Ongoing research at TU Delft focuses on recycling low-quality glass by casting it into volumetric elements, under the assumption that bulk flaws, and thus compatible contamination, have little influence on the volumetric component's strength. However, to validate the structural behavior of volumetric glass with significant bulk flaws, a corresponding uniform tensile testing method is needed that is not governed by the surface quality of glass and thus, can trigger bulk flaws. This research explores the applicability of the Theta-specimen (Durelli et al. 1962), as a method for measuring tensile strength of volumetric cast glass, while avoiding the main drawbacks of a direct uniaxial tensile test. Three theta-sample geometries are evaluated using FEA, and by mechanical testing of 4-5 CNC waterjet cut float glass specimens per geometry. Polarized light is used to visualize the development of stresses within each sample. Post-fracture fractographic analysis is performed to identify the origins of fracture, and to estimate failure stresses based of the fracture mirror radius using Orr's formula. The photo-elastic patterns closely match the FEA prediction, confirming that the largest tensile stresses occur within the intended central test strip. Polarized light reveals a sensitivity to eccentric loading for the two newly proposed designs, which can be minimized by introducing a neoprene interlayer between the sample and testing machine. All samples failed at multiple points; stress estimations based on fracture mirror size indicate that the lowest failure stresses consistently occurred within the test strip, further confirming that failure initiated within this area. It is concluded that the Theta-sample has potential as a uni-axial tensile testing method for brittle materials such as glass, though further research is required to fully confirm its reliability. ... Read more

Advances in structural glass have enabled a new paradigm in expressive and transparent architecture. Cast glass can further extend the possibilities of structural glass by allowing for more complex and sophisticated shapes than the current planar geometries of structural float glass. However, the use of cast glass is currently limited because of the lengthy annealing process, making massive component sizes impractical to fabricate. Topology optimization (TO) has been proposed as a solution to this problem, as it is known to generate structurally efficient designs with a low volume of material. If tailored appropriately, TO can reduce component sizes and thereby diminish the total annealing time needed, while intelligently placing material in the areas where it will be utilized most effectively. For TO of glass to be successful, algorithms must properly capture glass’s specific material behavior. This research proposes a suite of TO algorithmic frameworks that design specifically for structural glass. These algorithms are demonstrated in a 2D design space, and the resulting geometries are fabricated using cut float glass and tested for experimental comparison on a 4-point bending load case. The results of these experiments provide valuable insights into the development of TO for structural glass, and help inform future research in TO of large-scale cast glass structures. ... Read more

Um die Recyclingquote von Altglas zu steigern, experimentiert ein Team der Technischen Universität Delft mit neuartigen Gussglasverfahren. To increase the recycling rate of used glass, a team from Delft University of Technology is casting glass waste into volumetric, architectural glass components. ... Read more

Glass casting offers the potential to create complex, large-scale, monolithic structural elements with optimized stiffness and material use. However, glass's brittleness and lack of post-failure redundancy pose safety challenges, especially since conventional float glass safety strategies are difficult to apply to volumetric components. Inspired by reinforced concrete, this study explores embedding metal reinforcement in cast glass directly during the casting process to enhance ductility, redundancy, and recycl-ability by avoiding adhesives. The novelty lies in directly bonding metal to glass using materials with similar thermal expansion coefficients, further allowing contamination-free recycling. Building on previous TU Delft research, we investigate two material combinations with matching thermal expansion coefficients: (i) bor-osilicate glass with F15 Kovar and (ii) soda-lime silica glass with Titanium Grade 2 or 5. Kiln-cast glass beams with a longitudinal metal reinforcement are produced and tested under four-point bending using Digital Image Correlation to assess their mechanical performance. Borosilicate glass specimens reinforced with Kovar demonstrated effective glass-metal interaction but lower strength due to interfacial bubbles, with all specimens failing in shear, in a similar manner to reinforced concrete, while retaining most glass attached to the metal rod. Soda-lime glass specimens reinforced with Titanium exhibited higher failure loads, though specimens reinforced with Grade 2 Titanium failed in bending similar to unreinforced glass. Titanium Grade 5 reinforcement showed potential for strength enhancement and progressive failure, emphasizing the importance of proper reinforcement selection and dimensioning. Finally, we discuss the potential for mate-rial separation at end-of-life and the applicability of this technology for embedded connections in cast glass. ... Read more

