Anisotropic Behavior of 3D-Printed Concrete: Interlayer Bonding, Pore Architecture, Reinforcement Limitations, and Durability Mechanisms.

The structural use of three-dimensional concrete printing remains limited by the directional weakness introduced during extrusion and layer-by-layer deposition. Although 3DPC offers major advantages in formwork elimination, architectural freedom, and automated construction, its printed architecture produces interfaces, pore networks, and reinforcement discontinuities that do not exist in the same form in conventionally cast concrete. This review examines the anisotropic behavior of 3DPC by linking its architectural arrangement, physical interlayer mechanisms, and chemical durability-related processes. The analysis shows that anisotropy develops from the combined effects of filament orientation, interlayer bonding quality, pore morphology, cold-joint formation, mechanical interlocking, hydration continuity, and reinforcement limitations. Weak interlayer regions act not only as preferred paths for crack initiation and propagation under tensile, flexural, shear, and compressive loading, but also as transport channels that accelerate water absorption, chloride ingress, carbonation, sulfate attack, and freeze-thaw deterioration. The review further highlights that fiber, textile, FRP, and discrete reinforcement strategies can reduce some consequences of anisotropy, but their effectiveness depends on whether they bridge the weaker interlayer regions rather than merely reinforcing the filament direction. SEM-based observations confirm that microstructural discontinuities, fiber-matrix debonding, irregular hydration products, and connected pores provide the material-level basis for the directional response of printed concrete. Overall, anisotropy should be treated as a design-critical feature of 3DPC rather than as a secondary defect. Reliable structural application requires coordinated control of mixture rheology, deposition parameters, interlayer timing, curing, toolpath design, and reinforcement layout.