1.1. Introduction: the different facets of printing

Since its initial development in the early 2000s, 3D printing has always been a polymorphic process, depending on the type of material used and the robotic system employed. Initial developments such as Prof. Koshnevis’ Contour Crafting concept (Khoshnevis 2004; Khoshnevis et al. 2006; Buswell et al. 2018), based on an extrusion system, or Enrico Dini’s D-Shape process, based on a selective particle bonding printing system (Colla et al. 2013), illustrate this diversity, which has continued to increase ever since. 3D concrete printing has almost as many variations as there are printers and companies working on the subject.

The multifaceted nature of printing is reflected in ASTM F2792-12A (ASTM International 2012), which defines seven categories of additive manufacturing processes that can be used for any type of material. These categories can also be related to the different processes used today for concrete and cementitious materials.

Although there is a wide range of possible processes, 3D concrete printing techniques (which often involve mortar more frequently than concrete, as the presence of gravel requires large pumps, print heads and robots) share many points in common, particularly in the use of digital technology and robotics (Perrot and Amziane 2019).

The integration of robotics and digital technology means that the starting point is the design of a 3D model of the object to be printed, the drafting of instructions that the robot can understand and finally the production of the object by the robot.

In this chapter, we will start by presenting these stages, which are common to the printing process in general. We will then present the current applications of the 3D concrete and mortar printing, focusing on the materials used and alternatives to conventional cementitious materials.

Finally, we will present several classifications that help position a given printing process in the world of digital concrete manufacturing.

1.2. 3D printing: from digital model to physical object

1.2.1. From digital model to print

The first step in 3D printing is to design the part to be printed. This design stage always begins with creating a 3D model in CAD (computer-aided design) software.

Many 3D design solutions exist, but they differ according to the philosophy of the tool used and the complexity of the object to be modeled.

For modeling simple, perfectly vertical shapes, we may draw the deposition contours of a layer to be printed on the deposition plane and then extrude this surface in the direction perpendicular to the deposition plane (generally the vertical axis). This design and subsequent printing strategy is often referred to as 2.5D printing.

For more complex shapes with overhangs, it is necessary to work in three dimensions and use modeling solutions to represent lines and surfaces in a 3D reference frame.

Finally, in order to design architectural shapes with patterns, parametric 3D design is an extremely useful tool for obtaining shapes with repetitive patterns that provide added aesthetic value or additional functions (noise absorption, sun shading, etc.). The use of tools, such as Rhino/Grasshoppers, is among the most popular parametric design solution (Figure 1.1). This solution uses a graphical programming system to manipulate shapes and objects and repeat actions to create 3D patterns.

As part of a BIM project, geometric design in software compatible with the IFC format, the BIM exchange format may be of interest. The 3D model then needs to be exported in a format compatible with the software used to generate the robot trajectory. The goal is to export the file in a format that can generate a G-Code (Geometric Code), which is a file containing a sequence of instructions that drives a 3D printer. The .STL, .obj and .amf formats can be used for this purpose.

These export files are then imported into a “slicer”, which is used to program the trajectory of the print head and set several parameters, such as the start of extrusion, the position of the print head during deposition, the infill of a volume (pattern and density) or the start position of the print. The defined robot trajectory must then be optimized to ensure no collisions during printing and good contact between adjacent or superimposed deposits.

It is also important to note that it may be necessary to convert the G-Code to a language compatible with the printing system (such as six-axis robots).

1.3. 3D concrete printing – application examples

1.3.1. Prefabrication

1.3.1.1. Support-free printing

The use of 3D concrete printing in the prefabrication of concrete parts, which are then assembled on-site to increase precision and/or production rates (Volpe et al. 2021), or to create specific functions with unique or complex geometries (such as artificial reefs, stormwater basins, voussoirs for a bicycle bridge, prefabricated staircases, etc.; Salet et al. (2018); Ly et al. (2021)) is now commonplace. Among the additive manufacturing methods used for concrete, some do not require supports, such as extrusion-deposition methods, which are particularly developed today.

Numerous companies have developed their own printing solutions and related businesses. Examples include XtreeE in France, Hyperion Robotics in Finland, Weber, TAM and Vertico in the Netherlands, and Sika in Switzerland.

All these companies use six-axis robots to print precast-printed concrete elements. Some of these companies also use a rail on which the robot is placed to increase the available printing volume (Figure 1.2). A system for preparing and transporting the material by pumping is also a standard feature of printing plants.

Prefabrication is often used to manufacture small parts or parts with a small cross-section. As a result, the length of the printing contour (the length of the filament of a single printed layer) imposes very short deposition times, requiring a rapid rate of elevation of the structure being printed. This fast printing rate induces rapid loading of the structure, which is not compatible with the structuring kinetics of traditional cements (Reiter et al. 2018).

