Measurement and modelling of fibre distribution and orientation in UHPFRC

Ultra High Performance Fibre Reinforced Concretes (UHPFRC) combine outstanding protective (extremely low permeability) and mechanical (high tensile strength and significant tensile strain hardening) properties. These materials can be used in composite UHPFRC-reinforced concrete members to provide long term durability and improved structural performance. However, their processing at fresh state and geometric boundary effects can induce significant anisotropic fibre distributions that must be considered when evaluating material characterisation test results or designing structural members to reliably benefit from their strain hardening properties. The fibre distribution and the coefficient of orientation have been measured in UHPFRC tensile specimens cast in forms with different casting directions, and fibre length. A large-scale UHPFRC-reinforced concrete composite structural member has also been investigated to link tensile specimen test results and structural element performance. Relationships have been found between the fibre distribution and orientation as a function of the form size vs. fibre length ratio, casting direction and the resulting mechanical properties. Furthermore, an original model for characterizing the fibre orientation from the determined number of fibres within three orthogonal cuts was developed, tested and successfully employed.