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This research investigates a new digital fabrication method for large-scale polychromatic glass elements. Glass elements with locally differentiated properties usually require manual labor or are limited to film applications of secondary materials that are incapable of producing material texture and relief in glass. To create mono-material glass elements for buildings with customized color, opacity, and relief present in the same glass element, this research investigates a novel robotic multi-channel printing process for industrial float glass. Mono-material polychromatic glasses do not require any additional material and can be fully recycled. This paper presents a design-to-production workflow for the construction scale within feasible cost. Investigations include kilning and material considerations, multi-channel tool and fabrication setup, tool path generation, process parameter calibration, and large-scale prototyping. The co-occurrence of locally varying opacities, colors, material textures, and relief within one glass element enabled by the presented robotic fabrication method could allow for novel optical and decorative features in facades and windows.

Optimizing the shape of concrete construction elements is significant in reducing their material consumption and totalweight while improving their functional performance. However, the resulting non-standard geometries are difficult andwasteful to fabricate with conventional formwork strategies. This paper presents the novel fabrication method of mineralfoam 3D printing (F3DP) of bespoke lost formwork for non-standard, material-efficient, lightweight concrete elements. Manyinnovative formwork studies have shown that stay-in-place formwork can help to reduce waste and material consumptionwhile adding functionality to building components. Foams are particularly suitable for this application because of their highstrength-to-weight ratio, thermal resistance, and good machinability. F3DP allows the waste-free production of geometricallycomplex formwork elements without long lead times and production-specific tooling. This paper presents the materialsystem and robotic F3DP setup with two experimental case studies: a perforated facade panel and an arched beam slab. Bothcases use concrete as structural material and strategically placed custom-printed foam elements. In this first preliminarystudy, concrete savings of up to 50% and weight reduction of more than 60% could be achieved. This is competitive withstandardized solutions such as hollow-core slabs but, in contrast, allows also for non-standard element geometries. Additionalfunctionality, such as programmed perforation, acoustic absorption, and thermal insulation, could be added through thestay-in-place formwork. Moreover, the challenges and future developments of F3DP for sustainable building processes arediscussed. Further studies are required to verify the findings. However, considering the urgent need for resource-efficient,low embodied-carbon solutions in the construction industry, this work is an important contribution to the next generation ofhigh-performance building components., Architecture, Structures and Construction, 3 (2), ISSN:2730-9894, ISSN:2730-9886

This paper presents an innovative design and fabrication workflow for a lightweight composite slab prototype that combines mineral foam 3D printing (F3DP) and concrete casting. Non-standardized concrete elements that are geometrically optimized for resource efficiency often result in complex shapes that are difficult to manufacture. This paper extends the research in earlier studies, showing that F3DP can address this challenge. F3DP is used to construct 24 stay-in-place formwork elements for a lightweight, resource-efficient ribbed concrete element with a 2 × 1.3 m footprint. This advancement highlights the improved robotic F3DP setup, computational design techniques for geometry and print path generation, and strategies to achieve near-net-shape fabrication. The resulting prototype shows how complex geometries that were previously cost-prohibitive can be produced efficiently. Discussing the findings, challenges, and future improvements offers useful perspectives and supports the development of this resourceful and sustainable construction technique., International Journal of Architectural Computing, ISSN:1478-0771, ISSN:2048-3988

Graph Neural Networks (GNNs) have become a popular toolkit for generative floor plan design. Although design variation has improved greatly, few studies consider non- geometrical characteristics, such as building energy performance, in the generative design process. This paper presents a GNN-based approach to predict the energy performance for floor plan customization (energy-aware design). The approach lays the foundation for a performance-aware generative design using GNN. The results show that the GNN can achieve high accuracy in energy performance prediction., Computing in Construction, 3, ISSN:2684-1150, Proceedings of the 2022 European Conference on Computing in Construction, ISBN:978-88-7590-226-1