There are two solutions to this problem. The first is to develop single-component formulations, which have a period of rheological stability to enable mixing and transport to the print head, followed by a period of rapid structuring of the material to enable fast printing. The other solution uses two-component materials and consists of injecting a texturizing agent or a setting accelerator at the print head to enable rapid setting kinetic growth as soon as the material is deposited.

Each method has its advantages and disadvantages: the single-component solution does not tolerate interruptions to the printing process, as otherwise, there is a risk of curing in the pump lines and consequent blockage. The two-component solution, on the other hand, adds an in-line mixing stage just before deposition, which requires a careful choice of product to accelerate curing and the design of a more complex print head with an in-line injection and mixing system (active or passive) (Tao et al. 2021; Boscaro et al. 2022; Wangler et al. 2022).

A specific two-component process called dual pumping has been developed by Ghent University (Tao et al. 2022) and enables two fluid materials with equivalent volumes to be pumped, which, once mixed just prior to extrusion, undergo rapid mechanical structuring, enabling high printing speeds.

Robotic complexity also plays an important role in 3D printing (Figure 1.3). A three-axis robot (Cartesian, delta or a six-axis robot using only three-axis) will enable a structure to be printed with a cylindrical filament cross-section, a print head that is always vertical, and limited freedom of form. Switching to four-axis adds rotation around the vertical direction and enables the use of perfectly cylindrical layer cross-sections, which is of interest for obtaining smooth vertical surfaces. Finally, using the robot’s six-axis allows printing with changing printhead orientations, providing greater freedom of form and enabling more efficient construction of arches or domes (Carneau et al. 2020).

Printed shotcrete also makes it possible to achieve high printing speeds for precast elements, as shown by studies carried out at the Technical University of Braunschweig (Hack and Kloft 2020). 3D printing of shotcrete allows greater freedom of form.

The smart dynamic casting technique also requires no support to produce prefabricated parts such as columns or slender and slender walls (Lloret-Fritschi et al. 2017; Szabo et al. 2020). The use of a six-axis robot fitted with variable-section formwork enables these elements to be produced by adjusting the speed at which the formwork rises to match the speed at which the cementitious material is structured. If the material leaving the sliding formwork is too fluid, the structure collapses, whereas if it is too rigid, the friction at the formwork wall becomes greater than the tensile strength of the printed material section, which becomes brittle and breaks into two parts (Craipeau et al. 2021).

1.3.1.2. Printing with support

It is also possible to produce prefabricated parts using 3D printing techniques that use supports. These methods allow greater freedom of form, enabling the creation of parts with cantilevers that transition from a vertical to a horizontal structure.

The particle-bed printing technique can therefore be used to produce parts with complex shapes whose surface roughness depends on the size of the particles used. For example, the selective bonding 3D printing technique will produce parts with a rougher surface than the activation bonding technique.

Similarly, 3D printing by depositing material in a threshold fluid, such as carbopol or limestone suspensions, enables lattice structures to be produced, making it possible to use topological optimization methods for structural dimensioning (De Schutter et al. 2018; Vantyghem et al. 2020) and thus save material for the same mechanical function. It is also possible to print ultra-high-performance concrete in ordinary concrete for the same reasons of mechanical efficiency (Lowke et al. 2021).

Finally, methods for pouring concrete into printed formwork fall into this category. Since the Yhnova house printed in 2017 in Nantes with polyurethane formwork (Furet et al. 2019), numerous advances have been to reduce the weight and thickness of formwork while guaranteeing faster filling speeds, as in the In-crease (Lloret-Fritschi et al. 2022) or Eggschell (Burger et al. 2020) projects at ETH Zurich.

1.4. Classification of concrete printing processes

3D concrete printing is a polymorphous process that needs to be well described in order to be able to properly design the robotic complexity and the specifications of the printable cementitious material. Similarly, for a given concrete printer, we need to know what type of parts can be printed, on what scale and in what environment. A number of projects have focused on printing process classification systems that can be used to describe the specifications of a process and the possibilities available.

1.4.2. Classification proposed by RILEM

The work of RILEM’s Technical Committee No. 276 has led to the creation of a classification process (Buswell et al. 2020) that encompasses all types of manufacturing processes associated with “digital concrete”. This process takes into account a number of principles that allow all automated concrete construction processes to be encompassed.

One of the first principles is considering the printing environment: whether in the prefabrication plant or on-site. Also, the authors drew on pre-existing definitions of material shaping and assembly (Groover 2011) and incorporated standards relating to additive manufacturing (ISO 17296-3 2014; ISO/ASTM 52900 2021). Thus, the classification must account for variations in the size of printed parts and types of cementitious materials used. This classification also describes sub-processes corresponding to the manufacturing technology used (for example, particle beds, extrusion deposition for additive manufacturing). Complementary processes can also be described in the classification (surface treatment, reinforcements, etc.) that may occur sequentially, simultaneously or contiguously.

