## 4 Summary, Conclusion, and Outlook  
The results from Chapter 3 show that all three parameters examined have a noticeable influence on the heating of ultra-high performance concrete. Regarding the upper stress level, it can be said that the warming is faster with increasing values. At the same time, not only the heating speed is higher, but also the degradation under fatigue stress increases. This caused an earlier failure. Finally, the highest upper stress level does not condition the highest temperature difference at the same time. Tests with a load cycle between 10% and 70% of the mean concrete strength did not achieve the calculated values according to the Wohler curve of the fib model code 2010 for UHPC 1. The lowest tested maximum stress level at \(\text{S}_{\text{c,max}}=0.6\) showed only very low temperature increases. It can be assumed from this that there is a minimum maximum stress level below which no (or negligible) heating occurs in the specimen. One thesis to explain a minimum upper stress level is the approach of deformations in the elastic and plastic range. Only at a certain stress does the proportion of plastic deformation and thus irreversible structural changes become so large that heat can be generated by internal friction in microcracks. If the stress remains within the elastic range of the concrete, no significant heating is to be expected. As a differentiation upwards, the influence of a modified damage above 75% of the mean concrete strength can be decisive (see Section 3.2). Additional tests with smaller load steps between upper stress levels that were investigated until now will demonstrate this influence in more detail. For the parameter frequency, it was found that the increase of the load speed increases the achievable temperature difference. This applies to tests with failure as well as to the run-throughs that had reached the break-off criterion. At the lowest frequency, only a small heating was generated and in no case the expected value, which calculation takes no temperature influence into account, was not achieved. Therefore, a minimum speed can also be assumed for the frequency, which is required to generate heat under internal friction. However, between 3 Hz and 10 Hz the difference in the temperature increase is considerably greater than in the comparison between 10 Hz and 20 Hz. A decreasing influence can be seen. With further intermediate steps regarding the frequency, this should be checked to investigate whether there is an upper limit value above which the temperature generation is not further influenced by the frequency. The heating rate in relation to time increases with increasing frequency due to the more frequent load changes per time. This can only be detected per load change after the constant crack growth, the so-called phase II, has been reached [1; 4; 21; 22; 23; 24]. However, the average speed increases with increasing frequency. The influence of different maximum grain sizes was tested with two UHPC mixtures of approximately equal strength. The mixture with grains up to 1 mm warmed up faster than the mixture with 8 mm. The maximum temperatures reached, however, were on average almost the same. The only difference was that the maximum temperature was reached later due to the lower heating rate. On average, with the same configuration, it is possible to carry out a fatigue load twice as long with the larger maximum grain size. The comparison also showed that there is an influence on the fatigue strength due to the different temperature increase. Thus, only the UHPC 1 does not achieve the expected value according to [20] under rapid heating. An explaining approach is the larger inner surface between cement and aggregates with a smaller maximum grain size. This greater friction potential could have a decisive influence if the components matrix and aggregate have different stiffness. On this thesis we will focus in our further investigations.