Directional pore structure governs compressive anisotropy in 3D printed concrete

3D printed concrete produced from an identical mixture exhibits distinct compressive strengths under different printing paths. This anisotropy originates from printing-induced pore alignment that alters load transfer. Scalar porosity and qualitative interlayer descriptors lack loading-direction resolution and cannot capture direction-specific strength penalties or in-plane strength re-ranking. To address this gap, a loading-direction-resolved pore directional index is developed to link pore elongation, area weighting, and orientation to direction-specific defect severity. Specimens produced with four representative printing paths and a cast control are investigated using a fixed mixture. Directional compressive strengths along x, y, and z are quantitatively linked to pore structure on x–z and y–z sections. This mechanism is validated using image-informed finite-element models of reconstructed pore-scale representative volume elements subjected to directional compression. The simulated strength ratios agree with experiments, with maximum relative errors of 13.69% (x–z) and 9.42% (y–z), demonstrating that pore structure controls printing path-dependent anisotropy.