Crumb rubber modified (CRM) binder is a material made by reducing scrap tyres or other discarded rubber into bitumen, which can then be used in asphalt or sprayed seals for roads. Traditionally, CRM is used for sealing roads, mainly in Victoria and New South Wales, and has limited use in asphalt. However, investigations into the possible applications of CRM on a larger scale are underway. Here, Erik Denneman, Director Technology and Leadership at the Australian Asphalt Pavement Association (AAPA), discusses the latest CRM asphalt trial in Queensland and the benefits it can provide the road industry.
What is Crumb rubber modified (CRM)?
Approximately 70 per cent of the weight of a standard tyre is made up of rubber and carbon black. Crumb rubber is produced by reducing scrap tyres down to these basic materials and removing steel and fiber along with any other contaminants such as dust, glass or rock. The modification of crumb rubber, or CRM binder, improves the properties of bitumen so that it is less prone to bleeding, cracking and stone loss.
CRM binder has enhanced elastic properties, which results in more durable asphalt and sprayed seal road surfacing than those currently in use today. It also offers a much improved cracking resistance compared to asphalt and seals with unmodified bitumen.
Mr Denneman said investigations into the use of CRM binder for asphalt on a large scale in Australia has encountered some barriers that have so far prevented wider adoption of the technology after initial activities in the 1990s.
In order to further analyse the use and practicality of CRM binder, Mr Denneman was part of the team that organised a trial of the use of CRM binder in open grade asphalt in Queensland.
Open grade asphalt trial
The CRM binder trial was constructed in 2017 and aimed to analyse and overcome any perceived barriers while monitoring both harmful emissions during the installation and continuing to monitor its overall performance.
Mr Denneman said open graded asphalt has important safety benefits in terms of reducing splash and spray in wet weather.
“An issue with open graded asphalt is that it doesn’t last as long as dense asphalt mixes. The open structure means it is more prone to oxidative aging. The use of crumb rubber modified binder increases the durability of open graded asphalt because it allows a thicker binder coating.”
Mr Denneman said that there were no difficulties with the installation compared to the conventional product that was placed as a control.
“The section is continuing to be monitored and a second section has been installed in the Gold Coast in January 2018,” Mr Denneman said.
The question of emissions
One of the biggest barriers preventing the large-scale uptake of CRM is the perception that its use may produce harmful emissions during installation.
“The AAPA will continue to advocate for further studies into harmful emissions from crumb rubber construction. Extensive studies have been done internationally, but we have an obligation to verify the outcomes in Australia,” Mr Denneman said.
“The results show that for this particular trial the emissions from crumb rubber were similar to that of asphalt with conventional binder.
“The trial also showed the benefits of producing the product at as low a temperature as possible. Odour, fuming and emissions from asphalt increase rapidly with increase in temperature, so in general there is a benefit in trying to reduce production temperatures.
“This has possible sustainability benefits as well in terms of reducing CO2 emissions.”
The barriers for CRM uptake
Mr Denneman said other challenges for CRM include cost and technological changes.
“Some bitumen rubber products are more expensive than conventional products due to the higher binder content. This may be offset by improved durability, but needs to be taken into consideration in the procurement process by road agencies,” Mr Denneman said.
“There are technological challenges as well, but these are easily overcome as demand grows. For instance, from a technical and operational perspective it may be preferable to blend crumb rubber and binder in a blending unit at an asphalt plant, rather than transporting it pre-blended from elsewhere.
“Such blending units are available on the international market, but obviously industry needs to be certain of demand before it invests in such technology.”
The future for CRM in Australia
The pathway to CRM becoming the standard solution for road asphalt and spray seal surfacing will depend on road agencies, the industry and the public allowing space to propose and trial application of the technology.
“I don’t see a reason why it would not become the standard solution. Asphalt as a 100 per cent reusable construction product is already a good news story in terms of sustainability. The combination with recycled tyre rubber just adds to that appeal,” Mr Denneman said.
“There is a whole plethora of technologies available for use of crumb rubber in bituminous products. It is therefore important that industry and road agencies work together on developing permissive specifications that allow different technologies to