## 3 Results and Discussion  
### Validation of the Test Setup  
#### 3.1.1 Impact of the Frequency of the Alternating Current  
In the first series of six specimens, one of each fibre content and electrode array, with fibre contents of 0, 40 and 80 kg/m\({}^{3}\), the test setup was assessed in combination with the FEM model, where the electrical conductivity of the electrodes was set to that of titanium (2.5 \(\times\) 10\({}^{6}\) S/m). The corresponding geometry factors are given in Tables 1 and 2 in Appendix A. The specimens were left covered with foil in the formworks and were tested at 9 days and 38 days of age. The different test dates were chosen to see whether changes in the pore structure of the concrete, based on hydration, show significant influences on the results of the electrical resistivity measurements. The age of 9 days represents a very early stage of the concrete, where the hydration process has not yet been finished but an initial investigation of concrete structures would be possible because partial demoulding could be performed, depending on the structure itself. The age of 38 days was chosen because it was assumed that the hydration process would be almost complete and the drying process of the concrete should not have taken place, so a comparison of the results of both ages will provide information about the influence of the hydration process and will not be affected by the different saturation of the specimens. For each specimen, the electrical resistivity in the horizontal and vertical direction was determined with several electrode configurations and the five possible frequencies of the alternating current (100 Hz, 120 Hz, 1 kHz, 10 kHz, 100 kHz). As presented in several previous studies (see, e.g., [5; 33]), the frequency of the alternating current influenced the electrical resistivity, where a higher frequency led to slightly lower resistivity values. While for the PC there was almost no influence and a change in frequency from 100 Hz to 100 kHz only resulted in a decrease in the resistivity of about 2 %, for SFRC, depending on the fibre content, a decrease of 10 to 20% was observed (see Figure 5). To identify effects based on the specimen's direction, the age of the specimen and the number of electrodes inside the specimen, the calculated resistivity of the concrete, based on a measurement with an alternating current frequency of 1 kHz, was used throughout the study. With this approach, we were able to exclude polarisation and induction effects. Afterwards, the results of all frequencies were evaluated according to Section 2.4 to estimate the fibre content and the orientation of fibres inside the specimen. 3.1.2. Electrode Array with Angular Distances of 180\({}^{\circ}\)  
The results of the three specimens with an electrode array of only six electrodes with angular distances of 180\({}^{\circ}\) show that a higher fibre content results in a lower electrical resistivity of the composite material (see Figure 6), which is in line with several studies (see, e.g., [30; 39; 40; 41]). This behaviour is the same for every direction that was analysed within this study. In the horizontal direction, the maximum area that was possible to analyse was investigated using a parallel connection of the electrodes at all three height levels on each side of the specimen (blue column). Compared to the measurements of single electrodes in the horizontal direction (red column) which means that the electrical resistance between both electrodes at height level one, level two and level three was measured, there is almost no difference in the electrical resistivity of the plain concrete, while for the fibre reinforced concretes, higher differences were observed. This can be explained by the inhomogeneity of the concrete and fibres and the anisotropy of the fibres, which are of a higher consequence if a smaller volume of materials is analysed.