Recommendations

Based on the main findings of this research, this chapter discusses the limitations of the applied research and provides recommendations for the further investigation of cast glass for structural members and systems

Unveiling the third dimension of glass

Glass as a material has always fascinated architects. Its inherent transparency has given us the ability to create diaphanous barriers between the interior and the exterior that allow for space and light continuity. Yet, we are just starting to understand the full potential, properties and characteristics of glass as a material. Only in the last decades did we discover the structural potential of glass and started to use it, besides as a cladding material, also for load-bearing applications thanks to its high compressive strength. Indeed, at present the structural applications of glass in architecture are continuously increasing, yet they are dominated by a considerable geometrical limitation: the essentially 2-dimensionality imposed by the prevailing float glass industry. Although glass panels can stretch more than 20 m in length, the maximum monolithic thickness by this manufacturing method remains a mere 25 mm. As a result glass structures are currently dominated by virtually 2-dimensional, planar elements and confined to the limited shapes that can be achieved by those. This research focuses on the exploration of cast glass as a promising, 3-dimensional construction material in architecture. The main aim of this research is therefore to investigate the potential, as well as the constraints, of cast glass components for the engineering of transparent, 3-dimensional glass structures in architecture. By pouring molten glass into moulds, solid 3-dimensional glass components of virtually any shape and cross-section can be made. Owing to their monolithic nature, such components can form repetitive units for the construction of freeform, full-glass structures that are not sensitive to buckling. Such structures can take full advantage of the high compressive strength of glass, sparing the necessity of additional supporting elements. To achieve cast glass structures, it is essential to use an intermediate material between the individual glass components that contributes to the structure’s stiffness, ensures a homogeneous load distribution and prevents early failure due to concentrated stresses triggered by glass-to-glass contact. To maximize transparency, this intermedium should be colourless and any additional substructure should be minimized. Accordingly, the main scientific contribution of this research work is the design, development and experimental investigation of two distinct systems for selfsupporting envelopes of maximized transparency: An adhesively bonded glass block system, using a colourless adhesive as an intermedium and a dry-assembly, interlocking cast glass block system, employing a colourless dry interlayer. Although, in this work, both systems have been developed for self-supporting envelopes, the results can be used as a guideline for further structural applications of cast glass components in compressive elements, such as columns, arches and bending elements, such as beams and fins. At present, the load-bearing function of cast glass in architecture remains an uncharted field. Discouraging factors such as the lengthy annealing process required, the to-date non-standardized production and the corresponding high manufacturing costs, have limited cast glass to only a few realized architectural applications. As a result, there is a lack of engineering data and a general unawareness of the potential and risks of employing cast glass structurally. Hence, in order to accomplish the research goal, all pertinent aspects of a cast glass structure should be tackled, ranging from cast glass’s production method to practical implications when building with cast glass. These distinct aspects are addressed through the formulation of the research sub-questions, which in turn define the different chapters of this dissertation. Accordingly, the presented work is divided in four parts. Part I provides the Introduction to the Research, and aims at giving a brief summary of the involved challenges, identify the research gap and introduce the research questions and the research methodology.  Part II focuses on the Theoretical Framework of the Research. It lays the foundations for this dissertation and contributes to the scientific field of structural glass by providing the first comprehensive literature review and state-of-the art overview of cast glass structural applications. Initially, the material compositions and production methods for solid cast glass components are explored. Then, to address both possibilities and limitations in the size and form of cast glass components, an overview and critical assessment of the largest produced monolithic pieces of cast glass is made. Given the limited published scientific output on this specific field, an extensive field research was conducted in order to derive the relevant data. The discussed examples, although coming from different fields of science and art, provide great insight into the practical implications involved in casting as a manufacturing method. Subsequently, a separate chapter gives an overview of the state-ofthe- art in cast glass structural applications in architecture. Aiming on providing the reader with an holistic overview of the structural