Effects of amorphous silica on mechanical contribution of coarse aggregates in UHPC: From micromechanics to mesoscale fracture and macroscopic strength

Despite the enhancement of amorphous silica on Ultra-High Performance Concrete (UHPC), the influence of amorphous silicas on the mechanical contribution of coarse aggregates (CA) in UHPC from the perspective of multiscale is still unclear. Herein, the mechanical contribution of CA owing to the incorporation of silica with different morphology (i.e. nano-silica and silica fume) is studied from micromechanics to mesoscale fracture and macroscopic strength. The results show that, on the premise of guarantee flowability, silica fume generates more C–S–H but more microcracks owing to the higher content, while nano-silica results in a higher percentage of high-density C–S–H. The fracture across CA in pure tensile cracking is controlled by the competition between high-density C–S–H content and microcracks, whereas, the fracture across CA in pure shear cracking is governed by microstructure compactness. That is, the high-density C–S–H benefits more the fracture of CA in the case of tensile condition, while the microstructure compactness benefits more the fracture of CA in the case of shear condition. Consequently, in terms of the macroscopic strength, silica fume benefits more the mechanical contribution of CA in compressive loading because of the denser microstructure, while nano-silica is more beneficial to that in flexural loading owing to the higher content of high-density C–S–H.