Simultaneous and contiguous integration of mineral-bonded carbon reinforcement into additive manufacturing with concrete
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Neef, Tobias
Mechtcherine, Viktor
3D concrete printing is becoming more and more popular, not only in research but also in practice of construction. One of the main challenges however remains the integration of reinforcement. This publication focuses on the use of continuous fibers for this purpose. The special feature of the new type of carbon yarns applied is the mineral impregnation used instead of the conventional polymer one. Such reinforcement not only yields excellent mechanical properties, but also offers many advantages, specifically with respect to processing flexibility and is therefore very well suited for additive manufacturing. The concrete for 3D printing to be reinforced with such yarns must meet particular requirements, especially when the finest concrete filaments are printed. This is also a subject of the article at hand.
 Hi, my name is Tobias Neff. I'm a research assistant from the Institute of Construction Material. I'm dealing with additive manufacturing, or better known as concrete 3D printing. Today I want to show some aspects about cementenous and conditious integration of mineral carbon reinforcement into additive manufacturing with concrete. I want to start with the special reinforcement we are using. The next part we are dealing with the integration of the... with the integration of the reinforcement in the printing process and the obtained mechanical results. Finally, I will give you an expression how this technology can be downsized for fine filament printing with a high reinforcement grade. Typical concrete structures are reinforced with steel rebars and the elasticated non-metallic reinforcements are in common. One advantage is that the non-metallic reinforcement requires only a very thin concrete cover for the bond. They are not prone to corrosion like standard reinforcement. The thin cover leads to material saving and so is the more environmentally friendly construction. For the full activation of the yarn, coating is required, which is normally polymer based. This solution leads to a lot of temperature sensitive mechanical load bearing and adhesive behavior. This problem is avoided with mineral impregnation. Significantly higher temperature resistance than polymeric impregnation materials can be reached. One advantage is that the easy formability of the freshly impregnated yarn in the first four hours. You can see in the video the yarn can be deposed in any geometry without strain. So the full potential of the geometric freedom of 3D printing can be used. The fresh and fresh technology generates an intensive bond between the reinforcement and the concrete. The mineral impregnation we use is based on cement. To do the grain size distribution of ordinary cement and the diameter of the carbon fiber filament, it was found that the yarn does not fit together. Both have nearly the same dimensions, so the distance between the carbon filament is too high. To prevent this, we use two ultra fine cements and a micro silica suspension. The impregnation slurry has a water binder ratio of 0.8 and the flowability is adapted by a superplasticizer. In the figure low right, you can see the grading curves of the constituents of the suspension in comparison to a typical ordinary cement. In the picture of the scanning electronic microscope, you can see that the solution works. Before we start with the fiber integration, I would like to introduce you to the concrete printing process developed at the Teotristen. In this video, we can see the printing process of a wall corner with a width of 50 cm and a layer height of 5 cm. But processes like these are not comparable to ordinary reinforcement for concrete. They compete to the machinery to the missing reinforcement. To overcome these disadvantages, the integration of reinforcement is necessary. Today, reinforcement technologies parallel to printed concrete filament are in focus of the presentation. The easiest way is to lay down the carbon yarn on top of the concrete filament. Based on that step by step production, it is classified as a continuous process. The extrusion of the concrete filament is followed by the deposition of the carbon growings onto it. Then subsequently covering the reinforcement yarn by extrusion a new concrete filament. The advantage of this method is that the placement of the concrete and the reinforcement are decoupled in time. This video shows us the acrylic prototype status. The number of deposition reinforcement between the concrete filaments can be varied according to the expected loading. This reduces the cost of reinforcement and promotes environmentally friendly and resource-saving construction. One disadvantage is that the placement of the reinforcement in the joint between two concrete filaments is the weakening of the porous region. Transfer tensile stress resulting from the bond between yarn and concrete can lead to splitting along the bonded joint. The second possibility is the reinforcement integration through a simultaneous process. The carbon reinforcement is embedded in the concrete filament and deposed with it. The advantages of this method are the completion of the yarn without weakening. The joint in an efficient and time-saving manner. This disadvantage is lack of intersection and overlaps of reinforcement layers. This can make the transfer of tensile force into compression zones difficult. The second disadvantage is that for a simultaneous process the degree of reinforcement in the concrete filament can be adopted by using special nozzle geometries. Our actual printing nozzle can take up to six row rings which are integrated simultaneously. The next steps, the mechanical parameters of both production