# Non-Destructive Multi-Method Assessment of Steel Fiber Orientation in Concrete +
## 4 Discussion
The results are shown in Table 2. It is striking that the two samples with longitudinally aligned fibers (Q80 || | and Q40 || |) show approximately the same and, compared to the other fiber reinforced samples, somewhat lower amount of large pores. This could, however, be due to the better radiolucency of the fibers and the associated reduction of artefacts that are incorrectly recognized as pores by the automatic analysis software. In the sample without fibres, the amount of large pores is only slightly lower. The results may, therefore, be not very reliable due to the interference caused by the steel fibres. Strictly speaking, the pore content can only be reliably compared if the fibre orientation is the same. Thus, it is not possible to deduce the effect of the casting process on the volume of large pores using the measurement data listed here. However, given that the total porosities measured using MIP did not vary strongly in relation to fiber distribution or orientation characteristics, it appears unlikely that porosity variations had a major influence on the results of the SIP and US investigations. The SIP measurements showed that our results support the findings of [16], which showed that the main orientation of steel fibers in test specimens coincided with the direction of the lowest resistivity. In the present study, we measured, however, not only the resistivity, which has previously been evaluated by [16], but also the frequency dependency of the complex resistivity by evaluating the polarization behavior of our samples. Our data indicate that the phase spectra might be an even more sensitive quantity for measuring fiber orientation than the amplitude spectra as the differences between samples with parallel and perpendicular oriented steel fibers were even more pronounced. The works of [17; 18] both focus on the influence of fiber dosage (not their orientation) on the electrical resistivity for samples prepared with random fiber distributions. The fibre dosage of our samples (for instance, 0.5 vol% for Q40--, Q40 || | and Q40*) is at the lower end of what the other \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **X-ray CT-Por.** & **MIP-Por.** & **MIP-Por.** & **MIP-Por.** \\  & \(\geq\) **160 \(\upmu\)m** & \(\geq\) **160 \(\upmu\)m** & \(\geq\) 3.6 nm & **BET Surface Area** \\ \hline Q 80 || & 1.9 & 0.6 & 12.22 & 3.89 \\ Q 40 || & 1.8 & 0.5 & 15.19 & 5.05 \\ Q 40* & 3.6 & 0.4 & 13.25 & 4.43 \\ Q 40— & 3.4 & 0.8 & 12.14 & 3.44 \\ Q 0F & 1.5 & 0.6 & 12.98 & 4.20 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Porosities determined by X-ray CT and MIP in vol.% and BET surface area in m\({}^{2}\)/g. Figure 10: Pore throat size distributions of all five samples determined with MIP. working groups used and it is conceivable that by evaluating both resistivity amplitude and phase, we could extract information about fibre orientation and content at the same time. The observed changes in the ultrasound signal are not very strong and experience has shown that this can also be caused by changes in the coupling. The test specimens are relatively small compared to the ultrasonic wavelength. Due to reflections of the ultrasonic waves at the sides of the test specimen and the superposition of different wave modes, the results are not clear. Thus, further ultrasonic measurements on larger specimens are necessary to confirm these initial promising results. The measurement of steel fiber content and/or orientation with ultrasonic dry contact probes would have the advantage that the concrete surface does not have to be prepared for a measurement. Other influencing variables like the grading curve, the pore structure, cracks or variations in the moisture content of the samples were not regarded in this study. All these parameters can significantly influence the results and sensitivity of SIP or US measurements. Therefore, further research is needed.