## 3 Results
### Specim with Thermal Insulation  
In [15], two possible effects of temperature on early failure of high-strength grouted concrete were presented, which may also apply to UHPC. On the one hand, for all specimens with temperature sensors in the concrete (such as Figure 3a), which showed a temperature increase until failure, an increasing difference between the internal and external temperature was determined in [22,24] in analogy to [15]. This gradient can cause constraint stresses and change the force flow through the specimen so that the effectively loaded area becomes smaller and the real related upper stress increases. A further effect can be the reduction of the static compressive strength with increasing temperature [31]. Insulated tests were carried out to verify the hypotheses. The aim was to minimize the temperature gradient in the cross-section through a casing with an insulation pipe. The insulated and non-insulated test specimens are shown in Figures 3 and 4. Figure 10 shows the temperature curves (10(a)) and the heating rate (10(b)) for the different specimen preparations. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Designation**} & **Max. Heating Rate I** & **Max. Heating Rate II** & **Difference in Heating Rate** \\  & [**K/lc]-10\({}^{-3}\)** & [**K/lc]-10\({}^{-3}\)** & [**K/lc]-10\({}^{-3}\)** \\ \hline PK 1T & 0.83 & 1.54 & 0.71 \\ PK 2T & 0.85 & 1.39 & 0.46 \\ PK 3T & 0.87 & 1.59 & 0.72 \\ PK comparison [24] & 0.80 & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Maximum heating rate at first and second cyclic load. Figure 9: Heating rate for the first and second fatigue load inside the specimen. The uninsulated samples PK 5T and PK 6T both show a growing temperature gradient over the duration of the experiment (Figure 10a). These gradients differ in their size. In the test shown in light blue, the difference reaches a value of 12 K over the cross-section. In the test shown in purple, this difference is only half as large, at a maximum of 6 K. Almost as large as the relative difference of the gradients is also the difference of the number of load cycles achieved until failure. At the small gradient, 122,095 load cycles, and at the large gradient, only 69,370 load cycles were achieved. Thus an influence of the gradient seems plausible. In the insulated attempts (PK 7 TI and PK 8 TI), the goal of a significantly reduced gradient was achieved. Only shortly before failure differences of once 2 K and once 4 K were measured. However, a higher number of load cycles than in the uninsulated tests was not achieved as expected. The samples failed after 66,186 resp. 52,462 load cycles even earlier. The gradient can therefore not be the sole  
Figure 10: (**a**) Temperature development and (**b**) heating rate of test specimen with (red and orange) and without thermal insulation (purple and light blue), test conditions: frequency f\({}_{\text{P}}\) = 10 Hz, relation between lower and upper stress level S\({}_{\text{c,min}}\)/S\({}_{\text{c,max}}\) = 0.1/0.7. main influencing factor. In addition to the changed differences, the insulated samples also showed a stronger temperature increase. Thus, the temperatures reached inside and on the surface were higher than in the non-insulated tests. In the centre of the test specimen, at around 55 K, about 10 K more were achieved, and at the surface, at around 52.5 K, the difference was even nearly 15 K. These results indicate that it is mainly the maximum temperature reached in the test specimen that has an influence on the test specimen failure. The differences in the temperature curves can also be seen in Figure 10b. On the one hand, the insulated tests show good agreement for the heating rate inside and outside and, on the other hand, the curves are different from the uninsulated curves. The starting values of the cooling speed of all four tests lie between \(0.75\times 10^{-3}\) K/lc and \(1.00\times 10^{-3}\) K/lc and thus in a corresponding range.