Quantitative assessment of interfacial fracture properties in 3D printed alkali-activated recycled sand concrete based on a closed-form fracture model

This study focuses on 3D printed alkali-activated recycled sand concrete (AARSC), a material that offers environmental benefits and design flexibility. The 3D printing process creates interfaces between filaments and layers, which may become potential weak points and increase interfacial inhomogeneity and discontinuity. Therefore, evaluating fracture resistance of interfaces is vital for ensuring the reliability of 3D printed materials. This work develops a model that accounts for discontinuity and inhomogeneity, enabling the determination of more realistic interfacial tensile bond strength ( f t ) and fracture toughness ( K IC ) for 3D printed AARSC. The model establishes a linear relationship between the maximum load ( F max ) and interfacial fracture parameters, allowing the determination of size-independent f t and K IC through analytical solutions. Three-point bending tests were conducted on 3D printed AARSC beams with different interface types and recycled sand replacement ratios to systematically evaluate the effect of recycled sand content on the interfacial fracture properties. The results show that the predicted interfacial f t and K IC exhibited size-independence as intrinsic interfacial properties. Moreover, f t values of the inter-layer interface were slightly higher than those of the inter-filament interface for the same recycled sand content, with the largest observed difference at 100% recycled sand replacement, being 22.7%. When the replacement ratio increased from 60% to 100%, the decrease in f t became significant, with the reduction rates increasing to 32.0% and 26.4% for Type I and Type II interfaces, respectively. Additionally, a positive correlation was observed between the recycled sand content and the anisotropy in fracture behaviors of 3D printed AARSC.