Tool developments for additive manufacturing processes with anchor for the modern flow production

In modern flow production, the focus is on the continuous digitally controlled tools within the context of the digital twin. Spray extrusion, especially in combination with a thin layer of shotcrete and ribbed supporting structures, for example for facade elements, leads to very efficient components with high load‐bearing capacity and high quality surfaces. The developed combined spray extrusion nozzle offers the possibility of both: spraying and extruding, adapting mortar and concrete. In addition, automated integration of anchors is the focus of flow production, for which anchors and tooling systems have been developed. Based on the determination of characteristic values, the practicability and the increase in productivity could and will be proven. This data forms the basis of a digital twin in preparation for Industry 4.0. This increase in productivity in additive manufacturing leads to positive economic effects, applications of high strength lightweight structures in construction are becoming more attractive and enable the series implementation of free form modular constructions.


Introduction
Rapidity, precision, economics and resource saving are the most important requirements of the construction industry of the future.The digital twin serves as a basis for the simulation, evaluation and direct control of real processes.In the context of Industry 4.0, the Asset Administration Shell (AAS) is seen as a global open source standard.Modern lightweight construction technologies can implement these requirements and add the aspects of design freedom, efficiency and sustainability.Therefore, the development of new manufacturing technologies for highly stressable, durable and free-forming prefabricated concrete components with low material and energy input is the focus of the research group "Lightweight Constructions in Civil Engineering" (LBW) at the Chemnitz University of Technology.
The aim of current research work is to fundamentally investigate the interactions of process and material parameters of a new flow line for the continuous precision manufacture of modular elements.The single and double curved basic modules consist of thin-walled and fiber reinforced concrete with a support structure and integrated anchoring elements, appropriate to the flow of forces for attachment to existing buildings, for example facade elements.The production takes place in a robot assisted flow production process under use of concrete spraying and extrusion technologies (3d concrete printing) in combination with an integrated fully automated insertion system for fastening anchors.

Figure 1 scheme of production process
According to the state of the art, the robot assisted processes are always carried out with one tool for one work step.This leads to the limitation of production possibilities and reduce significantly the variety of products.
In particular, for the design and expansion of the robot assisted individual construction stages, a novel nozzle tool "spray extrusion multitool nozzle" (SEMT; Step 1 -3) was developed, with which a spray mist can be generated only by controlled activation of compressed air (concrete spraying) and a roving for reinforcement is concentrically embedded in the concrete layer (extrudate with reinforcement).[1] Another focus of this research work is the development of an integration device for a robotic arm that inserts anchors into the concrete matrix in the flow production process.The development was based on a DIN EN 1992-4 suitability test with regard to the results of pull-out tests and evaluation of the fracture pattern.[2] With these two developed tools (incl.digital twin in the AAS) the material utilization and economic efficiency in modern flow production processes can be improved and the lightweight construction principles can be implemented.

Test field
A future-oriented lightweight technology in construction, especially in precast concrete construction, is the additive manufacturing process (colloquially also called "3D concrete printing") with mineral-bound materials.This innovative manufacturing technology is considered futureoriented with regard to "Industry 4.0" in the construction industry and stands for a continuously digitally controlled value chain.The system used consists of two identical robots (KUKA KR 90 R2700 pro; figure 2).Robot A is for concrete processing, for example for spraying and extrusion, because the concrete conveyor (PFT BOLERO FC-230 V; Qmax 2,5 l/min) is nearby and the hoses are already installed [3], [4].Robot B is mainly used for special tools [5], [6].The programming and virtual simulation of the robots working in a team is carried out with the help of the software packages Rhinoceros, Grasshopper and KUKA|prc.Here, the layers are deposited using defined travel paths of the robots.The required outlet volume and speed are controlled by the conveyor system.

Material
The material composition is based on a complex of technological restrictions and requirements for the precast concrete element.For example, a high strength and durable fine grained concrete with short fiber reinforcement was developed, so these could be processed using spraying and extrusion.When characterizing the material, the green strength as well as the material and component related strength and failure behavior under compressive and flexural stress were of particular interest.
In addition to high strength in the use state, the concrete for this process combination of spraying and extrusion also requires application specific properties in the uncured state, such as high green strength, thixotropy, low shrinkage, adjusted processing time and good bonding behavior of the individual layers, as well as the contact zone between the sprayed and extruded layers.
To evaluate the durability, cracking and failure behavior of the reference components, climate change tests and angle-dependent insertion pull out tests were also carried out.Furthermore, the influence of the recipe and the process parameters on the material characteristics in the fresh and hardened state were examined and described.
Another focus was on the exposed concrete properties of the sprayed concrete shell for facade elements.For this purpose, the air void content in the fresh concrete, the exposed concrete surface according to the DBV/VDZ data sheet in porosity class and deviations from evenness and color requirements according to DIN 18202 were determined and evaluated [7,8].
The concrete recipe was determined empirically with constant consistency testing and is summarized in table 1.
The mixing process was carried out with an Eirich R05T.

