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.@en

This work develops a computational method that produces algorithmically generated design forms, able to overcome inherent challenges related to the use of cast glass for the creation of monolithic structural components with light permeability. Structural Topology Optimization (TO) has a novel applicability potential, as decreased mass is associated with shorter annealing times and, thus, considerably improved manufacturability in terms of time, energy, and cost efficiency. However, realistic TO in such structures is currently hindered by existing mathematical formulations and commercial software capabilities. Incorporating annealing constraints into the optimization problem is an essential feature that needs to be accommodated, whereas the brittle nature of glass invokes asymmetric stress failure criteria that cannot be captured by conventional ductile plasticity surfaces or uniform stress constraints. This paper addresses the approximation problems in the evaluation of principal stresses while concurrently incorporating annealing-related manufacturing constraints into a unified TO formulation. A mass minimization objective is articulated, as this is the most critical factor for cast glass structures. To ensure the structural integrity and manufacturability of the component, the applied constraints refer both to the glass material/structural properties and to criteria that ensue from the annealing and fabrication processes. The developed code is based on the penalized artificial density interpolation scheme and the optimization problem is solved with the interior-point method. The proposed formulation is applied in a planar design domain to explore how different glass compositions and structural design strategies affect the final shape. Upon extraction of the optimized shape, the structural performance of the respective 3D structures is validated with respect to performance constraint violations using the Ansys software. Finally, brief guidelines on the practical aspects of the manufacturing process are provided.@en

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.@en

Editorial@en

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.@en

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.@en

Interlocking cast glass assemblies are a promising solution for architectural cast-glass applications aiming for high transparency and a reversible structure that allows the reuse of the glass components (Oikonomopoulou et al.,2018; Oikonomopoulou,2019b). In such a system, an interlayer material between the glass elements is essential, to assist the homogenous stress distribution and account for the surface microasperities of the glass elements. Towards circularity, this material should be dry (and not an adhesive), allowing for the eventual disassembly of the system. Previous experimental work by (Aurik at al.,2018; Oikonomopoulou at al.,2019b) has focused on the use of PU and PVC interlayers as suitable candidates; the focus in those studies has been solely placed on the mechanical performance of the interlayer material. This research provides a review of potential material candidates suitable for interlayers of an interlocking cast glass assembly based on a set of revised design and performance criteria that are divided into primary and secondary. Furthermore, the impact their unique material properties have on the potential application of the interlocking system is examined. The whole process, from fabrication to construction of the entire assembly, based on an assumed building scenario, is presented in a chain reaction manner, whose starting point is the interlayer itself. After defining the design criteria the interlayer should adhere to, the proposed candidates are: PETG sheets (Vivak®), Neoprene, Aluminum, Laminated Polyurethane (PU) and a Soft-core aluminum interlayer. The unique properties and fabrication challenges of all five proposed interlayers are considered, as well as their properties in relation to assembly, which leads to the development of two distinct assembly sequences. The main distinction concerns the interlayers that risk creeping and those that do not. The research concludes with a comparison between the interlocking assembly and the other glass block assemblies currently applied.@en

This research revolves around the design, fabrication and testing of tubular glass columns, with particular focus on their redundancy and fire-safety mechanisms; moreover, addressing aspects such as: the column shape; cleaning and maintenance; end connections; geometric tolerances in the glass and demountability. Two alternative circular hollow (tube) column designs are initially developed and engineered to address these aspects, namely: the MLA (Multi Layered with Air) and the SLW (Single Layered with water). In both concepts the main load-bearing structure consists of two concentric laminated glass tubes. Thus, in order to explore the manufacturing challenges and structural potential of these concepts, the prototyping and experimental work focuses on six 300 mm long samples with 115 mm outer diameter that are laminated and fitted into customized, engineered steel end-connections. Particular attention in terms of manufacturing is paid to the lamination process and associated bubble formation, the possible fracture of the glass by internal resin-curing stresses and the interface between the glass tube and the steel end-connections. All samples are laminated with Ködistruct LG 2-PU component. Three samples are assembled using DURAN® (annealed) glass and the other three are using DURATAN® (heat-strengthened) glass. Subsequently, the six samples are tested in compression until failure to investigate the behaviour of the interlayer material, the post-fracture behaviour of the designs, the differences between annealed and heat-strengthened samples, the capacity of the glass tubes and the performance of the end connections. Initial cracks appeared between 95-160 kN (compression strength of 30-50 MPa) in the DURAN® samples and between 120-160 kN (compression strength of 37-50 MPa) in the DURATAN® samples. These loads are lower than the ones estimated by calculations; in specific, the first cracks occurred at 34-64% of the calculated load. Nevertheless, the samples are found to be robust, with a considerable load-bearing capacity beyond the first cracks, leading to a maximum nominal compression strength capacity of up to 152 MPa for the DURATAN® samples and up to 233 MPa for the DURAN® samples.@en

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.@en

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.@en

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. […]@en

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.@en

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.@en

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.

