# A New Method to Determine the Steel Fibre Content of Existing Structures--Evaluation and Validation
## 4 Conclusions  
This paper presents the results of the evaluation phase of a newly developed non-destructive test setup for the determination of the fibre content and orientation of steel fibres in concrete based on electrical resistivity measurements by the investigation of drilling cores. The basic test setup was already developed, and the FEM model was constructed in the first part of this research project (see [2]). This part of the study contains the validation process, which was performed on drilling core samples to enable statistical analysis and, of course, conclusions regarding the accuracy of the method. The main findings of this work can be summarised as follows:  
Figure 16: Calculated fibre orientation of the drilling cores with a fibre content of 40 kg/m\({}^{3}\).  
Figure 17: Calculated fibre orientation of the drilling cores with a fibre content of 80 kg/m\({}^{3}\).  
* In contrast to AC-IS, the used method, based on electrical resistivity measurements, is much easier to use. However, similar models can be adapted to calculate both fibre content and orientation. One limitation in the application is the small amount of data that is used to calibrate the equations for the calculation of the fibre content for drilling cores which are needed to analyse structural elements or buildings.
* Based on the data gained in this study, a coefficient dependent on the expected value was found to calculate an adjusted fibre aspect ratio as a new coefficient for the literature's model for specimens with small dimensions where probably a huge number of fibres are truncated through the drilling process and thus the effective fibre length is much smaller than the original one.
* With this coefficient, the fibre content of drilling cores can be estimated in a satisfying way, and thus the basis for the analysis of the fibre content of existing structures in an easy way has been provided. Independent of such a factor, the orientation of the fibres inside a specimen can be calculated comparative from the electrical resistivity in different directions, which can be measured with the cylindrical test setup very easily and fast.
* The results show that the newly developed method is suitable for rapid and non-destructive structural diagnosis based on drilling cores using electrical resistivity measurements.
* In further studies, the authors will focus on the verification of the correlation between the fibre content and the coefficient for adjusting the aspect ratio, especially by a variety of specimen size and fibre lengths. Additionally, different concretes with deviant compositions will be analysed to see if those calculations are applicable in a universal way or if additional adjustments are needed in some cases. Finally, the test setup and the models will be adjusted for several geometries in case of precast elements or existing structural elements.  
Conceptualisation, methodology, validation, formal analysis, investigation, data curation, visualisation, 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.  
Geometry factors k for the drilling core analysis, calculated by Comsol with an electrode array with angular distances of 90\({}^{\circ}\) and electrical conductivity of the electrodes in Comsol model of stainless steel: 1.4 \(\times\) 10\({}^{6}\) S/m.