Located in Apple Park in California (USA), Mirage is an outdoor sculpture of 448 solid cast glass cylindrical columns, each 210cm high and 15cm in diameter, made from 70 different desert. sands. The sculpture by Katie Paterson and Zeller & Moye involved a large team of artists, architects, geologists and scientists. This paper discusses the experimental work conducted by TU Delft on devel-oping glass compositions of the desired colour, hue and tint, as a function of foreign, impure sand addition to the basic batch recipe. XRF and XRD analyses of the desert sand samples revealed their chemical composition and crystallographic structure, purity level and presence of undesired con-taminants. Repetitive melting experiments led to establishing relationships between the sand character-istics and the colour hue, tint and intensity of the resulting glass, concluding to 0.02-17% desert sand content as substitute of the original, low-iron sand in the batch recipe. Iron oxides were identified as the prevailing colour agents, yielding a colour palette of blue, aquamarine and green hues based on their oxidation state. The melting experiments led to the formulation of a glass recipe prediction model and to modifications of the standard batch, towards achieving the desired colour gradient while preventing critical defects in the glass. The vertical casting of the columns by the glass studio gave a distinct surface pattern with recess lines and a high bubble content. To identify if the strength is governed by the surface quality, the bubble content or the glass's chemical composition and to attain representative strength data, 4-point bending tests were conducted on 37.5x51x635mm beam specimens produced by the same foundry. Fractography analysis of the tested specimens showed that the characteristic surface pattern governs the design strength. Mirage was inaugurated in May 2023, showcasing the potential and versatile beauty of using a wide range of impure sands in glass making. ... Read more

Glass’s high compressive strength makes it ideal for compressive-only structures such as vaults. Float glass, the most common type in architecture, is limited by its planar form, often resulting in buckling-induced tensile stresses that undermine glass’s compressive potential. 3D-printing and casting are alternative fabrication methods that enable the production of volumetric glass components that can better utilize glass’s compressive capacity. Due to fabrication limitations, both 3D-printed and cast glass assemblies in building-scale require segmentation, calling for specialized joinery solutions. Existing built projects rely on permanent adhesives. However, towards circular construction, a reversible connection is needed that can transfer the desired loads, enable customization, allow for disassembly, and preserve recyclability. Accordingly, this research investigates two novel, reversible joinery methods for dry-stacked glass vaults composed of either cast or 3D-printed interlocking units: (i) a Velcro-inspired, polymer interlayer, directly 3D-printed onto the glass and (ii) a dry, laser-cut expandable metal interlayer. We first assess the fabrication constraints of cast and 3D-printed glass bricks and their implications for joinery design. The two joinery methods are then evaluated based on criteria linked to manufacturability and structural performance. Finally, we present preliminary feasibility testing and discuss the practical challenges and potential of each connection type in relation to both glass fabrication methods and overall vault design. ... Read more

Glass is highly sensitive to damage accumulation during its service life, leading to a significant reduction in strength. Annealed glass used in glazing units of low-rise buildings can experience a 71–85% strength reduction after 20–30 years of natural weathering, which can be detrimental in structural applications. Despite these considerable reductions in strength, there are currently no well-established methods for repairing glass components, with hypothetical repair methods primarily limited to resin injection and little evidence on their durability or their efficiency in preventing water diffusion and subcritical crack growth. As glass panels increase in size, complexity and cost, the standard approach of replacing damaged components with new glass becomes unsustainable. This paper develops effective and durable thermal healing methods for damaged glass components. A systematic experimental investigation is undertaken involving controlled artificial aging of annealed glass, followed by thermal healing, microscopy and destructive flexural tests to assess the effectiveness of the repairs. Different thermal and hydrothermal profiles are explored showing that thermal treatment has potential for strength recovery. In fact, thermal healing for the flaws in this study, at high temperatures in the order of 300–500 °C, can fully restore and even increase the design strength of glass beyond new as-received strength. This suggests that thermal healing can support and promote repair and reuse of end-of-life glass, enhancing circularity in the architectural glass industry. ... Read more