The proposed classification takes into account:

• the type of material and its service life properties once cured;
  
• the printing environment (in-plant, on-site, mixed site-plant, on-site but under a protected and regulated area);
  
• the printed element (size, element to be assembled, resolution and geometric tolerance, etc.);
  
• process limitations, installation and printing phasing.

The description of a printing process can thus be gathered under the acronym MAPP for Materials, Application, Product, Process, which describes the context and technical details of the printing process.

The processes are classified according to Figure 1.4 reproduced from Buswell et al. (2020). This classification is based on a core group of processes related to automated concrete placement. These include assembly and surface treatment. Sub-processes must be distinct and identified from the main concrete forming and assembly process.

It is noteworthy that a construction process using 3D printing often involves a series of manufacturing and assembly sequences representing the sequencing of each stage, along with elementary processes and sub-processes over time, which enables a clear representation of the studied process and facilitates comparisons with others. Examples of manufacturing processes are shown in Figure 1.5 for a 3D printing process using two-component extrusion/deposition reinforced with continuous glass fibers, known as anisotropic concrete (Bos et al. 2017; Ducoulombier et al. 2020), and another for 3D printing using selective particle bonding (Pierre et al. 2018; Lowke et al. 2018), in line with the classification of processes proposed by RILEM (Buswell et al. 2022).

In some cases, a process combines additive manufacturing processes with other formative and subtractive processes involving cast concrete and CNC machine tools for cutting, milling or even welding operations. In such cases, the classification accommodates hybrid processes. This is the case, for example, with the Mesh–Mould process, which is a succession of digital placement of a digital network followed by concrete casting and surface finishing by smoothing, or when a metal reinforcement is welded during concrete 3D printing (Hack and Lauer 2014).

1.5. Printing concrete with alternative binders or without cement?

Concrete printing raises the issue of the carbon impact of printed materials, even if the use of printing means that the structure can be optimized. This carbon impact is all the greater when the quantity of ordinary Portland cement in the composition is high, due to the decarbonation of limestone during cement manufacture and the potential fossil energy used to reach the 1,400°C required to bake the raw materials (Habert et al. 2020).

To ensure material pumpability and adapt to the size of most of the robots used as printers, mortar rather than concrete is used as the printing ink. This increases the quantity of paste and, therefore, of cementitious binder, which carry a significant carbon weight.

Fast-setting cements such as natural prompt cement, calcium aluminate cements or sulfo-aluminate cements are potentially interesting binders to replace traditional cements, as they have a lower carbon weight and high stiffening speeds in line with rapid impression. They are also commonly used as set gas pedals in ordinary Portland cement-based systems. However, researchers have less experience of the durability of these less alkaline systems than traditional cements (Kim et al. 2021; Kleib et al. 2021). It is also important to note that the price of these binders remains higher, which limits their industrial-scale use to date.

The recent NF EN 197-5 standard (Elkhaldi et al. 2023) governing the use of a performance-based approach for low-carbon cements paves the way for the development and use of low-carbon cements containing reduced levels of traditional Portland cement, such as LC3 (limestone–calcined clay–cement). Work is already underway on the use of these new low-carbon binders for 3D printing applications.

The activation of aluminosilicate pozzolanic materials such as blast-furnace slag or calcined clays by more or less concentrated alkaline solutions enables the formation of geopolymers which, in their hardened state, offer strengths of the same order as traditional cement concretes (Duxson et al. 2007). The use of these geopolymers in 3D printing has already been the subject of numerous studies using several different printing methods, such as particle bed printing techniques or the extrusion and filament deposition method (Zhong and Zhang 2022). In these studies, the authors highlight the reactivity of these systems, which enable rapid printing, and the reduced carbon impact associated with the non-use of Portland cement. For example, a recent study (Zhong and Zhang 2022) indicates a reduction in carbon footprint by a factor of between 2 and 5, and a 50% reduction in the energy required for the printing process compared with printing using cementitious material.

Finally, recent work has focused on the use of soil to create 3D-printed structures as a substitute for cementitious materials in non-structural applications, or where expected levels of mechanical resistance remain low (small buildings, small dwellings, etc.) (Gomaa et al. 2022). 3D soil printing is at a crossroads between several inspirations: robotized methods adapting vernacular techniques such as bauge and pisé, on the one hand (Schweiker et al. 2021), and deposition extrusion printing methods inspired by developments made for cementitious materials, on the other hand (Perrot et al. 2018). It is also important to note that soil can present a wide range of consistencies – fluid, plastic or firm (in this case, the state of the soil is in a so-called wet state as the particles are wetted to a water content insufficient to saturate all the pores between the grains and form a homogeneous paste). For each type of consistency, several manufacturing processes can be imagined, and the techniques developed are manifold, as shown in Figure 1.6.

There are major differences between these two philosophies: the technique inspired by vernacular techniques, such as the robotized pisé technique developed at Braunschweig Technical University, uses only raw soil and no other binders or stabilizers. For this technique, the soil is compacted in a wet state by a robot using a pisoir and mobile formwork. The compaction is sufficient to confer adequate strength to the printed structure, which remains stable and gains in strength as it gradually dries over time, before being put into service.

