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This paper presents an experimental investigation on mechanical properties (including compressive strength, tensile splitting strength and fracture energy) of ultra-high performance concrete (UHPC) with recycled steel fiber, compared with none fiber and industrial steel fiber reinforced UHPC. Moreover, the microscopic observation of fracture energy was carried out. All specimens were prepared at 0.18 water /binder (W/B) ratio and the dosage of steel fiber was controlled at 60 kg/m3. The results indicate that recycled steel fiber has a significant effect on enhancing strength and toughness of UHPC. And owing to the crimped shape, higher tensile strength (1800-2000 MPa) and appropriate diameter (1 mm) of recycled steel fiber, the steel fibers of UHPRSFRC will not immediately be pulled off and necking phenomenon is distinct.

In order to improve the durability of concrete, hydrophobic technology is used to isolate the contact between concrete and external water. The addition of hydrophobic admixture is a popular method, but most of the hydrophobic agents mixed with cement sacrifice the partial strength of concrete to improve the hydrophobicity. In this paper, a novel and multifunctional hydrophobic admixture named YREC was prepared, which has a positive effect on strength, impermeability, and durability under freeze-thaw attack. The chloride resistance permeability of concrete mixed with YREC increases by 5.3 times. Water contact angles up to 108 ± 2 were achieved on the concrete. The action mechanism of YREC was investigated using multiscale pore size distribution by means of nitrogen adsorption and mercury intrusion porosimetry, scanning electron microscope, X-ray diffraction testing, and thermogravimetric analysis (TG). The results indicate that YREC particles can accelerate cement hydration based on TG-DTG analysis. YREC can be more effectively applied in practical engineering due to the simpler preparation process and cheaper raw material.