Auxetic Formwork

Researchers at the University of Exeter are using grids with unique properties to create structurally efficient concrete domes

Imagine a garden trellis. Pull the ends apart to stretch it, and the grid’s squares will turn to diamonds, making the trellis longer and thinner. This seems natural: after all, if you stretch dough or melted mozzarella, it too becomes thinner.

But not everything becomes slimmer when stretched. Replace the squares of the trellis with a grid formed from tessellated bow-tie shapes, for example, and something unexpected happens. The shape thickens when stretched, and becomes thinner when compressed. Grids or meshes with geometries that exhibit this effect are described as “auxetic”.

“Of course compression is a key part of how concrete works in structures,” says Dr Raffaele Vinai at the University of Exeter. “So, a few years ago, we started to think about whether we could use the extra confinement coming from auxetics to make concrete structures that are stronger, or more efficient in terms of material and carbon usage.”

A project funded by the UK Engineering and Physical Science Research Council gave the research team – made up of Dr Vinai, Dr Prakash Kripakaran and Prof Ken Evans at the University of Exeter, and Prof John Orr at Cambridge University – the opportunity to explore these concepts.

Vinai’s first experiments concerned embedded auxetic reinforcement grids: “But we found that concrete is too brittle to allow the grid to move sufficiently to have an auxetic effect. Interestingly, though, we found that if we used Miscrete [a building material developed by Dr Vinai and his colleagues using miscanthus fibres and lime-based binder] its extra squeezability meant that auxetic reinforcement doubled the load-bearing capacity of bow-
tie-shaped blocks compared with standard prismatic blocks.”

But bio-fibre blocks, like Miscrete or Hempcrete, are not structural materials, and are used mainly as an eco-friendly form of insulation. “So we looked at other ways we could use auxetics,” says Vinai, “particularly in formwork.” In addition to becoming thinner when compressed, auxetic grids have another unique feature: “If you bend an auxetic grid one way, say north-south, the grid will automatically curve in all other directions. So a
flat square becomes a perfect dome.

Domes are structurally very efficient, but are seldom constructed because they are very labour-intensive. Making formwork for concrete domes is challenging, but because auxetic grids bend synclastically – identically in all directions at once – they can form them easily. Our goal now is to use auxetics to develop deployable supporting structures for domes.”

Vinai’s team have already proved it can be done using an auxetic steel grid that had been water-cut from a solid steel sheet. But this is inefficient in terms of wasted steel. So their first full-scale steel grid is made with thousands of individual components that can be assembled like Meccano, resulting in cells about 150mm long. “At 25mm thick, and 2m square, we think it might be the world’s largest-ever auxetic grid.”

The team carried out a major test at FP McCann’s R&D facility in Knockloughrim, Northern Ireland, where they deformed the 120kg grid by hanging 800kg worth of concrete slabs from it. “The grid bent into the expected shallow dome, although we observed that the joint design and load repartition need further development to fully deliver the concept we envisage in a structurally sound way.”

What can you do with a 2m2 dome? “Domes work under static compression, so can support far more load per tonne than a rectangular slab or beam, and could ideally work without steel reinforcement. This significantly reduces the material needed. We use prismatic beams and slabs because they are cheap to produce, not because they are efficient.”

Domes could be used structurally for material-efficient flooring or roofing systems, or naturally self-shading facade panels, he suggests. “But there are many possibilities.” Auxetic grid formwork, says Vinai, could do more than create simple dome shapes. The team is now working on the prototype of a complex-shape casting bed: “By connecting several servo motors to an auxetic grid, it could be deformed into all kinds of interesting shapes by pushing and pulling at various points. So one casting bed could produce infinitely variable curved shapes. Combine this with 3D printing, and the potential for automation is quite exciting.”

Designing complex, curving elements has become relatively straightforward, he adds. The hard part is making them. “Auxetics is still a fairly niche area of research – but we hope it could be the answer to making complex concrete shapes efficiently and economically.” 

Interview by Tony Whitehead

Photos Paul Burroughs