Researchers from ETH Zurich have built a prototype of an ultra-thin, curved concrete roof using innovative digital design and fabrication methods. The formwork system will be used in an actual construction project in 2018.

The shell is part of a roof-top apartment unit called HiLo that is planned to be built on the NEST, the living lab building of Empa and Eawag in Dübendorf. Researchers led by Philippe Block, Professor of Architecture and Structures, and Arno Schlüter, Professor of Architecture and Building Systems, want to put the new lightweight construction to the test and combine it with intelligent and adaptive building systems.

The self-supporting, doubly curved shell roof has multiple 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, onto which thin-film photovoltaic cells are installed.

Eventually, thanks to the technology and an adaptive solar façade, the residential unit is expected to generate more energy than it consumes.

The prototype, which has already been dismantled to make space for future experiments, was 7.5 m high with a surface area of 160 m² (covering an "area in plan" of 120 m²). The concrete has an average thickness of 5 cm that varied between 3 cm along the edges of the roof to 12 cm at the support surfaces.

Instead of formwork using non-reusable custom-fabricated timber or milled foam, the researchers used a net of steel cables stretched into a reusable scaffolding structure. This cable net supported a polymer textile that together functioned as the formwork for the concrete. This not only enabled the researchers to save a great deal on material for construction, they were also able to provide a solution to efficiently realize new kinds of design. Another advantage of the flexible formwork solution is that during the concreting of the roof, the area underneath remains unobstructed, allowing interior building work to take place at the same time.

The cable net is designed to take on the desired shape under the weight of the wet concrete, using a calculation method developed by the Block Researcher Group and their collaborators in the Swiss National Centre of Competence (NCCR) in Digital Fabrication.

The algorithms ensure that the forces are distributed correctly between the individual steel cables and the roof assumes the intended shape precisely. The cable net weighs just 500 kg and the textile weighs 300 kg. Wth a total of 800 kg of material, the 20 tons of wet concrete are supported.

The HiLo unit makes use of an innovative lightweight floor system and a building technology with a positive energy balance. Energy is exchanged between the individual units in the NEST building. The HiLo unit is required to produce more energy than it consumes. As compensation, it can use waste heat from the other building units, as well from buildings in NEST’s district network.