Enhancing ductility of cementitious composite via hierarchical bridging network of multi-scale steel fiber: Fracture resistance and hybrid reinforcement effect

Fracture in cementitious composite is a multi-scale process, evolving from microcrack initiation to macrocrack propagation. Mono-scale steel fiber provides limited macrocrack bridging only and cannot effectively prevent brittle failure. To address this limitation, three distinct scales of steel fiber are hybridized into cementitious composite, aiming to enhance fracture crack control capability and promote ductile behavior. Three-point bending test on notched beam, combined with double-K analysis and digital image correlation, was conducted to explore the effect of multi-scale steel fiber on fracture performance. The hybrid incorporation of multi-scale steel fiber increases the initial fracture toughness of cementitious composite by 67.8 % compared to pristine composite. The fracture mode of the composite shifts from localized brittle cracking to multiple cracking failure, and the crack mouth opening displacement at peak loading decreases by 65.3 %. It is revealed that distinct scales of steel fiber form a hierarchical bridging network within cementitious matrix, controlling crack development at different stages and leading to superior energy dissipation. Moreover, a multi-scale steel fiber hybrid reinforcement function is proposed, establishing a quantitative relationship between fiber characteristic parameters and hybridization effect of multi-scale fiber on mechanical strength and toughness of the composite. This study offers a novel and effective approach to enhance toughness of cementitious composite via multi-scale steel fiber, promoting the development of ductile composite material for safe and durable construction.