Predicting the tensile strength of ultra-high performance concrete: New insights into the synergistic effects of steel fiber geometry and distribution

Steel fiber size and fiber distribution synergistically influence the tensile behavior of ultra-high performance concrete (UHPC). The suitable aspect ratio of steel fiber can improve crack-bridging capacity, while the oversized ratio results in poor fiber distribution, degrading tensile performance substantially. Therefore, identifying the synergistic effect of fiber geometry and distribution is key for superior tensile properties, which is quantitatively evaluated here. It is found that the aspect ratio determines the domain of stress distribution, while fiber distribution affects the overlapping domain, leading to variations of tensile properties, which is revealed as the "pile group" working mechanism of fibers. Furthermore, a novel model was proposed to accurately predict the uniaxial tensile strength of UHPC by combining the Gradient Boosting (GB) and statistical approaches. The matrix tensile strength was obtained using the GB algorithm, and the relationship among fiber factor, fiber distribution, and tensile strength was established by the statistical regression method. The findings provide unique insight into how the fiber geometry and distribution govern the tensile properties and inspire a new design scheme of UHPC toward excellent tensile behavior.