Concrete is the most used human-made material in the world, and it is responsible for around 8% of the total greenhouse gas emissions worldwide. Hence, efficient concrete construction methods are one of the main foci of research in architecture, civil engineering, and material science. One recent development that promises to achieve this goal is the use of digital fabrication for building components. Most investigations focus on direct extrusion 3D printing with concrete, which has already been covered in several review articles. Conversely, this article reviews a different approach, which focuses on the indirect digital fabrication of concrete through 3D printed formworks. This approach is under investigation for structural and nonstructural, as well as for on- and off-site applications, with a number of projects having already been built, but a comprehensive review of 3D printed formworks has not yet been compiled to synthesize the findings. This article provides a comprehensive map of the state-of-the-art of five different 3D printing technologies used for the fabrication of formworks so far. The aim is to serve as a fundamental reference for future research, provide a basis for consistent language in this field, and support the development of construction standards. The article further discusses the new geometric possibilities with 3D printed formworks and their potential for making concrete construction more sustainable. In addition, the opportunities and challenges of 3D printed formworks are evaluated in the context of other traditional and digital fabrication tools. A synthetic classification in five functional typologies is proposed and illustrated with 30 representative case studies. Finally, the article concludes with a brief reflection on the role of 3D printing in the broader context of formwork innovation and a possible outlook for this technology.

This research investigates robotically fabricated polychromatic float glass for architectural applications. Polychromatic glass elements usually require labor-intensive processes or are limited to film applications of secondary materials onto the glass. Previous research employs computer numerical control (CNC) based multi-channel granule deposition to manufacture polychromatic relief glass; however, it is limited in motion, channel control, and design space. To expand the design and fabrication space for the manufacture of mono-material polychromatic glass elements, this paper presents further advancements using a UR robotic arm with an advanced multi-channel dispenser, linear and curved-paths granule deposition, customized color pattern design approaches, and a computational tool for the prediction and rendering of outcomes. A large-scale demonstrator serves as a case study for upscaling. Robotic multi-channel deposition and tailored computational design tools are employed to facilitate a full-scale installation consisting of eighteen large glass panels. Novel optical properties include locally varying color, opacity, and texture filter light and view. The resulting product constructs sublime architectural experiences through light refraction, reflection, color, opacity - beyond mere transparency.

In recent years, the architecture, engineering and construction industry is increasingly embracing digital design approaches and robotic production methods. This is as well noticeable in the façade design sector, especially when it comes to architectural design tasks dealing with complex geometries or adaptive components. Although glass is mainly used in facades due to its transparency, it often determines significantly the aesthetic appearance of a building and has to control the transmitted light and the visibility between exterior and interior. Recently, a novel method for producing polychromatic glass panels by fusing granular glass powder of different colours onto the surface of annealed float glass in a kilning process was developed at ETH Zurich. By using a multi-channel tool attached to a robotic arm for placing the granular powder onto the glass surface, digitally designed patterns can be realised precisely and repetitively. This paper focuses on the structural behaviour and the micro-structural characteristics of this novel type of coloured glass panels for façade applications. The bending strength of such glass panels was determined based on four-point-bending tests and was compared to that of annealed float glass. A non-negligible decrease in the bending strength was observed. Furthermore, the reasons for this reduction in structural performance were analysed based on microscopic investigations. It was observed that both the kilning process and the density of deposited granular powder had an influence on the surface microstructure. The presented results are essential for the production of polychromatic kilned glass as well as for future applications of this type of glass panel in façades., Glass Structures & Engineering, 8 (1), ISSN:2363-5142, ISSN:2363-5150

While digital fabrication technologies with concrete have focused primarily on process development and compressive strength in terms of performance, more recently studies evaluating the durability performance of 3D printed cementitious materials have started to appear. To date, however, no field performance assessments have been published regarding 3D printed cementitious materials. In this study, we present the condition of bespoke 3D printed columns that have been exposed to the high alpine environment of Riom-Parsonz, Switzerland, for a period of two years. Damage levels are variable from column to column, often appearing as vertically oriented cracks that can also track along weak layer interfaces. The damage is conjectured to be linked primarily to freeze-thaw damage and/or thermal or differential shrinkage strains. The link between column design and damage is discussed in depth, in particular as it relates to the current predominant use case of 3D printed cementitious materials as a lost formwork. ISSN:2211-0844

