How 3D printing concrete saves material and CO2

Concrete is the most widely used building material in the world. It can be used in many ways, can be produced locally and is very durable. Its environmental performance, on the other hand, is the subject of critical debate. In particular, the production of the cement that makes up integral concrete emits a lot of CO2. “If we want to make concrete construction more sustainable and climate-friendly, we have to work on new concrete formulations and at the same time use concrete more purposefully and intelligently”, summarizes Andreas Trummer. Together with Stefan Peters, Trummer conducts research on lightweight construction methods with concrete at the Institute for Structural Design of TU Graz. Originally, the civil engineer focused on wood construction, but: “As soon as climate compatibility comes into play – and this is indispensable – we must turn to mineral building materials. And this is where the very large potential CO2 savings can be increased.”

The researchers gained their first practical experience within the framework of the Atelierdach project at the Schloss Seehof in Lunz. It is owned by Austrian sculptor, graphic designer, photographer and media artist Hans Kupelwieser. “It was the first application outside the lab. We had a lot of time and space, which is certainly not the case on a ‘real’ construction site,” says Trummer. The first use of precast lightweight concrete elements in real construction site conditions was the ceiling of an underground car park exit for a housing estate in Nördlingen, Bavaria. The project was completed in cooperation with Eigner Bauunternehmung GmbH in just a few weeks. For the first time, the production of the printed concrete parts was carried out directly by the execution company. The TU Graz team took care of the planning, design and facilitation of the project. “It was a very valuable collaboration for us, because there are many subtleties that can only be recognized in the construction process“, describes Trummer.

He and his team are currently advising on a similar building project in Vorarlberg. In the university laboratory there is time and space to delve into the details and work with great precision. During construction, however, things look different: “The clock is ticking, the schedule is tight, the construction staff are very busy. This is where it has to be practical and work.” It is therefore important that all stages and interfaces of the construction process are precisely defined. This also includes deviations and tolerances. “The accepted tolerances and deviations to which 3D printed items need to be remanufactured have yet to be worked out and negotiated. This is only possible with a step-by-step practice facilitated by us.”

In the meantime, it has been possible to introduce the technology of 3D concrete printing into construction practice. Nevertheless, there are still open questions, such as the question of reinforcing the floor slabs with integrated stamped concrete elements. A smooth, conventional concrete slab is reinforced with geometrically simple steel bars or grids. In the case of light slabs printed with intersecting ribs, this is much more complex and therefore more expensive. “There are just a lot more hand movements needed. It was a great achievement in our institute,” says Andreas Trummer. The Institute is therefore working with the company AVI (Alpenländische Veredelungs-Industrie GmbH) based in Graz within the framework of a “City of the Future” project financed by the FFG (3D printing of concrete and reinforcement of load-bearing structures low emission in building construction) on a new principle of reinforcement for concrete slabs of this type. The aim is to facilitate assembly on site. Ideally, in the future, the perfectly fitting reinforcement can be ordered directly from the concrete 3D printer for each individually planned light slab.

The Institute of Structural Design team also managed to add filigree steel reinforcement to the printer’s concrete strand. “We can embed a thin steel cable directly into the printed concrete lines (PCL; the extruded concrete strands) so that the reinforcement is directly integrated into the printing process.” This makes 3D printed constructions even more robust and resistant. At the same time, the heart of the architect rejoices, because: “This way you can also be active in terms of design and think of filigree panels for the facade, for example. This is where freedom of design and constructive advantage meet. explains Robert Schmid, who discusses this key area in his doctoral thesis. Baumit has already used this reinforcement tool, which was developed at TU Graz, as a prototype on three machines.

The dossier “Sustainable construction with concrete” gives an overview of the research activities of TU Graz on more climate-friendly construction with concrete.

In the same “City of the Future” project, the institute is working with BOKU on the logistical issues surrounding pre-printed concrete elements: how do precast elements arrive on site? Where and how are they stored before that? How do you get to the right place at the right time on the construction site? “Experience has shown that it is in high demand on construction sites. And these new filigree components are relatively sensitive and must be dealt with on site as quickly as possible.” points out Trummer, who also points out that “So far, none of these components have been damaged, so they can handle a lot.”

Another specialty of the Institute is cellular concrete. In this process, air bubbles are introduced into the concrete by means of a protein foam and stabilized. This also significantly reduces the amount of material used and at the same time gives the concrete better thermal insulation properties. Although aerated concrete is already processed by some companies, says Trummer: “We were able to send this aerated concrete through the 3D printer for the first time.” Where concrete needs to be load-bearing, it needs to be heavy and dense. In other parts of the structure, more thermal insulation may be necessary; here, aerated concrete from the 3D printer could be used to reduce petroleum-based insulation materials. “Many things are possible here and the Institute is a pioneer in this field,” Trummer said. He is referring here to the collaboration with the Carinthian company Mai International, which manufactures, among other things, mortar pumps for 3D printers.

Source and top image: Graz University of Technology