The Smart Slab enhances the excellent structural properties of concrete with a radical new aesthetic enabled by the 3D-printed formwork.

Innovation

The pioneering construction method of the Smart Slab uses 3D-printed formwork for casting and spraying concrete in geometrically complex shapes. 3D printing overcomes the geometric limitations of traditional formwork fabrication methods. Furthermore, it enables the construction of integrative concrete elements with elaborate, free-form and highly detailed surfaces and smart construction details. 3D printing has the added benefit that geometric complexity and differentiation come at no additional production cost.

Context

The Smart Slab is part of the DFAB HOUSE, a collaborative demonstrator of the NCCR Digital Fabrication for the NEST building of Empa and Eawag. Within this building project, eight ETH Zurich chairs and industry experts investigate how digital fabrication can change architecture. The Smart Slab is at the core of the demonstrator, resting on the double-curved Mesh Mould wall and supporting the two-story robotically-assembled timber units above. In addition, on the perimeter, it interfaces with the 15 Smart Dynamic Casting facade mullions.

LightWeight Structure

The Smart Slab is a 78-square-meter prestressed concrete slab discretized into eleven 7.4-metre-long segments. Each segment is unique and prefabricated with special interface features which facilitate on-site connection through post-tensioning tendons.

The geometry of the Smart Slab is structurally optimized for its challenging load-case, involving cantilevers of up to 4.5 meters. The material is distributed in a hierarchical grid of curved ribs, which vary between 30 and 60 cm in depth. In addition, the interstitial surfaces stabilize the grid and are only 1.5 cm thick. Consequently, the slab only weighs 15 tonnes, almost 70% less in comparison to a conventional solid concrete slab.

Computational Design

The Smart Slab is a fully functional structural element which showcases an exquisite digitally designed geometry, with a deeply folded surface and millimetre-precise details. The computational design process uses the structural grid as a starting point to generate a basic mesh geometry with several dozen faces. After several iterations of selective subdivision sequences and parametric smoothing based on the relative position of the vertices within the slab, the articulated final surface is defined with around 13 million mesh faces.

3D Printing

3D printing is used for the most resource-intensive process in concrete construction: fabricating the formwork. Beyond economic benefits, 3D printing enables several types of geometric features which are a significant challenge for other fabrication methods. Undercuts, sharp inner edges and micro-structures are difficult to achieve with CNC milling or hot-wire-cutting. Therefore, for the formwork of the Smart Slab, different 3D printing technologies were used to efficiently take advantage of their unique capabilities. Binder jetting was used for the most part, while fused filament deposition was used for locally integrating building services within the slab.

Supplementary, laser-cut plywood formwork panels were integrated to define the geometry of the upstand ribs. This is because the upstand ribs only contain flat surfaces which do not require the high-resolution of 3D printing.

Integration

This extensive and tolerance-free fabrication freedom for concrete has further applications, beyond the new and radically expressive aesthetic. It enables the precise integration of the complete suite of building services and structural features necessary for a working building. The formwork facilitates the accurate provision of functional voids within the slab for electrical conduits, water ducts, light fittings, fire sprinklers and rebar form ties, as well as for the accurate spatial curving of the post-tensioning ducts. The integration of building services during prefabrication streamlines assembly on site and considerably reduces construction tolerances.

These results have implications beyond the Smart Slab itself, showcasing how the material strength of concrete can be advantageously combined with the geometric freedom of 3D printing in a new construction method for free-form load-bearing elements.