Rate-and direction-dependent compression and tensile behavior of 3D-printed PP-fiber concrete

This study investigates the anisotropic quasi-static and dynamic behavior of a 3D-printed polypropylene (PP) fiber-reinforced concrete (3DPC) produced by extrusion-based gantry printing. Cylindrical and disc specimens were tested in compression and splitting tension under quasi-static loading and high strain rates using a Split Hopkinson Pressure Bar (SHPB) apparatus. Cast reference specimens were compared with 3D-printed specimens oriented along the three principal material directions, namely along the filaments within a layer (X), transverse to the filaments within a layer (Y), and through the build direction across layers (Z), and more-interface (MI) and less-interface (LI) configurations were introduced so that the critical stress or crack path intersected predominantly filament cores or inter-layer/inter-filament regions. Digital image correlation (DIC) and scanning electron microscopy (SEM) were employed to elucidate damage evolution and failure micromechanics. Quasi-static compressive strengths of 3DPC in X and Z directions were comparable to the cast reference ( f c ≈ 17 MPa ), while Y-loaded specimens showed a modest reduction. In splitting tension, LI configurations in all directions reached strengths close to the cast discs ( f t ≈ 1.7 –1.9 MPa), whereas MI specimens — especially Z-MI (interlayer tension) — dropped to about 45% of the cast reference. Under SHPB loading, both cast and printed concretes exhibited pronounced rate strengthening, with dynamic increase factors up to DIF c ≈ 3.5 in compression at ɛ ̇ ≈ 1.7 × 1 0 2 – 3.0 × 1 0 2 s − 1 and DIF t ≈ 4.0 in splitting at ɛ ̇ ≈ 3.5 – 6.5 s − 1 . At a given rate, MI configurations remained systematically weaker than LI by roughly 25% in the Y-direction and 45% in the Z-direction. Directional strength-reduction factors (SRFs) and dynamic increase factors (DIFs) were embedded in a concrete damaged-plasticity (CDP) model with weakened SRF strips representing inter-layer/inter-filament bands. Finite-element simulations reproduced the measured compressive and splitting-tensile strengths of all configurations within about 10% and captured the observed anisotropic cracking patterns, providing a compact framework for design-oriented analyses of 3D-printed fiber concrete under static and impact loading.