# A New Method to Determine the Steel Fibre Content of Existing Structures--Evaluation and Validation
## 3 Results and Discussion
### Testing of Drilling Core Samples
#### 3.2.4 Estimation of the Fibre Orientation  
Although the determination of the fibre content seems to be problematic when no information about the concrete is available, the fibre orientation can be calculated without changes to any equations based on the electrical resistivity in different directions. As presented in Figure 16, for fibre content of 40 kg/m\({}^{3}\), an unexpectedly high orientation factor in a vertical direction (mean value of 41%) is observed, while for each specimen, the factor in a vertical direction is above 1/3, so most of the fibres are detected in a vertical direction. In comparison to the cubic specimens, this behaviour is contrary but could be based on the large dimensions of the concrete plate where the drilling cores have been extracted from. The large dimensions lead to a high volume of concrete and thus a lower effectivity of the compaction by the vibration table, which for smaller specimens is the most influencing parameter for the fibre orientation. In addition, one can see a high scatter in orientation that reaches from 34% to 50% for the vertical and 20% to 38% for the horizontal direction. One interesting fact can be identified by comparing Figures 15 and 16. Where a fibre orientation in a vertical direction of more than 40% is calculated, the calculated fibre content is significantly lower than the expected value of 40 kg/m\({}^{3}\), which also is the mean value of all specimens. The same can be seen for specimen 8, which has the highest calculated fibre content and the lowest fibre orientation coefficient in a vertical direction. This correlation leads to the assumption that either the calculation of the global fibre content based on the three directions is slightly inaccurate, or the higher vertical orientation coefficient is a result of lacking horizontal fibres inside of single specimens. Again, a small influence of the specimen position inside the concrete plate could be assumed by looking at the results of specimens 7 to 9 with lower edge distances that show a higher horizontal orientation factor. Based on the compaction process, perhaps some fibres tend to orientate themselves on the walls of the formworks and thus are detected as horizontal fibres in the specimens near the formwork. Vertical fibres near the formwork are not detected because the distance between the drilling core and the formwork with half of the fibre length was chosen high enough not to be influenced by the walls.  
For the specimens with a higher fibre content of 80 kg/m\({}^{3}\), the expected orientation of more than 2/3 of the fibres in the horizontal direction is detected with maximum values up to 44% and minimum values of about 28% (Figure 17). In addition, the uniformity of the specimens is much higher, and only specimen number 5 shows a significantly lower vertical orientation coefficient. A correlation of the calculated fibre content and orientation factors was observed for the lower fibre content. In this case, it cannot be seen, probably based on the more uniform fibre distribution and orientation and thus the better acceptance of the calculated global values. Compared to the orientation of separately cast SFRC specimens, the difference in the fibre orientation is clearly visible which means that the orientation of fibres inside SFRC in contrast to the fibre content cannot be determined on such specimens and a direct analysis of structural elements is necessary. Here, no big influences of the specimen's position inside the concrete plate can be observed because of the relatively homogenous results in total. Only specimen number 5, which is in the centre of the plate,shows a significantly lower vertical fibre orientation factor, which can be explained by the flow process of the concrete through the compaction.