Adhesive Bond between UHPFRC and construction materials

The excellent advantages of Ultra High Performance Fibre Reinforced Concrete (UHPFRC) are durability, strength and ductility. Thus, various possibilities for new developments in structural concrete arise. On the one hand the outstanding mechanical properties of hardened UHPFRC allow slender and light building elements, which almost reach the aesthetic occurrence of steel structures. On the other hand its good rheological properties in the initial liquid state can easily be utilised in order to get effective bond to other materials. Hence, mixed building technologies can be developed, where each material contributes its beneficial properties. An impressive example for such developments is the glass-concrete-technology, which combines the robustness of reinforced concrete and the elegance end transparency of glass. Mixed building technologies (Composite Structures) are generally based on adhesion between the different components. Adhesion depends on numerous parameters like chemical composition, wetability, roughness and morphology etc. which also lead to several different bond mechanisms. These complex interdependencies are not studied systematically yet. The current models only consider the roughness characterised by the mean roughness depth and the strength of the concrete. This proposal presents a coherent multi-disciplinary approach to study these phenomena. Investigations of the interfacial zone with electron microscopy and computer tomography will give information about the chemical and the geometrical mechanisms of adhesion. Intermolecular forces will be considered by a thermo dynamical approach. Information about the amount of these forces is expected to get by experimental determination of the surface energy of both materials. The theory proposed bases on the correlation between true contact surface and adhesive power. Therefore, special attention will be given on the measuring of the geometry of the surface in a micro scale. The method of evaluation of the measured data will depend on the wetting property of the applied UHPFRC. An extensive mechanical testing program will complete the necessary data for the finding of a reliable calculation model for adhesive bond strength as well as a meaningful constitutive law. The results of the project will provide the scientific base for safe design of innovative composite constructions with UHPFRC. Moreover, the theoretical model will be formulated by simplified but really existing phenomena, which should provide the possibility for an easy transfer to other materials.

Ultra High Performance Concrete (UHPC) is a young, mineral material, which has allowed significant development in architecture as well as in sustainability of infrastructure buildings like bridges. Preferably UHPC is used together with components made of other materials, for instance steel bars, glass panes, steel and glass fibres. Therefore, its adhesion to those materials is very important. The well known gluing effect brought about by the cramping of long chain molecules to a micro roughness does not exist in UHPC. Thus, its specific mechanisms as well as the power of adhesion are explored in that project, studying the micro and macro scale. It turned out that the adhesion to smoothly polished steel is strong to an extent that loading leads to fracture of UHPC. This happens although only intermolecular attraction forces account for the adhesion. That level of adhesion can only be obtained, if the shrinkage of UHPC, which always occurs during hardening, is sufficiently limited. Otherwise, cracks arise a few hours after the making of the contact between the pasty UHPC and its partner surface. Until this moment UHPC can easily be deformed and as a consequence no enforcement will come into being. But, in the further course of the hardening process each crack becomes an origin for stresses in the interface. They try to separate the UHPC from its partner surface. For practical application the project provides all the findings required for making theoretical predictions about the decrease of adhesion due to shrinkage enforcements. From a chemical point of view the adhesive power between UHPC and glass could even exceed that of steel. But due to the high surface energy of glass water of the UHPC mixture concentrates on the glass surface immediately after wetting. Therefore the distance between the surface and the molecules coming into existence during the hardening process is greater than it is in the case of steel. The theory shows that a few tenth of a nanometre suffices for a reduction of adhesion to 50% which is observed from experiments. An experimental series with different loading directions with respect to the interface leads to the finding that the well known law of friction can also be applied to the molecular force interaction in the interface. It can be shown that the friction coefficient of steel is valid for both, the nano scale of the molecules and the macro scale representing the dimensions of real things. Building on these findings the effectivity of rough surfaces can be explained with regard to physical and geometrical aspects. It is surprising that the friction between smooth glass and UHPC turns out to be stronger in the nano scale. This outcome has indeed not been proved in the macro scale so far, but from a theoretical point of view the higher effectivity resulting from the roughening of glass surfaces verifies that fact. The preoccupation that water, which is physically embedded in the pores of UHPC, is accountable for the existence of adhesion could fully be allayed. This is a very important fact with regard to the sustainability of such adhesive connections in building practise.