Setting new standards for 3D-printed concrete construction

Reinforcement that grows

“The reinforcement was a big challenge indeed,” says Kaufmann, ETH Professor of Structural Engineering. As the printer’s robotic arm cannot move around or between rebars, the researchers developed a concept that combines horizontal reinforcement in the form of rings placed at regular intervals during printing, along with vertical longitudinal rebars placed after the print is completed. A particularity of these rebars is that they are inserted into vertical channels integrated in the digital design. Moreover, to ensure that this assembly behaves as closely as possible to conventional reinforced concrete, these channels are ultimately filled with a self-compacting mortar that essentially locks the rebars into place when it hardens. In the tower’s upper floors, the longitudinal reinforcement is also pre-stressed to minimise the risk of cracking. Together with Dillenburger’s group, the ETH spin-off Mesh AG and the company Zindel United, the insertion of the steel rings was even automated and carried out by a robot during the final stages of the project.

Safe dimensioning of printed structures

As standards and guidelines for the dimensioning of 3D-printed buildings do not yet exist, Kaufmann’s group had to develop new test procedures to verify the structural safety of the tower. “The most relevant difference to cast concrete is the layering of the 3D print,” Kaufmann says. “As a result, the material doesn’t have the same properties in all directions.” To find out the degree to which the joints between layers weaken the concrete, Kaufmann’s group established a modified slant shear test, which involves applying uniaxial compression to a material sample until it shears off. By repeating such experiments with the layer joints arranged at different angles they obtained crucial information on the mechanical impact of interlayer joints.

“A key breakthrough was finding a simple, reliable way to test the strength of 3DPC, with results that fit existing concrete models with only minor adaptations. That gave us the confidence to advocate 3DPC as a load-bearing material for Tor Alva,” says Senior Assistant Dr Alejandro Giraldo Soto, who has led the research projects on the structural performance of 3DPC and the full-scale testing of the V-columns.

“Based on these results, we can now reliably dimension reinforced 3D-printed concrete,” explains Kaufmann, who is working on the international recognition of this slant shear test. This makes designing reinforced 3DPC structures almost as simple as for conventional reinforced concrete, where the dimensioning of the concrete is also derived from a uniaxial compression test. It makes the structural design of 3D printed structures much more accessible to ordinary engineering bureaus, removing an important barrier to the adoption of this technology in practice.

For Tor Alva, the researchers also experimentally investigated the load-bearing behaviour of the columns on a 1:1 scale. Supplementary tension tie tests provided information about the bond between the 3D-printed concrete, the mortar and the post-installed longitudinal reinforcement. In terms of durability, a particular concern is the higher risk of corrosion for the transverse reinforcing rings, both because of the higher interlayer porosity of the printed concrete and because of the thinner cover layer. For this reason and to limit the number of unknowns in this already challenging project, the researchers opted for stainless steel.

Innovative material and an automated process

Tor Alva couldn’t have been built without the preparatory research that Robert Flatt’s group, the Chair of Physical Chemistry of Building Materials, began in 2017 in collaboration with the Chair for Digital Building Technologies. In 3D printing, fresh concrete is pumped through a nozzle on the arm of an industrial robot. It must therefore be fluid enough to be pumped and yet harden fast enough as it is deposited not to flow under its own weight. More importantly, it must then develop enough strength over time to support the weight of the layers added as printing progresses. Flatt’s concrete mix achieves this with the help of two additives mixed in line with the concrete just before the nozzle. One of these components, whose exact mode of action is currently being researched by a doctoral student, controls the consistency of the freshly deposited concrete, while the other assures enough strength development thereafter.

“Tor Alva is proof that we can print with a reliable quality and on a large scale,” Flatt says. One major milestone was the automation of the concrete feed, which his group developed together with Dillenburger’s group and industry partners BASF and Knauf. This eliminated the need for repeatedly mixing and adding concrete manually, making way for an automated mixing and pumping of a material delivered in premixed big bags. This also constitutes an important step in making 3D printing more ready for adoption by industry.
 

Sustainability is the Achilles’ heel

Tor Alva is taking the world of digital construction to a new level, but research into load-bearing 3D-printed components is far from complete. “We want to find alternatives that work better in 3D printing and are easier to automate,” Kaufmann says. Initial projects using reinforcements made of fibres or wires between the printed layers have already been carried out.

Flatt is currently investigating the emissions associated with load-bearing 3D-printed concrete, such as the material used in Tor Alva's columns. Unlike the more common practice of 3D printing columnar structures, where only the infill concrete is considered in the structural design to bear the loads, the 3D-printed concrete in this case serves as a load-bearing component rather than merely as non-structural lost formwork. This reduces the amount of used material, but there is a penalty to pay owing both to the stainless steel and the higher amount of carbon intensive 3D printed concrete. Together with Guillaume Habert, ETH Professor of Sustainable Construction, he is now calculating the outcome of the trade-offs between the different scenarios for structures as Tor Alva. An interesting aspect is that printed concrete reabsorbs CO2 from the air much faster than ordinary concrete. While for ordinary construction this would constitute a corrosion risk for the reinforcement, the use of stainless steel in Tor Alva suddenly makes CO2 (re)capture an important part of the life cycle assessment.

For Flatt, one thing is clear: “We still need to improve the sustainability of reinforced 3D-printed concrete – for example by using recycled stainless steel, but also lower carbon concrete mix designs.” The researchers have already reported successful printing with mixes having half the carbon footprint, but did not implement them for Tor Alva because of their grey colour and because once again, they did not want to add too many uncertainties to this project. But Tor Alva anyway already makes important steps towards sustainability. Indeed, it’s entire design is aimed at allowing it to be dismantled after its building permit in Mulegns expires, so that it can be rebuilt elsewhere.

With the support of the Albert Lück Foundation and under the umbrella of Design++, Flatt’s group is also researching new methods for the quality control of 3D-printed structures, including the use of augmented reality. Flatt believes that this is another important step towards increasing trust in digital fabrication and thereby allowing it to gain a foothold in the market.

This text builds upon an article that was featured in Globe, the magazine of ETH Zurich and ETH Alumni, and highlights the key contributions coming from D-BAUG that have enabled the construction of Tor Alva. Text: Stéphanie Hegelbach.