Tool for concrete processing
A spray-extrusion multitool nozzle (SEMT) was developed and implemented for the production of the concrete modules in the robot-assisted flow production in order to avoid changing tools in the production process when switching from spraying to extrusion technology (figure 3, [12,13]).This allows, on the one hand, a fine material application by concrete spraying and, on the other hand, the application of a defined concrete extrudate can take place.In this way, facade modules are created that consist of a very thin, fiber reinforced shotcrete surface in fair faced concrete quality and a support structure made of extruded concrete that is applied in the green state directly to the shotcrete.
The new type of nozzle allows the concentric, fiber reinforced extrusion of a concrete layer and the generation of a spray mist through the controlled activation of compressed air and thus the shaping by means of concrete spraying.The SEMT is attached to the existing robot arm using a quick coupling.As a result, several process steps (spraying, extrusion, endless fiber reinforcement) can be implemented in an automated process with the SEMT without changing the tool or position of the component.This results in advantages for the user in terms of acquisition costs (only one nozzle for several work processes), set-up and maintenance costs and the transport route to several production stations is eliminated.Parallel to the development of the nozzle, a glass fiber reinforced concrete mixture was assembled, which allows both, extrusion and spraying through targeted adjustment of the material parameters (e.g.plastic viscosity, yield point and green strength).
To determine the compressive strength (fck, cube, 28 d), cube-shaped specimens with an edge length of 150 mm were produced and stored under water for 28 days (according to DIN EN 12390-2 [11]).The compressive strength was determined in accordance with DIN EN 12390-3 [12].A four-point bending tensile test was chosen to determine the load properties, because in contrast to the three-point flexural test, there is a constant bending moment between the upper force application points.In this way, the material parameters are determined more realistically, especially in the case of plate-shaped samples, and these are less influenced by possible inhomogenities in the sample.For the determination of the four-point bending tensile strength, based on DIN EN 12390-5 [13], were prismatic specimens (extrudate) manufactured and stored under water.In addition to the classic parameters, there is a focus on the bond between the sprayed and extruded layer.For assessment, a 15 mm thin layer was sprayed and a 40 mm thick layer of extrudate was built up on top of this [14].
After the 28 days of hardening, the samples were prepared so that this contact zone could be subjected to tensile loads.Here drill cores with a depth of 5 mm were created, which protruded into the sprayed layer.

Automated integration of force introduction elements
To complete the robot-assisted flow production process, force transmission elements (anchors; figure 4) were developed, which can be automatically integrated into the extruded concrete matrix in the uncured green state according to the load.After determining the preferred variant, an integration device for a process integrated industrial robot was implemented and tested (figure 5).This tool for robot B was developed in cooperation with the professorship of Assembly and Handling Technology (Chemnitz University of Technology).

Figure 5 Integration system
The device contains a magazine loaded with nine anchors.
The anchors are separated and transferred to the gripper.The gripper picks up the anchor in the internal thread and applies it automatically to the concrete using feed cylinders.With the help of the new device, anchors can be integrated into the concrete with almost no defects under infinitely variable vibration frequencies.The system can be scaled by integrating a larger magazine and adapting the software.The developed integration process can be used for the precise vertical integration of alternative connecting elements.
To assess suitability as a fastening anchor, pull-out tests were carried out according to DIN EN 1992-4.Here the anchors are inserted into a plate-shaped form filled with concrete.
After 28 days of storage in the water bath, the pull-out tests were carried out on a ZwickRoell Z250 and a device (figure 6 left).The fracture cones were recorded with the 3D measuring system GOM Core Atos (figure 6 right).The square samples (l= 370 mm; h= 50 mm) were fixed into the device.The hold-down device is ring-shaped (d= 350 mm) and concentric to the anchor.The anchor was pulled vertically until fracture at a test speed of 5 mm/min.The test force and distance were recorded.

