The team used fibres extracted from the textile reinforcement commonly embedded into tyres Adding these fibres to the concrete mix was shown to reduce the concrete’s tendency to spall – where surface layers of concrete break off – explosively under the intense heat from a fire.
The fibres melt under the intense heat from a fire, leaving networks of tiny channels. This means that moisture trapped within the concrete is able to escape, rather than becoming trapped, which causes the concrete to break out explosively.
“Because the fibres are so small, they don’t affect the strength or the stiffness of the concrete,” said lead author of a paper on the research in the journal Fire Technology Dr Shan-Shan Huang. “Their only job is to melt when heat becomes intense. Concrete is a brittle material, so will break out relatively easily without having these fibres help reducing the pressure within the concrete.”
Protecting the concrete from fire spalling means that steel reinforcements running through the concrete are also protected. When the steel reinforcements are exposed to extreme heat they weaken very quickly, meaning a structure is much more likely to collapse. The Liverpool Waterfront Car Park suffered this kind of damage during a fire in 2017, leading to the entire structure eventually having to be demolished.
The group also collaborated with Sheffield firm Twincon to develop a method for reclaiming the fibres from the used tyres. This involved separating the fibres from the tyre rubber, untangling the fibres into strands, and then distributing them evenly into the concrete mixture.
Using man-made polypropylene (PP) fibres to protect concrete structures from damage or collapse if a fire breaks out is a relatively well-known technique but the Sheffield study is the first to show that these fibres do not have to be made from raw materials, but can instead be reclaimed from used tyres.
“We’ve shown that these recycled fibres do an equivalent job to ‘virgin’ PP fibres which require lots of energy and resources to produce,” explained Dr Huang. “Using waste materials in this way is less expensive, and better for the planet.”
The team now plans to continue testing the material with different ratios of the fibres to concrete, and different types of concrete. It also plans to find out more about how the materials react to heat at the microstructure level by scanning the concrete as it is heated.