Glass casting displays great forming potential allowing for the realisation of three-dimensional glass elements of virtually any shape and size, as showcased in glass art. Disposable mould technology seems to be ideal for the fabrication of such customised and complex geometries, including for architectural and structural cast glass components deriving from structural topology optimization, since it offers great shape freedom and cost effectiveness. However, currently, glass casting on disposable moulds faces the major drawback of a resulting rough and opaque glass surface quality, requiring considerable post-processing to yield a glossy, smooth surface. This in turn results in a compromised dimensional accuracy and on increased time and production costs. If the surface remains unprocessed, it can greatly affect not only the visual but also the mechanical properties of the cast glass element. Aim of this research is to improve the surface quality of complex glass components cast in disposable moulds, directly during demoulding, reducing in this way the need for post-processing. To achieve this the research focuses on exploring ways to pre-process disposable moulds. In specific, the research focuses on series of kiln-cast laboratory experiments at various maximum firing temperatures / annealing schedules involving the use of two different types of disposable moulds, 3D-printed sand moulds and silica plaster moulds (Crystalcast®), and the application of refractory coatings, coating combinations and protective layers. The experimental work conducted thus far indicates that the best results are obtained at the lowest maximum temperature tested (870 °C), with the combination offering the best finishing quality to be a synthetic (ceramic) sand mould coated with Crystalcast® and Zirkofluid® (6672, 1219). Scaling-up of the kiln-cast prototypes unveils a complex correlation between the maximum dwell time at the maximum firing temperature and the casting effectivity/ performance of mould materials and coatings. ... Read more

Cast glass is an excellent candidate for achieving fully transparent arch, vault and dome structures. By casting, voluminous, free-form glass components can be produced that fulfil the complex geometry requirements, offer increased compressive strength and maximize the incoming sunlight. Nonetheless, the application of cast glass in such structures is seldom seen; manufacturing challenges, uncertainties over the strength of the material, missing guidelines on the assembly methods of the components, lack of engineering norms, and the associated research and development costs to overcome those barriers, are some of the key discouraging factors. This paper explores several realised case studies of small-scale, cast glass arches, vaults and domes, where the TU Delft Glass Research group was involved in the R&D process, as well as unrealised case studies developed by the group (Fig. 1). The main challenges and developed solutions for each project are described, focusing on three main aspects: (i) the manufacturing challenges linked with achieving the desired curvature, (ii) the assembly method and mechanical validation of the system, and (iii) the construction ease of the system. Based on the comparative study of the selected projects, the paper aims to provide a design methodology for future projects employing cast glass curved structures. ... Read more

Recent research at TU Delft has highlighted the potential of using structural Topology Optimization (TO) for designing large monolithic cast glass structures of maximized stiffness with minimal mass. The mass efficiency of these structures results in considerably shorter annealing times and, consequently in improved manufacturability in terms of time, energy and cost efficiency. Nonetheless, the geometrical complexity and customization of the resulting forms renders them challenging in terms of fabrication. Exploring the manufacturability of such intricate glass structures, in this paper we discuss the different possible fabrication methods for three-dimensional glass structures of complex and customized geometries, via a review of existing literature, experimental work and prototyping. Specifically, with the aim of addressing all possible manufacturing solutions, we look into the following fabrication methods: (i) casting in disposable moulds; (ii) waterjet cutting and lamination of float glass panes and; (iii) additive manufacturing of glass. We assess these methods based on a set of criteria linked to the structural performance, visual quality, fabrication limitations and sustainability. Accordingly, we discuss the potential, challenges and practical limitations of each fabrication method for real-world applications of TO glass structures. Subsequently, we propose the integration of alternative constraints into the TO formulation, so that customized TO tools that better reflect each fabrication method can be created. ... Read more

