Key parameters in optimizing fibres orientation and distribution for Ultra-High Performance Fibre Reinforced Concrete (UHPFRC)

•UHPFRCs with high flowability and different viscosities are developed.•Effect of casting length, height and viscosity on fibre orientation is studied.•Effect of casting length, height and viscosity on fibre distribution is studied.•Mechanism of fibre dynamic movement during UHPFRC casting is analyzed.•The fibre orientation state in the UHPFRC flowing procedure is proposed. This study addresses some key parameters in optimizing fibres distribution and orientation for Ultra-High Performance Fibre Reinforced Concrete (UHPFRC). Based on a materials efficient application point of view, a densely compacted UHPFRC cementitious matrix is designed by using the modified A&A (Andreasen & Andersen) model. Several UHPFRCs with different viscosities are produced by adjusting the proportions of silica fume to metakaolin. Then, some key influence factors are considered and studied to clarify their effect on fibres distribution and directions in UHPFRC, including concrete casting length, casting height and mixture viscosity. At last, an image analysis tool is applied for further prove the fibres orientation and distribution in the hardened UHPFRC. The obtained experimental results indicate that the fibres orientation can be strongly affected by the UHPFRC casting length, casting height and the mixture viscosity. The optimized fibre orientation can be obtained when the UHPFRC with 1000 mm length, 150 mm cast length and 4.409 pa·s viscosity. Moreover, compared to the effect of casting length and mixture viscosity, the casting height is more important to produce a homogeneous UHPFRC with optimized fibre distribution.