Providing reinforcement is essential for the structural integrity of concrete elements and for safely handling, transporting, and assembling prefabricated concrete parts. However, the integration of reinforcement is a persisting challenge in 3D concrete printing with extruded concrete. This paper presents a production process consisting of 3D printing around pre-installed reinforcement. The reinforcement is composed of conventional reinforcing steel bars, which can be pre-assembled in cages independently of casting, boosting the specialisation and efficiency in production. This approach was used to produce a 3.4 m span T-beam with optimised topology, consisting of three segments connected with matching surfaces. The beam segments were printed upside down, with an open web on top of the flange. Each segment featured reinforcing steel installed in the flange and web. After printing and assembling the segments, a conventional reinforcing bar was inserted in the web as bending reinforcement and grouted subsequently. The structural performance was assessed in a six-point bending test. The fabrication and structural testing of this case study beam showed that pre-installed reinforcement imposes several challenges to the extruder precision, the precision of the bent reinforcement, and – if applied – the casting after printing., RILEM Bookseries, 37, ISSN:2211-0844, Third RILEM International Conference on Concrete and Digital Fabrication, ISBN:978-3-031-06115-8, ISBN:978-3-031-06116-5

The issue of reinforcement remains a difficult one for digital fabrication in concrete construction, with many solutions offered on how to best incorporate it with the technology in a practical manner. While certain technologies, such as digital casting systems, allow more seamless introduction of traditional steel reinforcement bars, the dominant technology of extrusion-based 3D printing still shows great limitations, with the most typical application being that of a lost formwork for traditionally cast concrete, or as unreinforced masonry. The use of both conventional and unconventional reinforcement materials, and their incorporation into processes, is a large and active area of research with the digital concrete community. In this work, we present a 3D printing platform and manufacturing concept in which a 3D printed form serves as a vertical substrate for a reinforcement, either textile or conventional steel. The structural concrete is then either applied by hand or sprayed around the reinforcement. ISSN:2211-0844

This paper presents the design and fabrication of a lightweight composite concrete slab prototype using 3D printing (3DP) of mineral foams. Conventionally, concrete slabs are standardized monolithic elements that are responsible for a large share of used materials and dead weight in concrete framed buildings. Optimized slab designs require less material at the expense of increasing the formwork complexity, required labour, and costs. To address these challenges, foam 3D printing (F3DP) can be used in construction as demonstrated in previous studies for lightweight facade elements. The work in this paper expands this research and uses F3DP to fabricate the freeform stay-in-place formwork components for a material-efficient lightweight ribbed concrete slab with a footprint of 2 x 1.3 m. For this advancement in scale, the robotic fabrication and material processing setup is refined and computational design strategies for the generation of advanced toolpaths developed. The presented composite of hardened mineral foam and fibre-reinforced ultra-high-performance concrete shows how custom geometries can be efficiently fabricated for geometrically complex formwork. The prototype demonstrates that optimized slabs could save up to 72% of total concrete volume and 70% weight. The discussion of results and challenges in this study provides a valuable outlook on the viability of this novel fabrication technique to foster a sustainable and resourceful future construction culture.

An important research perspective for optimization of concrete structures today can be recognized in the concept of controlled spatial grading of concrete elements. Spatial grading aims to control the volumetric distribution of concrete within an element based on the internal stresses, which enables a more efficient structural performance with less material without changing the overall geometry of a structural part. Due to high geometric complexity, the fabrication of spatially graded structures are typically based on additive manufacturing techniques such as extrusion or voxel-based 3D printing. However, for large-scale production and fabrication, 3D printing is ultimately constrained by speed and cost. This article presents the potential of regular ice as aggregate for casting spatially graded concrete components. Prefabricated ice aggregates of varying size and geometry are carefully placed into a mould to form regions with lower and higher relative densities, into which self-compacting concrete is poured. We investigate various parameters of customized ice aggregates including geometry, scale and relative densities. The presented experiments demonstrate how ice aggregate can be used to efficiently fabricate spatially graded concrete elements with high geometric complexity, while improving the structural performance by concentrating concrete only where it is needed. In addition, the intricate patterns of voids created by ice aggregate result in unique visual and lighting qualities that would be difficult to achieve with other fabrication methods. This article addresses the fundamental research questions and provides a ground for further research in digitization and automation of the ice aggregate production method., RILEM Bookseries, 37, ISSN:2211-0844, Third RILEM International Conference on Concrete and Digital Fabrication, Digital Concrete 2022, ISBN:978-3-031-06115-8, ISBN:978-3-031-06116-5

