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The Circle House research project is billed as Denmark’s first circular housing scheme – an alternative way of building with precast concrete that enables the components to be disassembled and reused in new structures. 

It has been developed by 3XN, with social housing provider Lejerbo and a 60-strong team of collaborating companies, with the aim of disseminating new knowledge throughout the industry. After extensive testing of a prototype, construction of 60 units has just started on site at Lisbjerg outside Aarhus. The expectation is that 90% of the materials will be able to be reused without losing their value.

As housing in Denmark is generally built with precast concrete, the team were keen to develop a circular way of building that used this type of structure. ‘It’s a testbed done under a normal social housing framework but with circularity built in,’ explains Lasse Lind, 3XN partner and head of consultancy in its GXN innovation unit. Rather than a piece of architecture, the result is a system conceived as a “way of thinking”, that can incorporate many different precast concrete products. It can also be configured and reconfigured in different ways, he adds, and has built-in adaptability, so that the house is easier to change over its lifetime. 

After analysing the different components of a house and their lifespans, the architects realised that the key to circularity lay in the way the structural components were joined together. So instead of using cast-in-place cement-based mortar connections, they chose to fix the beams and flooring slabs with standard mechanical joints of bolts and screws. For the final Circle House design, the contractor modified the elements slightly so that the joints can be solved with very simple steel plates and bolts. The walls of the 1:1 prototype took just one hour to erect. 

Crucially, the connections are cast with a lime-based mortar that protects the elements against air, sound and fire but bears no load. This can be pressure-hosed out to enable the system to be unscrewed and the components reused. During the research, different strengths of mortar were trialled and left for two months before being hosed off. 

As well as the concrete wall and floor components, the system includes steel-plate connectors, facade hangers and threaded rods and concrete foundations, all using the same disassembly strategy. Altogether, there are six different precast concrete components including the foundations, with 9300mm and 15200 length beams, and slabs sized 4800 x 1200mm and 3600 x 1200mm. This structural system is combined with a lightweight facade design, mounted on the exterior using screws or mountings to allow reuse. 

The prototype remained in place for three years as a demonstrator exhibit, and will now be dismantled and reassembled as part of the first build-out of the system in Aarhus. Meanwhile the practice has moved to the client side to monitor the project and review its circularity. The housing will be built in a mix of two and three-storey terraced houses, and five-storey tower blocks.

Lind says that in principle the system is inherently scalable, and hopes this approach will be picked up by industry: “It’s doable. It takes industry involvement – there have been a lot of stakeholders involved. Changing anything takes a joint effort,” he says. “It’s a new way of thinking about buildings – not as products but as a service. And as materials that have multiple lifespans, not just one.”

Project Team

Client

Lejerbo Housing Association

Architects

3XN, GXN Innovation, Lendager Group, Vandkunsten

Contractor

MT Højgaard

Precast concrete supplier

Peikko

Photos

Tom Jersø; 3XN