Anisotropic Fracture Behavior of PAN Fiber Reinforced 3D Printed Concrete (2026-04)¶

3D printed concrete (3DPC) offers high construction efficiency and geometric flexibility, yet its brittle fracture behavior and pronounced mechanical anisotropy induced by layer-wise deposition limit structural applications. This study investigates the anisotropic fracture behavior and toughening mechanisms of polyacrylonitrile (PAN) fiber-reinforced 3DPC with fiber volume fractions ranging from 0% to 0.3%. Three-point bending tests on pre-notched beams were performed under X, Y, and Z loading directions, combined with digital image correlation (DIC) and double-K fracture analysis to quantify crack evolution and energy dissipation. The results indicate that the incorporation of PAN fibers improves the fracture resistance of 3DPC, facilitating the transition from brittle to quasi-ductile behavior. At a fiber content of 0.3%, compared to fiber-free specimens, the peak load and unstable fracture toughness under Y-direction loading increased by 35.6% and 111.7%, respectively, while the fracture energy increased by 52.9%. Fiber bridging was observed to prolong the stable crack propagation stage and expand the fracture process zone, particularly when the crack plane was orthogonal to the fiber alignment direction. However, the introduction of fibers further amplified the directional dependence, with fracture performance consistently following the trend of Y > Z > X. As the fiber content increased from 0.1% to 0.3%, the anisotropy index of unstable fracture toughness increased by approximately 15%. Scanning Electron Microscope observations of specimens with 0.3% fiber content suggested that extrusion-induced fiber alignment played a role in the directional selective toughening behavior. These findings, based on combined fracture mechanics and microstructural analyses, provide insights into the stage-dependent and anisotropic toughening mechanisms of fiber-reinforced 3DPC, contributing to the knowledge base for designing high-toughness 3DPC structures.