Techniques inspired by cementitious materials for extrusion printing and filament deposition must use a binder that causes rapid rigidification of the material in order to print quickly (Perrot et al. 2018). This binder can have several origins, cementitious or hydraulic binders (Faleschini et al. 2023), biopolymers such as alginate or hemoglobin extracts (Biggerstaff et al. 2021a, 2021b; Autem et al. 2023). It is also possible to use a binder with thermo-dependent behavior that liquefies on heating for the transport and extrusion phases, then causes the material to stiffen after deposition when it returns to ambient temperature (Christ et al. n.d.; Jacquet and Perrot 2023). In this case, natural waxes or animal gelatins can be used.

1.6. Conclusion

This chapter illustrates the diversity of methods, applications, scales and robotic systems in the field of 3D concrete printing. Today, prefabrication or on-site printing and the manufacture of small parts or entire buildings are all possible realizations by printing.

This multifaceted nature has necessitated a number of classifications, which are also presented in this chapter.

Finally, this chapter explores the possibility of printing alternative materials, which could reduce the carbon impact of printed structures used in construction.

1.7. References

1. Ahmed, G.H. (2023). A review of “3D concrete printing”: Materials and process characterization, economic considerations and environmental sustainability. Journal of Building Engineering, 105863.
   
2. ASTM International (2012). ASTM F2792-12a. Standard terminology for additive manufacturing technologies. Standard, ASTM International, West Conshohocken.
   
3. Autem, Y., Bourgougnon, N., Guihéneuf, S., Perrot, A. (2023). Comparative study of effects of various seaweed parietal polysaccharides on rheological, mechanical and water-durability properties of earth-based materials. Materials and Structures, 56(6), 108.
   
4. Barnett, E. and Gosselin, C. (2015). Large-scale 3D printing with a cable-suspended robot. Additive Manufacturing, 7, 27–44.
   
5. Biggerstaff, A.O., Fuller, G., Lepech, M., Loftus, D. (2021a). Determining the yield stress of a biopolymer-bound soil composite for extrusion-based 3D printing applications. Construction and Building Materials, 305, 124730.
   
6. Biggerstaff, A.O., Lepech, M.D., Loftus, D.J. (2021b). Evaluation of a biopolymerbound soil composite for 3D printing on the lunar surface. In Earth and Space 2021. ASCE, Reston.
   
7. Bos, F.P., Ahmed, Z.Y., Jutinov, E.R., Salet, T.A. (2017). Experimental exploration of metal cable as reinforcement in 3D printed concrete. Materials, 10(11), 1314.
   
8. Boscaro, F., Quadranti, E., Wangler, T., Mantellato, S., Reiter, L., Flatt, R.J. (2022). Eco-friendly, set-on-demand digital concrete. 3D Printing and Additive Manufacturing, 9(1), 3–11.
   
9. Burger, J., Lloret-Fritschi, E., Scotto, F., Demoulin, T., Gebhard, L., Mata-Falcón, J., Gramazio, F., Kohler, M., Flatt, R.J. (2020). Eggshell: Ultra-thin three-dimensional printed formwork for concrete structures. 3D Printing and Additive Manufacturing, 7(2), 48–59.
   
10. Buswell, R.A., De Silva, W.L., Jones, S.Z., Dirrenberger, J. (2018). 3D printing using concrete extrusion: A roadmap for research. Cement and Concrete Research, 112, 37–49.
    
11. Buswell, R.A., Da Silva, W.L., Bos, F.P., Schipper, H.R., Lowke, D., Hack, N., Kloft, H., Mechtcherine, V., Wangler, T., Roussel, N. (2020). A process classification framework for defining and describing digital fabrication with concrete. Cement and Concrete Research, 134, 106068.
    
12. Buswell, R.A., Bos, F.P., da Silva, W.R.L., Hack, N., Kloft, H., Lowke, D., Freund, N., Fromm, A., Dini, E., Wangler, T. (2022). Digital fabrication with cement-based materials: Process classification and case studies. State-of-the-Art Report, RILEM TC 276-DFC.
    
13. Carneau, P., Mesnil, R., Roussel, N., Baverel, O. (2020). Additive manufacturing of cantilever – From masonry to concrete 3D printing. Automation in Construction, 116, 103184.
    
14. Chen, H., Zhang, D., Chen, P., Li, N., Perrot, A. (2023). A review of the extruder system design for large-scale extrusion-based 3D concrete printing. Materials, 16(7), 2661.
    
15. Christ, J., Perrot, A., Ottosen, L.M., Koss, H. (n.d.). Rheological characterization of temperature-sensitive biopolymer-bound 3d printing concrete. Construction and Building Materials, 411, 134337.
    