Suitability for concrete processing
The practical proof of the suitability of the newly developed nozzle tool for the production of reinforced and nonreinforced extrudates and sprayed layers is based on pressure, flexural and tensile adhesion tests.The samples with the designation "mixer" are characterized by the fact that the concrete was taken directly from the concrete mixer.The samples with "spraying" and "extrusion" were sprayed or extruded directly into the corresponding mold.
The following figure 7 summarizes the compressive strength results.In comparison to the compressive strengths, the sprayed "spraying" and the extruded "extrusion" samples characterize significantly high values.It is assumed that the particle acceleration minimizes the defects.A comparison of the bending tensile strengths of the individual batches shows figure 8.Because of the compilation, it can be stated that all sample batches produced with the new tool can generate high flexural strengths with little deviation.Similar to the pressure tests, the sprayed samples also showed the highest strength.
To assess the bond between the sprayed layer and the extruded layer, the area was subjected to a tensile test.The mean value is 2 MPa (Min.1.9 MPa, Max.2.2 MPa).
The fracture took place in the extrudate.According to DIN 18555-6 it is a cohesive fracture.This means that the contact zone is more resilient than the joining partners are.
Overall, it can be stated that the extrudates produced with the new tool achieve a very high level in terms of strength and deviations.

Suitability anchor integration
For example, in the case of facade construction with concrete elements, the focus is on the fastening elements on the building.In addition to the substructure, the focus is on integrating the anchors into the concrete.
According to the specifications from the DIN EN 1992-4, the fracture elements should also be classified and assessed in addition to the values.Figure 9 shows the individual curves of the pull-out tests together with the fracture cones.The adaptation of the developed procedure to the integration of vertical reinforcement bars and stirrups for the reinforcement of extruded concrete structures offers further research potential.

Preparation of the digital twin
In modern flow manufacturing processes, the focus is on a high degree of automation together with a digital twin.The results obtained were entered into the AAS-Explore and the digital twin was made usable (figure 11).The digital twin serves as the basis for the simulation, evaluation and direct control of real processes, starting with the dimensioning, through the robot-controlled concrete shaping and integration of the anchors, to the assembly.

Conclusions
In summary, fundamental scientific questions regarding the continuous additive manufacturing of thin walled fiber reinforced concrete elements for free form modular constructions could be solved.The process combination of concrete spraying and extrusion provides an automated additive technology that has great potential in terms of resource efficiency, sustainability, design and cost effectiveness due to the new tool development (SEMT and anchor integration device) and technology-oriented material development.Based on the scientific principles, further optimizations, new references and a transfer to the precast concrete industry are planned.
The goal of combining the high-performance and at the same time lightweight material concrete with new automated manufacturing processes within a digital value chain was achieved and confirmed by a large number of results.Thus, an important milestone for the establishment of lightweight construction in concrete construction could be set.This innovative contribution includes the immense material and weight reduction, the development of high loadsupport structures and further application-specific methods in terms of design and functional integration (damage prediction, structural monitoring, etc.).In addition to cement, other important resources such as sand and additives can be saved during production.
Directly related to the technology results are free form design and weight reduction through additive manufacturing.In addition, the lighter construction enables lower transport costs and thus a better CO2 footprint.Furthermore, formwork resources can be significantly minimized in various production scenarios.The effort for space capacities and work processes is reduced.Economic and ecological aspects (e.g.reduction of CO2 emissions) as well as the guiding principle of sustainability ("green building") are also of particular importance for the necessity of lightweight construction solutions.
This project serves to establish lightweight construction and thus, in addition to the aspect of environmental protection, also offers considerable market potential for numerous companies in the field of ecological lightweight construction solutions.

Figure 2
Figure 2 Test field "LBW" at Chemnitz University of Technology

Figure 6
Figure 6 Pull out Test and GOM Core System

Figure 7
Figure 7 compressive strength results

Figure 8
Figure 8 flexural strength results

Figure 9
Figure 9 Pull-Out test results: diagram and sample pictures According to the curve of the individual samples and the fracture patterns, there is little scatter and therefore a high level of reproducibility.The values can be used as a basis for calculating component dimensioning.Another specification for the design is the diameter of the fracture cone.This determines the exact position of the anchors to the edge of the component and to each other.After the 3D measurement, the fracture cone had an average diameter of 78 mm (figure10 left).In order to verify that no defects occur during installation in the concrete, a cross-section was determined (figure10 right).

Figure 10
Figure 10 3D scan (left) and cross-section of fracture cone

Figure 11
Figure 11 Flow production scheme in the context of Industry 4.0