## 3 Test Results and Discussion
### Heat-Transfer Behaviour  
The heat transfer from the fire-exposed side to the fire-unexposed side was monitored with thermocouples TC1-10. Considering the requirements of structural members, which should serve as fire walls, the mean temperature on the fire-exposed side must not exceed 140 \({}^{\circ}\)C, while the maximum temperature on the side not exposed to fire must not exceed 180 \({}^{\circ}\)C above the initial temperature. Although testing standards do not set requirements  
Figure 8: Temperature in the plane between the insulation and the concrete wythe not exposed to fire. Figure 9: The mean temperature increases on the side not exposed to fire. for the temperature increase within the cross-sections of walls, knowledge about the temperature gradient through the wall to the side not exposed to fire enables a better understanding of the thermal and mechanical performances of insulation and connectors. A sudden increase in temperature to around 100 \({}^{\circ}\)C at the depth of the insulation and non-fire-exposed concrete wythe interface was observed (see Figure 8). This indicates that after the temperature of 100 \({}^{\circ}\)C was reached in the fire-exposed wythe, the moisture escaping the concrete and insulation increased the temperature throughout the thickness of the insulation in a very short time of ca. 5-10 min. The observed plateau at around 100 \({}^{\circ}\)C can be explained by the evaporation of free water at the interfacial depth, as a similar effect was observed in the fire test on sandwich panels in [15]. This phenomenon should be taken into consideration, especially in the case of temperature-sensitive insulating materials, where sudden increases in temperature up to 100 \({}^{\circ}\)C can entirely eliminate the insulating layer from the structural contribution to the flexural stiffness of sandwich panels. The faster increases in temperature in the case of Specimen 2 (after 18 min) were caused by the thinner concrete wythes protecting the insulation from heat, indicating that the thickness of fire-exposed concrete wythes can have a crucial influence on the fire resistance of heat-sensitive insulation materials in sandwich walls. The small decreases in the temperature of the interface, from 100 \({}^{\circ}\)C to 96 \({}^{\circ}\)C, recorded in Test 2 may have been caused by the evaporation of water from the insulation layer and the removal of the main heat-transferring agent at this stage of the fire. The mean temperature increase measured as a mean value of the temperatures recorded with thermocouples TC1 to TC5 in each test is presented in Figure 9 (TC1-5). Specimens 1 and 3, which differed only in their insulation thickness, showed similar heat transfer behaviour. In both tests, the intensive temperature increase phase started approximately 50 min after the furnace ignition and, therefore, at the end of the tests, almost the same temperature (difference of \(\sim\)1 \({}^{\circ}\)C) was reached. However, the maximum mean temperature in the case of Specimen 3 reached only 51.0 \({}^{\circ}\)C (only 37% of the maximum allowable mean temperature). The mean temperature increased more quickly in the case of Specimen 3, which had a thicker insulation layer. This indicates that the thickness of the thermal insulation does not play a leading role in terms of the heat transfer under fired conditions when boiling water needs to escape the wall. The relatively small difference in heating rate between Test 1 and 3 could have been caused by a difference in moisture content in the concrete and the insulation. The moisture content was not measured; however, a few litres of condensed water steam leaked out of Specimens 1 and 3 outside the furnace, and intense evaporation was visible on the perimeters of the walls. In the case of Sandwich 2, where both the facing wythe and the insulation were located directly in the furnace, the temperature increase started 5 min sooner, at 40 min after the furnace ignition, and reached 29.7 \({}^{\circ}\)C, only 21% of the maximal allowable mean temperature.