Crumb rubber: a legacy investigation

Main Roads WA is working with the Australian Road Research Board and Tyre Stewardship Australia to validate the use of recycled asphalt pavement that contains crumb rubber. Roads & Infrastructure reports.

In Western Australia, crumb rubber consumption has doubled in the past two years, from between 800-1000 tonnes to nearly 2000 tonnes. A collaborative project with Main Roads WA and the Australian Road Research Board (ARRB), supported by Tyre Stewardship Australia (TSA), has been undertaken to investigate the recyclability of crumb rubber asphalt.

Alongside the movement to use a variety of recycled materials in asphalt, road base and spray seal, the road construction industry has been increasing its capability to recycle asphalt itself.

As asphalt is 100 per cent recyclable, contractors are increasingly opting to mill out roads, save the materials and reuse them which is a material well known as Reclaimed Asphalt Pavement (RAP).

However, one of the major unanswered questions in the industry is; can these mixes containing recycled products then be recycled as RAP when the pavements reach their end of life?

When Zia Rice, Senior Professional Leader at ARRB, sat down to investigate literature on this topic she found there wasn’t much evidence to answer this question.

“The initial conception of this work began on an Australian Asphalt Pavement Association (AAPA) study tour in South Africa that was sponsored by TSA. Crumb rubber has been used widely in asphalt and spay seals in South Africa for many years, however, it seemed the country had small issues with recycling crumb rubber asphalt due to its sticky properties,” she says.

“When we got back to Australia, where crumb rubber being used in asphalt is relatively new, we wanted to find out what the barriers might be when turning crumb rubber asphalt into RAP.”

The project was part of the Western Australia Road Research and Innovation Program (WARRIP), involving Main Roads and ARRB and was supported by TSA through additional funding to allow a deeper dive into this crumb rubber question.

“To start the project, we had feedback from the study tour but we needed some more information. To get this we undertook a literature review and that was when we found there hadn’t been much work done yet in this area globally,” Rice says.

“The second step was to plan and undertake a practicality study which would help us to identify key steps industry would need to take from reclaiming the asphalt, processing it, mixing it and repaving.”

ARRB and Main Roads worked with industry partners Fulton Hogan and Asphalt Recyclers Australia to perform those steps in a controlled environment in order to identify any issues, while at the same time performing additional laboratory testing.

“When we make asphalt mixes that contain RAP if we are at percentages greater than 20 per cent, we need to characterise the RAP viscosity so we can tailor the new mix to a target viscosity. So, we did a laboratory investigation to see how the RAP characterisation was affected by the presence of crumb rubber alongside the practicality testing,” Rice explains.

First, the team recovered 10 tonnes of crumb rubber RAP and re-incorporated 10 per cent into a new mix. There were not any issues during this stage when claiming, processing, mixing and repaving the RAP.

“We decided to put the mix through a batch plant which is what we would consider to be the least suitable type of plant for sticky RAP material. We didn’t see any issues with the batch plant so we didn’t think it necessary to test it out in a drum plant, we just looked at increased tonnages,” she says.

For the second stage the team looked at 20 tonnes of the crumb rubber asphalt and at this stage the team did see some issues with build up during processing which could reduce efficiencies.

“Despite the slight build up during processing of larger tonnages, we could see this is something that could be easily rectified by industry if they understand that when they are going through high volumes of this mix, they might need to tailor their plans,” Rice says.

The laboratory investigation also returned interesting results.

“We found it difficult to characterise the viscosity of a crumb rubber RAP binder due to the presence of the rubber particles. Through our current methods it’s hard to get a representative answer so further investigation is needed there,” Rice says.

“This could facilitate a new design process for level two and three RAP mixes with high percentages, perhaps we will look at an alternative performance indicator rather than target viscosity.”

From here, the next steps will be to investigate how to design new recycled asphalt with crumb rubber and the importance of RAP traceability and separation.

“I think this initial project has been really good. It provides industry and asset owners the confidence of the adoption in this new technology at the beginning, without compromising performance or perpetual recyclability issues,” Rice says.

“At this stage I can’t see the collection and processing efforts from crumb rubber RAP to be very different from conventional RAP.”

Eventually industry guidance will need to be created which could be a new specification for crumb rubber RAP or the mention of crumb rubber in the current specifications for RAP.

Liam O’Keefe, TSA’s Senior Strategy Manager, says road manufacturing is a major focus for the organisation with TSA funded infrastructure projects set to consume around 10,262 tonnes of tyre derived products yearly, which is estimated to add an additional $6 million in sales to Australian tyre processors.

“This project is an example of the great work happening in WA with Main Roads through both policy and industry. We have seen a real transformation in the WA market, and they are working to increase the use of crumb rubber exponentially,” he says.

TSA’s mission is to increase the consumption of crumb rubber across all sectors in Australia. O’Keefe says the team were aware of the perennial questions across the sector that inhibit some products from being sold, such as the lifetime legacy of crumb rubber in asphalt.

“TSA applauds the work Main Roads and ARRB are doing, undertaking this project for the benefit of the whole road construction sector. It’s helping to overcome an issue that is sector wide and it gives confidence to all users that environmental outcomes in the short term don’t mitigate those in the long term,” he says.

Rice says industry could already see performance benefits from modifying binders with crumb rubber and hopes it will encourage the sector to use crumb rubber more going forward.

“We’ve seen the whole of life cycle performance benefits in many projects and it’s just great for us to be able to really push the use of a product where we know we are not going to have any legacy issues down the track,” she says.

TSA will look to work with organisations that will progress this work to its future stages and work with industry leaders to increase the use of crumb rubber across the country.

“It’s not just about increasing use, but initiating on-going high demand in the roads sector will enable the local recycling market to invest in equipment and produce more highly refined products cost effectively for all markets,” O’Keefe says.

Throughout 2019/20 TSA approved 11 new recycled rubber projects. The successful delivery of these projects is expected to create market demand for the Australian resource recovery industry of 16,118 tonnes yearly.

The organisation is excited to work on these projects and more with industry partners throughout the 2020/21 financial year.

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