Reclaiming Australian asphalt pavement

Two asphalt suppliers have developed intermediate course asphalt mixes with higher RAP that were placed on two road construction projects in WA, in 2019.

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. 

One 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 up-keeping 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 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 amount of oxidisation that has occurred.

“Generally speaking, older pieces of RAP will most likely be more viscous as they will have oxidised more, but it depends on many factors such as how much bitumen, what type of bitumen, air voids, whether it is from the wearing course and so on,” Mr. Middleton says.

While there are examples of overseas practices in places such as Japan, this research is tailored to Australian circumstances.

The National Asphalt Pavement Association of the US found one possible reason for this average on a tour to Japan. In a report the association states that given the relatively small land area of the Japanese islands, with limited raw materials and even less space for waste disposal, conservation of natural resources and minimising waste are ingrained in the culture and society and designated in legislation.

“I think based on the amount of asphalt we do have in WA where the majority of roads are granular pavements with thin surfacings, the percentages that we are testing are appropriate for how much RAP currently is available,” Mr. Middleton says.

He says asphalt plants also require modifications if higher quantities of RAP need to be produced.

“For increased quantities generally, you have to add the RAP into the mixing process in different locations. This is mainly to ensure adequate heating and drying of the RAP.

“In WA currently, the majority of plants are able to incorporate 10 to 30 per cent RAP. However, some companies are looking into increasing their capacity,” Mr. Middleton says.

Mr. Middleton’s partner at Main Roads WA Les Marchant, Manager of Materials Engineering, agrees.

He says Main Roads WA’s 10 per cent limit was adopted due to research that shows generally there is no need to account for any material change when using only 10 per cent.

“One of the big reasons behind the current 10 per cent limit is that WA hasn’t had the same availability of RAP seen in other states. This is because we have only started to switch from granular to full depth asphalt pavements over the last 10 years or so,” Mr. Marchant says.

“There was a bit of contention as to whether the base number should be 10 to 15 per cent as it is in other states, but we based our decision on Austroads and other research.”

Mr. Marchant says that many in the industry have only been using three to four per cent RAP in recent years as field crews find the asphalt harder to work with. Main Roads WA has focused on increasing RAP to 10 to 25 per cent as part of the Western Australian Road Research and Innovation Program project, balancing supply and demand.

Following further workshops with the Australian Asphalt Pavement Association (AAPA) and industry partners, Main Roads WA is looking at rolling out an increased percentage for consistent ongoing use of RAP.

“Some limitations we see in WA are the high variability of RAP properties and availability of RAP. I would rather see RAP used continuously around the 20 per cent mark, rather than have 40 per cent used in big projects which then leaves none for further works,” Mr. Marchant says.

He says that Main Roads WA’s focus for now is on increasing the use of RAP in intermediate course mixes. Nevertheless, he does see the state moving towards its inclusion in wearing course mixes, but that will be two or three years away.

As the levels of RAP currently vary across the country, Mr. Marchant says Main Roads WA is supportive of harmonised specifications. However, he says the harmonisation needs to be flexible enough to cater to states individual needs.

“We are following the Austroads guidelines and research as our base documents, which is what we should all be harmonising to,” Mr. Marchant says.

He says Main Roads WA is happy with the results to date and plans to achieve increased use of RAP through its implementation of Austroads processes.

ASPHALT PLANT TECHNOLOGY

As the amount of RAP increases in a mix design, contractors have to look out for a range of factors. They have to look out for the binder’s viscosity, in addition to the quality and moisture content of the aggregate and RAP. To add another layer of complexity, they need to ensure the technology in their asphalt plants can handle the different material.

Paul Vandersluis, Managing Director of Ammann Australia, says once you begin to add more than 30 per cent RAP content into mix designs, additional plant technology is needed.

“You have to do gentle heating with RAP material once it goes above the 30 per cent mark, so anything from 30 to 100 per cent needs to have special heating technologies,” Mr. Vandersluis says.

A standard plant has one dryer for the virgin asphalt, and he says most plants have the capability to introduce cold RAP material up to around 30 percent.

“Anything beyond that normally involves a second dryer dedicated to the RAP materials. Then depending on which processing technology is used, that limit can extend to 60 or even 100 per cent,” Mr. Vandersluis says.

“It is a big investment to upgrade a plant to deal with high RAP levels as it essentially becomes two plants with two separate drying systems that feed into one mixer.”

Another important consideration is the documenting process for creating high RAP content asphalt mix designs.

“When dealing with RAP, temperature and moisture content is critical to the process, so an asphalt plant needs to have an advanced control system with the capability to monitor each part of the process to ensure consistency,” Mr. Vandersluis says.

