The holes in the concrete absorb the lower frequencies of traffic noise. And when plants grow in them, they can also filter out higher frequencies. | Photo: Ice Formwork

Imagine an acoustic concrete made up of cavities that not only absorbs urban noise, but also contributes to biodiversity by providing a habitat for small plant and animal species! This is the project of Vasily Sitnikov, a postdoctoral student at ETH Zurich. To bear this out, he pours concrete around an ice mould. As the ice melts, it leaves a hollow structure in the concrete.

Sitnikov has just launched the start-up Ice Formwork to develop the technique. He is currently testing its commercial potential with private companies in the construction, urban planning and environmental protection sectors.

It was in Sweden, at the KTH Royal Institute of Technology, that he first had the idea of using ice to create a hollow structure in concrete. Conventional processes did not lend themselves well to his project. Injecting gas into liquid concrete creates cavities that are too small to absorb low- and medium-frequency noise, which is particularly typical of urban environments. 3D printing, meanwhile, is not suitable for mass production.

“At the time, some of my colleagues were experimenting with synthetic wax to create cavities in concrete”, says Sitnikov. “I wondered whether we could use something more environmentally friendly than these petrochemical materials, and I thought of water. It was a bit of a mad gamble”.

Reducing the solidification temperature

The main challenge was to obtain a material capable of hardening below water’s freezing point. Conventional concretes solidify at a minimum of five degrees Celsius. At this temperature, the temporary ice structure would melt before the material had time to harden. Sitnikov therefore had to develop a series of chemical additives that would allow the concrete to harden at temperatures between -10°C and -5°C.

To modulate the acoustic properties of the material, he then played on the size of the hollows obtained in the final concrete. He targeted medium- to low-frequency sound waves, between 250 Hz and 4 kHz. “This is typically the range of frequencies produced by car traffic, such as the noise of a tyre on the road”. Measuring one metre square and 20 centimetres deep, the panels are acoustically tested with a set of microphones that record how the material absorbs and reflects sound waves.

Support for biodiversity

As this concrete does not filter out higher frequency sounds, which are still audible to humans, Sitnikov went on to realise that the hollows in the material could be used to grow vegetation, like a vertical garden. Indeed, one specific property of greenery is that it absorbs this type of noise.

In doing so, he was to discover a second, complementary and completely separate application for his concrete: a space for biodiversity. As well as plants, small animals could also thrive in the hollows of the material. It is an interesting property in cities, and even more so in marine ecosystems. “We are also working with biologists on a concept for artificial reefs”, he says, upbeat.

Although the process is primarily intended to fill a niche, its inventor insists that its industrial potential is real. It can be deployed on a large scale, in factories and even directly on building sites – provided there is enough space to set up a refrigerated container, says Sitnikov. “All you have to do is deploy equipment designed for refrigerated transport and install the production lines there”.