# A New Method to Determine the Steel Fibre Content of Existing Structures--Evaluation and Validation
## 3 Results and Discussion
### Testing of Drilling Core Samples  
#### 3.2.1 Effect of the k-Value Based on the FEM Model  
The first important step was to identify which parameters of the FEM model result in the most realistic and accurate geometry factors. On the one hand, simulating the electrodes with the electrical conductivity of stainless steel represents a good connection between concrete and electrode with no isolation. On the other hand, using the electrical resistivity of concrete represents an isolating effect by the wet sponge cloths that are soaked with pore solution and thus will have nearly the same resistivity as the concrete itself. Both possibilities have been tested, and the electrical resistivity of the PC was analysed with the help of both k-factors, given in Tables 11 and 12. As a measurement from the bottom of the cylinder to its top was performed with the electrical conduction of the whole upper and lower surface, where the k-factor can be calculated by the surface area and the specimen length, the real values of the electrical resistivity of the concrete are well known.  
Figure 10 shows the comparison of the vertical resistivity, calculated with the easily calculated k-factor, and the vertical and horizontal resistivities of the drilling cores, calculated of the results of the FEM analysis, with the electrodes, simulated with the electric parameters of stainless steel. It is clear to see that the electrical resistivity of the nine drilling cores in a vertical direction, measured with the basic test setup, is approximately 68 \(\Omega\)m with a very low deviation. The Comsol model with the electrodes' electrical conductivity set to the one of stainless steel with 1.4 \(\times\) 10\({}^{6}\) S/m results in calculated resistivities that are significantly higher, and additionally have a higher scatter, based on a large number of test directions and the fact that only a part of the inhomogeneous specimen is analysed. Due to the good conductivity of the electrodes in the model, in this case, the geometry  
Figure 9: Calculated fibre orientation of the cubic specimens with a fibre content of 80 kg/m\({}^{3}\), based on the electrical resistivity measurements on specimens with different concrete ages at a frequency of the alternating current of 1 kHz.  
factors that are calculated are too high and do not represent the realistic behaviour of the test setup and the specimen. In particular, the vertical resistivity and the horizontal one with single electrodes show the largest deviations, which can be explained by the higher number of possible measurement directions and the lower specimen volume that is observed each time.  
In contrast to this, setting the electrical resistivity of the electrodes in the FEM model to the one of concrete with 100 \(\Omega\)m gives perfectly fitting results of the PC specimens in comparison to the real values of the measurement from bottom to the top of the specimen (see Figure 11). The scatter of the results is comparable to the results with the other k-factors, which means that the variation is based on the measurement results and not on the FEM modelling. Based on those results, it seems that there is only a marginal influence of the electrodes that are connected to the surface of the specimen but not electrically conducted for single measurements. Thus, the lower k-factors of the model, where the electrodes were simulated as concrete, are used for the further examination of the results. This observed behaviour of the results with the newly developed test method shows the opportunity to directly measure the electrical resistance of drilling cores instead of preparing expensive specimens by high effort to determine the fibre content. Here, the full area measurement in a horizontal direction and the vertical resistance, measured by the side electrodes, have sufficient accuracy and can later be used to build an estimation model.  
Figure 10: Calculated electrical resistivity of PC drilling cores in different electrode configurations, simulated with the conductivity of steel.