As part of the Swiss National Centre of Competence in Research (NCCR) Digital Fabrication, researchers from the Block Research Group and the Physical Chemistry of Building Materials Group of ETH Zurich have joined forces to develop and test a novel knitted formwork system for building bespoke, doubly-curved geometries in concrete. Their first experiment resulted in a small-scale concrete bridge prototype.

The approach intelligently combines 3D-knitted technical textiles with special cement paste coatings to create a lightweight stay-in-place formwork for concrete. This combination promises to provide a material-saving, labour-reducing and cost-effective solution for the construction of complex concrete structures. Customised textiles informed by structural considerations – among other parameters – can be easily fabricated and used with great geometrical freedom.

Knitted textiles as waste-free, stay-in-place, self-supporting formwork

The forming system has been tested through a small-scale concrete bridge prototype, built using a custom-made, pre-stressed, knitted textile and bending-active hybrid structure that acted as a waste-free, stay-in-place, formwork which is also self-supporting as it is stiffened with the cement paste. Knitting makes it possible to fabricate 3D geometries with tailored material properties and integrated solutions for inserting additional elements, all in a single process. To have a low tensioning of the textile and also minimise deformations during casting without intermediate support from below, strength is built up by gradually applying thin layers of concrete.

First, a custom knit pattern is algorithmically generated based on the desired 3D geometry. This pattern is produced by a knitting machine. This textile is tensioned into the desired shape with glass fibre rods and cables. After tensioning a thin cement paste coating is applied. Once the textile is hardened, a mortar layer is sprayed onto it. This package can now be used as formwork for casting concrete in a traditional fashion.

Great advantages for complex and standard concrete elements

This textile-concrete hybrid results in an ultra-lightweight formwork that is easily transportable and significantly reduces the need for falsework, support and scaffolding, which also has many advantages on the construction site. In the specific case of the bridge prototype, the knitted textile itself weighs only 434 grams and is small enough that it could be transported to the construction site in one single backpack! While the approach was developed with doubly-curved geometries in mind, it can be easily applied to standard building elements. For such standard elements, it can additionally be used to create formworks such that concrete can be cast in a shape that is structurally optimised.

A customised Steiger Libra 3.130 industrial machine was delivered at the beginning of the month. The new setup allows the researchers to further apply the process for architectural scale building components using a fast, reliable, industrial process. Future explorations will focus on the tailoring of material properties, the production of spacer fabrics, rib-stiffeners, and the inclusion of inlaid elements as reinforcement or further functionalisation of the textile. The process will be further developed to an easily deployable, on-site system.

For detailed information about the novel concrete building system, you can consult the project website or read the published journal paper.

Image caption: NCCR PhD researchers Mariana Popescu and Lex Reiter jumping on the small-scale knitted bridge prototype.
Image credits: Block Research Group and Physical Chemistry of Building Materials Group, ETH Zurich
Video credits: Block Research Group and Physical Chemistry of Building Materials Group, ETH Zurich