Fiber distribution and orientation characterization in UHPFRC elements using a magnetic non-destructive testing method

Abstract

Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) is a promising material for different applications such as bridge deck strengthening due to its improved strength and durability properties. Especially the tensile strength and ductility is heavily dependent on the distribution and orientation of the steel fibers within the element. This fiber behaviour can be unpredictable, causing a large scatter in material properties. This scatter is inconvenient for predicting the material properties. The novelty of the material is the main reason that UHPFRC is not included in the Eurocode. Certain countries like Switzerland and France have developed their own UHPFRC norms.

In order to increase the knowledge and understanding of the fiber distribution and orientation, a non-destructive testing method has been developed. Two types of magnetic probes have been developed: the C-shaped ferrite probe and the single ferrite probe. These probes are based on other probes developed at the University of Milan and Porto. The C-shaped ferrite probe is most suitable for measuring the total fiber content and fiber orientation. The single ferrite probe is able to take fiber content measurements closer to the edge making it more efficient for fiber distribution measurements. This can result in a reliable prediction of the peak-tensile strength. The addition of UHPFRC specific design rules and this non-destructive testing method will make UHPFRC and its applications more attractive.

In this research, 27 different UHPFRC plates were cast with the same dimensions of 10x200x200 mm3. These plates were cast horizontally or vertically with either 1%, 2%, 3% or 4% fiber content. Seven plates were vibrated after casting in order to analyse the influence of vibration. Each fiber content resulted in a relatively homogeneous fiber distribution and random fiber orientation. Horizontal casting of UHPFRC plates usually result in a more homogeneous fiber distribution and more random fiber orientation compared to vertically cast plates. The best distribution and orientation was reached when the elements were not vibrated at all. Vibration can result in severe fiber segregation along the element height and vertical orientation of the fibers. These conclusions hold for longer UHPFRC elements where the element height and the element thickness remains constant.

One of the main pitfalls in this research is the small thickness of the plates. The length of the fibers exceeds the plate thickness, which makes casting very difficult. During casting, fibers will get stuck at the top of the mould, which also influences the results. Moreover, it is important to continuously keep mixing the UHPFRC mixture in order to equally divide the fibers over all plates and to use a mixture that is castable yet not too flowable. High workability mixtures are more prone to fiber segregation.