## 3 Results and Discussion
### Validation of the Test Setup
#### 3.1.4 Evaluation of the Electrode Arrays  
When comparing both electrode arrays, it can be noticed that for the plain concrete, the calculated resistivity of the array with angular distances of 90\({}^{\circ}\) was slightly higher, while for the fibre reinforced concretes, lower values were determined. Since the setup with more electrodes was able to describe the concrete in more detail and thus, different measuring directions were able to be analysed, the variations of the single electrode measurements, especially, but also of the full area measurements were higher than for the setup with a  
Figure 6: Electrical resistivity in different directions depending on fibre content at a specimen age of 9 days and an electrode array with angular distances of 180\({}^{\circ}\). lower number of electrodes (see Figures A2-A5 in Appendix A). The differences in the total results could be based on the variations of the single specimens but could also imply that the FEM model is not perfectly befitting of the test setup. A comparison of the single results of the measurements of both series was carried out to evaluate whether the differences between both test series could be a result of the FEM model. In evaluations up to now, it was assumed that all electrodes have a very good connection to the concrete and thus, no initial resistance occurs, but only the resistivity of the composite material has an influence on the measured electrical resistance. Figure 8: Electrical resistivity in different directions depending on fibre content at a specimen age of 9 days and an electrode array with angular distances of 90\({}^{\circ}\). Figure 7: Electrical resistivity in different directions depending on fibre content at a specimen age of 38 days and an electrode array with angular distances of 180\({}^{\circ}\). To decide whether there is a general problem with the FEM model, the comparison of two different specimens can indeed offer a hint but cannot be seen as an absolute validation. So, the fact that, for the 90\({}^{\circ}\) electrode array, all values for the fibre reinforced concrete specimens were lower than for the other electrode array is no proof that the FEM model is not suitable just because the opposite behaviour is identified in the plain concrete. As both electrode arrays consisted of three height levels of electrodes, the easiest way to identify the possible weaknesses of the model is to analyse the resistivity in the vertical direction. Here, three possibilities of the current flow are feasible: from height level one to height level two; from height level two to height level three; and from height level one to height level three. In this way, the percolated volume of the measurement from height level one to height level three should contain both volumes of the other measurements and thus, the resistivity will be approximately the mean value of the resistivities of both other combinations. For the specimens with angular distances of the electrodes of 180\({}^{\circ}\), the expected behaviour was almost identified for the PC and perfectly identified for the SFRC with a steel fibre content of 40 kg/m\({}^{3}\) (see Figure 9). In addition, the electrical resistivity from height level one to height level two, which represents the lower part of the specimen, was higher than the other values. This can be explained by an increased fraction of large aggregates and a lower porosity, based on the rise of water in the wet condition of the specimen as part of a small segregation process, even if the mixture was very stable. Only the specimen with a high fibre content the resistivity that was measured from the lowest to the highest height level of electrodes did not fit the expectations, but based on the high fibre content, it can be presumed that vertically orientated fibres in the middle area of the specimen could have a huge impact on the resistivity and thus, explain this result.