# A New Method to Determine the Steel Fibre Content of Existing Structures--Evaluation and Validation
## 2 Materials and Methods  
### Concrete Mix Design  
The concrete mix design was already used in earlier studies of the authors such as [1]. The design focussed on sufficient workability for plain concrete (PC) and fibre contents up to 80 kg/m\({}^{3}\) in steel fibre reinforced concretes (SFRC). The concrete mix design presented in Table 1 enables the analysis of both PC and SFRC without the addition of a superplasticizer. For the SFRC hooked end, macro steel fibres with a length of 60 mm and a diameter of 1 mm were added to the plain concrete after mixing. The fibres thus had an aspect ratio of 60 and were produced out of steel wire.  
The mixing process was performed according to the following scheme: First, the solid components, cement and aggregates, were mixed in a compulsory mixer with a nominal volume of 170 L for 30 s to obtain a homogenous mixture. In the next step, the water was added to the solids while the mixing process was still ongoing, followed by a mixing phase of two minutes. After a visual inspection of the concrete, adhering components were removed from the mixer walls, followed by a final mixing process of one minute. On the final PC mixture, fresh concrete tests, such as the flow table test as well as the density and the air content in accordance with EN 12350-5 [30], EN 12350-6 [31], and EN 12350-7 [32], were assessed. In the case of the SFRC with fibre contents of 40 kg/m\({}^{3}\) and 80 kg/m\({}^{3}\), the fibres were added after the fresh concrete tests, and the concrete was mixed for another minute.  
In total, three concrete batches were produced. Out of each concrete batch three cubic specimens with an edge length of 150 mm as well as one plate with dimensions of \(500\times 500\times 212\) mm\({}^{3}\) were cast, each in two layers with subsequent compaction on a vibrating table. A height of 212 mm was chosen to enable grinding of both the bottom and surface of the later extracted drilling cores and gain specimens of a height of 200 mm. The specimens were covered with foil to inhibit dehydration of the surface. After 24 h, the specimens in steel formworks (cubes) were demoulted. Moreover, the upper surface was  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Unit** & **32-60-300-00** \\ \hline CEM 1 32.5 R & kg/m\({}^{3}\) & 300.0 \\ Water & kg/m\({}^{3}\) & 180.0 \\ Sand 0–4 mm & kg/m\({}^{3}\) & 845.5 \\ Gravel 4–16 mm & kg/m\({}^{3}\) & 1004.0 \\ Water/cement ratio & - & 0.60 \\ Grain size distribution & - & A/B16 \\ Steel fibre type & - & Macrofibre 60 mm \\ Steel fibre content & kg/m\({}^{3}\) & 0, 40, 80 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Concrete mix design of the basic concrete.  
ground to ensure a flat surface with a good connection to the electrodes and afterwards stored underwater. The plates were left in the formwork to simulate a hardening of structural elements such as floor plates or walls and were stored at a climate of 20 \({}^{\circ}\)C and 65% relative humidity. After 14 days, nine drilling cores with diameters of 100 mm were extracted out of each concrete plate. Thereby two different edge distances were used. The first six specimens were drilled with edge distances to the formworks of 60 mm, appropriate to the length of the fibres, while the last three cores were drilled with edge distances of 30 mm, appropriate to half of the fibre length. A schematic figure of the arrangement of the drilling cores is presented in Figure 1. After drilling, the lower and upper surfaces of the cores were ground analogue to the cubic specimens and the specimens were stored underwater.