## 3 Results  
### Temperature Decrease During Dwell Time in the Testing Machine  
After heating of the specimens PK 1T to PK 3T, they were placed directly into the testing machine. The measuring equipment was connected so that the cooling down could be recorded. The diagram in Figure 5 shows the temperature curves at all four measuring points of one specimen (right y-axis) and the associated cooling rate (left y-axis) over time. The temperature rate was calculated using the derivative of the temperature curves. For the colour assignment of graphs and sensors, compare with the sketch of the test specimen on the right. The differences of the sample temperatures to 50 \({}^{\circ}\)C at the start of the recording can be explained by the test procedure. The measurement started as soon as the specimen was clamped in the machine under minimum load. The lower side (blue sensor) of the concrete cylinder stood on the metal loading plate from the beginning of placement into the machine and showed therefore the highest temperature loss until the start of measurement. In contrast, the surface in the middle and at the top of the cylinder cooled down to a similar extent until the calotte was fully in contact with the upper surface. The measurement inside the concrete shows only a small difference to 50 \({}^{\circ}\)C due to the greatest  
Figure 4: Sample with thermocouples and thermal insulation. distance to the outside environment until the start of measurement, and therefore also the highest starting value. The curves clearly show that the temperature reduction is strongest directly at the beginning of the recording near the load introduction plates which are made of steel. So they dissipate the temperature much faster than the ambient air. After about 10 min, the temperatures at the top and the bottom were already 10 K lower than in the middle. Due to the significantly lower absolute temperature, the discharge speed also decreased now and became lower than that for the sensors in the middle. This can be confirmed by the data from [20], according to which the absolute temperature has a decisive influence on the thermal conductivity of concrete. Another interesting aspect of the recorded data is that in all tests the cooling of the inside of the test specimen was faster than the surface cooling in the same cross-section even at the same absolute temperatures. One explanatory approach is that the steel load introduction plates affect the heat dissipation from the centre of the sample, even if they are three times as far from the centre of the sample as the nearest sample surface. In order to check whether the faster cooling inside the test specimen is really caused by the metal plates, the test on PK 4T was carried out with insulating plates. Figure 6a shows the temperature development of the test specimen when it was clamped between two Styrofoam insulating boards. Because the sample PK 4T was clamped in the test machine a bit faster than the PK 2T after heating, the temperatures at the beginning of the measurement were very similar. For PK 4T, the temperature could only be derived via the surrounding atmosphere. The curves are very similar for all sensors. Especially between the measuring points at half height and at the top and the bottom, the difference is no longer as large as under standard test conditions. The lower cooling speed inside at the beginning of measurement results from the further distance to the surface. Diagram 6b shows in detail the speed of temperature release, displayed via the cooling rate, from PK 2T to allow the comparison with PK 4T. The time required for cooling is significantly higher in the insulated test, as rapid release over metal surfaces is prevented. While the PK 4T achieves a higher cooling rate at the beginning, this is also due to the higher absolute temperature. If we compare the rate at same temperatures, we see that, at 40 \({}^{\circ}\)C, for example, the cooling rate under standard conditions is \(0.75\times 10^{-2}\) K/s and with insulation only \(0.55\times 10^{-2}\) K/s.