## 3 Test Results and Discussion
### Composite Action  
The mechanical connection of concrete wythes provided by insulation and connectors is crucial for the fire resistance of sandwich walls for two reasons: First, the composite action provided by the shear transfer between the concrete wythes increases the flexural stiffness of the wall, and second, burning debris or pieces of the facing wythe must not fall off the load-bearing wythe of the sandwich wall applied as a fire wall. The effectiveness of the connection of the two concrete wythes was monitored with LVDT6 in Test 1 and 3,which measured their relative displacement. A general assessment of the composite action was performed on the basis of these data and of observations of the wall's deformations during exposure to fire. The failure of the connectors should result in a displacement downwards, since the shear stiffness of the core layer is weakened by exposure to fire. On the other hand, the thermal bowing of the entire wall and the rotation of the cross-section at the wall base should result in the upward displacement of the bottom edge. The displacement recorded with LVDT6, which was the result of the two described effects, showed that the facing wythe's bottom edge was moving upwards (see Figure 11). This indicates that the displacement of the edge resulting from the rotation caused by the heat-induced bowing was clearly of a larger magnitude than the potential sagging of the facing wythe under its weight supported by the core layer. As schematically depicted in Figure 12, the thermal elongation of the fire-exposed side of the wall induced bowing in the direction of the furnace. The edge of the facing wythe following the curvature of the deforming load-bearing wythe was moving upwards, since the rotation point was at the bottom edge of the load-bearing wythe. The small initial downward displacement (\(-0.2\) mm) in the case of Test 3, visible in Figure 11, was caused by the applied vertical load. This phase was skipped in the case of the Test 1 (where the initial displacement was 0.3 mm) due to problems with the furnace controller resulting in a minor initial pre-heating of the wall. In both tests, the displacement of the facing wythe began an intensive growth phase 9 min after the furnace ignition, when the walls underwent heat-induced bowing. The final relative displacement of the facing wythe's edge was larger by 1 mm in Test 3 due to the thicker cross-section of the wall and, therefore, larger temperature differences between the concrete wythes, leading to larger horizontal deflection (see Figure 7) and larger rotation at the bottom edge of the wall. As indicated in [15] the facing wythe is assumed to provide thermal and non-structural protection to the load-bearing layer in the presence of fire. This function was fulfilled throughout the duration of the tests, even on Test 2. Despite the local damage to the connectors, the facing wythe and the insulation stayed in place. The observed behaviour of Specimens 1 and 3 indicates that the degree of composite action was sufficient to follow the curvature of the deforming wall by both concrete wythes and keep them in a steady relative position. In Tests 1 and 3, in the visible cross-section of the facing wythes sticking outside of the furnace, a few regularly distributed curvature-induced cracks with spacing of approximately 500 mm were observed. The cracking of the unexposed wythe was interpreted to mean that the connectors and insulation provided the wythes with a connection  
Figure 11: Relative displacement of the facing wythe. that was sufficient to ensure the equal curvature of the deformed wythes. The mechanical coupling of both wythes was provided, and both facing and insulation layers supported the flexural strengthening of the load-bearing wythe. In this case, the flexural stiffness of the deformed unexposed wythe imposed by the thermal extension of the exposed wythe acted counter to the deflection caused by the fire loading, resulting in the reduced deflection of the entire wall.