1 Introduction

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. 1
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Principles 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. 2
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Brittle 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. 3
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Examples 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. 4
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Fiber 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. 5
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Tension 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. 6
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Load–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.

2 Process requirements

Typical 3D-printing machines, such as gantries (Cobod 2021), cranes (APIS Cor 2020) or rope systems (WASP Project 2020), are characterized by three degrees of movements that do not allow the three-dimensional rotation of a tool. Industrial robot arms allow for six degree of freedom to move and rotate the tool in space. For spatial concrete extrusion, the usability of industrial robots is still be restricted. As seen in Fig. 7, the tool, which is the print head in 3D printing, is characterized by a long hose to penetrate the liquid suspension. Robotic paths must consider the orientation of the nozzle and reachability of the tool path within the case of the supporting medium. Such constraints are accompanied by an additional inertia of the robot movements and restrictions to the work space of the robot. Therefore, any effector built for three-dimensional extrusion or aligned fiber-reinforced concrete requires an actuator with an improved nozzle control system.

Fig. 7
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Limits of reachability of industrial robots with nozzle, TCP, rotations

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For the aligned fiber spatial extrusion in suspension, the requirements to the dynamic print head can be defined as follows:

  • Rotation radii are minimal to allow for a good accessibility in restricted work space

  • Axis must be water resistant and robust against surrounding support material.

  • Conveyor hose needs continuous curvatures to prevent clogging and fiber deformations of a continuous concrete flow

  • Print head should be modular, allowing for integration with any robot system such as gantries, cranes and industrial robot

  • Capable of use in a wide bandwidth of additive manufacturing applications

Based on these requirements, the actuator for a dynamic print head was designed and developed to increase the rotation and reachability for a three-dimensional alignment of fiber-reinforced strands in spatial additive manufacturing.

3 Process development

3.1 Concept and design

The concept of the aligned strand extrusion for fiber-reinforced concrete is illustrated in Fig. 8. The core component is a dynamic print head, which is attached to a robot system and positioned according to the programmed path of the building element. The dynamic print head is characterized by two perpendicular rotation axis: axis 1 rotates vertically turning the whole print head around the z-axis, while axis 2 rotates horizontally aligning the nozzle upwards and downwards. Attached to the print head a flexible conveyor hose is attached to prevent buckling and squeezing in the material flow. Its continuous curvature allows the flow and orientation of the fiber-reinforced concrete leading to preferential fiber alignment with the extrusion flow.

Fig. 8
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Concept for the aligned spatial extrusion of fiber-reinforced strands in a suspension

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The dynamic print head is attached to the conveyor hose, or pumping system, which facilitates a continuous material flow for extrusion. In a support material, the print head can be aligned with less rotation movement compared to conventional industrial robot. Hence, its nozzle rotation is facilitated in restricted work spaces increasing its reachability and accessibility.

The actuator design of the print head is shown more detailed in Fig. 9. The kinematic chain bases one horizontal axis on top of the print head and a vertical axis at the nozzle. The distance between axes must be set to the necessary penetration depth, which is needed for printing into a suspension container. Both axes are fixed to a configurable mounting system which connects end effector to a wide variety of robotic systems. A conveyor hose made of a flexible pipe can adjust to the rotation without buckling, while allowing for easy customization of length. A bearing with unlimited rotation decouples the rotation of the horizontal axes allowing the print freedom of movement in the continuous printing of aligned reinforcement strands.

Fig. 9
figure 9

Kinematic chain and axis arrangement of the dynamic print head

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3.2 Prototype development

Figure 10 shows the prototype of the dynamic print head at a laboratory scale. The main structural part is an aluminum mounting (2) that connects a continuous material feeding system and the nozzle actuator by an adapter flange (4) to the robot system. As a material feeding system a cartridge press (3), which is driven by a stepper motor (1) with a rotation force up to 8 Nm, was chosen to ensure a continuous feeding rate at high pressures suitable for stiff materials.

Fig. 10
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Components of the lab-scale dynamic print head

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The nozzle actuator combines a vertical axis (5) and a horizontal axis (13). The vertical axis is driven by a Nema 17 stepper motor and a 3D-printed gear (6) that rotates the conveyor hose by a tight sleeve bearing (7). The horizontal axis is driven by a servo motor (9) on the top of the penetrating part of the rotating nozzle. The motor drives the axis by a wire around the rotation joint (13) attached to the nozzle (14), which is the end of the flexible conveyor hose.

The power supply of the second axis is ensured by a sliding contact as seen in Fig. 11. The contact consists of conductive slip rings on the one side and sliding contacts on the other side. It ensures the power supply for the four-pole servo motor is independent of the rotation of the vertical axis.

Fig. 11
figure 11

Sliding contact for power supply of the second axis

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The lab-scale prototype achieves a weight of 6 kg, fully loaded, allowing industrial robots with low payload to use the system for material and structural experimentation. The following research utilized a KUKA agilus KR 6 with a reach of 900 mm capable of lifting as much as 6 kg. The aluminum profile construction of the print head is stable, resistant to the suspension medium and robust against external impacts. The reduced size of the nozzle facilitates its movement in a resistant liquid. Since the flexible conveyor hose is a common area for printing problems to appear, it is mounted in an easily detachable fashion, thereby reducing the effort needed for cleaning or repair.