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Beginning of November 2020, five years after realizing the famous Crystal House concept with architectural firm MVRDV, the Glass Research Group of the Faculty of Architecture and the Built Environment of the Technical University of Delft was contacted by architect Konstantin Ikonomidis with the request to provide glass expertise for a small - yet challenging - project.@en

The emerging interest in the architectural applications of cast glass components reveals a knowledge gap on the mechanical properties of cast glass. Apart from its chemical composition, cast glass is characterized by its manufacturing history and thermal profile, often inheriting a set of defects that define its properties. The role that inhomogeneities in the bulk of voluminous glass components have on the strength of the final product is also uncertain. Systematic testing is therefore necessary for the safe structural application of cast glass. Towards this direction, the presented research aims to experimentally investigate the fracture resistance of cast glass under sharp contact loading, by means of a customized splitting test using a sharp linear indenter. Cubic specimens with 50 mm sides are kiln-cast at low forming temperatures, employing a variety of silicate-based cullet and firing schedules and their inherent defects are documented. The results of the splitting tests show that the borosilicate specimens fail at the highest splitting force, followed by the soda lime float specimens, while the fused or porous specimens have a significantly lower resistance to fracture. The strength order of the various glasses, as this results from the splitting tests- is opposite to that found earlier in four-point bending tests, due to the different fracture mechanisms activated. The fracture resistance of a glass specimen is governed, first by its ability to deform around the indenter to relief the developing stresses and then by its bond strength to resist crack propagation. Thus, a good balance between glass network flexibility and high bond dissociation energy is required, explaining why the tested homogeneous borosilicate and soda lime glasses are more resistant than the modified soda lime compositions with high alkali content. In addition, the fractographic analysis indicates that the non-stress inducing flaws in the bulk have a negligible contribution to the fracture resistance of the specimens.@en

This paper explores a novel restoration approach for consolidating historic structures using structural glass to substitute for the missing elements, preserving at the same time the structural integrity of the structure. A restoration design is proposed for the hypothetical consolidation of the remaining tower of Schaesberg Castle, in Limburg, The Netherlands. The masonry walls suffer from a significant loss of material and a temporary steel structure currently prevents the tower from collapsing. It is proposed to use stacked float glass to fill the missing parts of the wall. The connection between the old masonry and new glass is the most challenging aspect given the different physical and mechanical properties of the materials, which need to work together in a coherent way. Shear tests of various connecting materials are carried out in order to evaluate the performance of this connection with respect to aspects of compatibility and feasibility.@en

Cast glass has great potential for diverse load-bearing, architectural applications; through casting, volumetric glass components can be made that take full advantage of glass’s stated compressive strength. However, the lack of engineering, production and quality control standards for cast glass and the intertwined ambiguities over its mechanical properties-particularly due to the variety in chemical compositions and the lack of understanding of the influence of flaws occurring in the glass bulk-act as an impediment to its wide-spread application. Addressing the above uncertainties, this work studies a total of 64 silicate-based glass specimens, prepared in 20 * 30 * 350 mm beam size, either by kiln-casting at relatively low forming temperatures (970–1120 ∘C), or by modification of industrially produced glass. For the kiln-casting of the specimens, pure and contaminated recycled cullet are used, either individually or in combination (composite glasses). The defects introduced in the glass specimens during the casting process are identified with digital microscopy and qualitative stress analysis using cross polarized light, and are categorized as stress-inducing, strength-reducing or harmless. The Impulse Excitation Technique is employed to measure the Young’s modulus and internal friction of the different glasses. Differential Scanning Calorimetry is used on a selection of glasses, to investigate changes in the glass transition range and fictive temperature of the kiln-cast glasses due to the slower cooling and prolonged annealing. The four-point bending experiments are shedding light upon the flexural strength and stiffness of the different glasses, while the fractographic analysis pinpoints the most critical defects per glass category. The experiments show the flexural strength of cast glass ranging between 30–73 MPa, according to the level of contamination and the chemical composition. The measured E moduli by both methods are in close agreement, ranging between 60–79 GPa. The comparison of the flexural strength with prior testing of cast glass involving shorter span fixtures showed a decreasing strength with increasing size for the contaminated specimens, but similar strengths for pure compositions. The results highlight the versatile role of defects in determining the glass strength and the complexity that arises in creating statistical prediction models and performing quality control.@en