## 5 Results and Discussion
### Concrete Testing in Elevated Temperature  
The cubic samples were heated at 300 \({}^{\circ}\)C, 500 \({}^{\circ}\)C, and 700 \({}^{\circ}\)C, in a special furnace: PK 1100/5 (Termolab S.C., Warsaw, Poland), powered by electricity. The station is equipped with a dedicated ThermoPro program that allows you to program the heating process. The temperature distribution in the tested element was monitored with the use of NiCr-Ni thermocouples, which meet the requirements of the standard [81]. Before starting the annealing, all samples were dried to constant weight in a laboratory dryer. The test was performed according to the \"temperature-time\" standard curve. To determine the heating time at each particular temperature, pilot tests were performed on specially prepared samples in which special holes were made for measuring the thermocouples. After reaching the assumed temperature, the samples were annealed for another 30 min, which was aimed at equalizing the temperature on the outer surface of the sample and throughout its entire volume. The temperature distribution was monitored using four thermocouples. Holes for heating the thermocouples were made in the center of the sample (T3), 25 mm from the edge of the base (T2), and 10 mm from the edge of the base (T1). The control thermocouple (TR) was introduced through the back wall and located near the stove ceiling. The drilling depth was 50 mm. The increase in temperature weakened thestructure of the material. Surface cracks and scratches were visible on the sample surface. The actual example of the temperature distribution is shown in Figure 11, which makes it possible to observe the behavior of the concrete in a fire situation. Figure 12 shows the results of the average compressive strength of the concrete samples after annealing at different temperatures. The studies performed on the influence of high temperatures confirmed that it is possible to use fly ash from the incineration of sewage sludge in concrete subjected to thermal loads. The annealing itself was carried out in accordance with the assumed schedules (Figure 12). It should be noted, however, that the slope of the curves, and thus the time needed to achieve the set temperatures, was much longer than the time in which these RABT and RWS temperatures are achieved. This was due to the technical limitations of the research furnace used. The achieved characteristics are, however, similar to the so-called standard curve, which is used to test the fire resistance of load-bearing structural elements. No explosive damage to the concrete samples was recorded during the tests. This is due to preparing the samples for the thermal load tests by drying them to a constant mass. Under normal conditions in tunnels, during a fire, explosive spalling is likely to occur due to the elevated dampness of the reinforced concrete linings of the tunnels, which is caused by environmental conditions as well as the conditions of use. The compressive strength tests performed showed that, regardless of the amount of the additive in the form of fly ash from sewage sludge incineration, high temperatures reduce the strength. In each case tested, the temperature of 700 \({}^{\circ}\)C caused a reduction in strength. Notably, in relation to the temperature of 20 \({}^{\circ}\)C, the increase in the amount of ash increases the strength of the samples. Figure 11: Temperature distribution in the pilot sample—300 \({}^{\circ}\)C Heating Curve. Figure 12: Average compressive strength of thermally loaded samples. In the case of ordinary concretes with silicate or limestone aggregate, when heated at 300 \({}^{\circ}\)C, the decrease in compressive strength is up to 10% in relation to the normal temperature. The results of our tests showed that for BZ and P5% samples the strength increased. It is difficult to clearly define what factors caused this phenomenon; perhaps it is influenced by the type of aggregate used.