A recent tech talk at RMIT in Melbourne discussed the state-of-play for the use of tyre-derived crumb rubber in asphalt for roads in Australia.

The use of crumb rubber from end-of-life tyres for Australian roads is nothing new – it has been used in Victoria since 1975.

Originally used in sprayed seals, the viability of crumb rubber as a component of asphalt has been an area of increasing interest – from both sustainability and performance perspectives.

For the uninitiated, asphalt typically comprises the aggregate – a mix of rock, sand, and other granular elements; and the binder – the sticky bitumen that holds it together. The coarseness of the aggregate and the recipe of the mix can be adjusted based on a range of variables, including the location and application of the road.

Crumb rubber can be added to asphalt in either ‘dry’ process – crumb rubber particles incorporated as part of the aggregate during the mix, or ‘wet’ – crumb rubber particles added to the binder prior to the mix.

Regardless of the process, studies have demonstrated that crumb rubber-modified asphalt can yield roads that are more durable, quieter, and safer.

The Australian Flexible Pavement Association (AfPA) recently hosted a tech talk at RMIT in Melbourne, outlining the state-of-play of crumb rubber in asphalt mixes in Australia.

Sponsored by Tyre Stewardship Australia (TSA), along with bitumen suppliers Puma Bitumen and Boral Australia, the talk featured speakers involved in tyre recycling, ongoing research, national specifications, and practical trials.

Crumb rubber in Australia

Lina Goodman, TSA Chief Executive Officer, used Victorian data to provide a snapshot of where Australian end-of-life tyres are currently ending up.

According to TSA figures, Victoria generates on average an estimated 120,000 tonnes of end-of-life tyres each year. Of that, 71 per cent is recovered and processed for reuse.

The catch is, 80 per cent of that material – 85,000 tonnes worth – is exported, meaning just 20 per cent makes it back into the local market.

“We really need to focus our attention on utilising this recovery rate to our benefit,” Lina said. “It’s being collected, it’s being processed. Now we need the market to be able to consume the material.”

One of the main challenges for the TSA is fostering increased demand for products containing tyre-derived material, such as crumb rubber.

Lina presented preliminary research by the TSA on the greenhouse gas emissions associated with the use of tyre-derived materials. This included the emissions profile of crumb rubber use in asphalt mixes, and the significant reductions it can offer.

Early figures indicate that crumb rubber used as 15 per cent of an asphalt binder in a wet process could yield a 10 per cent reduction in emissions compared with traditional polymer-modified bitumen mixes.

This jumped to 12 per cent when used as 27 per cent of an asphalt binder in a dry process, compared with conventional asphalt.

“This is based on a life-per-life basis,” Lina said. “But we know crumb rubber content in roads does increase the life of the roads. So, if you consider an increased service life, the emissions reductions improve exponentially.”

These emissions reductions could amount to 40 and 41 per cent respectively if a three-year service life increase is factored in.

Anna D’Angelo, AfPA Executive Director Technology and Leadership, offered a deeper dive into the process of recovering crumb rubber from tyres.

AfPA is involved in multiple projects to generate further knowledge around crumb rubber use. These include a national study with AustRoads and the TSA into sourcing crumb rubber from passenger tyres as well as other non-truck tyres, such as those from the mining and agriculture sectors.

The second, in partnership with the Southern Sydney Regional Organisation of Councils (SSROC) and the TSA, is a multi-council demonstration project testing the performance of various crumb rubber asphalt mixes across various sites.

Erik Denneman, Puma Bitumen Global Technical Manager, followed up with an outline of the AfPA crumb rubber asphalt specification, which came about as a collaboration between industry and road agencies.

The specification is based on international best practice, particularly that used in the United States. One important element of the AfPA specification is the mandated maximum mix temperature to keep emissions and fuming down.

“If you use a warm mix, you reduce the temperature,” Erik said. “For every 12 degrees you reduce the temperature, you halve the emissions.”

Crumb rubber in action

Carl Topp, Boral Asphalt Technical Manager, summarised the company’s experience with trialling crumb rubber modified asphalt on the Sunshine Coast in Queensland in mid-2021 – a project which used the equivalent of 22,000 passenger tyres.

He said that Transport and Main Roads Queensland (TMR), which specified the project, was looking for something with better performance than dense-graded asphalt, and with the benefits of recycled material.

As well as some technical details of the project’s mix design, Carl outlined the overall logistical challenges Boral faced in adapting its plants and fleet for the production and transport of the modified asphalt. Boral also monitored the reduced emissions offered by use of the warm crumb rubber mix – something Carl said was of interest to TMR.

Feedback from the workers on the project was positive, with the product’s significant reduction in fumes improving comfort on the job.

John Esnouf, of VicRoads, outlined a crumb rubber asphalt trial on East Boundary Road in East Bentleigh, which ran from March 2020.

These trials focused on long-term durability in roads modified with crumb rubber, with test areas featuring experimental and control sections to measure the effectiveness of different processes.

“We decided we’d give industry a chance to innovate – with freedom from our specifications,” John said.

“A real job where a whole lot of different mixtures could be trialled. The design, the production, and the placement were all up to the individual companies.”

Testing was conducted throughout the project, including monitoring for emissions, with overall positive results.

Local and global research

Filippo Giustozzi, Associate Professor – Roads, Railways and Airports at RMIT University, said while there has been extensive research into road durability from a traffic wear-and-tear perspective, very little attention has been given to the ageing effects of sun damage on roads.

Comparing ultraviolet radiation levels across the globe, he noted the relatively high dosage Australia is subjected to, and how that can accelerate the degradation of the nation’s roads.

His team’s research involved using RMIT’s UV machines to simulate long-term UV exposure to bitumen samples containing different crumb rubber ratios, revealing its highly beneficial “sunscreen” effect.

Closing out the tech talk, United Kingdom-based Daru Widyatmoko, Technical Director/Pavement Materials Research Team Leader at AECOM, outlined the UK’s experience with crumb rubber in asphalt.

In a pre-recorded presentation, Daru said the urgency for pursuing sustainable road paving solutions was crucial, but that a balance must be struck between sustainability and performance.