processes can be compared. The graph on the right hand shows both techniques. It achieves nearly the same load bending but the stiffness is even lower in the contiguous process. To do the difference of the static height, the results are not really comparable. The more significant difference is the failure mode. In case of the continuous process the specimen failures are as expected in the center of the first printed layer. Then the crack went in horizontal direction and delimination occurs. For the other production technologies the simultaneous process shows the typical failure mode of a bending specimen. Besides the bending test, only actual tensile tests were performed additionally to the LVDTS. A photochromical measurement system was used to become ideas on the crack development. The experimental results show a fine and well distributed crack pattern with narrow crack openings. The mechanical properties show meaningful results but there is still some potential for future optimization. One methodology to generate better insight is the CT scan. In one cooperation a specimen with three embedded yarns was scanned. Following these insights we found four different optimization potentials. On the left we see the ligament is not perfect. The more over some air inclusions become visible. Additionally the bond between the yarn and the symmetric matrix can be improved. The next challenge we are working on is downsizing the filament. The step will increase the reinforcement ratio and lowers the material needed. This leads to a more environmental friendly material and reduces the carbon footprint. In this development step we are facing new challenges like pumpability, small cross sections with high demand on surface quality and continuous concrete deposition, other consistencies of concrete and robotic control. In the first step we learned how to control the robot with different geometries and additionally what happens when we like to produce more than one specimen with regard to the collision of other issues. In the next step we focused on the buildability of the chosen concrete material. To do the lap scale we had a fast over build which gives a relatively high stress in the layer below. Without any chemical helps we are able to reach a height of 25 to 30 cm. To overcome this border we use an accelerator which is sprayed on the surface. You can see the results here. In the next step we like to automate the spraying process. To improve the pumpability, the particle optimization of the concrete matrix was conducted and additionally the mixing energy was observed. This gives some hints regarding the efficiency of the optimization process. After this optimization step we performed re-scale tests to see the pumpability, extruability and buildability. With this we focused on reproducibility and processing time, placing multiple elements, printing with a second batch or integrate short fibres to prevent shrinkage cracks. This experimental work shows stable results so that the next development step is to indicate the mineral coated yarns in the concrete filament. With this final slide I would like to show you the ideas of how the integration can be performed for a stable flow of the concrete and an external integration of the mineral bonded carbon yarn is planned. Thank you for your attention and on the right side you can see our printed specimens.
 Hi, my name is Tobias Neff. I'm a research assistant from the Institute of Construction Material.  I'm dealing with additive manufacturing, or better known as concrete 3D printing.  Today I want to show some aspects about cementenous and conditious integration of mineral carbon reinforcement into additive manufacturing with concrete.  I want to start with the special reinforcement we are using.  The next part we are dealing with the integration of the...  with the integration of the reinforcement in the printing process and the obtained mechanical results.  Finally, I will give you an expression how this technology can be downsized for fine filament printing with a high reinforcement grade.  Typical concrete structures are reinforced with steel rebars and the elasticated non-metallic reinforcements are in common.  One advantage is that the non-metallic reinforcement requires only a very thin concrete cover for the bond.  They are not prone to corrosion like standard reinforcement.  The thin cover leads to material saving and so is the more environmentally friendly construction.  For the full activation of the yarn, coating is required, which is normally polymer based.  This solution leads to a lot of temperature sensitive mechanical load bearing and adhesive behavior.  This problem is avoided with mineral impregnation.  Significantly higher temperature resistance than polymeric impregnation materials can be reached.  One advantage is that the easy formability of the freshly impregnated yarn in the first four hours.  You can see in the video the yarn can be deposed in any geometry without strain.  So the full potential of the geometric freedom of 3D printing can be used.  The fresh and fresh technology generates an intensive bond between the reinforcement and the concrete.  The mineral impregnation we use is based on cement.  To do the grain size distribution of ordinary cement and the diameter of the carbon fiber filament,  it was found that the yarn does not fit together.  Both have nearly the same dimensions, so the distance between the carbon filament is too high.  To prevent this, we use two ultra fine cements and a micro silica suspension.  The impregnation slurry has a water binder ratio of 0.8 and the flowability is adapted by a superplasticizer.  In the figure low right, you can see the grading curves of the constituents of the suspension  in comparison to a typical ordinary cement.  In the picture of the scanning electronic microscope, you can see that the solution works.  