In an increasing effort to address the environmental challenges caused by the currently linear economic paradigm of “produce, use, and discard”, the construction industry has been shifting towards a more circular model. A circular economy requires closing of the loops, where the end-of-life of a building is considered more carefully, and waste is used as a resource. In comparison to traditional building materials such as timber, steel and concrete, mycelium-based materials are renewable alternatives that use organic agricultural and industrial waste as a key ingredient for production, and do not rely on mass extraction or exploitation of valuable finite or non-finite resources. Mycelium-based materials have shown their potential as a more circular and economically competitive alternative to conventional synthetic materials in numerous industries ranging from packaging, electronic prototyping, furniture, fashion to architecture. However, application of mycelium-based materials in the construction industry has been limited to small-scale prototypes and architectural installations due to low mechanical properties, lack of standardisation in production methods and material characterisation. This paper aims to review the current state of the art in research and applications of mycelium-based materials across disciplines, with a particular focus on digital methods of fabrication, production, and design. The information gathered from this review will be synthesised to identify key challenges in scaling up applications of mycelium-based materials as load-bearing structural elements in architecture and suggest opportunities and directions for future research., Biomimetics, 7 (2)

Increasing demand for complex concrete shapes in architecture has triggered the use of digital fabrication methods for producing non-standard formworks. To this end, polymer extrusion 3D printing has already shown promising results in a range of research projects. However, 3D-printed polymer formworks can suffer from sudden failures during casting, which are generally associated with the pressure exerted by the wet concrete on the thin plastic formworks. Such failures can however be substantially accelerated by the corrosive effects of the wet concrete paste on some 3D-printed thermoplastics, a phenomenon that has not been studied in depth so far. This paper first reviews the state of the art of 3D-printed polymer formworks and their applications in architecture. It focuses on the failures caused by formwork pressure and synthesizes the various solutions to this problem reported in the literature. Beyond this, additional results are introduced to provide a guideline for thermoplastic selection in 3D-printed formworks. Common thermoplastics are presented, and their behaviour during concrete casting is documented. This behaviour can be analysed through ultimate strength and creep tests performed in three-point bending on 4 × 10 × 100 mm 3D-printed polymer samples, both in dry state and exposed to an alkaline solution of pH 13, typical for cementitious materials. The tests show that contact with the alkaline solution leads to a rapid strength loss in some thermoplastics. This effect is particularly pronounced with PLA and its blends, for which the strength loss over 10 min is more than 80 % compared to dry samples. Other thermoplastics, such as ABS, ASA, HIPS, Nylon, PVA and ABS/PC have shown more limited or in some cases no deterioration under the same conditions. The material-specific mechanism of deterioration is environmental stress cracking in contact with the alkaline solutions and it occurs in all 3D printing orientations and loading directions where tensile stresses occur., Additive Manufacturing, 58, ISSN:2214-8604

The pursuit for complex geometries in contemporary architecture is driving innovation towards an unconstrained fabrication freedom for building components. Concrete is a building material with excellent structural and architectural qualities, which has the theoretical capacity of being cast into any shape. However, in practice, concrete is generally limited by the formwork manufacturing industry to solid, planar shapes. The aim of this research is to overcome the fabrication limitations for formwork and, indirectly, for concrete. To achieve this aim, the objective of this research is twofold: a) enable the fabrication of building-scale concrete components through 3D-printed plastic formworks and b) develop computational design and optimisation methods suitable for this fabrication method. The resulting design and construction method takes advantage of the load-bearing capacity of concrete and relies on the fabrication freedom inherited from the 3D-printed formwork, thus making complex topologies and precise details possible for concrete structures. The research method for demonstrating this fabrication process focused on the design to fabrication steps of skelETHon a functional four-meter-long concrete canoe which was designed, built and raced in a regatta on the Rhine river (Figure 1). The main research achievement was the fabrication of the canoe, an optimised truss-like concrete component with members as thin as 15 mm in diameter. Such slender geometric features are difficult to produce in concrete with other known formwork systems., Gestão & Tecnologia de Projetos, 14 (1), ISSN:1981-1543

ZusammenfassungEin verzweifelter Patient kommt in die Praxis. Laut Aussage des Arztes hat er mehrere „Baustellen“ in seinem Körper und es scheint so, als gäbe es nur einen operativen Ausweg. Mittels eines strukturierten Vorgehens der modernen Manuellen Therapie lernt der Patient schlussendlich, mit seinem Problem adäquat umzugehen.

This chapter discusses the meaning of intuition, creativity, and intelligence in the context of architecture and how information technology can potentially support designers and augment their own intuition. It starts with looking at how technology has been employed to support the design process throughout history, even before computers were invented. The general introduction sheds light on different creative processes, asking what creativity is and how it can be classified. It is followed by a description of two case studies conducted by the authors. Although these projects are very diverse in type, application, and scale, they showcase the potential and promising applications of artificial intelligence (AI) that enhance architects’ creativity and allow them to encompass the synthetic approach for architectural development from the design to its physical realization. The chapter reflects on the current and categorical limitations of AI and concludes with an optimistic outlook on various possible creative applications of AI in the domain of architecture.

