June 20, 2019
Additive Manufacturing with Cement
This paper will outline current trends in additive manufacturing as they relate to using cement materials, along with current and future research on additives to the cement and how they relate to its strength properties and strength testing, lastly I will examine construction 4.0 which is a name given to the family of technology including automation and digital construction made possible with AM technologies and the internet of things.
Current Methods of Cement Additive Manufacturing.
There are currently two main paths using cement in AM. One type is extrusion-based cement AM (Buswell, Leal de Silva, Jones, Dirrenberg 2018) which operate much like traditional desktop 3d printers except instead of using meltable thermoplastics these machines extrude liquid cement. An AM machine is setup with an extruded head that travels on a preset tool path from computer aided manufacturing or CAM software that gets its geometry from a computer aided design or CAD software that has been designed digitally and can be transferred to the machine to build its product in a single layer that goes up vertically each time a new layer is to be added.
This process can have issues with concrete splitting or tearing in the concrete which can influence the stability of the concrete if the mixture is not liquid enough (fig. 1). There is discussion if this cracking happens because the curing time for concrete out of extrusion-based systems can dry at uneven times and could be a result of traditional batch processing that is used in commercial concrete pouring where a large amount of concrete is mixed at once to be used quickly. To try and minimize the cracks teams are exploring using micro batches of concrete so that they can control the curing times more precisely, as illustrated in (fig 2) the curing time is very important to this process With concrete extrusion-based systems there is need for support material just like extrusion-based AM with thermoplastic materials, with cement AM they use tools that can be printed over and then removed after the material has set.
The other popular process is particle bed cement AM which works very similar to binder jetting systems. In this system a material chamber full of small particles of cement are rolled over a build plate that has cinder material added a layer at a time that is initiate the curing process (Lowke, Dini, Perrot, Weger, Gehlen, 2018). This process has a much higher resolution than extrusion-based systems but is limited by the size of the build chamber. Although the strength of these systems may have a more uniform structure, they are hindered by the need to have drain holes for the cement powder to drain out of and due to their small size of printing area the remainder of this paper will focus on extrusion-based systems.
Additives and Materials Used in Strengthening Cement Based AM
There is a team of researchers working on using AM to create 3D printed metallic supports that act like rebar and are formed like a machine with a tig welding tip attached (Mechtcherine, Grafe, Nerella Spaniol, Hertel, Fussel 2018). This team has shown (fig 3) that when they use bond lengths of metallic deposits at 32 mm, they can outperform traditional steel bars in a stress strain pullout test. The steel bars have a higher sheer stress displacement to begin with but quickly fail when displaced over 4 mm. This technology shows the possibility of being able to add structural stability to cement AM construction. Since this technique can operate at the same time as a extrusion-based AM machine they could both build their material and internal supports one layer at a time and allow for a continuous steel reinforcement to be created during the construction process. This process also uses what they refer to as an adaptive process for AM (fig 4) to be able to make their reinforcements print in a concentric circle that will maintain its tolerance and size.
Another additive research teams are studying in concrete AM is using fiber reinforcements. This has previously been shown to add strength in thermoplastics, the proces is similar where the paste is pushed through and extruder and as this happens the fiber grains align (fig 5). In a study published in Cement and Concrete Composites (Hambuck, Volkmer, 2017) study the influence on strength that infill patterns have on fiber reinforced concrete. This study shows the added strength that comes when the infill pattern follows straight paths along the stress areas (fig 6). You can see that when the infill pattern of a rectilinear test part is in line with the stress applied to it the deformation curve goes but by almost two times the original cement with no additives. When the test is performed on a cube shape the deformation is still two times as strong but the deformation benefits from having the grain completely aligned is not as much and shows that the longer and less even a part is the more it will benefit from having aligned grain structures of the fiber reinforcement. In my opinion there is room for future research to study how different infill patterns at specific layer heights may help to create a structure that can have flexural strength in both X and Y directions where strain could be applied. This additive will help the layer strength but still doesnt help to resolve the anisotropic stress in the Z direction.
Geopolymers have the possibility of creating a more ecologically responsible additive for cement-based AM construction,
The term geopolymers was introduced in 1978, characterizing a new class of materials with the ability to poly-condense at low temperatures like polymers. This process involves the chemical reaction of aluminosilicate materials with Na- or K- based alkali activators (Panda, Singh, Unluer, Tan, 2019, pp. 610).
In their paper Panda et al. show that it is possible to use industrial waste byproducts as the solid binding agent for cement AM with allowing the material enough time to bond to itself with at least a 60 second cooling time to not displace the layers they are printed on. Since concrete production is one of the largest creators of environmental pollution any solution that minimizes the amount of raw material and energy needed to create it is a step in a positive direction.
