## 4 Conclusions  
In summary, the test setup, consisting of a breadboard and a LCR meter, can be used for the analysis of both plain concrete and fibre reinforced concrete, and the FEM model provides the possibility to calculate the resistivity of the material and thus, to compare the different directions of the specimens. The evaluation of the specimens, included in formworks with electrodes, showed that the FEM model fits the real current flow inside the whole test setup, although the small number of specimens did not allow for a detailed analysis. While the age of the specimen, as well as the frequency of the alternating current, showed an impact on the calculated fibre content, neither parameter have any effect on the fibre orientation. The results show that with the increasing age of the concrete, the electrical resistivity also increases. For plain concrete, it increases from about 40 \(\Omega\)m to 70 \(\Omega\)m from 9 days to 38 days, but the proportion between steel fibre reinforced concrete and plain concrete is not affected because the electrical resistivity of the steel fibre reinforced concrete shows a proportionally smaller increase for a fibre content of 80 kg/m\({}^{3}\), based on the conductivity of the fibres.  
Since the setup with angular distances of 90\({}^{\circ}\) allowed the analysis of specimens in at least three directions without the repeated mounting and demounting of the specimen, it was decided to use this number of electrodes and check which electrical resistivity of the electrodes showed the best results in the FEM model for further tests with drilling core  
Figure 12: Calculated fibre orientation of the concrete specimens with different electrode arrays depending on fibre content at a specimen age of 9 days.  
samples. The electrical parameters of the electrodes should be varied between the electrical resistivity of steel, to represent a good connection between the electrode and concrete, and the electrical resistivity of concrete, so a low connection could be analysed. With help of the developed test setup and the FEM model, a comparison between cubic specimens and drilling cores is part of current investigations being conducted by the authors [34] to validate the suitability of the setup for in situ analyses of existing structures.  
Conceptualization, methodology, validation, formal analysis, investigation, data curation, visualization, writing--original draft preparation: S.C. Review and editing, supervision and advice: T.M. and M.R. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to ongoing project work.  
The authors declare no conflict of interest.  
Figure 14: Electrical resistivity in different directions depending on the fibre content at a specimen age of 38 days and an electrode array with angular distances of 90\({}^{\circ}\).  
Figure 13: Coefficient of variation of the electrical resistivity in different directions depending on the fibre content at a specimen age of 9 days and an electrode array with angular distances of 90\({}^{\circ}\).  
Figure 6: Calculated fibre content of the concrete specimens with different electrode arrays depending on the fibre content and frequency of the alternating current at a specimen age of 38 days.