16. Colla, V., Dini, E., Canessa, E., Fonda, C., Zennaro, M. (2013). Large scale 3D printing: From deep sea to the moon. In Low-Cost 3D Printing, for Science, Education & Sustainable Development, Canessa, E., Fonda, C., Zennaro, M. (eds). ICTP, Trieste, 127–132.
    
17. Construction 3D (n.d.). [Online]. Available at: https://www.constructions-3d.com/.
    
18. Craipeau, T., Toussaint, F., Perrot, A., Lecompte, T. (2021). Experimental approach on a moving formwork. Construction and Building Materials, 270, 121472.
    
19. De Schutter, G., Lesage, K., Mechtcherine, V., Nerella, V.N., Habert, G., Agusti-Juan, I. (2018). Vision of 3D printing with concrete – Technical, economic and environmental potentials. Cement and Concrete Research, 112, 25–36.
    
20. Dörfler, K., Dielemans, G., Lachmayer, L., Recker, T., Raatz, A., Lowke, D., Gerke, M. (2022). Additive manufacturing using mobile robots: Opportunities and challenges for building construction. Cement and Concrete Research, 158, 106772.
    
21. Duballet, R., Baverel, O., Dirrenberger, J. (2017). Classification of building systems for concrete 3D printing. Automation in Construction, 83, 247–258.
    
22. Ducoulombier, N., Demont, L., Chateau, C., Bornert, M., Caron, J.-F. (2020). Additive manufacturing of anisotropic concrete: A flow-based pultrusion of continuous fibers in a cementitious matrix. Procedia Manufacturing, 47, 1070–1077.
    
23. Duxson, P., Fernández-Jiménez, A., Provis, J.L., Lukey, G.C., Palomo, A., van Deventer, J.S. (2007). Geopolymer technology: The current state of the art. Journal of Materials Science, 42, 2917–2933.
    
24. Elkhaldi, I., Rozière, E., Loukili, A. (2023). To what extent does decreasing the proportion of clinker in cement production effectively decrease its carbon footprint? Materials Today: Proceedings [Online]. Available at: https://www.sciencedirect.com/science/article/pii/S2214785323042190.
    
25. Faleschini, F., Trento, D., Masoomi, M., Pellegrino, C., Zanini, M.A. (2023). Sustainable mixes for 3D printing of earth-based constructions. Construction and Building Materials, 398, 132496.
    
26. Furet, B., Poullain, P., Garnier, S. (2019). 3D printing for construction based on a complex wall of polymer-foam and concrete. Additive Manufacturing, 28, 58–64.
    
27. Gaël, G., Phelippe, L., Amziane, S. (2023). 3D concrete printer. Google Patents, FR1751938A.
    
28. Girmscheid, G. and Moser, S. (2001). Fully automated shotcrete robot for rock support. Computer-Aided Civil and Infrastructure Engineering, 16(3), 200–215.
    
29. Gomaa, M., Jabi, W., Soebarto, V., Xie, Y.M. (2022). Digital manufacturing for earth construction: A critical review. Journal of Cleaner Production, 338, 130630.
    
30. Groover, M.P. (2011). Introduction to Manufacturing Processes. Wiley, Hoboken.
    
31. Habert, G., Miller, S.A., John, V.M., Provis, J.L., Favier, A., Horvath, A., Scrivener, K.L. (2020). Environmental impacts and decarbonization strategies in the cement and concrete industries. Nature Reviews Earth & Environment, 1(11), 559–573.
    
32. Hack, N. and Kloft, H. (2020). Shotcrete 3D printing technology for the fabrication of slender fully reinforced freeform concrete elements with high surface quality: A real-scale demonstrator. In Second RILEM International Conference on Concrete and Digital Fabrication: Digital Concrete 2020. Springer, Heidelberg.
    
33. Hack, N. and Lauer, W.V. (2014). Mesh-mould: Robotically fabricated spatial meshes as reinforced concrete formwork. Architectural Design, 84(3), 44–53.
    
34. Hass, L. and Bos, F. (2020). Bending and pull-out tests on a novel screw type reinforcement for extrusion-based 3D printed concrete. In Second RILEM International Conference on Concrete and Digital Fabrication: Digital Concrete 2020. Springer, Heidelberg.
    
35. Heidarnezhad, F. and Zhang, Q. (2022). Shotcrete based 3D concrete printing: State of art, challenges, and opportunities. Construction and Building Materials, 323, 126545.
    
36. ISO (2014). Additive Manufacturing – General Principles. Part 3: Main Characteristics and Corresponding Test Methods. Standard, ISO 17296-3.
    
37. ISO (2021). Additive Manufacturing – General Principles – Fundamentals and Vocabulary. Standard, ISO/ASTM 52900.
    
38. ISO (2023). Additive Manufacturing for Construction: Qualification Principles – Structural and Infrastructure Elements. Standard, ISO/ASTM 52939.
    
