## 2 Experimental Program
### Specimen Configuration  
The test programme was developed to focus on the fire resistance of sandwich wall and its performance in terms of potential use as a fire wall. Three fire tests were conducted, where the side of the panel being exposed to fire and the thickness of the core layer were the investigated parameters (see Figure 3). Two representative limiting cases were tested: thin and thick insulation layer, which were 60 and 200 mm, respectively. Different insulation thicknesses were deemed to cause different temperature increases in the connectors over their lengths and, therefore, different mechanical performances. Additionally, since the concrete wythes were of a different thickness, the side of the wall being exposed to fire could potentially impact the behaviour of connectors. In order to obtain more complete information on the fire resistance of sandwich wall, the two specimens were tested in two configurations, differing in terms of the side of the wall being exposed to fire. This was performed for a wall with 60 mm insulation layer, because in this way, the more severe case could be tested. Specimens with insulation layer of 200 mm were tested only with load-bearing wythe being exposed to fire. The thickness of the insulation layer in contemporary PCSPs used in many countries varies between 50 and 75 mm, while in France and Germany, panels with thicknesses up  
Figure 3: Vertical test set-up and location of the eccentric vertical loading. Figure 2: GFRP connectors used as a shear connection between the two concrete wythes, before assembly in the wall (**a**) and after the fire test—Test 3 (**b**). to 200 mm are produced [10]. On the one hand, thicker insulation layers lead to better thermal properties in walls in normal situations. On the other hand, assuming that the cross-sectional geometry and properties remain constant, an increase in length results in a decrease in shear performance. The lower shear stiffness of connectors with longer free lengths results in a decrease in the degree of composite action generated in walls. Therefore, Specimens 1 and 3, differing in the thickness of their insulation layers, were designed to demonstrate this parameter's influence on wall flexural stiffness in the presence of fire. Thicker thermal insulation, as in the case of Test 3, can cause a longer part of the connector to reach high temperatures. The deteriorated mechanical properties in longer connectors can lead to more severe damage to the shear connection of both concrete wythes than would be the case with short connectors. The heat-induced decrease in connector stiffness and temperatures above the melting point of the resin could result in highly reduced or entirely lost composite action between the concrete wythes under fire conditions. Furthermore, this could lead to impairment of wall integrity and the facing layer falling off the load-bearing layer. This condition, being inacceptable for a fire wall, should be investigated. The geometries of Specimens 1 and 2 were identical in order to determine full fire resistance of the sandwich wall exposed to fire on each side. The facing wythe is usually of a smaller thickness than the load-bearing wythe in order to reduce its weight. As a result, its insulating properties are much lower. Therefore, in sandwich walls with discrete connectors, where the DCA is relatively low, the structural contribution of the facing wythe to the flexural stiffness is typically neglected [10]. Nevertheless, the structural contribution to both the stiffness and the DCA present can potentially be reduced much more quickly when the facing wythe is exposed to fire. Excessive increases in temperature in connectors can lead to potentially dangerous fall-off of the facing wythe. In this case, any degradation of concrete by fire- or heat-induced spalling of concrete also leads to significant losses in connectors' anchorage strength. It should be mentioned that the conditions in the test configuration in which the facing layer was exposed to a fully developed fire are less likely to occur.