Large-scale additive manufacturing for construction has gained momentum during the last two decades as a promising fabrication technology that can save materials, labor, and costs. Although foams are a significant material group in construction and explored in 3D printing (3DP) studies, no comprehensive review about this field exists to date. Consequently, the aim of this review is to define the field of foam 3DP (F3DP) in construction and provide an overview of relevant developments, challenges, and future research. Based on the analysis of more than 150 peer-reviewed articles and research reports, three major themes within the academic debate about F3DP could be identified: developments in material composition and material design, printing and processing technologies, and future challenges in application and material processing development. This review brings together promising advancements in F3DP for construction into a systematic overview and opens new horizons in research and development for sustainable construction processes., Automation in Construction, 130, ISSN:0926-5805

We present a method to design non-planar layered print paths for robotic fused deposition modeling (FDM) printing of single-shell surfaces. The advent of robotic arms has created great potential in the 3D printing industry for the realization of non-planar print paths that allow transitioning between different orientations during the print. However, this potential is often not fully realized due to the various challenges associated with the design of feasible non-planar print paths. Inspired by the ubiquitous key-framing technique in animation, where an input of limited degrees of freedom is used to describe a complex behavior, we propose a method to generate non-planar layered print paths subject to the input of the designer, by specifying a series of target curves on the surface of a mesh. Our method generates intermediary print paths with a direction that interpolates that of the targets while respecting the sequence and distances between neighboring paths imposed by fabrication constraints. The resulting print paths have variable layer height, and their realization relies on variable end-effector orientation. We present several examples and fabricated prototypes and make qualitative comparisons to planar slicing, to showcase the capabilities of our method.

With 3D printing, material consumption can be reduced: It allows to place material only where needed, therefore structurally optimized building parts or formworks can be printed. Currently this technique uses an epoxy based resin to glue layer by layer an inert sand bed. However, this material choice holds a large improvement potential from an environmental and health perspective. It was shown previously, that the organic glue, that releases unhealthy volatile organic compounds, can be replaced by a mineralic binder, namely a geopolymer. With geopolymers and alkali activated materials, the embodied energy can be reduced, especially when built from waste materials. In this study, we focus on the replacement of the sand, which is becoming a scarce resource. The waste from a local gneiss quarry in Ticino (Switzerland) could be a good alternative. The powder bed of the 3D printing is made of aggregates of crushed quarry waste mixed with an aluminosilicate powder. The printing liquid is an alkaline solution that activates the aluminosilicate and reacts to a geopolymer. Droplet penetration experiments on different powder mixes were performed to adjust the binder composition. With a custom built powder bed 3D printer, samples with varying compositions and porosities were printed and tested on compression. It could be shown that samples printed with quarry waste perform as well as samples printed with silica sand in terms of compression strength and accuracy. This new material system is promising: with 3D printing of geopolymers and quarry waste, we can combine the environmental benefits of a new building technique with a low carbon intense material. The application of this technique may help the sustainable development of the local quarry sector by consuming the volumes of waste that causes storing and ecological issues and keep small quarries running., IOP Conference Series: Earth and Environmental Science, 588 (4), ISSN:1755-1315, ISSN:1755-1307

Additive Manufacturing techniques enable an unprecedentedly fine-grained control over the material they process, both in terms of its geometric distribution and its very composition. With the rapid development of printing methods suitable for the production of large-scale objects together with their respective material systems, applications in the construction industries showcase an expanding portfolio of practical examples. However, considering the more and more critical demand for minimizing the environmental impact due to construction activities, the aim for mechanical stability of a material system is no longer sufficient and needs to be flanked by attentive considerations over the sustainability of its components.

