Non-destructive assessment of fibre content and orientation in UHPFRC layers based on a magnetic method

The tensile response of the ultra-high performance fibre reinforced cementitious composite (UHPFRC) placed in a given structural element is strongly dependent on the final fibre distribution and orientation with respect to the stress direction. In the present paper, an NDT method based on the magnetic properties of the steel fibres is developed for the in-situ detection of these two quantities in UHPFRC layers. A magnetic probe was designed for performing inductance measurements and the equations relating the inductance with the fibre content and orientation are derived based on an analogy between the magnetic and electrical circuits and on a simplified physical model of the composite material. The performance of the method has been validated on UHPFRC specimens produced with varying matrix formulation, fibre length, fibre content and orientation. Based on the theoretical formulation and supported by the collected experimental evidence, two reliable indicators of the fibre content and orientation are proposed which require only two inductance measurements on each zone. The relevance of this method for quality control and for the design with UHPFRC is highlighted.