Composite behaviour of wide sandwich panels with high performance concrete thin wythes with and without Thermomass shear connectors

To partly address the future sustainability of energy use in buildings, there is a programme in Europe to retrofit domestic and commercial buildings with non-load bearing insulated re-cladding and over-cladding pre-cast concrete sandwich panels. Recent research work has shown that limited composite action is possible depending on the type of non conductive shear connector used and the stiffness of the insulation between the inner and outer wythes. This paper reports on the development of a sustainable mix design incorporating recycled aggregates to produce high strength 20mm thick fibre reinforced concrete wythes. These were incorporated into the manufacture of wide sandwich panels with five silicone-bonded layers of XPS insulation, making up a panel which was 220mm thick. These model panels were then tested in flexure using displacement control tests to establish the effectiveness of the composite action and the extent of post cracking toughness of the panels in resisting flexural loads. During
the observation of good composite action in the elastic range for the shear-connected panel, it was noted that the lower wythe cracked first due to its stiff support on the end bearings while the upper wythe flexed less due to effectively sitting on the insulation acting as an elastic foundation. This panel exhibited considerable post-cracking toughness, with observable interleaf shear
movement in the insulation. On the other hand, in the absence of shear connectors, the top and bottom wythes both cracked under
low load while a small residual strength was offered by the fibres pulling out and the shear sliding of the insulation, albeit with considerable flexibility.