Structure Meets Ornamentation: Printed Concrete Ceiling at DFAB House

Researchers from ETH Zurich have manufactured a lightweight ceiling of concrete for an experimental building in the Swiss town of Dübendorf. The ceiling measures 78 m². According to details provided by ETH Zurich, it is the first time that concrete formwork of 3D-printed sandstone has been used in a building at real scale.

At its thinnest points, this "smart slab" is a mere 20 mm thick and weights only around half as much as a conventional ceiling of the same load-bearing capacity. It is a core element of the experimental apartment building known as DFAB House, which was assembled last summer at the NEST research and innovation building in Dübendorf. Altogether, the 15-tonne ceiling consists of eleven individual segments. It transfers the load of the two upper storeys, both of wood-frame construction, to a sigmoid concrete wall on the ground floor. The complex geometry of this wall, which is about 12 metres long, arises from the fact that the ceiling lies on it only in the middle.

"Printed" sandstone is made of fine sand and a polymeric binding agent; it is transformed into casting moulds and formwork in industrial 3D printers. This technique is familiar from metal casting; now it is becoming increasingly prevalent in concrete building as well. Among other projects the manufacturer, Voxeljet, is currently testing 3D-printed concrete formwork for the construction of the mushroom-shaped columns at the new main railway station in Stuttgart.

Above all, the Zurich researchers expect to see cost advantages in the manufacture of 3D-printed formwork for concrete building components. Furthermore, they hope there will be the potential to save materials in concrete building, for the geometry of the components corresponds exactly to the loading conditions of a particular project. This means that the parts can be implemented in built reality without too much effort.

For the smart slab, the sand formwork was used for the finely ribbed underside of the ceiling, to reinforce the ceiling itself and to enhance the acoustics in the space below. Most of the loads from the upper storeys are borne by prestressed ribs on the upper side of the ceiling, which were made with CNC laser-cut wooden formwork.

Altogether, around 180 3D-printed formwork elements were used for the smart slab. In order to simplify transport and save printing costs, each form was the size of a palette. In making the ceiling, a thin coat of fibre-reinforced sprayed concrete was first applied to the printed formwork. Subsequently, the ribs were cased from concrete according to the usual method.