Glass-rigid foam composite for innovative concrete sandwich elements

In building envelopes, sandwich elements with facings made of glass currently require either adhesives or mechanical connectors. The avoidance of any connectors seems to be favorable in terms of resource and energy savings both in production and in building envelopes. The present studies are part of the development of a glass-rigid foam-concrete sandwich element without additional adhesives and mechanical connectors. This paper reports on the structural bond behavior between polyurethane rigid foam and float glass with different surfaces with or without applying a bonding agent. Tensile bond and shear tests show, that a sandblasted toughened glass surface results in cohesive failure of the insulation layer. The two production-related surfaces of float glass are defined as atmosphere and tin side. Both surfaces offer an adhesive failure between the insulation layer and glass. Test specimens of glass and insulation layer without bonding agent show no significant differences between the atmosphere and the tin side. Overall, the test specimens with bonding agents achieve higher levels of adhesive tensile bond and shear strength. Light and electron microscopic studies of fractured surfaces show, that the bonding agent has a significant influence on the wetting and pore formation of the liquid polyurethane.


| State of the art
Building envelopes with opaque glass as the final surface layer combine the requirements of design, durability, and weather protection in facade construction. 1 Panes of opaque, colored, or coated glass suggest themselves as surface layers (Figure 1). Glass also offers the option to use solar power by integrating photovoltaics. 2 Typical applications in concrete constructions are sandwich wall elements, which comprise a load-bearing shell made of reinforced concrete, an insulation layer made of rigid foam, and a reinforced concrete facing shell fixed to the load-bearing shell by mechanical connectors. 3 The sandwich wall element in Reference 4 combines the conventional build-up with a facing made of glass panes, which are joined to the reinforced concrete facing shell by polymer cement adhesive mortar (Figure 2A).
Lightweight steel-rigid foam sandwich panels, which use a direct bond between steel sheets and a core of polyurethane rigid foam as a proven method, are also often applied in building envelopes. In Reference 5, this structural member has been enhanced by using glass as an additional layer ( Figure 2B). The steel-rigid foam sandwich panels are fitted with an additional layer of heat-strengthened glass 6 which is adhered to the steel surface. However, installing connectors or applying bonded joints are labor and cost-intensive. The technical innovation of the new glassrigid foam-concrete sandwich element presented, provides loadbearing composite joints, which ensure load transfer only by structural adhesion between the individual layers with no additional adhesives or connectors (Figure 2c). Without these connectors the design, which is applied and analyzed for the first time in this context, proves to be more economical with regard to material saving and manufacturing requirements. Furthermore, due to the elimination of penetrating mechanical connectors and resulting thermal bridges, higher thermal insulation is possible with a lower component thickness.

| Objective
Elimination of adhesives or mechanical connectors in the new glassrigid foam-concrete sandwich element requires knowledge of the composite behavior between concrete and rigid foam, and between the glass and rigid foam. A detailed review and parameter study on the bond of various types of rigid foam and concrete of different strengths can be found in Reference 7 and 8, demonstrating the suitability of such composite joints without any adhesives or mechanical connectors for precast concrete sandwich elements. In Reference 9 and the present study, the technical feasibility of a glass-rigid foam sandwich element without adhesives and mechanical connectors is demonstrated by characterizing the composite behavior of glass and freshly applied polyurethane foam.
Furthermore, these first results show the general suitability for application as a facing layer with integrated insulation in sandwich elements with precast reinforced concrete load-bearing shells. In this context, the glass layer requires sufficient tensile bond and shear strength to the rigid foam for various loads, such as dead or wind loads. Therefore, the focus of the tests for the manufacturing process is primarily on the application of the rigid foam to the glass surface.
Subsequent investigations include the determination of the tensile bond and shear strength in order to characterize the load-bearing behavior of the composite joint under tensile and shear stress. The mechanical tests are supplemented by microscopic analysis of the glass and rigid foam fractured surfaces.

