# A New Method to Determine the Steel Fibre Content of Existing Structures--Evaluation and Validation
## 3 Results and Discussion
### Testing of Drilling Core Samples
#### 3.2.2 Statistical Analysis of the Test Results  
After optimising the FEM model, the electrical resistivity of the concrete specimens in different directions, as well as the variation for different specimens, were analysed. As already seen for the cubic specimens, an increase of the fibre content results in a lower electrical resistivity (see Figure 12). In contrast to the prior results, the difference between the results of different fibre contents and the PC is much lower for the drilling cores. As one can see, a fibre dosage of 40 kg/m\({}^{3}\) only decreases the electrical resistivity by about 20 \(\Omega\)m, while another 40 kg/m\({}^{3}\) lead to a decrease by about 10 \(\Omega\)m. For the cubic specimens cast together with the plates, where the drilling cores were extracted, the electrical resistivity was determined to 29 \(\Omega\)m for a fibre content of 40 kg/m\({}^{3}\) respectively 18 \(\Omega\)m for fibre content of 80 kg/m\({}^{3}\). This fact is expected to lead to a lower accuracy of discrimination of different concrete based on resistivity measurements. Since the electrical resistivity of the PC is exactly in the same range for the drilling cores as for the cubic specimens, and the results of the basic test setup fit those of the vertical advanced setup, both factors, the test setup as well as the k-values by the Comsol model, can be eliminated as reasons for the unexpected high resistivity values. A possible reason could be the unknown fibre distribution inside the plates and so local deviations in the fibre content, which would lead to higher resistivities based on lower fibre contents for some specimens. On the other hand, in this case, there should also be some specimens with very high fibre contents and thus very low electrical resistivity what is not the case here. The second possible explanation of the high results could be the size of the specimens compared to the fibre length and thus the high probability of truncated fibres by the drilling process, while for the earlier tests, only complete fibres were present. Another thing that can be seen in the results is a differing fibre orientation, compared to the cubic specimens, that can be estimated by the similarity of the resistivity in a horizontal and vertical direction for the SFRC drilling cores. Regarding the variations of the different specimens, it was expected that the variations for PC would be the lowest because, for SFRC, the inhomogeneity and anisotropy of both concrete and fibres can be superpositioned and thus result in a higher variation. As presented in Figure 13, the expected behaviour can be observed, especially for a fibre content of 40 kg/m\({}^{3}\), which shows the highest variations with up to 16%, while the PC samples only show COVs of less than 8%. For specimens with a higher fibre content of  
Figure 11: Calculated electrical resistivity of PC drilling cores in different electrode configurations, simulated with the resistivity of concrete. 80 kg/m\({}^{3}\), the inhomogeneity of the fibres is tendentially lower than for lower fibre content, and thus the COV is also lower, but still not on the level of PC. Comparing the variations in different directions of the specimens, it can be concluded that the variation in the vertical direction is slightly higher than for the horizontal direction. An explanation could be the possible segregation processes through the compaction even for robust concrete compositions, which result in small differences of the local water-cement ratio in different heights of the specimens as well as in a higher number of large aggregates in the lower zone of the specimen. In the horizontal direction for full area measurements and single electrode measurements, no significant differences regarding the variations can be detected, but it has to be considered that for the single electrode configuration, the  
Figure 12: Calculated electrical resistivity of drilling cores depending on the fibre content.