39. Izard, J.-B., Dubor, A., Hervé, P.-E., Cabay, E., Culla, D., Rodriguez, M., Barrado, M. (2017). Large-scale 3D printing with cable-driven parallel robots. Construction Robotics, 1, 69–76.
    
40. Jacquet, Y. and Perrot, A. (2023). Evolutionary approach based on thermoplastic bio-based building material for 3D printing applications: An insight into a mix of clay and wax. In Bio-Based Building Materials, Amziane, S., Merta, I., Page, J. (eds). Springer, Cham.
    
41. Ji, G., Xiao, J., Zhi, P., Wu, Y.-C., Han, N. (2022). Effects of extrusion parameters on properties of 3D printing concrete with coarse aggregates. Construction and Building Materials, 325, 126740.
    
42. Keita, E., Bessaies-Bey, H., Zuo, W., Belin, P., Roussel, N. (2019). Weak bond strength between successive layers in extrusion-based additive manufacturing: Measurement and physical origin. Cement and Concrete Research, 123, 105787.
    
43. Khoshnevis, B. (2004). Automated construction by contour crafting – Related robotics and information technologies. Automation in Construction, 13(1), 5–19.
    
44. Khoshnevis, B., Hwang, D., Yao, K.-T., Yeh, Z. (2006). Mega-scale fabrication by contour crafting. International Journal of Industrial and Systems Engineering, 1(3), 301–320.
    
45. Kim, H., Son, H.M., Lee, H.-K. (2021). Review on recent advances in securing the long-term durability of calcium aluminate cement (CAC)-based systems. Functional Composites and Structures, 3(3), 035002.
    
46. Kleib, J., Aouad, G., Benzerzour, M., Zakhour, M., Abriak, N.-E. (2021). Effect of calcium sulfoaluminate cements composition on their durability. Construction and Building Materials, 307, 124952.
    
47. Kloft, H., Empelmann, M., Hack, N., Herrmann, E., Lowke, D. (2020). Reinforcement strategies for 3D-concrete-printing. Civil Engineering Design, 2(4), 131–139.
    
48. Le, T.T., Austin, S.A., Lim, S., Buswell, R.A., Gibb, A.G., Thorpe, T. (2012). Mix design and fresh properties for high-performance printing concrete. Materials and Structures, 45, 1221–1232.
    
49. Lim, S., Buswell, R.A., Le, T.T., Austin, S.A., Gibb, A.G., Thorpe, T. (2012). Developments in construction-scale additive manufacturing processes. Automation in Construction, 21, 262–268.
    
50. Lindemann, H., Gerbers, R., Ibrahim, S., Dietrich, F., Herrmann, E., Dröder, K., Raatz, A., Kloft, H. (2019). Development of a shotcrete 3D-printing (SC3DP) technology for additive manufacturing of reinforced freeform concrete structures. In First RILEM International Conference on Concrete and Digital Fabrication–Digital Concrete 2018. Springer, Heidelberg.
    
51. Lloret-Fritschi, E., Reiter, L., Wangler, T., Gramazio, F., Kohler, M., Flatt, R.J. (2017). Smart dynamic casting: Slipforming with flexible formwork-inline measurement and control. In HPC/CIC Tromsø 2017, 27.
    
52. Lloret-Fritschi, E., Wangler, T., Gebhard, L., Mata-Falcón, J., Mantellato, S., Scotto, F., Burger, J., Szabo, A., Ruffray, N., Reiter, L. (2020). From smart dynamic casting to a growing family of digital casting systems. Cement and Concrete Research, 134, 106071.
    
53. Lloret-Fritschi, E., Choma, J., Scotto, F., Szabo, A., Gramazio, F., Kohler, M., Flatt, R.J. (2022). In-crease: Less concrete more paper. RILEM Technical Letters, 7, 199–208.
    
54. Lowke, D., Dini, E., Perrot, A., Weger, D., Gehlen, C., Dillenburger, B. (2018). Particle-bed 3D printing in concrete construction – Possibilities and challenges. Cement and Concrete Research, 112, 50–65.
    
55. Lowke, D., Talke, D., Dressler, I., Weger, D., Gehlen, C., Ostertag, C., Rael, R. (2020). Particle bed 3D printing by selective cement activation – Applications, material and process technology. Cement and Concrete Research, 134, 106077.
    
56. Lowke, D., Vandenberg, A., Pierre, A., Thomas, A., Kloft, H., Hack, N. (2021). Injection 3D concrete printing in a carrier liquid – Underlying physics and applications to lightweight space frame structures. Cement and Concrete Composites, 124, 104169.
    
57. Lowke, D., Mai, I., Keita, E., Perrot, A., Weger, D., Gehlen, C., Herding, F., Zuo, W., Roussel, N. (2022). Material-process interactions in particle bed 3D printing and the underlying physics. Cement and Concrete Research, 156, 106748.
    
