An innovative concrete formulation combined with novel reinforcement strategies and design methods, all developed by researchers at ETH Zurich, have paved the way for the large-scale 3D printing of load-bearing components. Starting in May, these can be admired as the building blocks of the world's tallest 3D-printed tower.
Buildings fabricated digitally - for example using 3D printers - promise to revolutionise the construction industry by offering reduced material consumption, lower costs, greater occupational safety, higher efficiency and almost unlimited architectural freedom. So far however, the adoption of such technologies remains only marginal, largely due to the lack of suitable methods for reinforcing printed concrete.
An interdisciplinary research team of engineers and architects has now overcome this hurdle. In May, they will unveil a world first on the Julier Pass in the Swiss Alps: a 30-metre tall tower built with printed reinforced concrete. As the tallest building ever printed, this tower, called Tor Alva (white tower in Romansh), sets new standards in digital construction. With its 36 ornamented and branching columns, it not only explores a new architectural design language, but also pioneers load-bearing 3D printing at this scale.
The visionary project, which is the result of a cooperation between ETH Zurich and the cultural foundation Nova Fundaziun Origen, was developed by ETH Professor Benjamin Dillenburger from the Chair for Digital Building Technologies and the computational architect Michael Hansmeyer. Structurally dimensioned by the engineering firm Conzett Bronzini Partner, Tor Alva was particularly challenging to materialise and required numerous innovations by the different contributing teams. Key contributions coming from the Department of Civil, Environmental and Geomatic Engineering were provided in terms of structural engineering, material science and processing through the chairs of Professors Walter Kaufmann and Robert Flatt.
"Based on these test results, we can now reliably dimension 3D-printed reinforced concrete."Prof. Walter Kaufmann
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. 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. "Based on these results, we can now reliably dimension 3D-printed reinforced concrete," says Kaufmann, who is working on the international recognition of this slant shear test. This makes calculating 3D structures almost as simple as for conventional reinforced concrete structures, 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 belt 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 shear 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 at 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.
"Tor Alva is proof that we can print at a reliable quality and on a large scale."Prof. Robert Flatt
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 looking into the emissions of load-bearing 3D printing. In contrast to the dominating 3D printing of columnar structures, Tor Alva elements are not filled with concrete that ultimately carries the load. This reduces the amount of used material, but there is a carbon footprint 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 3D printing reinforced 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 extensive engineering research that has enabled the construction of Tor Alva. Text: Stéphanie Hegelbach.
