01/10/2018
Fibres (coloured by orientation) within concrete samples cast

Fibres (coloured by orientation) within concrete samples cast individually by hand (left) and cored from within a slab (right)

Source: BAM, Division Micro Non-Destructive Testing

Fibre-reinforced concretes (FRCs) offer significant improvements in tensile strength and durability compared to most other concrete mixes. However, for safe and efficient use of FRC in large structures, anisotropy of fibre orientation needs to be understood and properly controlled. For the analysis of this phenomenon, BAM was provided with a dataset by the United States Army Engineer Research and Development Center (ERDC) that included X-ray computed tomography (CT) images of both cored samples extracted from a FRC slab and FRC samples cast individually in small moulds. These X-ray CT scans provided 3D images of the internal material structure of the samples. Given the high X-ray attenuation characteristics of the steel fibres in comparison to other concrete components, the fibres could be identified and separated through the imposition of greyscale thresholds within the images. Using analysis algorithms developed at the BAM, quantitative measurements of fibre orientation and anisotropy could then be extracted from the resulting data and compared.

These results showed that fibres within samples extracted from the slab were highly anisotropic in orientation while fibres in individually cast samples exhibited much more heterogeneous orientations. This indicates that fibre-orientation is dependent on the casting method and the flow pattern of the material (including the effects of component geometry and internal obstacles, such as reinforcing bars). A detailed understanding of these phenomena is critical for numerical modelling, structural design and construction practice since fibres oriented parallel to cracks tend to create zones of weakness, rather than strengthening the material. The results also provide evidence that traditional material characterization and quality control methods, which are carried out using only individually cast samples, may be insufficient for evaluating the strength and performance characteristics of FRC in structural components.

A methodology for quantifying the impact of casting procedure on anisotropy in fiber-reinforced concrete using X-ray CT
Tyler Oesch, E. Landis, D. Kuchma
Materials and Structures (2018) 51:73
BAM Department Non-destructive testing, Division Micro Non-Destructive Testing