Effect of adhesive joint stiffness on optimal size of large-format cladding comparison of artificial and real environment

Abstract The efficiency and lifespan of bonded facade joints can be increased and extended by designing the optimal size of the facade cladding. In the case of large-format cladding, the maximal possible size is always desired. The authors of this paper have scrutinised a relationship between the size of the facade cladding and the stiffness of the support, i.e. the bonded joint. In general, the higher the stiffness of the joint, the smaller the size of the cladding can be used in the design. After the experimental analysis of the material properties, very simple numerical approach with consideration of a linear stress-strain behaviour was chosen to determine the optimal size of the facade cladding. This paper investigates the material properties of selected adhesive systems, two of which are polyurethane-based and two are based on silyl modified polymers. The effect of the stiffness of the adhesive joint was assessed in combination with four different facade claddings (Cetris Basic, multilayer solid wood panel, solid timber plank and wood plastic composite) with a relatively high thermal and moisture expansion. It was, therefore, necessary for the bonded joint to be flexible and to adapt to these dimensional changes without creating greater internal tension. Moreover, the experimental part included artificial as well as real weathering conditions which allowed to compare the reliability of commonly used laboratory methods. While in combination with Cetris Basic the recorded data did not show any significant differences and the weathering method was less important than the chemical composition of the adhesive system, the results obtained in combination with solid wood panel demonstrated exactly the opposite. The joint stiffness with PU-I recorded after weathering in the real environment had dramatically increased, which led to the strength and adhesive elongation reduction by more than 50%, this also caused a premature failure of the bonded joint, and it greatly affected the cladding size that had to be reduced by 46%. Similar phenomenon was observed in combination with both MS-based adhesive systems after artificial weathering. Moreover, the size of the cladding had to be reduced by up to 56% in combination with MS-I after real weathering. On the other hand, the rigidity of PU-II and MS-II tested in the real environment had considerably decreased which allowed an enlargement of the cladding size by more than 30%.