Multi-scale mechanical behaviors of ultra-high performance fiber-reinforced concrete influenced by ultra-fine mineral additives: A hierarchical perspective on toughness gain modulation

Enhancing the toughness gain of ultra-high performance fiber-reinforced concrete (UHPFRC) through fundamental unit (e.g., nanostructure) optimization remains a challenge. This work explores the multi-scale mechanical behaviors of UHPFRC under four-point flexural loads, incorporating silica fume (SF) and ultra-fine fly ash (UFFA) as the ultra-fine mineral additives. SF and UFFA promote the formation of C( A )SH with high Si/Ca and Al/Ca ratio, altering the structural characteristics of both cement matrix and fiber-matrix interface. At the nanoscale, SF enhances the C( A )SH modulus through higher cohesion force, while UFFA elevates its friction coefficient; although both additives decrease C( A )SH hardness by reduced intrinsic modulus, their synergism improves C( A )SH stiffness. At the micro/macroscale, the stiffness of cement matrix and modulus of fiber-matrix interface dominate the strain-hardening behavior before fiber debonding, whereas the stiffness and friction coefficient of interface control the strain-softening process during fiber pulling-out. These insights highlight the hierarchical pathway to toughness modulation in UHPFRC.