The design of the print head actuator allows for an endless rotation around its vertical axis. Around its horizontal axis, its allows a smooth rotation between a vertical, horizontal and upwards alignment of up to 60° as seen in Fig. 12. The flexible conveyor hose stands out due to the rotational freedom and improved capability to orient fiber-reinforced material.

Fig. 12
figure 12

Rotation freedom of the prototype

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3.3 Control system and CAD/CAM implementation

The print head was designed to be integrated in any computer controlled positioning system and is implemented as a demonstrator in this development for industrial robots. This demonstrator utilizes the programmable CAD-interface Grasshopper for Rhinoceros3D® and its industrial robot control plugin KUKA|prc, for the parametric design of robot ready code capable of large-scale additive manufacturing (Willmann et al. 2019).

Figure 13 shows the overall process control environment of the industrial robot and the print head device. The CAD input is generated in Rhinoceros3D® and processed by Grasshopper. via KUKA|prc. Robot movements and commands are generated by the path of the geometry. The unique orientation of the additional alignment axes is directly extracted from the tool path of the robot. Robot execution is controlled by mxAutomation, a Siemens® robot-command-interpreter, which allows a real-time and positioning feedback and command adaptation of robot execution (Braumann and Brell-Cokcan 2015).

Fig. 13
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Process control environment and integration to a programmable CAD environment

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A python script was developed for the print head control, which communicates the axis movements simultaneously with the positioning of the robot. UDP (User datagram protocol) communicates the axis rotation to an Arduino Uno, which interprets the axis positions by stepper libraries to accurately drive the axis motors. The rotation commands for the print head were calculated in the Python script implementation according to the initial geometry of the CAD-object in dependence to the positioning of the industrial robot.

The calculation of the rotation angles is done in the python plugin in based on an initial geometry that represents the structure of the strand. The procedure is explained visually in Fig. 14:

  1. 1.

    The strand structure is modeled in CAD by a continuous curve. The section type is generated by an extrusion along the curve to illustrate the model. During modeling the start and end-point of the curve must be defined. Collisions must be avoided by section wise build-up of the structure.

  2. 2.

    Several points are extracted along the curve. The curve is divided by a specific distance into a list of Cartesian points with x, y, and z-coordinates. The number of points represents the resolution and accuracy of the path. The smaller the subdivision of the curve the better the curve is mapped.

  3. 3.

    Vectors between the points are set along the curve. The vectors (ui) are defined by its direction and length from one point (Pi − 1) to the following point (Pi).

  4. 4.

    The rotation angles in horizontal and vertical direction of each vector are determined for the axis constellation of the print head. The horizontal rotations αi are referred vector by vector starting with the reference y-direction. The rotation direction is expressed positive or negative according to its alignment. The vertical rotation βi is referred for each vector to the X/Y-plane. The angles βi are positive for an upwards and negative for a downwards movement.

  5. 5.

    The print head control angles for each vector are calculated by accumulated vertical rotations φ1,i and the horizontal rotation φ2,i. Thereby, k represents the number of vectors:

    $${\text{for}}\;k \epsilon [1,n],$$
    $${\mathrm{Vertical \; rotation}: \varphi }_{1,i}={\alpha }_{k}= {\alpha }_{k-1} +{\alpha }_{i},$$
    $${\mathrm{Horizontal \; rotation}: \varphi }_{2,i}={\beta }_{k}=90+{\beta }_{i}\mathrm{ with }\beta \left[-60, 60\right].$$
  6. 6.

    The robot trajectory is determined on the basis of the axis constellation of the print head and the geometric trajectory. The geometry trajectory extracted by the position x/y/z of the points along the curve is transformed by the horizontal and vertical distances to the robot end effector position xR/yR/zR:

    $${x}_{\mathrm{R}}=x+\mathrm{sin}\left({\varphi }_{1,i}\right)\cdot (\mathrm{cos}\left({\varphi }_{2,i}-90\right)\cdot {l}_{2})$$
    $${y}_{R}=y+\mathrm{cos}\left({\varphi }_{1,i}\right)\cdot (\mathrm{cos}\left({\varphi }_{2,i}-90\right)\cdot {l}_{2})$$
    $${z}_{R}=z+{l}_{1}+\mathrm{sin}\left({\varphi }_{2,i}-90\right)\cdot {l}_{2}.$$
Fig. 14
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Process control determination for the dynamic printhead and robot trajectory

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By the simultaneous positioning of the robot and the rotation of the print head three-dimensional trajectories can be executed with an automated alignment of the nozzle. The robot trajectory planning is restricted to x, y, z positioning without any orientation. Hence, the system can also be applied to any gantry or crane system with at least three degrees of freedom.

