## 3 Experimental Methods
### Experimental Setup and Testing Procedure
#### 3.2.3 Tensile Strength Tests on CTRC  
To determine the tensile behavior of CTRC under electrical and ambient heating, tensile strength tests were conducted on rectangular CTRC specimens, depicted in Figure 6. The specimen dimensions were 500 \(\times\) 60 \(\times\) 20 mm\({}^{3}\) (L \(\times\) W \(\times\) T), both produced with a constant mortar cover using SRM-A4 on both sides and a single layer of CTR. The specimens were hand-laminated using a ratio of solid to water weight content of 1:0.13. Subsequently, the fresh-mortar properties were determined according to [29; 30; 31] and standard prism sets were prepared to determine the flexural and compressive strengths for the respective test dates of the tensile tests in accordance with [32]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{
\begin{tabular}{c} **Target** \\ **Temperature** \\ \end{tabular} } & \multicolumn{2}{c}{**CTR-EP-Sand**} & \multicolumn{2}{c}{**CTR-P**} \\ \cline{2-5}  & **Terminal Voltage** & **Current** & **Terminal Voltage** & **Current** \\ \hline \([^{\circ}\)C] & [V] & [A] & [V] & [A] \\ \hline
40 & 7 & 0.4 & 3.9 & 0.7 \\
60 & 17.5 & 0.7 & 5.5 & 1.1 \\
80 & 16.7 & 0.9 & 3.7 & 1.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Mean electrical parameters for heating the FS tensile specimens [10]. Figure 5: FS tensile strength test setup: (**a**) schematic drawing; (**b**) overview of EH test setup; (**c**) overview of AH test setup; (**d**) exemplary fiber strand CTR-P.  
For the electrical heating experiments, the ends of the specimens were connected using ferrules and terminal strips, and then linked to a laboratory power supply. The target temperature levels were set by adjusting the voltage. Comparative investigations were carried out using an identical test setup in a TEE 26/LN2 climatic chamber with JULABO CF 41. The temperature was observed using the Flir E96 thermal imaging camera on the surface of the CTRC uniaxial tensile specimen and additional thermo elements directly on the grid, as depicted in Figure 6. In each batch, the core temperature development was analyzed using thermal elements in the initial test, subsequently leading to the determination of the heating period's duration. The thermal elements were positioned relative to the lower edge at 20 mm, 250 mm and 480 mm. The surface temperature was checked before each test using a thermal imaging camera, and if the target temperature was not achieved, the heating period was extended. Table 5 provides an overview of the electrical parameters used to set the target temperatures and the duration for the conditioning period within the climatic chamber based on the thermal camera and core temperature. For testing, the rectangular specimens were clamped in steel plates in the upper and lower load introduction areas over a length of 150 mm, cf. Figure 7. A torque of 25 Nm was applied. The tensile strength tests were carried out using a Zwick Z150 TL 150 kN universal testing machine, maintaining a displacement control rate of 2 mm/min until specimen failure. The load, crosshead displacement and temperature of the climatic chamber as well as the core temperature were recorded in the software TestXpert(r). To analyze the full-field deformation of the surface during the tests, the contact-free optical 2D/3D-measuring system ARAMIS(r) from GOM GmbH was simultaneously used. This system determined the exact load- and time-dependent measurements of crack formation and development. Therefore, a stochastic pattern was applied on the measuring area of CTRC uniaxial tensile specimens. With 5M pixel cameras, the measurement system was calibrated with a measuring volume of \(210\times 180\times 175\,\mathrm{mm}^{3}\), a mean calibration deviation of 0.099 pixel and a scale deviation of 0.001 pixels. For the deformation analysis, 5 images per second were taken. Based on the results from [34], the facet size was set to \(15\times 15\) pixels and the pixel overlap to 3 pixels, and the threshold for major strain was defined as 0.5%.