Evaluation on the fracture properties of alkali-activated slag/fly ash ultra-high-performance concrete

Although traditional ultra-high-performance concrete (UHPC) exhibits excellent mechanical and durability properties, it consumes substantial amounts of cement. Consequently, there is an urgent need to develop low-carbon and environmentally friendly alkali-activated ultra-high-performance concrete (AA-UHPC). However, the presence of initial defects and pronounced brittleness exacerbates the cracking risk. This study aimed to quantitatively evaluate the fracture properties of AA-UHPC. Through three-point bending tests conducted on 155 notched AA-UHPC beams with varying fibre volume fractions, heights, and notch lengths, the fracture mechanism of AA-UHPC was comprehensively elucidated. Subsequently, the characteristic meso-structure parameter ( C ch ) and two discrete coefficients ( β and C ) were introduced to characterise the material’s heterogeneity and discontinuity, respectively. C ch proved to be the average fibre spacing values on the fracture surfaces of AA-UHPC specimens and was determined using image analysis. Using the experimentally measured maximum fracture load ( F max ), the size-independent tensile strength ( f t ) and fracture toughness ( K IC ) for AA-UHPC were obtained based on the boundary effect model. Normality analysis was employed to determine the 95% confidence intervals for f t and K IC of AA-UHPC with varying fibre volume fractions. Parametric sensitivity analysis of C ch , β and C verified the stability of the f t and K IC predictions. Moreover, as the fibre volume fraction increased from 1% to 1.5%, f t and K IC increased by 30.4% and 13.0%, respectively; and as it increased from 1.5% to 2%, f t and K IC increased by 28.4% and 9.1%, respectively. The fracture resistance of AA-UHPC was comparable to that of traditional UHPC with the same strength grade.