The next generation of 3D printed buildings under construction in Denmark
On a construction site in Brøndbyvester near Copenhagen, concrete workers have cast what could be called the next generation of 3D printed buildings.
The future common house for the Kirkebjerg Søpark neighbourhood is to be built based on the experience that the contractor 3DCP Group has gained from the prototype building they built in Holstebro in 2021, and the company’s other projects in Guatemala and most recently in Ukraine, where 3DCP printed the shell structures for a school building in Lviv.
“When we made the prototype, both the architects and the engineers were faced with a completely new technology. Therefore, solutions were made that would have been used in ordinary construction. But that caused us a number of issues,” says Mikkel Brich, who has established 3DCP Group together with Hasan Alsofi, SAGA Space Architects, and the engineering company Bygkontrol.
Already while the prototype was being printed, a lot of cracks appeared.
“But we quickly fixed that by adding fibre to the concrete mix. It made the shrinkage cracks disappear.”
However, when the prototype was completed, a new problem emerged.
“When it rained, water ingress occurred in several places on the back wall. The water simply seeped in through the outer wall. And we had granular paper wool in the walls, so it wouldn’t be long before we had issues with mold.”
Impregnation of the facade did not work because the printed facade is so uneven, but a plaster system solution proved to be able to keep the water out. However, 3DCP Group wanted a solution that did not require finishing the outer walls.
“Therefore, we have tested five different wall structures and carried out simulated climate tests at the Danish Technological Institute,” Mikkel Brich says.
“Triple wall” to keep moisture out
So, in the new building in Brøndbyvester and in the 36 student residences that will be built in Holstebro next year, 3DCP Group will use a completely new external wall structure.
“In traditional heavy construction, you have a front wall and a back wall. In such a structure, you typically prevent moisture penetration either by making a completely tight front wall or by creating a ventilated cavity between the inside of the front wall and the insulation. In addition, there must be vertical seams at the bottom of the front wall. We cannot solve this 1 to 1 in 3D printed construction, but we can mimic a ventilated cavity by printing a ‘triple wall,’” Andreas Slot Tanderup from Bygkontrol explains.
The triple wall consists of a 50-millimetre-wide front wall, a ventilated cavity, a 50-millimetre middle wall, an insulated cavity, and a 50-millimetre rear wall.
“It’s a solution we’re quite proud of,” Mikkel Brich says.
The triple wall will—3DCP Group hopes—also make it possible to overcome another challenge that plagues 3D printed construction: “Even though we print with concrete, which has a much higher strength than what you can achieve in masonry—and even with many traditional types of concrete—we cannot count 3D printed walls as load-bearing. In order to get the buildings approved in accordance with norms and standards, we must instead erect a lot of traditionally reinforced and cast columns, which are not really needed at all,” Andreas Slot Tanderup explains.
In the 38 m2 prototype in Holstebro, where the height of the roof structure also varied, the walls thus hid a total of 16 columns. A huge waste of time, material, and CO2, according to Mikkel Brich.
“We spend almost 50 percent of our time printing the columns, because we first have to print a shell into which we can fill reinforcement, and then it has to be poured manually with concrete and vibrated. About 30 percent of the concrete and 80 percent of the reinforcement ends up in the columns. And it’s by no means necessary.”
With the triple wall structure, 3D printed construction approaches the dimensions and principles known from masonry structures. Therefore, 3DCP Group and the consulting company Bygkontrol, which is responsible for the design, hope that they can be allowed to design future buildings based on Eurocode 6 for masonry structures.
“There is currently no separate standard for 3D printed structures. But 3D printed structures are in principle somewhat reminiscent of masonry structures. Instead of binding bricks together with layers of mortar, it is layers of concrete that are bound together. That’s why we’re trying to carry out some experiments and tests, where we adhere to the masonry standard, so that we can hopefully be allowed to rely on 3D printed walls in the same way as we rely on masonry walls today,” explains Jesper Veber Jeppesen, owner and CEO of Bygkontrol.
The strength and load-bearing capacity that the reinforced concrete columns contribute can thus be dispensed with in whole or in part, if one is allowed to factor in the strength and load-bearing capacity of the 3D printed walls.
Three layers increase CO2 load
However, the triple wall has a negative effect—it increases both construction time and material consumption. It is bad for the building’s CO2 accounts. On the other hand, the future buildings will be printed with Aalborg Portland’s product Futurecem. In Futurecem, limestone filler and calcined clay replace such a large amount of the clinker in the cement that the cement’s CO2 emissions are 30 percent lower than in traditional cement products.
“It’s a bit more troublesome to work with, because Futurecem reacts more slowly than regular cement, and we need a fast-reacting concrete mix that can support the next concrete layer ten minutes after the first layer is laid,” Mikkel Brich explains.
In addition to Futurecem, 3DCP Group also uses fly ash, which replaces part of the cement, and thus pulls the CO2 accounts in the right direction. However, neither 3DCP Group nor Bygkontrol are able to state how much the CO2 load can be reduced at this time, because the projects’ details are not finalised.
Regardless of where the CO2 accounts end up in the two ongoing projects, they are confident that they can get them even further down.
“We do not think it is an advantage that we have to make a triple wall, because it increases the amount of material, but we are still in a learning process where we are constantly developing new solutions,” Jesper Veber Jeppesen from Bygkontrol says, while Mikkel Brich envisions a future without any cement in the 3D printers.
“We are sure that 3D printed construction is the future. But 3D printed concrete is not the future. However, switching to new materials will take time. You can see all the challenges we are now dealing with when we print with concrete. Then try to imagine where we would be if we wanted to print with some bio-based material. We have to take small steps at a time,” he says.
The National Building Fund was decisive
The caution is appreciated by the housing company Nordvestbo, which is behind the student housing project in Holstebro worth DKK 44 million, for which the municipality’s council has just approved a subsidy.
“We think that 3D printing is a super exciting technology, and there was also interest in building with the method in the municipality. But we have to be sure that things have been tested and that the solutions we want to use are tested in practice and examined by agencies that have an understanding of this sort of thing,” says Michael Sand Pedersen, operations and project manager at Nordvestbo.
As a general rule, social housing developers cannot use untested methods. Therefore, it was crucial for Nordvestbo that the National Building Fund approved the project.
“It was our first and most important condition that National Building Fund would be involved. They are our insurance,” Michael Sand Pedersen explains.
Fortunately, it turned out that the National Building Fund was on board with the idea. On the other hand, they have also followed the project more closely than Michael Sand Pedersen has previously experienced on other projects.
“Normally we hardly come into contact with them, but on this project, they sit with us and look through all the details and solutions and visit construction sites with us.”
The caution also means that Nordvestbo ended up adding more details and reservations to the contract with 3DCP Group than they normally do.
“We have made some additional reservations regarding schedules and finances to secure ourselves. And then we have very carefully described the standards we expect the building to meet, so we can say exactly what quality we will receive.”
At the same time, however, Nordvestbo is prepared that, over time, some other challenges may appear with the student housing than those they have encountered in their existing housing.
“It’s not because we’ve set aside a lot of money for maintenance, because apart from the walls, everything is standard construction. But it may well be that the wall surfaces will look different and that algae will find it easier to grow on them. But then you just have to live with it or polish them up on the outside, and then they just look like ordinary concrete walls,” Michael Sand Pedersen says.
Construction in Brøndbyvester has started, and construction in Holstebro is expected to start next year.
Illustration: Ulrik Andersen.
Illustration: 3DCP Group.
Illustration: Ulrik Andersen.