Before we start with the fiber integration, I would like to introduce you to the concrete printing process developed at the Teotristen.  In this video, we can see the printing process of a wall corner with a width of 50 cm and a layer height of 5 cm.  But processes like these are not comparable to ordinary reinforcement for concrete.  They compete to the machinery to the missing reinforcement.  To overcome these disadvantages, the integration of reinforcement is necessary.  Today, reinforcement technologies parallel to printed concrete filament are in focus of the presentation.  The easiest way is to lay down the carbon yarn on top of the concrete filament.  Based on that step by step production, it is classified as a continuous process.  The extrusion of the concrete filament is followed by the deposition of the carbon growings onto it.  Then subsequently covering the reinforcement yarn by extrusion a new concrete filament.  The advantage of this method is that the placement of the concrete and the reinforcement are decoupled in time.  This video shows us the acrylic prototype status.  The number of deposition reinforcement between the concrete filaments can be varied according to the expected loading.  This reduces the cost of reinforcement and promotes environmentally friendly and resource-saving construction.  One disadvantage is that the placement of the reinforcement in the joint between two concrete filaments is the weakening of the porous region.  Transfer tensile stress resulting from the bond between yarn and concrete can lead to splitting along the bonded joint.  The second possibility is the reinforcement integration through a simultaneous process.  The carbon reinforcement is embedded in the concrete filament and deposed with it.  The advantages of this method are the completion of the yarn without weakening.  The joint in an efficient and time-saving manner.  This disadvantage is lack of intersection and overlaps of reinforcement layers.  This can make the transfer of tensile force into compression zones difficult.  The second disadvantage is that for a simultaneous process the degree of reinforcement in the concrete filament can be adopted by using special nozzle geometries.  Our actual printing nozzle can take up to six row rings which are integrated simultaneously.  The next steps, the mechanical parameters of both production processes can be compared.  The graph on the right hand shows both techniques.  It achieves nearly the same load bending but the stiffness is even lower in the contiguous process.  To do the difference of the static height, the results are not really comparable.  The more significant difference is the failure mode.  In case of the continuous process the specimen failures are as expected in the center of the first printed layer.  Then the crack went in horizontal direction and delimination occurs.  For the other production technologies the simultaneous process shows the typical failure mode of a bending specimen.  Besides the bending test, only actual tensile tests were performed additionally to the LVDTS.  A photochromical measurement system was used to become ideas on the crack development.  The experimental results show a fine and well distributed crack pattern with narrow crack openings.  The mechanical properties show meaningful results but there is still some potential for future optimization.  One methodology to generate better insight is the CT scan.  In one cooperation a specimen with three embedded yarns was scanned.  Following these insights we found four different optimization potentials.  On the left we see the ligament is not perfect.  The more over some air inclusions become visible. Additionally the bond between the yarn and the symmetric matrix can be improved.  The next challenge we are working on is downsizing the filament.  The step will increase the reinforcement ratio and lowers the material needed.  This leads to a more environmental friendly material and reduces the carbon footprint.  In this development step we are facing new challenges like pumpability, small cross sections with high demand on surface quality and continuous concrete deposition,  other consistencies of concrete and robotic control.  In the first step we learned how to control the robot with different geometries and additionally what happens when we like to produce more than one specimen with regard to the collision of other issues.  In the next step we focused on the buildability of the chosen concrete material. To do the lap scale we had a fast over build which gives a relatively high stress in the layer below.  Without any chemical helps we are able to reach a height of 25 to 30 cm. To overcome this border we use an accelerator which is sprayed on the surface.  You can see the results here. In the next step we like to automate the spraying process.  To improve the pumpability, the particle optimization of the concrete matrix was conducted and additionally the mixing energy was observed.  This gives some hints regarding the efficiency of the optimization process.  After this optimization step we performed re-scale tests to see the pumpability, extruability and buildability.  With this we focused on reproducibility and processing time, placing multiple elements, printing with a second batch or integrate short fibres to prevent shrinkage cracks.  This experimental work shows stable results so that the next development step is to indicate the mineral coated yarns in the concrete filament.  With this final slide I would like to show you the ideas of how the integration can be performed for a stable flow of the concrete and an external integration of the mineral bonded carbon yarn is planned.  Thank you for your attention and on the right side you can see our printed specimens.
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