The modeling approach is suited for mesh or spatial frame structures that become valuable with spatial material extrusion in term of printing time, material costs and building element weight. For solid geometries as usual in 3D printing, the approach is not suitable, since its design effort is not competitive with automated slicing techniques.

The real-time command interpretation via UDP and mxAutomation allows for a simultaneous control of the robot and print head (Braumann and Brell-Cokcan 2015). In the parametric programming environment, control features such as the speed of the movements and the rotation of the axis can be adapted during the operation of the process. Real-time process optimization can be integrated by the use of sensors such as rotary encoders and external camera devices.

The real-time digital simulation and execution is shown in Fig. 15. The robot and print head movements are simulated in the digital environment and executed with the physical robot. Rotary encoders update the positioning and adapt to the digital process environment. Sliders for rotation speed, angles and off-sets allow for a real-time process adaption in terms of collisions and quality in accuracy.

Fig. 15
figure 15

Real-time control and simulation of the integrated system

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4 Process application

The spatial extrusion is shown as follows in a translucent polymer suspension, Fig. 16. It visualizes that continuous curved and edged strands of fiber–cement pastes can be aligned in space. This increases the stability of the structure through the custom alignment of the reinforcement fibers within the printed material.

Fig. 16
figure 16

Extrusion examples of spatial aligned strand extrusion for parabolic and edged geometry

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Stripped elements as seen in Fig. 17 show the applicability for complex curved and filigree concrete structures. With the extrusion in suspension grids, unsupported spatial shapes, complex tool paths can be made out of fiber-reinforced cement pastes. With a diameter of 8 mm and length of 200 cm, the spirals require high bending and strain stability. This has not been possible until the integration of the aligned extrusion process. Carbon fibers with a length of 3 mm and 1.0 vol% were shown to lead to an intense increase of stability.

Fig. 17
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Spirals of fiber–cement pastes with uniform and non-uniform extrusion rate

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Another application potential is the spatial strand extrusion for multi-material elements. As seen Fig. 18, this new process allows for the 3D extrusion of strands into a casted material. Thereby, the performance of casted materials can be selectively improved by the controlled application of three-dimensional fiber-reinforced strands. Hence, multi-material applications with complex geometries are possible that can be used for functional, esthetical or structural purposes.

Fig. 18
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Example of aligned 3D extrused strand for a multi-material element

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For structural applications, the major requirement is high tensile load capacity of the strands. With the use of mineral fiber-reinforced cements, the process holds the potential for further benefits. Chopped fibers can be recycled, are heat- and corrosion-resistant and show high durability in terms of carbon fibers. Furthermore, they are light weight showing very high specific strengths compared to metallic materials.

Through the spatial extrusion of high-performance fiber-reinforced strands, the production of individualized and complex structures are enabled with some examples illustrated in Fig. 19. Custom configurations of structurally optimized reinforcements could be made including filigree meshes for double curved components, baskets for voluminous components or combined vertical and horizontal strands tailored to individual geometries.

Fig. 19
figure 19

Disruptive design ideas of fiber-reinforced strands for multi-material elements

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5 Conclusion

This research details initial studies for the application of aligned fiber-reinforced concrete for spatial strand extrusion. Alignment of fiber reinforcement increases the tensile strength of the spatially printed concrete allowing for increased freedom of geometry. This advancement in material is integrated with robotic spatial printing through the creation of dynamic print head, which enables additional axes of reinforcement orientation. Prototypes were developed and tested for an automated three-dimensional nozzle capable of custom rotation in two additional axes while ensuring smooth material flow and easy maintenance. The print head is characterized by two perpendicular axes that rotate the nozzle vertically and horizontally and a flexible conveyor hose that ensures a continuous material flow. For spatial extrusion, this feature increases the rotation radii and accessibility of the robot system to control the 3D alignment of fiber-reinforced strands.

The application of the process was tested through geometric studies that pushed the limits of shapes previously possible in spatial extrusion. Inspired by the innovative application potentials, the researchers propose filigree strands structures designed to customize reinforcement of concrete structures with spatial printing of bespoke grids or cages. Furthermore, the process allows for multi-material application and is demonstrated for use in alternative material processes such as cast building components. For structural purposes, complex flat meshes, baskets or modular strand structures are presented, which demonstrates the innovation in terms of material, process and design.

This paper documents initial research into spatial strand extrusion for fiber-reinforced concretes and seeks to abstract material research knowledge so that others can apply the potential of this approach. The process, characterized by a dynamic nozzle to align reinforcement extrusion, has the potential to allow experimentation in structural optimization of filigree-reinforced isotropic structures through a mineral material, which is lightweight, widely available, durable and capable of entering the recycling stream. In future work, this research plans to continue improving the process automation, material science and industrialization of the innovative end effector. In this scope, the system will be scaled for large-scale structures enabled by endless material flow by cavity pumps, which are already common in practice for 3D concrete printing and increased nozzle diameters. In working towards the goal of a more integrated design to optimized production process, this research builds tools that more closely connect digital planning and automated construction, contributing in some small way to design freedom, efficiency and sustainability in production.