Quantitative evaluation for fracture properties of 3D printed ultra-high-performance concrete loaded in different directions

The integration of ultra-high-performance concrete (UHPC) with 3D printing technology introduces the revolutionary potential, offering exceptional mechanical properties, enhanced design flexibility, and automated construction processes. However, without additional reinforcement, the fracture performance of 3D-printed UHPC (3DP-UHPC) becomes critical to the crack resistance structures. To address the gaps in existing research, this study developed a closed-form fracture model to evaluate its fracture properties in varying loading conditions and copper-plated straight steel fiber dosages. The fracture mechanisms of 3DP-UHPC under different loading directions were systematically analyzed using fracture tests on 155 beams. By introducing the meso-structural characteristic parameter (C_ch) and discrete coefficients indicating the heterogeneity and discontinuity of 3DP-UHPC, the fracture model was developed allowing for determining size-independent tensile strength (f_t) and fracture toughness (K_IC). The results revealed that C_ch proved to be the average aggregate size for specimens loaded aligned with the printing direction and the average fiber spacing in other loading directions. The fracture properties of 3DP-UHPC exhibited pronounced directional dependency, with f_t and K_IC significantly higher when the specimens were loaded in the vertical direction of the printing compared to parallel loading. The fibers substantially improved the fracture resistance, particularly at the 1.5 % dosage, where fibers aligned perpendicular to the cracked section contributed most to crack resistance, achieving f_t of 49.43 MPa and K_IC of 5.28 MPa∙m^1/2. The reliability of the model was statistically validated by incorporating results of specimens with varying notch-to-height ratios and heights into the normality analysis, confirming the size independence of the derived fracture parameters.