Abstract: The aim of this study is to explore the feasibility of replacing industrial steel fiber (SF) with recycled steel fiber (RSF) in ultra-high performance concrete (UHPC). With SF as a comparison ,the effect of RSF on workability of freshly mixed ultra-high performance concrete was analyzed by means of fluidity test, rheological test and steel fiber dispersion test. The effect of RSF on mechanical properties of hardened ultra-high performance concrete was also studied by means of flexural and compressive test. Statistical analysis results indicated no significant difference of fluidity between freshly mixed RSFUHPC and SFUHPC. The yield stress and plastic viscosity of RSFUHPC were greater than those of SFUHPC, which can be attributed to the uneven aspect ratio of RSF. However, the shear thickening degree of RSFUHPC was significantly lower than that of SFUHPC, and the dispersion of RSF in freshly mixed and hardened UHPC was significantly greater than that of SF. Using the same mixed amount of RSF as that of SF, a P-value larger than 0.05 was obtained, indicating that, compared with SF, RSF had no significant effect on the load-displacement curve, flexural strength, or compressive strength of UHPC. Therefore, the addtion of RSF would ensure good dispersion in UHPC and excellent mechanical properties of UHPC, and would also reduce the construction difficulty associated with artificial vibrations in actual engineering structures and improve the construction quality.

Key words: ultra-high performance concrete, recycled steel fiber, industrial steel fiber, workability, mechanical properties