RECLAIMING AUSTRALIAN
ASPHALT PAVEMENT
ROADS & INFRASTRUCTURE EXPLORES THE USE OF RAP IN AUSTRALIA AND SPEAKS TO INDUSTRY EXPERTS AND CONTRACTORS ABOUT THE FACTORS CONTRIBUTING TO ITS LIMITATIONS AND THE POSSIBILITY FOR ITS INCREASED USE IN THE FUTURE.
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ne of the unique things about asphalt pavements when compared to other construction materials is the potential for the end product to be completely recycled. As the effects of climate change and social responsibility continue to influence the industrial sector across the world, the road construction industry has been investigating how it can better process its asphalt materials. For example, Japan uses an average of around 40 per cent reclaimed asphalt pavement (RAP) in asphalt mixes. RAP not only reduces the pressure on virgin aggregates but in some cases alleviates material carting requirements, all while relieving the industry’s reliance on bitumen products. Jurisdictions around the country have varying specifications for the percentages of RAP that can be used in mix designs. While upkeeping high quality standards, many contractors and road authorities are undertaking trials with increased levels of RAP to help increase standard limits. These trials and demonstrations are hoped to signal the industry’s ability to produce high RAP mixes at least to the quality and standard of virgin asphalt. WA TRIALS INCREASED RAP PERCENTAGES As part of the Western Australian Road Research and Innovation Program, Main Roads WA and the Australian Road Research Board (ARRB) are working to update WA’s engineering guidelines and specifications to increase the use of RAP in full-depth asphalt pavements. Currently the limit for use of RAP in WA sits at 10 per cent, and anything over must be assessed by Main Roads. Main Roads WA wants more RAP in asphalt through implementation of the Austroads processes to ensure reliable end products. Austroads has developed processes in the test method AGPT/T193 to measure RAP 24
ROADS MARCH 2020
properties, including the viscosity of the RAP binder. This determines the blended binder viscosity for manufacturing asphalt with high RAP contents. The design inputs, such as RAP quantity and virgin binder viscosity, can be varied to achieve a target viscosity for the blended bitumen. Steven Middleton, ARRB’s Senior Professional Leader, is working with Main Roads to implement the use of the processes in the AGPT/T193 test method. Two asphalt suppliers have developed intermediate course asphalt mixes with higher RAP that were placed on two road construction projects in 2019. Mr. Middleton says it is important to understand the Austroads methods and what is required to implement them in order to use increased percentages of RAP. “At this stage there is some variability of the RAP that we are finding in WA and we need to get a better handle of the properties of the RAP being used,” he says. “If a small fluctuation in the properties of the RAP occurs, and the more you increase the RAP content, the more it will impact the properties of the asphalt produced. Therefore, adequate management of the RAP is required to successfully manage this variability.” The findings of this work are expected by the end of this financial year. The new engineering guideline for using higher amounts of RAP on Main Roads projects is likely to be finalised in mid 2020. It will have three levels of RAP mixes. Level one will allow up to 10 per cent, as per the current specification. Level two will allow 11 to 25 per cent RAP content, requiring a separate mix design. Level three will allow 26 to 40 per cent RAP content, requiring a separate mix design and infrastructure that can warm the RAP before it is mixed in to produce asphalt. “The major benefits of RAP include reduced used of virgin material because we can reuse
aggregate and binder in the RAP in the new asphalt mix,” Mr. Middleton says. To successfully incorporate RAP, Mr. Middleton says there are certain considerations that need to be addressed to ensure the asphalt performs as if it were good as new. “As the RAP content increases, the oxidised binder contained in the RAP needs to be addressed. This oxidised binder has a relatively high viscosity and therefore by blending it with a virgin binder that has lower viscosity, the target viscosity is required. This follows the process described in AGPT/T193,” Mr. Middleton says. An increase in the stiffness of an asphalt mix as a result of the unaccounted for aged binder in the RAP can result in the finished road fatiguing quicker and being subject to earlier repairs. The WA work is hoped to inform the methods needed to account for the increase in RAP and, consequently, pavement performance. Main Roads WA is looking exclusively at the use of RAP in the intermediate course layers of asphalt. Mr. Middleton says the intermediate layers are not as exposed to the elements, so this was the right place to start. “The work is currently not investigating a higher RAP content in the wearing course but if it was accounted for in the right way, there isn’t really any reason why it couldn’t be included in any layer,” he says. However, Mr. Middleton recognises the risk increases in the top layer as it receives the most oxidation by being exposed to the elements, as well as often containing polymer modified binders which add further complexities. “When using RAP, it is crushed and screened it to make sure it complies with the grading of the new asphalt and does not contain conglomerates of aggregate held together by the binder,” Mr. Middleton says. The viscosity of the binder in each portion of RAP can also differ due to its age or the