potential of cast glass in architectural applications, this chapter includes the synopsis, feasibility assessment and comparison of not only the realized structural design systems but also of the adhesively-bonded and dry-assembly interlocking systems developed in this dissertation. Special attention is given to the advantages and disadvantages of the connection method of each -existing and developed in this dissertation- structural design system with solid glass blocks. Following the findings of the literature review and field research, Part III, consisting of four chapters, presents the design and experimental investigation of two distinct, novel structural systems out of cast glass components, developed for selfsupporting envelopes. Part III can be considered the main scientific outcome of this dissertation. Firstly, the research, development and experimental validation of an adhesively bonded system utilizing solid cast glass blocks is presented. Numerous full-scale prototypes are made and tested in order to comprehend the structural behaviour of the adhesively bonded glass assembly. A separate chapter explores the main challenges and innovations and defines the construction requirements necessary for the realization of the investigated system at the Crystal Houses Façade in Amsterdam. An important conclusion is that such an adhesively bonded system requires an extremely high dimensional accuracy both in the fabrication of the glass blocks and in the entire construction, and has an irreversible nature, which in turn results in a meticulous and unsustainable construction. Based on the aforementioned challenges, a new concept for glass structures out of dry-assembled interlocking cast glass components is developed that tackles the integral limitations of the adhesively-bonded system. An entire chapter is dedicated to the principles, the establishment of design criteria and to the preliminary exploration and assessment of different interlocking cast glass shapes that can yield an interlocking cast glass system of satisfactory structural performance. Following, the last chapter of this part concerns the experimental and numerical investigation of this second system. The effect of various parameters in the structural behaviour of the system is explored through the production of scaled prototypes and their experimental validation. A numerical model further explores the correlation of the various geometrical parameters of the interlocking geometry to the structural behaviour of the system. Finally, Part IV presents an integrated discussion of the research results, summarizing and discussing the main outcomes of the dissertation. Initially, responses to the research questions are given in order to assess the particular findings. Based on the conclusions, further recommendations are made, firstly for overcoming the limitations of the presented research, following by general suggestions on a wider range of the aspects of cast glass that can be explored and contribute to its structural applicability. The findings of this dissertation prove the feasibility of the discussed systems and can serve as solid guidelines for further applications. The research presented in this work has been positively received by the international architectural and engineering community. In specific, the presented adhesivelybonded cast block system, which was realized at the Crystal Houses façade, received numerous awards by the structural engineering community, including the Outstanding Innovation Award 2016 by the Society of Façade Engineers and the Glass Innovation Award 2016 from the Bouwend Nederland association. Still, the Crystal Houses façade is but the first real-scale prototype of the developed adhesively bonded system. The actual construction of the façade provided invaluable feedback on the engineering challenges and construction requirements involved in such a system, giving room for new suggestions. This triggered the development of the second presented system with interlocking glass blocks as a reversible, easily assembled solution. The interlocking cast glass block system, initiated within TU Delft and funded partially by a 4TU.bouw grant is yet to be applied in practice. Prototypes of this research, using recycled cast glass components, have been exhibited in international fairs such as the Venice Design 2018, the Dutch Design Week 2018 and Salone del Mobile 2019 and are currently displayed at the material collection of the Vitra Design Museum at the Vitra Schaudepot. The project was also nominated for the New Material Award 2018 under the title Re3 Glass. Even though cast glass has, so far, been rarely applied in structural applications, the development of new building systems and their experimental validation presented in this work provide a strong basis for further developments and applications in a range of compressive structures. At present, the most considerable drawbacks hindering the marketability of cast glass components are (a) the cost barriers imposed by their customized production and application and (b) the lack of standardized strength data and building guidelines. Thus, even if cast glass elements have proved to be suitable structural components, several economic aspects and logistics need to be tackled, and performance issues need to be further explored, in order to make cast glass a competitive manufacturing method to float production for structural components

