## 1 Introduction
Even though the insulation layer and facing wythe are not usually considered as load-bearing layers in a structural design, the mechanical coupling of both concrete wythes, which is caused by the transfer of shear forces between them through connectors and insulation, can significantly contribute to the overall stiffness and strength of the panel [12; 13; 14; 15]. The degree of composite action (DCA) describes the level of mechanical coupling of concrete wythes. The higher the DCA, the stiffer the sandwich panel and, in terms of stress distribution, its structural behaviour resembles that of a monolithic member [16]. High DCA is favourable in terms of the flexural stiffness and strength of panels and enhances their stability. As mentioned in work on the fire performance of steel sandwich panels [17], due to their various constituents, sandwich-panel walls exhibit different behaviour in the presence of fire to their pure components and should therefore be assessed as a whole system regarding their flammability. However, according to the authors' knowledge, no design codes and very limited work [13; 15] addressing the behaviour of sandwich panels and their degree of composite action under fire load can be found. Since high DCA activates the facing wythe and insulation layer in flexure, causing them to be more stressed, its influence on structural performance at elevated temperatures should be investigated in depth. Therefore, the accurate assessment of the contribution of the temperature-dependent core layer's shear stiffness would lead to the less conservative and more efficient design of PCSPs. The consideration of this contribution to the design of PCSPs could increase their predicted fire resistance or enable the reduction of concrete wythes' thickness while maintaining equal fire resistance. The potential use of sandwich walls as fire walls is of great significance for the construction industry. There is a gap in the normative regulations regarding the fire behaviour and fire resistance of sandwich walls. Furthermore, a classification concept for sandwich walls is lacking, hindering the wide use of PCSPs as fire walls. This paper addresses this knowledge gap by demonstrating the effect of fire loading on the fire resistance of PCSPs on three full-scale sandwich walls. Importantly, the data from these tests could form the experimental background against which to develop a validated classification concept and criteria for the estimation of the fire resistance of sandwich walls in the future. The insufficient experimental work performed on sandwich panels with energy-efficient connecting systems, such as GFRP connectors, and the difficulty in directly using standard testing methods, constitute barriers to the fire classification of sandwich panels. Addressing these issues, this experimental work involved the analysis of:  
* The potential use of a sandwich wall as a fire wall. The fire resistance of a sandwich wall and the fire behaviour of its components were tested and analysed, taking into account the REI-M [18] requirements for load-bearing fire walls. * The structural integrity of a sandwich wall under fire and impact load. The mechanical coupling of a facing wythe, unsupported at the base, to a load-bearing wythe must not deteriorate to the extent that it falls off during fire exposure time or impact loading. * The fire behaviour of GFRP connectors. The mechanical performance of connectors, e.g., their support for the facing concrete wythe and their contribution to the shear and flexural stiffness of the wall and anchorage safety in the presence of fire was investigated. Furthermore, their behaviour in terms of combustibility, fire spread and smoke evolution was analysed. Heat-damaged connectors can leave empty holes in wythes or enhance heat-induced concrete spalling, degrading the space-enclosing function of walls. The impact of these effects on the fire resistance of an entire wall was assessed. * The composite action of a sandwich wall in the presence of fire.