## 3 Test Results and Discussion
### Connectors and Insulation  
In a fire situation, the insulation layer should first ensure the thermal protection of the load-bearing layer, stay firm during the fire, and not contribute to fire spread. Further, specifically for sandwich walls, the effectiveness of shear transfer during fire provided by the insulation and connectors should be assessed, since this gives information about their contribution to the composite action. This was achieved based on the observations made on the wall's deformations and its general physical behaviour during and after its exposure to fire. No debonding of the mineral wool from the concrete on the visible outer edges of the walls in Tests 1 and 3 was observed. The deformation of the insulation followed the curvature of the bowed concrete wythes without creating any visible diagonal shear cracks. To the naked eye, upon physical examination, the insulation remained firm during and after its exposure to fire. However, the observation made on the cooled specimens after firing showed that the mineral wool crumbled and became brittle on the fire-exposed side to a depth of up to half of its thickness, especially in the middle of the wall, where its exposure to high temperatures was at its most intense. In Test 2, the fire-exposed facing wythe lost its mechanical connection to the load-bearing wythe in the vast middle area of the wall and, therefore, its bond to the thermal insulation. In this case, however, the insulation remained bonded to the load-bearing wythe and its thermal behaviour was similar to that in the two other tests (Figure 9). The post-fire condition of the connectors was investigated by cutting the specimens in half along their height (see Figure 10). Observations made with the naked eye showed that the connectors in the uppermost and the lowermost row (position A) remained in their original colour and shape, without revealing any visible signs of heat-induced damage. By contrast, the connectors of the inner rows (position B) experienced visible damage due to their exposure to high temperatures. The magnitude of this deterioration varied across the tests. The connectors in Specimens 1 and 3 in position B were substantially damaged along their free length, close to the fire-exposed wythe. In these parts, visible partial carbonation and the absence of resin on the perimeter of the connectors was observed, leaving some fibres unprotected. Similar, but much more severe damage was observed in Specim 3, where the connectors were partially carbonated on the longer part of their free lengths and the damage to the resin protection close to the fire-exposed wythe was larger. In both cases, while the connectors stayed firm and in their original shape, the glass fibres in their cores must have still been protected by the unmelted resin. The length of the described  
Figure 10: Location of the melted connectors in the cut sandwich wall after fire tests—example of Specim 2 and typical damage state of connectors. visible heat-induced deterioration was equal to ca. 20 mm and 70 mm for Specimens 1 and 3, respectively. This difference could be explained by the greater increase in temperature in the longer parts of the connectors in Specim 3 due to their significantly thicker thermal insulation (200 mm). In order to better identify the damage, the connectors were taken out of the concrete from the middles of Specimens 1 and 3, where they appeared to still be firm and rigid upon hand inspection, even along their length, and embedded in the fire-exposed concrete wythe. The partial fracture and exposure of the glass fibres visible in Figure 2 occurred unintentionally during the removal process, which may indicate that the connectors' mechanical properties significantly deteriorated. This observation goes far beyond the results reported in [19], where the GFRP rebars made from the same material as the tested connectors did not show any visible degradation of their external surfaces after reaching a temperature above the glass transition temperature \(\mathrm{T_{g}}\) of the resin.