## 4 Summary, Conclusion and Outlook
The reduction of the speed of the temperature increase after the start is in the range of 0.5 to \(0.75\times 10^{-2}\) K/s in the uninsulated tests (PK 5T/6T). In contrast, in the tests with insulation around the samples (PK 7T/8T) the maximum reduction was \(0.2\times 10^{-2}\) K/s (compare Figure 10b). To show that the cooling of the PK 4T with insulation between sample and loading plate represents the difference between the heatings with and without insulation around the specimen, a comparison can be made between, for example, PK 7 TI (yellow curve in Figure 10) and PK 5T (light blue curve in Figure 10). The test specimen, which is insulated on the surface, shows virtually no reduction in the temperature rate along its entire length. The test specimen without insulation reaches the minimum temperature rate with a heating of approx. 19 K. Compared to the start, the heating rate has been reduced by \(0.48\times 10^{-2}\) K/s. If the ambient temperature of 22 \({}^{\circ}\)C is added to the 19 K, the absolute temperature of PK 5T at the time of slowest heating is obtained. Finally, this temperature can be used to read the temperature rate lost over the surface in Figure 6a. This is approximately \(0.5\times 10^{-2}\) K/s. It can therefore be seen that the cooling rate of PK 4T describes the difference between PK 5T and PK 7 TI well. This may only be a first example, but taking into account the increasing thermal diffusivity of the concrete as the temperature rises (compare [20]), it can be assumed that the reduction in the rate of increase in uninsulated tests is mainly due to the increase in cooling. In summary, the following main conclusions can be made from the results of the presented test series:  
* During heating in the fatigue tests, the temperature is released most strongly via the load introduction plates. * The pre-damage has no significant influence on the cooling rate of the specimens. * A cyclical pre-damage leads to a faster temperature increase with a new cyclical load. * In addition to the temperature gradient in the cross-section, the absolute temperature has a strong influence on the early failure. With the above-mentioned main statements and the conclusions drawn concerning the relationships between heating, cooling, and damage influences, approaches were found with which the heating description in the fatigue test can be better understood. It is a further step to classify the heating phenomenon as a specific problem of the test technology. In order to get closer to the project aim, additional cooling experiments in the testing machine with insulated samples are planned, so that the proportion of the heat dissipation via the steel plates can also be considered separately. Knowledge of the temperature release capacity in tests up to 100 \({}^{\circ}\)C would also make it possible to add temperature-related heat released to the heating rate curves and to represent the realistically generated temperature at the heating speeds then reached. Furthermore, an extensive investigation of the concrete compressive strength of HPC and UHPC under the influence of temperatures in the range of the achieved test temperatures is currently being carried out. Conceptualization, M.D. and S.S.; methodology, M.D., S.S., and N.L.T. ; validation, S.S.; formal analysis, M.D. and N.L.T. ; investigation, M.D. ; data curation, M.D. ; writing--original draft preparation, M.D. ; writing--review and editing, S.S.; visualization, M.D. ; supervision, S.S.; project administration, S.S.; funding acquisition, S.S. and N.L.T. All authors have read and agreed to the published version of the manuscript. **Funding:** This research is funded by the Deutsche Forschungsgemeinschaft (DFG, German Research foundation).