Evaluation of Interfacial Fracture Resistance and Fracture Anisotropy in 3D-Printed Ultra-High-Performance Concrete (2026-04)¶

3D-printed ultra-high-performance concrete (UHPC) offers excellent mechanical properties and printability, yet weak interfaces formed during printing compromise structural integrity by facilitating crack propagation. To enable crack-resistance design in digitally fabricated UHPC structures, this study proposed a closed-form analytical framework for quantitatively evaluating interfacial fracture properties in 3D-printed UHPC. Three-point bending tests were performed on pre-notched 3D-printed UHPC beams with fiber volume fractions of 0%, 0.5%, 1.0%, and 1.5% to investigate the interfacial fracture behavior. By incorporating the meso-structural characteristic parameter (Cch) and discrete coefficient (β), the proposed fracture model effectively characterized the interfacial heterogeneity and discontinuity, and the resulting tensile strength (ft) and fracture toughness (KIC) were confirmed to be both size-insensitive and statistically reliable. The results revealed that the interfacial fracture resistance was primarily governed by the interlocking and bonding between aggregates and the surrounding matrix. When the fiber content remained within 1.0%, the inter-layer (IL) regions exhibited (12.4%–25.7%) greater ft and KIC compared to inter-stripe (IS) interfaces, owing to enhanced densification induced by extrusion pressure and gravitational compaction. However, when the fiber content increased from 1.0% to 1.5%, the fracture parameters for IS and IL interfaces decreased by 2.2% and 14.5%, respectively. Excessive fiber incorporation led to printability issues and interfacial imperfections, thereby diminishing the advantages of interlayer compaction. Furthermore, fracture anisotropy increased with fiber content due to weak interfaces and aligned fibers, necessitating careful consideration in structural design.