| Bond between the glass and rigid foam
Due to its constant material properties and surface quality, the investigations of the bond between the glass and rigid foam are done on industrially produced float glass (FG) made from soda-lime-silicate glass with a characteristic flexural tensile strength f g,kk of 45 MPa, and a Young's modulus of 70 000 MPa. 10 As a result of manufacturing in a float process, it has two different surfaces referred to as the atmosphere and tin sides. Sandblasting of one of the smooth sides of the float glass provides the third surface with higher roughness for the study. Thermally toughened glass as heat strengthened glass 6 or toughened safety glass 11 is initially not investigated during the course of this study since the flexural tensile strength of the glass is not relevant for the bond behavior.
Generally, the bond between two solids made by the adhesive is formed through adhesion and through micromechanical interlocking at the interface. 12 The quality of the bond largely depends on the effective surface. Due to the high porosity of the rigid foam, the surface available for load transfer consists of the cut cell walls, the number, and thickness of which can be increased through the use of bonding agents. 13 Increased surface roughness of materials results in an improved bond thanks to a larger contact area and the resulting improved interlocking of the joint partners. However, this requires the rigid foam to match the surface profile of the glass as good as possible. Otherwise, for instance, in the case of gas pockets included in Rigid foam insulation materials can be made from expanded polystyrene (EPS), extruded polystyrene (XPS), or foamed polyurethane (PUR). 14 The direct application of expanded polystyrene to glass is only possible at considerable expense with production facilities in the form of an expander and requires the production of metal negative forms which must be integrated into an existing production process.
After leaving the extruder, extruded polystyrene exists as a profile strand, the surface of which sets when it leaves the outlet and thus does not allow for a direct bond with the glass surface. By contrast, the process with foamed polyurethane offers a cost-effective and quick option for production, in which liquid polyurethane is applied to one side of the glass panes, sets into the rigid foam through the addition of pore inducers onto the glass surface, and allows for an adhesive bond. The following tests are therefore done with a polyurethane rigid foam. 15 2 | TEST OF THE COMPOSITE JOINT

| Production of the glass-polyurethane blanks
For the application of the polyurethane onto the glass surface, the glass panes are integrated into the common process for producing steel-rigid foam sandwich elements. In this process, the raw polyurethane mixture is foamed between two facing sheets made from steel.   Figure 4 shows the test set-up. The thickness of the polyurethane rigid foam on the glass panes is 40 mm. Test specimens with a 100 mm by 100 mm bonding surface are produced from the rigid foam. The glass pane is bonded with a carrier plate made from plywood and can therefore be braced against the testing machine. The tensile force is applied to the test specimen on the upper side via a steel plate which is screwed onto a plywood panel glued to the polyurethane rigid foam.
The load is applied displacement-controlled with a rate of 0.5 mm/min until fracture. Testing of the bond between polyurethane and the untreated surfaces of the atmosphere and tin side of the glass and the sandblasted surface is done in three series.
The same surfaces, but treated with bonding agent before application of the polyurethane, are tested in three further series. The five test specimens for each one of six surfaces results in a total of 30 tests.  For the atmosphere and tin sides without bonding agent a predominantly adhesive failure is identified. However, isolated planar polyurethane rigid foam residues are found on the glass surface, which means that a partial cohesive failure can also be assumed. The percentage of residues on the glass surface is smaller on the atmosphere side than on the tin side. The surfaces of the tin side with bonding agent and the sandblasted surface with and without bonding agent indicate a cohesive failure in the rigid foam.

| Electron micrographs and image analysis
The quality of the bond depends on the chemical, geometric, and mechanical properties of the surfaces of the bonded parts, among other things. 14  The results of the image analysis are summarized in Table 3  atmosphere sideshow a value which is 6% lower with bonding agent than without bonding agent. However, the coefficient of variation, at 28.2%, is relatively pronounced in comparison with the other results and shows a trend toward higher strengths in the individual results. It can therefore be assumed, that the connections from the image analysis with a higher number of test specimens also prove true for shear tests on the atmosphere side.
3.2 | Element detection on the surfaces of the glass and polyurethane rigid foam

| Light micrographs of the fractured surfaces
The quality, of the fractured surfaces of test specimens, which dem-

ACKNOWLEDGMENTS
The results of this research are based on a joint venture between the