Material compositions and production methods for solid cast glass components

Glass can be made by different manufacturing processes and by numerous of varied recipes that in return provide the material with different properties. Owing to their workability in lower melting temperatures and the corresponding decreased manufacturing costs, soda-lime and borosilicate glass types are preferred for cast glass applications in structures. Glass can be cast in two ways: primary and secondary casting. In primary casting, glass is molten from its primary raw ingredients, whereas in secondary casting, solid existing pieces of glass are re-heated until the (semi-) liquid mass can flow and be shaped as desired. The main process of primary casting is hot-forming (melt-quenching) and of secondary casting is kiln-casting. The principal difference between the two methods, besides the initial state of glass, is the required infrastructure. In hot-forming, molten glass from a furnace is poured into a mould and is then placed in another, second furnace for annealing. In contrast, kiln-casting employs a single kiln for the melting of the (already formed) glass into the moulds and for the subsequent annealing process and requires lower operating temperatures. In both methods the annealing process is similar. The annealing schedule is influenced by numerous factors that cannot be easily simulated as a complex non-linear time dependent multi-variable analysis is required. As a result the annealing schedule of large 3-dimensional cast units is commonly empirical. Different mould types, disposable or permanent, can be used for casting glass objects. The choice of mould mainly depends on the production volume and desired level of accuracy of the glass product, and is in practice usually cost and time driven. Currently, there is no standard to determine the design strength of solid cast glass objects for structural applications in architecture. Based on the assumption that the increased volume of cast glass can lead to a higher amount of randomly distributed flaws in the mesostructure, the bending strength of cast glass is expected to be comparable but slightly less than that of standard float glass

An adhesively-bonded cast glass system for the Crystal Houses façade

Chapter 4 provided an overview of the three structural systems utilizing cast glass components in architecture, including a brief overview of the work presented in this dissertation. This chapter presents the design principles and experimental work for the first of the two systems explored in this work: a transparent, adhesivelybonded glass block system designed for self-supporting envelopes. The proposed system was developed for the Crystal Houses façade in Amsterdam, designed by MVRDV Architects. The system is exclusively constructed by solid cast glass blocks, bonded with DELO Photobond 4468, a colourless, UV-curing adhesive. This allows for a system of an increased transparency, sparing the necessity of an opaque substructure. In contrast with previous realized projects, solid soda-lime glass blocks are used rather than borosilicate ones. Initially, several architectural prototypes, comprising glass elements of different tolerance ranges, are built to evaluate the visual performance and the thickness of the adhesive that allows for an even spread. The prototypes indicate that a homogeneous bond thicker than 0.3 mm cannot be obtained by the selected adhesive due to the latter’s flow properties and low viscosity. Based on the adhesive’s optimum application thickness, it is determined that the glass blocks’ top and bottom surfaces should be flat within 0.25 mm for guaranteeing an even adhesive layer of the highest strength. The structural verification of the system is demonstrated by physical testing of prototypes in compression, 4-point bending, hard-body impact and thermal shock. Compressive tests on individual blocks highlight the need for proper detailing and uniform load distribution of the system. Compressive tests on columns made of adhesively bonded glass blocks further confirm that strict size tolerances are essential for maximizing the load-bearing capacity of the system: specimens with larger size deviations fail in considerably lower stress values than specimens with smaller size deviations. Furthermore, series of 4-point bending tests on adhesively bonded glass beams demonstrate that the chosen adhesive enables the glass brick wall to behave monolithically under such loading when the adhesive is applied in a constant layer of the optimum thickness. Overall, the results show that the adhesively bonded glass block structure can provide the required structural performance, but only if strict tolerances are met in the geometry of the glass blocks so that the chosen adhesive can be evenly spread in a constant thickness

Unveiling the third dimension of glass:

Glass as a material has always fascinated architects. Its inherent transparency has given us the ability to create diaphanous barriers between the interior and the exterior that allow for space and light continuity. Yet, we are just starting to understand the full potential, properties and characteristics of glass as a material. Only in the last decades did we discover the structural potential of glass and started to use it, besides as a cladding material, also for load-bearing applications thanks to its high compressive strength. Indeed, at present the structural applications of glass in architecture are continuously increasing, yet they are dominated by a considerable geometrical limitation: the essentially 2-dimensionality imposed by the prevailing float glass industry. Although glass panels can stretch more than 20 m in length, the maximum monolithic thickness by this manufacturing method remains a mere 25 mm. As a result glass structures are currently dominated by virtually 2-dimensional, planar elements and confined to the limited shapes that can be achieved by those. This research focuses on the exploration of cast glass as a promising, 3-dimensional construction material in architecture. The main aim of this research is therefore to investigate the potential, as well as the constraints, of cast glass components for the engineering of transparent, 3-dimensional glass structures in architecture. By pouring molten glass into moulds, solid 3-dimensional glass components of virtually any shape and cross-section can be made. Owing to their monolithic nature, such components can form repetitive units for the construction of freeform, full-glass structures that are not sensitive to buckling. Such structures can take full advantage of the high compressive strength of glass, sparing the necessity of additional supporting elements. To achieve cast glass structures, it is essential to use an intermediate material between the individual glass components that contributes to the structure’s stiffness, ensures a homogeneous load distribution and prevents early failure due to concentrated stresses triggered by glass-to-glass contact. To maximize transparency, this intermedium should be colourless and any additional substructure should be minimized. Accordingly, the main scientific contribution of this research work is the design, development and experimental investigation of two distinct systems for selfsupporting envelopes of maximized transparency: An adhesively bonded glass block system, using a colourless adhesive as an intermedium and a dry-assembly, interlocking cast glass block system, employing a colourless dry interlayer. Although, in this work, both systems have been developed for self-supporting envelopes, the results can be used as a guideline for further structural applications of cast glass components in compressive elements, such as columns, arches and bending elements, such as beams and fins. At present, the load-bearing function of cast glass in architecture remains an uncharted field. Discouraging factors such as the lengthy annealing process required, the to-date non-standardized production and the corresponding high manufacturing costs, have limited cast glass to only a few realized architectural applications. As a result, there is a lack of engineering data and a general unawareness of the potential and risks of employing cast glass structurally. Hence, in order to accomplish the research goal, all pertinent aspects of a cast glass structure should be tackled, ranging from cast glass’s production method to practical implications when building with cast glass. These distinct aspects are addressed through the formulation of the research sub-questions, which in turn define the different chapters of this dissertation. Accordingly, the presented work is divided in four parts. Part I provides the Introduction to the Research, and aims at giving a brief summary of the involved challenges, identify the research gap and introduce the research questions and the research methodology.  Part II focuses on the Theoretical Framework of the Research. It lays the foundations for this dissertation and contributes to the scientific field of structural glass by providing the first comprehensive literature review and state-of-the art overview of cast glass structural applications. Initially, the material compositions and production methods for solid cast glass components are explored. Then, to address both possibilities and limitations in the size and form of cast glass components, an overview and critical assessment of the largest produced monolithic pieces of cast glass is made. Given the limited published scientific output on this specific field, an extensive field research was conducted in order to derive the relevant data. The discussed examples, although coming from different fields of science and art, provide great insight into the practical implications involved in casting as a manufacturing method. Subsequently, a separate chapter gives an overview of the state-ofthe- art in cast glass structural applications in architecture. Aiming on providing the reader with an holistic overview of the structural potential of cast glass in architectural applications, this chapter includes the synopsis, feasibility assessment and comparison of not