“The data from that technology is what gives contractors and authorities more faith in the process because they can see they are getting a consistent project of high quality.”

Ammann is an asphalt plant manufacturer with plants situated across the globe.

Some Ammann asphalt plants in Europe are producing asphalt of up to 100 per cent RAP and Mr. Vandersluis says over there, if you aren’t using RAP, you aren’t in the game.

He says that performance-based specifications in Europe have allowed for more innovation.

“Europe has more performance-based mix designs where the contractor is responsible for the road.

“This way, some authorities don’t mind if it has five or 100 per cent RAP content as long as it performs, but in Australia currently it is more legislated,” Mr. Vandersluis says.

He says he has seen some of the larger contractors in Australia already making the investment to purchase plants with the second drying process and high recycling technology.

“They are investing into a plant that is going to be used for the next 30 years.

“We are pretty certain that as the resources for fossil fuels deplete and the cost of bitumen is greater, instead of putting RAP in landfill, we will reuse it and reclaim as much bitumen as we can because it is a precious resource.”

Fulton Hogan supports higher RAP contents that are currently specified in circumstances where the stiffness of high RAP mixes would be a benefit. Image credit Fulton Hogan.

WORKING TO STATE SPECIFICATIONS

Fulton Hogan is a major contractor in Australia that uses RAP across the majority of its asphalt mixes nationally.

National Technical Manager Bevan Sullivan says the maximum amount of the RAP in asphalt designs that the company uses for state road authority works is dictated by their content limits.

“Many local government associations follow state road authority guidelines for the mixes used on their networks. This is generally an upper limit of 30 to 40 per cent for base course mixes and around 20 per cent for wearing course mixes,” Mr. Sullivan says.

The company has also developed a nearly 100 per cent reclaimed asphalt pavement product called RAPBASE. This is a bitumen-treated granular base material which uses a proprietary anionic bitumen emulsion as a binder, designed for use in base course and subbase layers.

“The RAPBASE material is designed to function as either a bitumen-treated base or modified unbound granular layer by controlling the material’s stiffness and strength to specified ranges,” Mr. Sullivan says.

Fulton Hogan has used RAPBASE with a number of local government associations.

The company supports higher RAP contents that are currently specified in circumstances where the stiffness of high RAP mixes would be a benefit. However, it also recommends caution with wearing course layers of asphalt pavement.

“The use of RAP at higher percentages in wearing course and in fatigue layers of pavements is not recommended without methods such as rejuvenation and/or blending with softer grade binders,” Mr. Sullivan says.

“That being said, it is viable, provided RAP supplies exist to design and produce higher RAP content mixes for both wearing and fatigue layers by using additional design and engineering controls.”

As the company incorporates RAP as a regular component of asphalt mixes, unless specifications prohibit its use, Fulton Hogan supports the idea to harmonise specifications across states.

“Harmonising RAP design and management across all states and local government areas would allow industry to invest and develop procedures for greater design and control mixes with higher proportions of RAP, ensuring the product is best designed and suited for its intended purpose,” Mr. Sullivan says.

HARMONISATION

Across Australia, the limits on the amount of RAP allowed in asphalt pavements without specialised mix designs varies between different states and territories.

This difference means that where contractors might be able to use up to 15 per cent RAP content in one state, another may only allow up to 10 per cent.

AAPA is the key industry body representing the flexible pavements industry in Australia. It has long called for harmonised specifications across all areas of asphalt production.

Anna D’Angelo, AAPA’s Executive Director of Technology, says the harmonisation of testing, delivery and specifications for any type of asphalt is paramount.

“What we currently have is each state or territory with their own tweaks and modifications to specifications. The consequence of this is a costly overhead for companies that have to design one product on one side of the border and a different product on the other side,” Ms. D’Angelo says.

Ms. D’Angelo says that even when people design plant equipment overseas and bring it into Australia they have to make small tweaks for different jurisdictions.

Every two years AAPA travels on international knowledge transfers to different countries to learn about their asphalt practices.

Ms. D’Angelo says they have seen harmonisation in other countries done successfully.

Ms. D’Angelo says Australia is a relatively small market for asphalt on the international stage.

“AAPA travelled to America where they demonstrated that in each state, they used very similar asphalt specifications. They produce 350 million tonnes every year compared to Australia, which produces around 10 million tonnes,” Ms. D’Angelo says.

“When AAPA went to America in 2010, they were using 10 per cent RAP, and on the next International Knowledge Transfer in 2014, [the Americans] had increased the limit to 15 per cent. When we asked them why, they simply put it down to more experience with the material across the country.”

In 2018, AAPA published its Reclaimed Asphalt Pavement Management Plan which details its recommendations for the use of RAP across the country.