An adhesively bonded, solid-glass brick pavilion has been designed by Konstantin Arkitekter as a landmark within the Aasivissuit – Nipisat UNESCO heritage in Greenland. The sculptural glass structure, measuring approximately 3.2 m in diameter × 2 m in height, faces a diverse set of engineering challenges compared to existing adhesively bonded glass brick structures. Placed in a remote location in the arctic circle, it has to withstand winter temperatures as low as -35 °C, and be built under a limited budget with the aid of the local population. Hence, key for the successful construction of the pavilion is finding an adhesive that satisfies the structural and aesthetic requirements of the project and simultaneously provides a simple and fast construction that spares the need for specialized building crew and sophisticated equipment, and is able to withstand the polar winter temperatures. Applicability and shear tests in (i) lab temperature conditions and (ii)) -5 °C lead to the final selection of: (a) 3M™ Scotch-Weld™ Polyurethane Adhesive DP610, which has a higher shear strength capacity, 1 mm gap filling capacity and is clear in colour, for bonding the bottom rows of the pavilion where higher strength is required due to the reduced overlapping of the bricks; and of (b) DOWSIL Experimental Fast Curing Adhesive developed by Dow Silicones Belgium particularly for this project, with a satisfactory shear strength, 3 mm gap filling capacity and white colour for the rest of the pavilion; its considerably larger gap filling capacity facilitates the ease of assembly as it can accommodate within the joint thickness the anticipated ± 1.5 mm standard size deviations of the soda-lime cast glass solid bricks and the possible accumulated deviations during construction. The paper further describes the application of the adhesive, first on a small-scale prototype, and then on site, and presents the encountered engineering and logistical challenges during the construction of the pavilion in Greenland. ... Read more

The Qaammat Pavilion, conceived and designed by architect Konstantin Ikonomidis, aims to create a landmark in the outskirts of Sarfannguit, a small fishing settlement of around 100 inhabitants within the Aasivissuit – Nipisat Unesco World Heritage Site, slightly north of the Arctic Circle. The adhesively-bonded glass block sculpture consists of two inclined semi-circular, perforated walls, roughly 2 m in height and 3.2 m in diameter. Comprising circa 1,100 solid cast glass bricks (240 × 110 × 53 mm), it is erected on top of arc-shaped steel plates, supported by stainless steel bars anchored through drilled holes in the rock below, a method borrowed from local housing architecture. […] ... Read more

Cast glass is a promising, three-dimensional expression of the material for architectural and structural applications, particularly for the creation of all-transparent, self-supporting structures and envelopes. Typically applied in the form of solid blocks, cast glass components can be used as repetitive units to comprise fully-transparent, cast glass masonry walls. To maximize transparency and ensure an even load distribution, the glass blocks are bonded together by a colourless adhesive. Currently, there is a lack of standardized structural specifications, strength data and building guidelines for such adhesively-bonded cast glass-block systems. As a result, any new application is accompanied by experimental testing to select the adhesive and certify the adhesively bonded system. Since the choice of adhesive is highly dependent on the prerequisites set for each case-study -such as the structural and visual performance, available budget, the structure’s geometry and climate conditions- the preselection of the most prominent adhesive family at an early project stage can prevent an excessive budget and construction complications. This paper, therefore, aims to shed light on the selection process of adhesives for cast glass assemblies by first providing an overview of the most suitable bonding media families for such systems; these include stiff adhesives, flexible adhesives and cement-based mortars. Following, the paper reviews the research & development process of the adhesively-bonded glass-block systems in three distinct built projects, in which the TU Delft team has been involved: The Crystal Houses façade (NL), the LightVault, a robotically assembled glass vault (UK) and the Qaammat pavilion in the arctic circle (GL). The adhesive requirements for each of the three case studies are discussed in terms of structural and visual performance and ease-of-assembly (constructability). These criteria are decisive in pointing out the most promising bonding media family per case-study. The final shortlist of adhesive candidates within that bonding media family is subject to the full list of performance criteria, but also to market availability. The shortlist of adhesive candidates are typically experimentally evaluated, first via application testing and then via strength tests in order to choose the most suitable candidate. Based on the above, the review concludes in proposing guidelines for the effective selection, design and experimental verification of adhesively-bonded cast glass assemblies. ... Read more