58. Lowke, D., Anton, A., Buswell, R., Jenny, S.E., Flatt, R.J., Fritschi, E.L., Hack, N., Mai, I., Popescu, M., Kloft, H. (2024). Digital fabrication with concrete beyond horizontal planar layers. Cement and Concrete Research, 186, 107663.
    
59. Ly, O., Yoris-Nobile, A.I., Sebaibi, N., Blanco-Fernandez, E., Boutouil, M., Castro-Fresno, D., Hall, A.E., Herbert, R.J., Deboucha, W., Reis, B. (2021). Optimisation of 3D printed concrete for artificial reefs: Biofouling and mechanical analysis. Construction and Building Materials, 272, 121649.
    
60. Mai, I., Lowke, D., Perrot, A. (2022). Fluid intrusion in powder beds for selective cement activation – An experimental and analytical study. Cement and Concrete Research, 156, 106771.
    
61. Mechtcherine, V., Buswell, R., Kloft, H., Bos, F.P., Hack, N., Wolfs, R., Sanjayan, J., Nematollahi, B., Ivaniuk, E., Neef, T. (2021). Integrating reinforcement in digital fabrication with concrete: A review and classification framework. Cement and Concrete Composites, 119, 103964.
    
62. Menna, C., Mata-Falcón, J., Bos, F.P., Vantyghem, G., Ferrara, L., Asprone, D., Salet, T., Kaufmann, W. (2020). Opportunities and challenges for structural engineering of digitally fabricated concrete. Cement and Concrete Research, 133, 106079.
    
63. Moretti, M. (2023). WASP in the edge of 3D printing. In 3D Printing for Construction with Alternative Materials, Bárbara, R., Ana, S.G., Jorge, L., Leonardo, S. (eds). Springer, Cham.
    
64. Nerella, V.N. and Mechtcherine, V. (2019). Studying the printability of fresh concrete for formwork-free concrete onsite 3D printing technology (CONPrint3D). In 3D Concrete Printing Technology, Sanjayan, J.G., Nazari, A., Nematollahi, B. (eds). Butterworth-Heinemann, Oxford [Online]. Available at: https://www.sciencedirect.com/science/article/pii/B9780128154816000166.
    
65. Nicolas, R., Richard, B., Nicolas, D., Irina, I., Temitope, K.J., Dirk, L., Viktor, M., Romain, M., Arnaud, P., Ursula, P. (2022). Assessing the fresh properties of printable cement-based materials: High potential tests for quality control. Cement and Concrete Research, 158, 106836.
    
66. Panda, B., Paul, S.C., Tan, M.J. (2017). Anisotropic mechanical performance of 3D printed fiber reinforced sustainable construction material. Materials Letters, 209, 146–149.
    
67. Perrot, A. (2022). M&S highlight: Le et al. (2012), mix design and fresh properties for high-performance printing concrete. Materials and Structures, 55(2), 42.
    
68. Perrot, A. and Amziane, S. (2019). 3D printing in concrete: General considerations and technologies. In 3D Printing of Concrete: State of the Art and Challenges of the Digital Construction Revolution, Perrot, A. (ed.). ISTE Ltd, London, and John Wiley & Sons, New York, 1–40.
    
69. Perrot, A., Rangeard, D., Pierre, A. (2016). Structural built-up of cement-based materials used for 3D-printing extrusion techniques. Materials and Structures, 49, 1213–1220.
    
70. Perrot, A., Rangeard, D., Courteille, E. (2018). 3D printing of earth-based materials: Processing aspects. Construction and Building Materials, 172, 670–676.
    
71. Perrot, A., Jacquet, Y., Rangeard, D., Courteille, E., Sonebi, M. (2020). Nailing of layers: A promising way to reinforce concrete 3D printing structures. Materials, 13(7), 1518.
    
72. Pierre, A., Weger, D., Perrot, A., Lowke, D. (2018). Penetration of cement pastes into sand packings during 3D printing: Analytical and experimental study. Materials and Structures, 51, 1–12.
    
73. Pott, A., Meyer, C., Verl, A. (2010). Large-scale assembly of solar power plants with parallel cable robots. In ISR 2010 (41st International Symposium on Robotics) and ROBOTIK 2010 (6th German Conference on Robotics). VDE, Berlin.
    
74. Ranjan, R., Kumar, D., Kundu, M., Moi, S.C. (2022). A critical review on classification of materials used in 3D printing process. Materials Today: Proceedings, 61, 43–49.
    
75. Reiter, L., Wangler, T., Roussel, N., Flatt, R.J. (2018). The role of early age structural build-up in digital fabrication with concrete. Cement and Concrete Research, 112, 86–95.
    
76. Roussel, N. (2018). Rheological requirements for printable concretes. Cement and Concrete Research, 112, 76–85.
    
77. Roussel, N. and Coussot, P. (2005). “Fifty-cent rheometer” for yield stress measurements: From slump to spreading flow. J. Rheol. (1978-Present), 49. doi: 10.1122/1.1879041.
    
