Outline of Testing and Evaluation Program Used in Selection of Adhesives for Transparent Adhesive Joints in All-Glass Load-Bearing Structures
ABSTRACT: Architects are increasingly demanding all-glass load-bearing structures with fully transparent adhesive joints. Usually, such structures are classed as nonregulated forms of construction. The Institute of Building Construction at Dresden’s Technische Universitat has now obtained the first individual approvals in Germany for all-glass structures with transparent adhesive joints for two buildings in Dresden and Grimma. In these cases, the loads are carried via load-bearing glued frames that rely on a material bond between the individual parts without any metal fixings. This solution is based on the results of many years of development. Currently, various frame corners covering a wide range of parameters are being studied in a follow-up project. The aim is to optimize the adhesive joints and the bonding technology. The requirements placed on the joint are being identified and corresponding adhesive systems researched. Preliminary studies of numerous material specimens form the starting point for determining the material parameters. Small-scale specimens are being tested under various boundary
conditions and aging scenarios to establish the strengths of a number of suitable adhesives. These results will enable prototypes to be designed for numerical simulation. Numerical calculations and experimental investigations are being carried out in parallel to optimize the geometry, load-carrying capacity, long-term reliability, and durability of the glued all-glass frame corners. Specimen components are loaded in a testing machine to study the structural effect of these glued glass frame corners. The findings will be incorporated in the design of the adhesive joint and in the development of a numerical simulation for the glued connection. Data for executing an adhesive joint, specific to each adhesive and crucial for the quality of the transparency, conclude the project.
Striving for complete transparency obviously also calls for load-bearing structures to be made from transparent materials, such as glass. Glass is a brittle material, which means that tried-and-tested methods of jointing, derived from structural steelwork and other engineering disciplines, cannot simply be transferred to this material. Glued connections enable a homogeneous flow of forces via the material bond between the components being joined and at the same time reduce local stress peaks in the glass. So, besides the customary mechanical connections, load-bearing adhesives are becoming very important as a form of connection, ideally suited to glass. For example, individual linear loadbearing elements made from glass can be glued together to form transparent load-bearing frames, which are addressed in this paper.
Appropriate adhesives for joining glass are entirely or partially based on polymeric compounds. Thus, their chemical structure and their material properties resemble plastics to a high degree. The aging behavior and durability of relevant types of adhesives have been examined in recent research projects focusing on glass-glass and glass-metal bonds. Usually, the experimental investigations comprise comprehensive testing on small-scale samples. Typically, those samples run through severe artificial aging scenarios, like climatic or thermal cycling, exposure to UV-light, high-humidity, corrosive media, or immersion in water or in a solution of a cleaning agent.
In general, thorough cleaning and surface pretreatments enhance the longterm stability and overall quality of the bond [1,2]. As an example, treating the glass surface with atmospheric plasma can increase the surface energy and remove specific organic impurities. The selection of a suitable adhesive is another crucial parameter for the design of bonded connections. Overend et al.
 propose an approach toward the characterization of the adhesive and the derivation of essential data for analytical and numerical models. Another substantial study  focuses on the ageing resistance of several UV – and lightcuring acrylates, which were used for glass-metal bonds. As a consequence, those adhesives could prove their suitability for load-bearing connections in glass applications under the condition that long-lasting exposure to moisture is avoided and temperature-dependent material behavior is taken into account for
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design calculations. Their tendency to significant creep deformations under permanent long-term loading  requires further improvement and careful consideration in the structural design.
Although durability testing has also been done on larger bonded elements, the dimensions of the weathering equipment generally limit the size and quantity of specimen components to be tested. Some research activities focus, for example, on the combination of glass with ductile materials to improve the structural performance after failure and expand the scope of applications beyond the initial limits. The so-called “reinforced glass beams" or “hybrid glass beams" are examined in Refs [6-8].
Nevertheless, the correlation of simulated conditions in the laboratory and outdoor exposure is still one of the main challenges of scientific research on adhesive bonding. Hence, testing on life-size glazing elements exposed to longterm outdoor conditions is often required to verify the assumptions made by scaling up the findings from small specimens. Experimental studies were done, for example, on a frameless glass shell , as well as on glued point fixings for glass canopies  or bonded glass lamellas .
Another significant step toward a more regular application of structurally bonded glass can be done transferring the knowledge achieved into innovative pilot projects. This paper presents experimental and numerical investigations that lead to the realization of a currently built glass corridor. The glass components are connected without using additional metal fasteners.