A group of researchers have been looking at destructive testing of cement AM in large scale tests to more closely resemble the size and scale of actual building structures (Bos, Wolfs, Ahmed, Salet, 2018). F Bos, et al used a straight piece of testing concrete to make tests easier to interpret and also used an accelerator to increase set time and reduce it from 2-3 hours down to 15 minutes and then sprayed a curing compound to avoid dehydration of the concrete, the researchers felt that this speed increase would not have a noticeable effect on the test pieces of concrete. The team used many tests including vertical flex tests, compression tests, and impact tests. In all of their tests the AM concrete met the requirements set for each loading condition with considerable margin (Bos, et al pp. 139). After analyzing their data the team found that although there may be small areas in the concrete that do not form perfect such as air bubbles or concrete or water flow inconsistencies the overall test results were positive and promising but would require further large scales testing to validate their results.
As spoken about in a research journal article from (Craveiro, Duarte, Bartolo, and Bartolo, 2019) construction 4.0 deals with AM being a possible industry disrupter of the traditional construction methods forcing workers to learn new skill and lead to a more streamlined and automated process of construction. This is accomplished through using automation and the digitization of construction methods., this can create a safer environment for workers since some of the more dangerous tasks can be automated with robotics, however this does mean that the current workforce in construction would need to learn new skills that would focus more on working with the machines and technology than the focus of becoming a skilled trades person doing the concrete and building construction and may disrupt the trade workers and create a skills gap where the technology is more advanced than a workforce can support. Construction companies would be wise to start exploring this technology while it is still early so that when the change to construction 4.0 happens, they are more ready to adapt to the new methods of construction. AM technologies are at the center of this process (fig 7) where the construction site of the future may operate with many integrated technologies such as laser scanning for building surveying, using drones to monitor construction progress and move small materials, having stronger materials through the use of advanced building materials that can even print with multi materials for an insulation layer, this would be accomplished with the use of two STL files like in traditional AM. When robots are doing the AM construction, they can be fitted with many sensors to fully validate and record the build process to study for anomalies. If a construction company uses smart materials with sensors built in connected to the internet of things than these buildings can be monitored long term and can send information that may require maintenance or service before a problem arises that would lead to a failure in the home or business that has been constructed.
This is a large area of research possible with using cement materials in AM. This presents a possibility to further study how to increase the strength of materials along with finding novel ways to create more strength in the Z direction results from the anisotropic nature of AM. When the materials to use AM technologies for digital construction can be validated and show a history of success there will most likely be a massive change in the way homes and business are constructed when designs can incorporate local materials and be designed shapes that can withstand high winds and environmental factors then the processes could lead to a new era in affordable housing that can be built quickly, affordably, reliably while incorporating sensors to keep these structures intact for longer periods of time than traditional construction methods.
Bos, F., Wolfs, R., Ahmed, Z., Salet, T., (2018) Large Scale Testing of Digitally Fabricated Concrete (DCF) Elements First RILEM International Confrence on Concrete and Digital Fabrication Concrete (pp. 129-147)
Buswell, R.A., Leal de Silva W.R., Jones, S.Z., Dirrenberger, J. (2018) 3D Printing Using Concrete Extrusion. Cement and Concrete Research 112 (pp. 37-49)
Craveiro, F., Duarte, J., Bartolo, H., and Bartolo, P., (2019) Additive Manufacturing as an Enabling Technology for Digital Construction: A Perspective on Construction 4.0 Automation in Construction 102 (pp. 251-267)
Hambuck, M., Volkmer, D., (2017) Properties of 3D Printed Fiber-Reinforced Portland Cement Paste. Cement and Concrete Composites 79 (pp. 62-70)
Lowke, D., Dini, E., Perrot, A., Wegener, D., Gehlen, C., (2018) Particle-bed 3D Printing in Concrete Construction Possibilities and Challenges. Cement and Concrete Research 112 (pp. 50-65)
Mechtcherine, V., Grafe, J., Nerella, V., Spaniol, E., Hertel, M., Fussel, U., (2018) 3D printed Steel Reinforcement for Digital Concrete ConstructionManufacture, Mechanical Properties and Bond Behavior. Construction and Building Materials 179 (pp. 125-137)
Panda, B., Singh, GVP. B., Unluer, C., Tan, M. J., 2019 Synthesis and Characterization of One-Part Geopolymers for Extrusion Based 3D Concrete Printing, Journal of Cleaner Production 220, (pp. 610-619)