Fracture resistance of UHPC-CA with amorphous silica: Competition between microstructure densification and shrinkage microcracking

Abstract

Amorphous silica is widely in Ultra-high Performance Concrete with coarse aggregates (UHPC-CA), however, the influence mechanism of amorphous silica on the fracture resistance of UHPC-CA with different notch positions has rarely been understood. Herein, a multiscale methodology, from microstructure quantification, mesoscale fracture surface analysis to macroscopic fracture resistance, is proposed to reveal the fracture resistance mechanism of UHPC-CA. The results indicate that higher amorphous silica contributes to more high-density calcium silicate hydrate (HD C-S-H) and ultra-high-density calcium silicate hydrate (UHD C-S-H), from 16.3% and 27.1% to 21% and 28.3%, but more microcracks from 0.018 mm/mm² to 0.031 mm/mm² and 0.042 mm/mm². More microcracks lead to less CA fracture in the case of UHPC-CA with central notch, whereas the denser microstructure and higher HD C-S-H and UHD C-S-H contribute to more CA fracture in the case of UHPC-CA with eccentric notch. Consequently, the peak fracture force of UHPC-CA with central notch firstly increases from 3420 N to 3426.6 N and then decreases down to 3166.7 N, attributed to the higher microcracks. Nevertheless, for that with eccentric notch, i.e., mixed-shear stress, the peak fracture force increases from 5073 N to 5765 N and 5510 N, thanks to the higher interlock of CA because of the denser microstructure. In the practical implications, 5% silica fume in binder system is recommended to balance the cracking resistance of UHPC-CA exposed to complex loading condition.