Fiber distribution and its influencing mechanism on properties in sprayed ultra-high performance concrete

A fiber-reinforced sprayed ultra-high performance concrete (SUHPC) with high consistency and suitable sprayed thickness was developed in this paper. Fiber distribution of SUHPC after pumping and spraying is the intrinsic mechanism for the changes in strength and toughness, especially with its high consistency, high fiber content, and no coarse aggregate is completely different from conventional shotcrete. Thereby a series of studies were performed including rheological behaviours, mechanical properties, and their influencing mechanisms. Results showed that the field sprayed thickness of SUHPC achieved about 30 mm when it had a flow resistance of 164.7 % of casted UHPC, and its deformation recovery was very weak due to its strong thixotropy. More bubbles were entrapped during spraying, the partially unbroken bubbles were easily trapped in the fresh matrix and transformed into irregular coarse pores (≥25 μm) and a 55.2 % increase in porosity, which will partially weaken the strength and toughness of SUHPC. Additionally, the fiber orientation in SUHPC was more parallel to the sprayed surface after spraying, mainly concentrated in 75 ° to 85 °, and the fiber lap rate was also improved by 28.2 % because of its approximate two-dimensional distribution. As the fiber content decreased, the consistency of fresh concrete, porosity, and strength all showed a reduction, and the fibers became more parallel to the sprayed surface. Overall, the short straight steel fiber is preferred by SUHPC and its content should not be less than 78 kg/m 3 . • Comparison between rheological and mechanical properties of sprayed and casted UHPC. • Fiber distribution and pore structure of sprayed UHPC were tested and calculated. • The effect of SUHPC fiber distribution on toughness was revealed for the first time. • Discussing internal relationship of fiber distribution, rheology, mechanics of SUHPC.