For many years there has been a goal of putting recycled rubber into concrete.

The aim is to make good use of a recycled product that has proven difficult to dispose of – and always will be a challenge for disposal, and add it to concrete to create a lighter end product without losing the characteristics of traditional concrete.

One of the challenges was that the rubber used was hydrophobic, so it did not readily mix – addressing that issue was critical to any project using tyre rubber. There were also issues with the end product in terms of strength and spalling when exposed to fire.

Now, engineers at RMIT University claim to have managed to replace 100% of conventional aggregates in concrete – such as gravel and crushed rock – with rubber from discarded tyres that meets building codes, promising a boost for the circular economy.

The team from RMIT University says the new greener and lighter concrete also promises to significantly reduce manufacturing and transportation costs.

Small amounts of rubber particles from tyres are already used to replace these concrete aggregates, but efforts to replace all of the aggregates with rubber have produced weak concretes that failed to meet the required standards – until now.

The study published in the Resources, Conservation & Recycling journal reveals a manufacturing process for lightweight structural concrete. The traditional coarse aggregates in the mix were entirely replaced by rubber from used car tyres.

Lead author and PhD researcher from RMIT University’s School of Engineering, Mohammad Momeen Ul Islam, said the findings debunked a popular theory on what could be achieved with recycled rubber particles in 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 that enhances the building material’s performance.” 

This advance builds on the breakthrough invention of this technique by fellow RMIT University Engineers Professor Yufei Wu, Dr Syed Kazmi, Dr Muhammad Munir and Shenzhen University’s Professor Yingwu Zhou. Several national phase patent applications are now filed to continue the protection of this technology.

Study co-author and team leader, Professor Jie Li, said this manufacturing process would unlock environmental and economic benefits.

“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,” he said.

Islam said that the team’s manufacturing process could be scaled up cost-effectively within a precast concrete industrial setting in Australia and overseas.

Following successful testing in the workshop, the team is now looking into reinforcing the concrete to see how it can work in structural elements.

The RMIT research team also includes Professor Yu-Fei Wu, Dr Rajeev Roychand and Dr Mohammad Saberian.