only the realized structural design systems but also of the adhesively-bonded and dry-assembly interlocking systems developed in this dissertation. Special attention is given to the advantages and disadvantages of the connection method of each -existing and developed in this dissertation- structural design system with solid glass blocks. Following the findings of the literature review and field research, Part III, consisting of four chapters, presents the design and experimental investigation of two distinct, novel structural systems out of cast glass components, developed for selfsupporting envelopes. Part III can be considered the main scientific outcome of this dissertation. Firstly, the research, development and experimental validation of an adhesively bonded system utilizing solid cast glass blocks is presented. Numerous full-scale prototypes are made and tested in order to comprehend the structural behaviour of the adhesively bonded glass assembly. A separate chapter explores the main challenges and innovations and defines the construction requirements necessary for the realization of the investigated system at the Crystal Houses Façade in Amsterdam. An important conclusion is that such an adhesively bonded system requires an extremely high dimensional accuracy both in the fabrication of the glass blocks and in the entire construction, and has an irreversible nature, which in turn results in a meticulous and unsustainable construction. Based on the aforementioned challenges, a new concept for glass structures out of dry-assembled interlocking cast glass components is developed that tackles the integral limitations of the adhesively-bonded system. An entire chapter is dedicated to the principles, the establishment of design criteria and to the preliminary exploration and assessment of different interlocking cast glass shapes that can yield an interlocking cast glass system of satisfactory structural performance. Following, the last chapter of this part concerns the experimental and numerical investigation of this second system. The effect of various parameters in the structural behaviour of the system is explored through the production of scaled prototypes and their experimental validation. A numerical model further explores the correlation of the various geometrical parameters of the interlocking geometry to the structural behaviour of the system. Finally, Part IV presents an integrated discussion of the research results, summarizing and discussing the main outcomes of the dissertation. Initially, responses to the research questions are given in order to assess the particular findings. Based on the conclusions, further recommendations are made, firstly for overcoming the limitations of the presented research, following by general suggestions on a wider range of the aspects of cast glass that can be explored and contribute to its structural applicability. The findings of this dissertation prove the feasibility of the discussed systems and can serve as solid guidelines for further applications. The research presented in this work has been positively received by the international architectural and engineering community. In specific, the presented adhesivelybonded cast block system, which was realized at the Crystal Houses façade, received numerous awards by the structural engineering community, including the Outstanding Innovation Award 2016 by the Society of Façade Engineers and the Glass Innovation Award 2016 from the Bouwend Nederland association. Still, the Crystal Houses façade is but the first real-scale prototype of the developed adhesively bonded system. The actual construction of the façade provided invaluable feedback on the engineering challenges and construction requirements involved in such a system, giving room for new suggestions. This triggered the development of the second presented system with interlocking glass blocks as a reversible, easily assembled solution. The interlocking cast glass block system, initiated within TU Delft and funded partially by a 4TU.bouw grant is yet to be applied in practice. Prototypes of this research, using recycled cast glass components, have been exhibited in international fairs such as the Venice Design 2018, the Dutch Design Week 2018 and Salone del Mobile 2019 and are currently displayed at the material collection of the Vitra Design Museum at the Vitra Schaudepot. The project was also nominated for the New Material Award 2018 under the title Re3 Glass. Even though cast glass has, so far, been rarely applied in structural applications, the development of new building systems and their experimental validation presented in this work provide a strong basis for further developments and applications in a range of compressive structures. At present, the most considerable drawbacks hindering the marketability of cast glass components are (a) the cost barriers imposed by their customized production and application and (b) the lack of standardized strength data and building guidelines. Thus, even if cast glass elements have proved to be suitable structural components, several economic aspects and logistics need to be tackled, and performance issues need to be further explored, in order to make cast glass a competitive manufacturing method to float production for structural components

