and X-directions (perpendicular to the lap gap) and lower in the Y-direction (perpendicular to the interlayer gap), while tensile strength followed the trend X > Y≈Z. Coefficient of variation (COV) analysis further indicates that printing speed significantly affects performance stability: the COV of density and wave velocity decreases with increasing speed, while the permeability coefficient remains high (0.3976); the compressive strength COV drops from 0.30 to 0.12, and the tensile strength COV peaks at 1.6 m/min (0.26), providing a quantitative basis for regulating performance uniformity through process parameters. X-CT reconstruction revealed horizontally distributed pores in the X-direction, mutually perpendicular pores in the Y-direction, and interconnected vertical pores in the Z-direction, accounting for the highest permeability in the latter. DEM simulations indicated that the bonding strength of both interlayer and lap gaps is significantly influenced by layer height and time intervals. When the gap direction is perpendicular to the loading direction, the effective load-bearing area is larger, resulting in higher overall strength; conversely, when gaps are parallel to the loading direction, stress concentration initiates earlier failure.