Smooshing wet cement with old tyres can create better concrete

Concrete – the grey, unassuming rock which invades most parts of our lives is just not good for the planet so any new research which replaces it in buildings is good news.

A massive 30 billion tonnes of concrete is used each year worldwide, and at least eight per cent of human global emissions come out of the cement industry.

With traditional concrete being made up of cement or asphalt, sand, rock and water, researchers around the world are trying to find ways of limiting the amount of sand used as well as replacing the steel or gravel with waste products like plastic waste, recycled construction waste and even face masks.

A new study in the Resources, Conservation & Recycling journal has looked at how to replace coarse aggregates concrete  with old tyres.

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Concrete mixing using recycled tyre rubber particles for the complete replacement of traditional coarse aggregates. Credit: Mohammad Islam, RMIT

“Discarded tyres are a major concern in developed countries. Billions of tons of vehicle tyres are generated each year globally. The available treatment methods, such as stockpiling, landfilling and burning are not feasible options for this living planet,” says lead author and RMIT University engineer Mohammad Momeen Ul Islam.

Read more: How to create living concrete

“We need to find alternative concrete materials which should be sustainable and renewable. Therefore, replacing gravel, a natural coarse aggregate, with waste vehicle rubber can be a  solution to address environmental issues.”

This is not the first time researchers have looked to concrete to dump old tyres but because of rubber’s flexible properties, it can create concrete that’s not as strong and doesn’t bond as well, sometimes ending up with cracks.

However, the new research has been able to replace their coarse aggregates with waste rubber particles, by pre-compressing the rubber particles in fresh concrete.

The ‘preloading’ method increased the compressive strength by up to 97 per cent, as well as increasing the flexural and tensile strength compared to normal concrete.

“We have demonstrated with our precise casting method that this decades-old perceived limitation on using large amounts of coarse rubber particles in concrete can now be overcome,” Islam said.

“The technique involves using newly designed casting moulds to compress the coarse rubber aggregate in fresh concrete which enhances the building material’s performance.”

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The RMIT team’s new casting technique generates structural lightweight concrete from used tyre rubber. Credit: Mohammad Islam, RMIT

The researchers also found that the carbon emissions were up to 60 per cent lower compared to a normal concrete sample.

“As a major portion of typical concrete is coarse aggregate, replacing all of this with used tyre rubber can significantly reduce the consumption of natural resources and also address the major environmental challenge of what to do with used tyres,” says RMIT civil engineer Professor Jie Li.

“As we develop the structural lightweight concrete, the cost related to transportation and materials can be reduced. As this is precast concrete, the construction time can be saved and labour costs can be minimized,” Islam says. 

The team is currently looking at reinforcing the concrete to see how it would work in the real world.

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