Zigzags For Greener Construction

Concrete is the most widely used construction material in the world. Made from cement, water, sand and gravel, this relatively cheap mixture can be shaped as needed and withstand high forces in compression. Yet it performs poorly under tensile stress, requiring steel reinforcement, and excessive amounts of material are often used. This fuels climate change, since producing concrete and steel generates significant carbon emissions.

Research by ETH Professor Philippe Block's team has consistently shown that it is possible to achieve structural stability while using less concrete and steel. One of the Block Research Group's (BRG) innovations is vaulted floor elements made of concrete . Through clever geometry, these floors achieve much thinner sections than their conventional counterparts and do not require any embedded steel reinforcement. This technology is now being commercialized through the ETH-spinoff external page VAULTED AG.

Considering sustainability at the first production step

Vaulted floors need a formwork system - moulds that give the poured concrete its delicate structure. However, these moulds can be bulky and are often made from petroleum-based materials like Styrofoam. Moreover, formwork for non-standard geometry is typically single-use, and its manufacturing process creates substantial waste. "Unfortunately, this cancels out some of the sustainability gains," explains Lotte Scheder-Bieschin. The doctoral student at the Block's research group has developed a foldable formwork system that can be reused and requires fewer resources to produce.

Massively reducing concrete and steel use

Unfold Form consists of thin, flexible plywood strips that are connected by textile hinges and can be unfolded like a fan. Four of these compact units can be rapidly assembled within a wooden frame to create a sturdy, zigzag shaped mould onto which concrete can be poured directly.

After the concrete cures, the formwork can be easily detached from underneath, folded away and stored for its next use. While the system used for the prototype weighs just 24 kilogrammes, it is can support up to a tonne of concrete.

"I was looking for a solution that would allow me to use strength through geometry not only to optimise the final structure but also the formwork itself," says Scheder-Bieschin. "This approach reduces material usage and makes the entire process more environmentally friendly." The formwork's distinctive geometric structure allows for reductions of up to 60 percent in concrete and 90 percent in reinforcement steel.

"The Unfold Form formwork can be produced and assembled without specialised knowledge or high-tech equipment," notes Scheder-Bieschin. One of her aims was to create a simple and robust system that could be used worldwide, even with limited resources. Currently, formwork for non-standard concrete shapes typically requires digital fabrication. "This creates barriers for sustainable concrete construction in developing countries, where the need for new buildings is especially high," she says.

The formwork can be produced cheaply. "The only things needed in addition to the materials are a template for the shape and a stapler," adds Scheder-Bieschin. The materials for the prototype cost only 650 Swiss francs in total.

The individual components are light and compact enough to transport easily. Scheder-Bieschin demonstrated the system's simplicity by assembling it herself during her pregnancy. "I wanted to ensure my design was simple enough for anyone to build, regardless of their circumstances," she says.

Zigzag ridges like seashells

How does this innovative formwork achieve both lightness and stability? During development, Scheder-Bieschin applied her expertise in bending-active structures - a topic she had worked on during her studies. This technique involves bending elastic materials like thin and long splines or plates of wood, where the resulting deformation creates stability and allows for curved, lightweight structures.

A key feature of Unfold Form is the zigzag-like arrangement of wooden strips. "This ribbing provides additional stiffness without significantly increasing the overall weight," explains Scheder-Bieschin. "You can find such articulated structures in nature, like in seashells."

The zigzag pattern strengthens both the formwork and the concrete poured onto it: "The concrete incorporates this design into a structural rib pattern, which assist in load transfer."

Strength through curvature

The interaction between the individual strips is crucial for the formwork's stability, Scheder-Bieschin explains: "When you bend a single strip or plate, it becomes very wobbly when loaded, and it's difficult to control which shape the board will bend into." However, when you connect two strips along a curved edge, you achieve much higher rigidity. "Under load, the strips deform minimally, and you can control the final shape via the design of these connection curves," she says. This technique, called curved-crease folding (CCF), has been around for some time and draws its inspiration from the art of origami.

Folding in general consistently progresses from larger to smaller sizes, beginning with a flat element that is gradually reduced through the folding process. This makes the folding technique unsuitable for use in construction. "Consider a vaulted concrete floor measuring two by three metres - the starting plate would need to be approximately three by five metres. From a transport perspective, this is of course highly impractical," Scheder-Bieschin points out.

From paper model to concrete prototype

Scheder-Bieschin was intrigued by the challenge of adapting this simple but ingenious CCF system for architectural purposes. She experimented with paper models at her desk, eventually devising a system that she calls curved-crease unfolding: "At some point, I started gluing the pieces together differently. And that's how I found a system that starts out as a stacked shape that can be fanned out - just like a hand fan. And at the same time, the curved shape is achieved."

The next challenge was to move from a thin sheet of paper to a structural material of a certain thickness. The doctoral student solved this tricky problem with the help of textile hinges.

She then developed a computer method for the simulation. "The initial prototypes already validated my concept," she recalls. "Using simple 2D prefabrication, I could create folded, compact panels that unfold easily and have the required rigidity to support concrete."

Real-life test in South Africa

In addition to the final 3-by-1.8-metre prototype, which is located in the Robotic Fabrication Laboratory (RFL) on the Hönggerberg campus, a twin concrete structure also exists in South Africa. It was built using the same formwork. Mark Hellrich, a scientific assistant and contributor to the Unfold Form project, transported the folded formwork to Cape Town using two surfboard bags. Working together with external page nonCrete , a local firm dedicated to sustainable construction and affordable housing solutions, they cast the second prototype.

This demonstrated three things at once: The formwork system can be reused without any loss of quality, it is easy to transport, and it works with different types of concrete. NonCrete used its bio-concrete based on shredded invasive vegetation from the area. "This shows that premium-grade concrete isn't necessary for creating sturdy floors with the new formwork," Scheder-Bieschin notes.

She says that the South African partner company was impressed by the results: "The goal is to use this innovative formwork system to construct high-quality, dignified and sustainable housing in South African townships."

Helping people help themselves

After completing her doctorate in a few months, the researcher will continue developing her technique as a postdoc at ETH Zurich, with plans to bring her product to market. Currently, the 33-year-old is working on the design for a market hall in a Cape Town township using her formwork system. The next step, however, is primarily about helping people help themselves: "We are planning training programs for the locals so they can build the formwork and the buildings on their own."