Experimental and numerical investigation of an interlocking system out of osteomorphic cast glass components

The previous chapter concluded that blocks following an osteomorphic geometry are the most promising in respect to the principles of interlocking and glass casting. Moreover, interlayers of the polyurethane (PU) family are considered the most suitable for a building application, without compromising the transparency of the resulting structure. Based on these findings, this chapter aims at investigating the structural behaviour of an interlocking assembly employing osteomorphic blocks and PU as an interlayer. Initially, experimental research is conducted in different PU interlayers available in the market, aiming on finding one that fulfils the established design criteria and mechanical properties, as discussed in chapter 7.6. Different readily available two-component PU interlayers, with a Shore Hardness ranging between 60A – 80A, are cast to follow the osteomorphic interlocking geometry and are tested under static compressive load between two half osteomorphic kiln-cast glass blocks in series of 3 specimens and in 2 different thicknesses (3mm and 6mm). The results suggest that the tearing strength of the interlayer is as important as its Shore Hardness; whereas the geometry of the interlocking form can further influence the overall resistance of the assembly against tearing. They also highlight that insufficient contact (mismatch) of the interlayer with the glass blocks, due to dimensional deviations, can lead to the eventual failure of the assembly even under static load due to peak stresses that are further increased by the lateral stresses occurring due to the creep of the interlayer. From the examined interlayers, PU with a shore hardness between 70A – 80A, are considered as the most suitable for the further experimental validation of the assembly. Following, to investigate the influence of the interlocking mechanism to the structural behaviour of the proposed Thus, to further explore the influence of the most crucial geometrical aspects of the interlocking mechanism, namely the height and amplitude of the glass components, to the overall structural performance under shear a numerical model65 is made. In accordance with the output of the experimental validation, here an osteomorphic block with multiple locks is tested, considering a 4 mm thick 70A PU interlayer. The results of the numerical model indicate that bricks of reduced height are more susceptible to failure by bending, whereas with taller brick variants shear lock failure is more critical. It is also confirmed that an increased amplitude of the interlock can be beneficial as it leads to an increased shear capacity and lower uplifting effects. It is also shown that specimens with increased height reach the failure stress limit with considerably smaller deformations, thus, requiring a higher manufacturing precision

Learning by building

Continuing from the experimental validation of the adhesively bonded system, this chapter presents the main challenges confronted and records the innovative solutions implemented during the consecutive construction steps of the adhesivelybonded cast glass façade. These include the manufacturing and quality control of the bricks, the set-up of the construction site, the levelling of the reference supporting beam, the bonding method used and the fabrication and installation of customized elements such as the architraves, window and door frames and the intermixing zone of glass with terracotta bricks. The experimental work on prototype elements, described in Chapter 5, resulted into the use of a colourless, UV-curing adhesive of the Delo Photobond family for bonding the solid glass blocks together. The tests indicated as well that the desired monolithic structural performance of the glass masonry system and a homogeneous visual result can only be achieved when the selected adhesive is applied in a 0.2-0.3 mm thick layer. In accordance, the bricks have to meet a strict dimensional tolerance of ± 0.25 mm. On the facade as a whole, this means that the overall size deviations will be limited to a few mm. The nearly zero thickness of the adhesive together with the request for unimpeded transparency introduced numerous engineering puzzles, addressed in this chapter. The fundamental difference between conventional brickwork and the developed glass masonry system is that a standard mortar layer compensates for the size deviations of the bricks, while the selected adhesive cannot. This manifests the level of complexity introduced by the manual bonding and the significance of constantly controlling the entire construction with high precision methods. Based on the conclusions of the research and the technical experience gained by the realization of the project, recommendations are made on the further improvement of the presented glass masonry system for future applications

Overview of realized examples in architecture using structural cast glass blocks

In view of the meticulous and lengthy cooling process discussed in the previous chapters, in architectural applications solid cast glass components have been commercialized up to the size range of standard masonry bricks. Owing to their large cross-sectional area, solid glass bricks are promising structural components that can fully exploit glass’s compressive strength. By forming repetitive components, self-supporting, 3-dimensional all-glass structures of undisturbed transparency can be achieved. Nonetheless, at present, little and rather sporadic exploration has been made in the use of casting as a manufacturing method for structural glass in architecture. To a degree, this is attributed to the existence of only a few realized examples of self-supporting structures made of solid cast glass elements. The, so far, limited demand has in turn led to the absence of a standardized manufacturing process, to a lack of consistent engineering data and to a general unawareness of the potential of cast glass in structural applications in architecture. Currently, including the contribution of this dissertation, there are 3 structural systems employed for creating self-supporting structures out of cast glass components: (a) with a metal substructure, (b) adhesively bonded blocks, (c) interlocking glass blocks. The former two have been applied in real structures, whereas interlocking glass blocks are for the first time introduced as a building system through this dissertation. In this chapter, the 3 concepts are briefly presented, analyzed and evaluated in terms of manufacturing, structural system, level of transparency, ease of assembly and disassembly