78. Salet, T.A., Ahmed, Z.Y., Bos, F.P., Laagland, H.L. (2018). Design of a 3D printed concrete bridge by testing. Virtual and Physical Prototyping, 13(3), 222–236.
    
79. Sati, A.S.E., Mantha, B.R., Dabous, S.A., de Soto, B.G. (2021). Classification of robotic 3D printers in the AEC industry. In ISARC. Proceedings of the International Symposium on Automation and Robotics in Construction. IAARC Publications, Lille.
    
80. Savić, A., Stević, M., Martinović, S., Vlahović, M., Volkov-Husović, T. (2020). Applying concept of 3D printing concrete in wind tower construction. In Proceedings of the 8th International Conference on Renewable Electrical Power Sources. Savez mašinskih i elektrotehničkih inženjera i tehničara Srbije (SMEITS), Beograd.
    
81. Schack, R., Krause, M., Näther, M., Nerella, V.N. (2017). CONPrint3D: 3D-concrete-printing as an alternative for masonry. Bauingenieur, 92, 355–363.
    
82. Schweiker, M., Endres, E., Gosslar, J., Hack, N., Hildebrand, L., Creutz, M., Klinge, A., Kloft, H., Knaack, U., Mehnert, J. (2021). Ten questions concerning the potential of digital production and new technologies for contemporary earthen constructions. Building and Environment, 206, 108240.
    
83. Szabo, A., Reiter, L., Lloret-Fritschi, E., Gramazio, F., Kohler, M., Flatt, R.J. (2020). Mastering yield stress evolution and formwork friction for smart dynamic casting. Materials, 13(9), 2084.
    
84. Tao, Y., Rahul, A.V., Lesage, K., Yuan, Y., Van Tittelboom, K., De Schutter, G. (2021). Stiffening control of cement-based materials using accelerators in inline mixing processes: Possibilities and challenges. Cement and Concrete Composites, 119, 103972.
    
85. Tao, Y., Rahul, A.V., Lesage, K., Van Tittelboom, K., Yuan, Y., De Schutter, G. (2022). Mechanical and microstructural properties of 3D printable concrete in the context of the twin-pipe pumping strategy. Cement and Concrete Composites, 125, 104324.
    
86. Tay, Y.W.D., Li, M.Y., Tan, M.J. (2019). Effect of printing parameters in 3D concrete printing: Printing region and support structures. Journal of Materials Processing Technology, 271, 261–270.
    
87. Vantyghem, G., De Corte, W., Shakour, E., Amir, O. (2020). 3D printing of a post-tensioned concrete girder designed by topology optimization. Automation in Construction, 112, 103084.
    
88. Volpe, S., Sangiorgio, V., Petrella, A., Coppola, A., Notarnicola, M., Fiorito, F. (2021). Building envelope prefabricated with 3D printing technology. Sustainability, 13(16), 8923.
    
89. Wangler, T., Lloret, E., Reiter, L., Hack, N., Gramazio, F., Kohler, M., Bernhard, M., Dillenburger, B., Buchli, J., Roussel, N. (2016). Digital concrete: opportunities and challenges. RILEM Technical Letters, 1, 67–75.
    
90. Wangler, T., Pileggi, R., Gürel, S., Flatt, R.J. (2022). A chemical process engineering look at digital concrete processes: Critical step design, inline mixing, and scaleup. Cement and Concrete Research, 155, 106782.
    
91. Weger, D., Pierre, A., Perrot, A., Kränkel, T., Lowke, D., Gehlen, C. (2021). Penetration of cement pastes into particle-beds: A comparison of penetration models. Materials, 14(2), 389.
    
92. Xiao, J., Ji, G., Zhang, Y., Ma, G., Mechtcherine, V., Pan, J., Wang, L., Ding, T., Duan, Z., Du, S. (2021). Large-scale 3D printing concrete technology: Current status and future opportunities. Cement and Concrete Composites, 122, 104115.
    
93. Zhang, X., Li, M., Lim, J.H., Weng, Y., Tay, Y.W.D., Pham, H., Pham, Q.-C. (2018). Large-scale 3D printing by a team of mobile robots. Automation in Construction, 95, 98–106.
    
94. Zhang, K., Chermprayong, P., Xiao, F., Tzoumanikas, D., Dams, B., Kay, S., Kocer, B.B., Burns, A., Orr, L., Choi, C. (2022). Aerial additive manufacturing with multiple autonomous robots. Nature, 609(7928), 709–717.
    
95. Zhong, H. and Zhang, M. (2022). 3D printing geopolymers: A review. Cement and Concrete Composites, 128, 104455.
    
96. Zuo, W., Dong, C., Belin, P., Roussel, N., Keita, E. (2022). Capillary imbibition depth in particle-bed 3D printing – Physical frame and one-dimensional experiments. Cement and Concrete Research, 156, 106740.