Digital Fabrication with Concrete (DFC) brings many new possibilities for the design and production of concrete structures, promising to revolutionise the concrete construction industry. While technological and material challenges have already been overcome to a large extent, there is still a lack of sufficiently mature reinforcement solutions. Therefore, most digital technologies encounter difficulties in producing load-bearing concrete members. Fibre reinforced concrete (FRC) is one of the most promising reinforcing strategies for DFC due to its capability for producing complex geometries. In conventional FRC, the fibres are dispersed randomly in the concrete matrix; for DFC applications this (i) forces to re-engineer the concrete processing (pumpability and rheology), and (ii) requires using very short and expensive fibres due to pumpability constraints. This paper presents a new reinforcement strategy for using FRC in layered DFC technologies that overcomes the stated limitations of conventional FRC. It consists in adding fibres right after the deposition of each layer of concrete in a controlled amount and orientation and providing a post-tensioning reinforcement in the perpendicular direction. The mechanical behaviour, as well as the potential and first implementation steps in the Concrete Extrusion 3D Printing and Eggshell technologies under development at ETH Zurich, are discussed. The mechanical results show a significant increase in tensile resistance of the aligned interlayer fibre reinforcement compared to conventional FRC. However, for large-scale applications, the main loads still need to be carried by post-tensioning reinforcement., RILEM Bookseries, 28, ISSN:2211-0844, Second RILEM International Conference on Concrete and Digital Fabrication. DC 2020, ISBN:978-3-030-49915-0, ISBN:978-3-030-49916-7

The construction industry is a significant emitter of CO2 and it uses more than 40% of the resources consumed worldwide, which are becoming scarce. 3D printing can help to reduce the material consumption by enabling the manufactur-ing of structurally optimized building parts and formworks since material is only placed where needed. To combine the reduction of material consumption with less CO2 emissions, the raw materials that are currently used in 3D printing in construction need to be replaced by materials with lower carbon footprint. Among the materials with high potential for this application, geopolymer is a good candidate to fulfill the required criteria in terms of performances and em-bodied energy as they can be built from waste materials such as slag, fly ash or glass waste as aluminosilicate source. The objective of this research is thus to define a mix design strategy to allow the implementation of geopolymers in powder bed 3D printing with various waste materials. In this technique, an aluminosilicate powder bed is activated through the deposit of an alkaline silicate solution. This selective binder activation ap-proach requires to master the spreading of the liquid into the powder bed and to ensure that the reaction takes place where it is needed. A series of experiments at the droplet level were performed to estimate the relationship between the initial liquid to solid ratio and the packing of the powder bed. Based on this a mix de-sign strategy for the printing solution was developed and proofed by SEM/EDX analysis. ISSN:2211-0844

Although slabs are major concrete consumers, they are mostly flat, oversized, monolithic boxes with significant embodied energy. The state of the art shows how computational design can lead to structurally efficient, lightweight, functionally integrated, and aesthetically accomplished slabs. However, these non-planar geometries are fabricated using complex formwork solutions involving multiple digital fabrication processes and manual concreting.

With the growth in popularity of 3D printing technologies, machinery is getting cheaper, while the variety of available materials is growing larger. However, 3D printing strategies for realizing architecture are still in their infancy. Direct 3D printing of building components is a very challenging task, as most printing technologies are, until now, not able to address the demanding requirements of architectural components, including scale or structural capacity.

This paper presents a novel application of volumetric modelling (VM) for the design of fabrication-informed three-dimensional deposition paths for in place spatial additive manufacturing (AM). VM offers modelling techniques for designing with great geometric flexibility using numeric data, as well as for managing fabrication constraints. To address the challenges and new design possibilities presented by in place spatial AM, we propose a set of tools to design deposition paths with embedded fabrication constraints, as well as methods to combine VM with curve and surface geometry to generate production data. As a case study, we implement this toolset to create wire arc additive manufacturing (WAAM) connections of standard elements. A comparison of virtual and physical results is presented to validate the approach. Finally, we discuss potentials and limitations of using VM tools for fabrication-aware design of in place spatial AM.

Non-standard stairs have an important role in architecture, but their complex details and three-dimensionality pose significant fabrication challenges. One of the preferred materials for custom stairs is concrete, which can be cast in complex shapes, the only limitation in this case being the fabrication of the necessary formworks.This paper reviews the opportunities and challenges of using 3D-printed formwork for fabricating custom concrete stairs with complex geometries (Fig. 1). 3D printing can unlock an entirely new vocabulary of shapes for concrete structures, previously unavailable with traditional formwork systems. Only a minimal amount of 3D-printed plastic is required to deliver a very thin, stable shell. With these formworks, complex topologies can be achieved in concrete elements. Such elements can optimize the structural performance or improve functional aspects, as well as introduce a radically different aesthetic.