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Construction Corner: Flexible formwork, why didn’t I think of that?

Korky Koroluk

Have you ever seen something somewhere — television, on radio, maybe in the paper — an idea that looks or sounds fantastic and leaves you muttering, “Why didn’t I think of that?”

When you write about technology, it’s not long before you start receiving emails from public relations firms touting the newest version of their client’s gizmo.

But for every one that looks like it might be worthwhile, you click through dozens of others.

Then you find one that leaves you wishing it was an idea that had been yours.

This leads to news from the Swiss Federal Institute of Technology (ETH Zurich) that a team of researchers has built a prototype of an extremely thin, vault-like concrete roof.

Getting the necessary curves meant using flexible formwork. The solution was to fashion a net of steel cables stretched into a reusable scaffolding structure. This cable net supported a polymer textile and the assembly became the form onto which concrete was sprayed.

The cable net was designed to take on the desired shape under the weight of the wet concrete, thanks to a specially-designed algorithm. Other algorithms ensure forces are distributed correctly between the individual steel cables and that the roof precisely assumes its intended shape.

The cable net weighs 500 kilograms; the textile weighs 300 kilograms. That means, the researchers say, that with the total of only 800 kilograms of material 20 tonnes of wet concrete are supported.

The doubly curved shell roof has several layers. The heating and cooling coils and the insulation are installed over the inner concrete layer. A second exterior layer of the concrete sandwich structure encloses the roof, under which thin-film photovoltaic cells are installed. The unit is expected to generate more energy than it consumes.

The researchers built their prototype in ETH Zurich’s robotic fabrication lab. But now that they’ve proven their concept, the prototype has been taken down to make space for another project.

The roof will take shape again, though, as part of a rooftop apartment unit called HiLo that is to be built next year atop NEST, the living lab building the Swiss government runs in Dubendorf.

The unit atop NEST will be larger than the prototype. That one was 7.5 metres high with a surface area of 160 cubic metres covering a plan area of 120 cubic metres. The thickness of the concrete varied between three centimetres along the roof edges to 12 centimetres at the support surfaces but averaged five centimetres.

Apart from the researchers working on the project, special expertise came into play at several stages.

For example, experts from Burgin Creations and Marty sprayed the concrete using a method developed specifically for this project, ensuring the textile could withstand the pressure at all times. And Holcim Schweiz experts were brought in to help with the concrete mix, which had to be fluid enough to be sprayed and vibrated, yet viscous enough that it wouldn’t flow off the fabric, even in the vertical spots.

As is common with a lot of research projects, this one took awhile. By the time the prototype was finished, almost four years’ work had been done.

Building a full-scale model at NEST is expected to take eight to 10 weeks. The individual components of the roof structure can be reused as often as needed. The cable net can be dismantled into a few parts, then can be reassembled and reused.

The idea of modular construction has had many false starts over the years and has always seemed like an idea that needed a boost. The work done at ETH Zurich may be that boost.

The prototype roof was built in a robotics lab. And robotic construction of all manner of building elements could be done indoors, as well, then hauled to the construction site.

All because of a simple idea: flexible formwork.

Now, why didn’t I think of that?

Korky Koroluk is an Ottawa-based freelance writer. Send comments to

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