ETH Zurich researchers aim to make construction more sustainable with robotic additive manufacturing.
Most engineers can probably recall building structures from clay or mud when they were young, but a team of researchers from ETH Zurich has taken that concept to the next level. They’ve developed a new, fast robotic printing process for building with earth-based materials that does not require cement.
Dubbed ‘impact printing’, the process involves using construction robots to shoot material from above, gradually building a wall. On impact, the depositions bond together with minimal additives and, unlike concrete 3D printing, there are no necessary pauses for solidification.
Making construction more sustainable
The researchers are aiming to increase the cost competitiveness of sustainable building materials through efficient and automated production. In contrast to layer-based 3D printing, this method is based on controlled, high-velocity deposition of dense material parts.
Depositing material at velocities up to 10 meters per second enables bonding and, according to the researchers, dry joints are not an issue. They also claim that, compared to typical extrusion-based 3D printing, the process deposits material in a more stable condition and it’s therefore less dependent on additives for mechanical strength in transitioning from a liquid to a solid state.
Custom robotics for additive construction
The team developed a custom printing tool that can be integrated on multiple high-payload robotic platforms. The idea with this approach is that it enables construction both onsite and offsite. The tool is integrated with a high-payload gantry system in the Robotic Fabrication Laboratory at ETH Zurich.
Alternatively, the hardware can be mounted on an autonomous legged excavator, also developed by the Robotic Systems Lab, and it has been used in this way to build structures that are almost 10 feet (3 meters) tall. In the future, this could enable impact printing in unstructured sites with variable terrain to build walls, acoustic barriers and other infrastructure.
While the walls produced using this method have a mottled surface texture, the researchers claim that methods for robotic surface finishing methods using ecological plasters can help achieve a high-quality surface finish.
Special admixture for 3D printing
The additive manufacturing process was specially developed for materials with low embodied CO2, such as earth-based and excavated materials. One mixture, developed at ETH Zurich includes locally sourced secondary material with a minimal amount of mineral admixture.
“For this project, we have developed a mix containing enough fine material to provide stickiness and interlocking capacity between parts and at the same time aggregates to be able to mix it without sticking too much to the mixer,” Lauren Vasey, postdoctoral fellow at ETH Zurich and lead author told engineering.com. “We would be able to have larger aggregates possibly in the future, but we will need to mitigate erosion of the extrusion and mixing hardware. The fresh performance of the mix can be achieved with a wide variety of excavation materials, however they might require adding either sand or clay to achieve an optimal granulometry curve.”
Constraints on 3D printed structures
Asked about the physical limitations on these structures, Vasey said, “The current custom mix design with mineral admixture used has a comprehensive strength of 1.9 MPa. This is slightly lower than unreinforced rammed earth which typically has a compressive strength of 2.3 MPa. Unreinforced rammed earth structures can be built up to several stories high, but they are typically extremely thick (0.2 to 0.4 meters wide) for several reasons: formwork-based production methods, increased thermal inertia, lower compressive strength than timber or concrete.
“The robotic surface finishing method can apply afterwards a plaster coating with higher mechanical properties. The commercially available clay plaster or mortar we are using has around 5 MPa compressive strength. This plaster can increase both durability and load-bearing capacity.
“In terms of height limitations, we have already printed a 3-meter tall structure with the Robotic Systems Lab autonomous excavator (HEAP) with a thickness of about 0.22 meters. This platform has a reach of 6 meters with our tool mounted, but the accuracy of the process can be affected by where the tool/payload is in the reach envelope. This platform is moving autonomously, therefore we could build much bigger structures.
“We have only printed overhangs that are supported by infills, so the column that you see had a temporary gravel support on the inside during construction that was later removed. The reason is due to the very high impact forces on the structure during construction. The material itself has very limited tensile capacity and is best used in pure compression forms.”
Vasey and her colleagues are planning to establish an ETH spin-off to commercialize the technology. For more information, see the article “Circular Robotic Construction” published in A Circular Built Environment for the Digital Age.
