# Non-Destructive Multi-Method Assessment of Steel Fiber Orientation in Concrete +
In the last case (red), an optimum utilisation of the fibers' cross section is expected, and hence the load-displacement response can exhibit substantial ductility and possibly higher strength than plain concrete, which would otherwise fail in a quasi-brittle manner after the crack initiation. The yellow fibers represent the random distribution, which can still exhibit cracking stress retention and, hence, ductility. In the most suboptimal (blue) case, fibers essentially do not contribute to crack bridging, and, hence, concrete tends to behave similarly to a plain unreinforced material. In fact, fibers in this latter orientation can even act as discontinuities in the matrix and accelerate cracking. The fiber distribution and orientation strongly depends on the casting conditions of the fresh mix as well as geometric constraints, and the effect of casting-induced variations in fiber orientations on SFRC structural performance has been shown in previous research [8; 9; 10; 11]. Some methods to control the fiber alignment in fresh concrete have also been proposed, e.g., by [12; 13]. The effect of the fiber orientations on the load-bearing performance of concrete is accounted for in current design guidelines [2; 14] by introducing a so-called fiber orientation factor, which reduces the nominal tensile strength value of fiber reinforced concrete, accounting for favorable and unfavorable fiber alignment effects as well as isotropy. Figure 1: Indicative alignment of fibers in relation to tensile stresses and cracking in concrete, from left to right: most favorable, with fibers (red) aligned with the tensile stresses; random (yellow), leading to isotropic performance; and unfavorable (blue) with no fibers (effectively) bridging the crack. Given the critical effect that fiber orientation and distribution have on the strength and failure pattern of SFRC structural components, NDT methods for evaluating fiber orientation and distribution are needed. In addition to measurement systems that can be applied in the lab on small samples, e.g., in [15], in situ testing techniques for large structures need to be developed. In this study, we focus on the application of methods, which have already shown some value in this regard but have not yet really been systematically applied on samples with defined amounts of fibers in different orientations. Previous research has indicated that electrical resistivity could be a suitable tool for the measurement of fiber orientations [16] and fiber-volume fractions in larger SFRC structural components [17; 18]. Dry cementitious materials are typically electrical insulators. However, conducting materials added to the concrete, such as steel fibers, significantly decrease its electrical resistivity. Nontheless, further research is still needed in order to precisely define quantitative values of steel fiber distribution and orientation based on electrical resistivity data. The frequency dependency of the electrical resistivity and, hence, the polarizability of SFRC in the low frequency range has not yet been studied. It is conceivable that looking at both the capability of a medium to conduct and to store electrical charges will give deeper insight for material characterization. This can, for example, be done when the spectral induced polarization (SIP) method is used for measuring the electrical properties of a medium. Recent research has also indicated that ultrasonic techniques may be useful for assessing fiber orientation characteristics [19]. If such an ultrasonic-based method could be validated, it would hold great potential, since ultrasound measurement techniques are already widespread within the civil engineering community and the capability of measuring ultrasonic properties up to several meters in depth has been successfully demonstrated [20]. However, further research is still needed to determine the sensitivity and accuracy of ultrasound for fiber orientation measurement, including ultrasound measurement quality and consistency for varying fiber types, structural shapes, and concrete mixtures.