AAPA’s recommendation for Australia is that using 15 per cent of RAP is acceptable, as this level does not require any significant change in the mix design.

Above 15 per cent RAP, some measures need to be taken to correct the stiffening effect of the RAP binder on the viscosity of the total binder blend. This could be the addition of a softer binder or the use of a rejuvenator agent to control the viscosity.

“What we are recommending across all the states is that they accept the principle of a grade bump, where you drop the bitumen grade down to allow the use of more RAP without having any negative or detrimental properties in the mix,” Ms. D’Angelo says.

To ensure the performance of an asphalt mix containing RAP meets that of a virgin mix, AAPA suggests control and accurate testing of the reclaimed asphalt is required.

“Once it has been taken off the road, it needs to be kept in a stockpile with an asphalt mix that has similar properties. The stockpile needs to be kept dry before the material is fractionated,” Ms. D’Angelo says.

“We then recommend testing each stockpile to understand its properties and fully comprehend what bitumen needs to be added to the mix.”

The National Centre for Asphalt Technology in the US told AAPA during their visit that they found less variability in some RAP stockpiles than with raw materials.

“They said because you are picking up material that has already been screened and put on the road. When you dig the road up, the material is in a very narrow band,” Ms. D’Angelo says.

Recovering binder properties in RAP is crucial as this is the best indicator of which virgin binder is needed to balance the mix.

“A test to determine the viscosity of the binder is one of the tests that we have included in the AAPA proficiency testing. It is important to understand the reliability of the mix in the lab before applying it in the field,” Ms. D’Angelo says.

In AAPA’s plan, RAP tests are outlined that allow producers to characterise the material and then decide what grade of bitumen to add.

“If the other state road authorities pick up this procedure and apply that against their material, they should be able to make similar conclusions for the RAP they have got,” Ms. D’Angelo says.

The Reclaimed Asphalt Pavement Management Plan was put together with the intention of harmonising practices across borders for Australian asphalt containing RAP.

Downer’s RAP processing facility from above.

PRACTICAL USE OF RAP

In early 2019, the City of Adelaide, in partnership with contractor Downer, resurfaced a road with completely recycled material.

Chatham Street in the city’s southwest was chosen as the site for the recycled surface made up of nearly 100 per cent RAP and recycled vegetable oil.

Downer’s General Manager – Pavements Stuart Billing says the council had a goal to be a carbon-neutral city.

Mr. Billing says this gave Downer the opportunity create a recycled pavement design that emulated the South Australian Government specification for virgin asphalt.

Downer used vegetable oil as a thinner material to restore the viscosity of the binder in the RAP.

The RAP used on Chatham Street was sourced from various local projects and Downer performed extensive testing to understand the grading, particles, aggregate sizes and bitumen content.

Mr. Billing said substantial work was undertaken by the team at Downer to understand how much rejuvenation the 100 per cent recycled mix required.

“We have been working to understand different bitumen rejuvenating products for the past six to seven years. Ultimately, we found that using vegetable oil is one of the better products and gives us the best performance outcome when combining it with the RAP binder,” Mr. Billing says.

“The basis of our design in terms of the performance of the asphalt was still centred on the standard Department of Planning, Transport and Infrastructure specification.”

The Downer team inspected the pavement in early February 2020, around one year after it was laid, and concluded the pavement was performing to standard.

Mr. Billing says the product is not at the stage where it could be used for every project continually, but it’s a fantastic demonstration that a quality product can be produced with high recycled material content.

“As we go forward, the more confidence our road authorities and customers get with using high-recycled products, the more we will see those specifications continually move and evolve, not just with RAP.”

Mr. Billing says it’s important for the industry to invest in the technology to ensure it is able to produce high-quality recycled content.

Downer asphalt plants in Adelaide and Melbourne have a dedicated RAP heater to successfully heat the RAP before it is combined with traditional asphalt. The company is also in the process of constructing another plant with RAP capabilities in Brisbane.

“Downer and the rest of the industry have a responsibility to demonstrate these capabilities to lead the way in a responsible manner with robust processes, so that we have the confidence that quality outcomes will be achieved,” Mr. Billing says.

THE FUTURE OF RAP

As the industry continues to work with and demonstrate quality asphalt pavements using RAP, the specifications will adapt.

AAPA will keep working towards harmonisation between states and territories, which each jurisdiction progressing towards its own advancements.

Main Roads WA will begin to look at the use of RAP in polymer modified binder mixes, which is not currently allowed. ARRB will also persist with its work to characterise RAP and advance the industry’s knowledge of RAP’s performance through documentation.

Contractors such as Downer and Fulton Hogan will continue to pursue demonstrations, with usage of RAP across road projects, and invest in new technology to advance the practice.


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