Abstract: In order to investigate the effects of fly ash and steel fiber on the abrasion resistance of ultra-high performance concrete (UHPC), this paper performed macroscopic mechanical test, underwater abrasion resistance test and microscopic characterization such as SEM and MIP to investigate the effects of fly ash and steel fiber on the workability, mechanical properties, microstructure, and abrasion resistance of UHPC. The results show that the replacement of cement by fly ash delays the emergence of hydration peak by about 3 h, and the cumulative heat flow at 3 d is reduced by 18%, significantly improving the workability of UHPC. The incorporation of fly ash reduces the mechanical properties of UHPC in the early stage and has no effect on the mechanical properties in the later stage. The incorporation of steel fiber improves the flexural strength and compressive strength of UHPC. The flexural strength and compressive strength are increased by 82% and 47% respectively due to the composite mixing of steel fiber and fly ash under steam curing. The addition of fly ash has little effect on the abrasion resistance of UHPC, while the addition of steel fiber improves the abrasion resistance of UHPC, and the composite mixing of steel fiber and fly ash increases the abrasion resistance of UHPC by 58%. The compressive strength and abrasion resistance strength of UHPC show a significant positive correlation. The incorporation of fly ash reduces the most available pore size, porosity and capillary pore content, making the internal structure of matrix more uniform and denser.

Key words: fly ash, steel fiber, ultra-high performance concrete, abrasion resistance, microstructure, mechanical property