This paper introduces the use of structural topology optimization (TO) as a new design approach that enables the creation of monolithic load-bearing cast glass components of substantial dimensions with significantly reduced annealing times, rendering such components viable in terms of manufacturing. Using topology optimization, the glass mass can be optimized to match design loads whilst maintaining high stiffness and a homogeneous mass for even cooling. Initially, the two main TO approaches are discussed in terms of suitability for cast glass. A strain-based optimization is eventually preferred over Von Mises optimization in the specific study. To explore the potential of TO for optimizing structural cast glass components, three distinct studies are analyzed in ANSYS workbench: (i) a structural glass node, (ii) a cast glass floor and (iii) a pedestrian bridge. These lead to the establishment of a set of design/input criteria, taking into account glass as a material, casting as a manufacturing method, addressing also the safety of the structure. The design studies also reveal the inherent challenges of using TO for load-bearing glass components, which, in turn, lead to the establishment of design guidelines for developing a TO tool specifically for glass. Towards the real-life applicability of such complex-shaped, customized components, possible manufacturing methods are also discussed. ... Read more

This paper presents the casting of volumetric glass components from glass waste as an alternative glass-recycling approach. The approach is characterized by its flexibility to accommodate a variety of compositions and ability to yield volumetric (solid or thick-walled) glass products that can tolerate higher contamination rates without a significant compromise to their properties. The novelty of the proposed glass-to-glass recycling method lies in the “as-received” recycling of glass waste, using relatively low forming temperatures (750–1200 °C). This reduces both the need for expensive, labour-intensive and logistically complex purifying, segregation and treatment (e.g. removal of coatings) techniques, and the required energy and CO₂ emissions for product forming. Aim of this paper is to provide an overview of the potential but also of the technical and supply-chain challenges and limitations that still need to be tackled, in order to introduce this recycling approach to the market. Addressing the supply-chain barriers of glass recycling, the principal challenges linked to the collection and separation of glass waste and the established quality standards for the prevailing glass production technologies are identified, in order to argue upon the potential of this new recycling approach. In continuation, addressing the technical challenges that are mainly linked to contamination, an overview is provided of the main experimental findings on the influence of cullet contaminants and casting parameters on the generation of defects, and how these affect the mechanical properties. The experiments study a broad variety of glass compositions, including soda-lime, borosilicate, aluminosilicate and lead/barium glasses, and different levels of cullet contamination, of embedded (e.g. frit, wire) or external (e.g. stones, glass ceramics) character. Based on the cullet characteristics and imposed firing schedules, different glass quality grades arise and critical defects are highlighted. Thereafter, the most promising glass waste sources that can be recycled via this novel recycling approach are distinguished and directions for future research are highlighted. ... Read more

Among the environmental factors affecting glass weathering are humidity, exposure time, temperature, and the presence of pollutants in the atmosphere. Notwithstanding that the weathering produced depends on numerous factors, the important weathering effect of high humidity may be specifically mitigated by using a good chemical composition for the glass. To evaluate this relationship, flat glass samples from three suppliers were studied. The chemical composition of the samples was determined and the variability in compositions was evaluated to verify to what extent these small differences can affect their chemical durability. The chemical durability of the samples was evaluated by determining the hydrolytic resistance of crushed glass powder and using a visual appearance evaluation of bulk samples. The results demonstrate that when the samples are frequently washed, the compositional differences found between the suppliers can cause a significant difference in durability. The samples possessing the highest molar concentrations of Al2O3, and alkaline earth oxides (MgO + CaO) exhibited the highest hydrolytic resistance and the least visual deterioration. Differences encountered for the weathering products of glasses of comparable bulk compositions highlight that the process parameters play a major role in the alteration of the surface compositions of the glasses. For the unwashed samples, no consistent correlation was found between hydrolytic resistance and visual deterioration. ... Read more

The shaping freedom of cast glass in combination with the robustness of the resulting voluminous components opens up new, exciting directions in the field of structural glass. Yet, cast glass components remain brittle, limiting their structural applications in hyper-static compressive structures designed with conservative safety factors. Stretching these limits, this work investigates the reinforcement of cast glass by incorporating metal bars during the casting process, in a similar principle to reinforced concrete. Aim is to increase the ductility of the composite glass component, provide a warning mechanism prior to ultimate fracture and secure a postfailure load-bearing capacity. The development of hybrid glass components involves kiln-casting experiments using different metal-glass combinations, of similar thermal expansion coefficients. The method of introducing the metal bar in the glass during casting, and the effect of the selected forming temperature are investigated. The resulting metal-glass interfaces are examined for micro-cracks using a digital microscope, and for internal stresses using cross-polarized light. Two material combinations are found successful; soda lime silica with titanium and alkali borosilicate with Kovar. A hybrid borosilicate-Kovar 30*30*240mm beam is further tested in 4-point bending until failure, while its displacement is measured by Digital Image Correlation. The flexural response of the composite component is compared to the performance of unreinforced cast glass beams of similar composition. Although reinforced and unreinforced specimens show a comparable flexural strength, the reinforced specimen exhibits a warning mechanism well before failure, a gradual fracture and a post-failure load-bearing capacity. These attributes encourage the further exploration of cast glass reinforcement. ... Read more

