Additive manufacturing of concrete has gained an increasing interest in the field of architecture and construction. This process can be utilized in the robot-controlled build-up of elements or buildings by a design to fabrication informed digital model. Over the past years, many different approaches for the additive manufacturing of concrete have been developed (Buswell et al. 2020). Extrusion-based techniques became the most prominent methods. The additive approach can be classified into two main process groups: layer-wise and spatial material extrusion.
The layer-wise material extrusion is the most common approach, having been intensively researched and developed over the past years (Mechtcherine et al. 2020; Khan et al. 2020; Hossain et al. 2020). As seen in Fig. 1, the premixed concrete, with grain sizes commonly under 4 mm, is extruded continuously using cavity pumps attached to gantries, cranes or industrial robots. The main applications are the production of architectural elements with complex shapes as well as the on-site fabrication of building-walls. The process provides a new freedom of geometry and profits from the efficiency of concrete without formworks. These benefits are offset by the challenges of overhangs which are limited by the stability of the freshly printed concrete. This physical constraint must be considered in the search for freedom of design and efficiency through automated fabrication.
Fig. 1Principles of a layer-wise and spatial concrete extrusion
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Compared to the layer-wise material extrusion, the spatial material extrusion with concrete is a less investigated approach. In contrast to spatial printing of plastics, which is well investigated and used for large space frame structures (Branch Technology 2020; Piker and Maddock 2020), concrete demands for a liquid or paste-like support material to enable the stability of the strand in space during hydration. The idea of printing in a liquid was first published by Mülhaupt et al. as a patent in 2003 (Hendrik et al. 2000). In 2014, this principle was demonstrated by researchers at Princeton in the project: Buoyant Extrusion (Foley et al. 2020). In 2016, further research in this direction was published as Rapid Liquid Printing by researchers at MIT (Hajash et al. 2017; Tibbits et al. 2018). The process was adapted for concrete by the founders of the startup Soliquid, who published research documenting their large-scale space frame structures. The potential of spatial application of concrete was researched and demonstrated preliminary investigations where material experimentation and parametric modeling led to the robotic fabrication of a spatially extruded space framed column, Fig. 1. Further research in spatial extrusion of concrete can be seen in the project Injection 3D Concrete Printing by researchers of TU Braunschweig (Hack et al. 2020).
The process of spatial concrete extrusion utilizes a printing medium liquid enough to allow for the movement of the print nozzle through the material, yet dense enough to hold concrete in a three-dimensional position without additional support. As seen in Fig. 1, left, the premixed the concrete is extruded into a support material, such as a liquid or an inert thixotropic suspension. It is held in a container and stabilizes the extruded material in fresh state while curing. After curing, the building elements can be stripped out, enabling structures with unlimited overhangs for filigree and ultra-lightweight space frame structures.
While the freedom of design can be increased significantly by spatial material application, its application requires consideration of the anisotropic and brittle nature of concrete behavior, as seen in Fig. 2. A reference space frame column with a height of 50 cm and a diameter of 33 cm and a C20/25 concrete collapsed at a load of 107 kg. While the strength is impaired by geometrical imperfections and intersection bondage, the stability is affected by the anisotropic strength and brittle concrete behavior itself that prevents this process from unreinforced use in construction.
Fig. 2Brittle collapse of a spatial extruded structure
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To develop this process for industrial use, the spatial concrete extrusion would require a concrete material with increased tensile strength and the ductility for the structural integrity of ultra-lightweight structures. By adding natural, synthetic or metallic fibers, the tensile strength and ductility can be improved without affecting the processing properties for additive manufacturing. The use of carbon fibers to strengthen concrete holds the potential for innovative material structures (Hambach and Volkmer 2017; Fischer et al. 2019). This research works to integrate fiber-reinforced concrete into the spatial extrusion process. The process is further advanced through a new process for a controlled alignment of the material carbon fibers within the structure of the 3D extrusion. The goal of this research is to create material processes for 3d spatially printed concrete structures reinforced with robotically aligned carbon fibers.
Fibers in a paste-like material tend to orient in the direction of the extrusion flow under different circumstances (Stähli et al. 2008). Aligned fibers provide better structural performance because they absorb tensile stresses with their alignment.
In pretests, the carbon fiber was mixed in a translucent substrate allowing for investigation into fiber orientation. It was shown that short fibers orient with the flow during the extrusion of material, according to fiber length to hose diameter ratio, matching the flow simulations findings of Kanarska et al. (Kanarska et al. 2019). As seen in Fig. 3, the short fibers with a length-ratio of 0.375 nearly completely orient with the flow of a liquid material with a respective fiber length of 3 and 6 mm.
Fig. 3Examples of oriented and not oriented fibers in extrusion flow
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During the process, the fibers must keep its alignment within the strands. In Fig. 4, the impact of perpendicular and parallel material application of fiber-reinforced pastes is compared. Carbon fibers with a length of 3 mm were mixed homogeneously in translucent Carbopol® suspension to make the fiber alignment evident. The fibrous material was extruded through hoses with a diameter of 8 (a) and 12 mm (b) and applied vertical and aligned onto a flat surface.
Fig. 4Fiber alignment in relation to the extrusion direction
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The results indicate the impact of extrusion orientation. By a vertical application, strands tend to deform during extrusion and fibers are reoriented inside the matrix. By an aligned application, the fiber orientation keeps structurally oriented inside the matrix. Reorientations due to swirls and deformation of the strand are prevented. Hence, the fiber alignment of fiber-reinforced strands must be ensured by the extrusion angle during material application. Hence, the alignment of the carbon fiber strands can be controlled to improve the material performance of the spatially extruded concrete.
To analyze the performance of aligned fibers, the influence of the application angle to the tensile strength of fiber-reinforced cement strands was investigated. Chopped carbon fibers with a length of 3 and 6 mm were mixed by a ratio of 1.0 vol% with a cement paste. The cement paste was made out of original Portland cement (1232 kg/m3), silica fume (414 kg/m3), superplasticizer (3.2 wt of cement) and stabilizer (0.5 wt of cement) and a water cement ratio of 0.2 in a planetary mixer.
The fiber-reinforced cement strands were applied vertically and aligned with a length of 20 cm. Material samples were load tested after 28 days. Hardened strands were embedded in concrete at support points and tensioned and tested as seen in Fig. 5. Reference specimen without fibers were too fragile for preparation and could not be tested.
Fig. 5Tension load test and broken strand specimen
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The results of vertically and aligned applied strands are shown in Fig. 6 as a load–strain diagram and tensile strength. Aligned fiber–cement strands increase the tensile strength and ductility compared to vertically applied strands. The aligned strands with 3 mm carbon fibers lead to a 90% higher tensile strength and 20% strain as the vertically applied ones. Thereby, the tensile strength of aligned specimen decreases with the length of the fibers according to mutual deflection in the extrusion flow.
Fig. 6Load–strain diagram and strength of fiber-reinforced cement strands applied vertically and aligned
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These results support the findings of the initial optical analysis and reinforce the research hypothesis that aligned fibers improve tensile strength while still allowing for a material capable of spatial printing. With the alignment of fiber orientation shown to improve the structural properties of the material, research continued in the development of an extrusion system capable of customizing the printing with consideration of reinforcement alignment. The following section details the prototyping process for a robotic end effector for spatial printing of concrete. This integrates the customization of carbon fiber alignment through additional axes allowing nozzle rotation and orientation while printing.
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