Old tyres and demolition rubble – the roads of the future

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RMIT researchers have shown how a blend of old tyres and building rubble can be used as a sustainable road-making material.

The zero-waste solution would boost recycling and support the circular economy.

Construction, renovation and demolition account for about half the waste produced annually worldwide and around one billion scrap tyres are generated globally each year.

The new material, developed by the RMIT University researchers, is the first to combine recycled rubble and rubber in a mix that is precisely optimised to meet road engineering safety standards. Designed for base layers, the blend is more flexible than current materials, making roads less prone to cracking.

“Traditional road bases are made of unsustainable virgin materials – quarried rock and natural sand,” says lead researcher Dr Mohammad Boroujeni. “Our blended material is a 100 percent recycled alternative that offers a new way to reuse tyre and building waste, while performing strongly on key criteria like flexibility, strength and permanent deformation.

“As we push towards a circular economy that can eliminate waste and support the continual use of resources, our recycled blend is the right choice for better roads and a better environment.”

Only 16 percent of Australia’s scrap tyres are domestically recycled. About 3.15 million tonnes of processed building rubble – known as recycled concrete aggregate (RCA) – is added to stockpiles each year rather than being reused. In 2019, federal and state governments agreed to ban the export of certain waste materials, with the aim of building Australia’s capacity to generate high value recycled commodities and associated demand. Whole used tyres will be banned from export by December 2021.

RCA can potentially be used on its own for road base layers, but adding recycled rubber can significantly enhance the finished product.

In previous research, the RMIT School of Engineering team has shown its rubble-rubber blend performs well when tested for stress, acid and water resistance. Its low shrinkage and good flexibility reduce the risk of cracking. The new study, published in Construction and Building Materials, looks at how the mix would withstand the pressures of being driven over by countless vehicles over its lifetime.

Researchers used special machinery to assess the blended material’s performance under frictional force, or shear stress, and compared different types of crumb rubber (fine and coarse) mixed into the RCA at different ratios.

The team identified an optimal mixture – 0.5 percent fine crumb rubber to 99.5 percent RCA – that delivered on shear strength while maintaining good cohesion between the two materials.

Chief investigator Professor Jie Li says while the recycling of construction waste and scrap tyres is growing, both industries continue to produce significantly more waste than is currently reused. “Solutions to our waste problems not only will come from reducing how much goes to landfill and increasing how much we recycle; developing new and innovative uses for our recycled materials is absolutely vital.”

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