Cast glass has great application potential in the architectural realm, yet, despite its possibilities and attractiveness, designers, engineers and developers are, from an early design stage, hesitant to employ it; the limited available and craft-based manufacturing facilities, the questionable quality of the product, the missing engineering data and quality control processes and the uncertainties linked with the structural application of cast glass, lead to discouraging high cost/ high risk solutions. Within the listed challenges and uncertainties, perhaps the most striking is our inability to answer the most obvious question: what is the strength of cast glass? A simple answer to this question does not exist, as the mechanical properties of cast glass vary from product to product, and are directly influenced by the employed chemical composition, thermal history, and casting process. Adding to this complexity, further questions over the mechanical properties and quality arise, once waste (contaminated) glass cullet and lower processing temperatures are used, in the efforts of reducing the environmental impact of cast glass components.

Focusing on this knowledge gap, the aim of this work is to develop an understanding of the effect of the casting parameters on the meso-level structure of cast glass, and thereupon of the relationship between this meso-level structure and the strength, stiffness and fracture resistance of cast glass components. Towards this aim, the dissertation adopts an experimental approach based on physical prototyping by kiln-casting, and destructive and non-destructive testing. The experimental work shows that by kiln-casting, a larger variety of chemical compositions can be cast, even at relatively low processing temperatures. As a consequence, a broad range of mechanical properties arises, especially when waste cullet is employed. Based on the casting parameters, combinations of different defects, grouped in meso-level structures, are commonly found in cast glass, yet these can often be tolerable when situated in the glass bulk. The dissertation highlights the potential of recycling-by-casting of currently challenging to recycle glass waste into reliable and aesthetically unique structural components, and the advantages of engineering composite cast glasses. It also underlines the need for manufacturing guidelines, test data, product certifications and quality control protocols, for the successful implementation of cast glass in the built environment.
... Read more

Up to now, fabricating cast glass components of substantial mass and/or thickness involves a lengthy and perplex annealing process. This has limited the use of this glass manufacturing method in the built environment to simple objects up to the size of regular building bricks, which can be annealed within a few hours. For the first time, structural topological optimization (TO) is investigated as an approach to design monolithic loadbearing cast-glass elements of substantial mass and dimensions, with significantly reduced annealing times. The research is two-fold. First, a numerical exploration is performed. The potential of reducing mass while maintaining satisfactory stiffness of a structural component is done through a case-study, in which a cast-glass grid shell node is designed and optimised. To achieve this, several design criteria in respect to glass as a material, casting as the manufacturing process and TO as a design method, are formulated and applied in the optimisation. It is concluded that a TO approach fully suited for three-dimensional glass design is as of yet not available. For this research, strain- or compliance based TO is selected for the optimization of the three-dimensional, cast glass grid shell node; in our case, we consider that a strain based TO allows for a better exploration of the thickness reduction, which, in turn, has a major influence on the annealing time of cast glass. In comparison, in a stress-based optimization, the considerably lower tensile strength of glass would become the main restrain, leaving underutilized the higher compressive strength. Furthermore, it is determined that a single, unchanging and dominant load-case is most suited for TO optimisation. Using ANSYS Workbench, mass reductions of up to 69% compared to an initial, unoptimized geometry are achieved, reducing annealing times by an estimated 90%. Following this, the feasibility of manufacturing the resulting complex-shaped glass components is investigated though physical prototypes. Two manufacturing techniques are explored: lost-wax casting using 3D-printed wax geometries, and kiln-casting using 3D-printed disposable sand moulds. Several glass prototypes were successfully cast and annealed. From this, several conclusions are drawn regarding the applicability and limitations of TO for cast glass components and the potential of alternative manufacturing methods for making such complex-shaped glass components. ... Read more