Technology Update

Austroads refines foamed bitumen designs

Austroads trial shows how foamed bitumen can be a viable option for major Victorian roads.

Austroads trial shows how foamed bitumen can be a viable option for major Victorian roads.The irony of degrading current roads as a consequence of fixing up sections hasn’t escaped Matthew Kovess, Stabilisation Manager at Downer Australia.

In a country as vast as Australia, Mr. Kovess says it’s a sad reality that local roads are damaged from the transport of crushed rocks from quarries, and asphalt from plants, to road maintenance sites.

“The less we can transport materials, the better,” he says. “Just think about the carbon output of quarrying and transporting rock. It’s a wasteful process, where our roads already contain high quality crushed rock. The more we can reuse of these current materials, the better.”

Fortunately, the practice of recycling current materials in the maintenance of our roads is growing, and the latest joint project initiated and funded by Austroads, with funding provided by AustStab and VicRoads, is helping promote that growth, by confirming the failure mechanism of the pavement. In Australia the failure mechanism is believed to be in fatigue.

The use of foamed bitumen in Australia dates back to trials in the 1960s and 1970s, via rudimentary, agricultural equipment. As technology improved in the 1990s, foamed bitumen became a more feasible project on Australian roads. Over the past five years, Kovess explains that road authorities in Queensland and Western Australia have stabilised many roads using the foamed bitumen process, appreciating the advantages over laying down new bitumen.

“A lot of it comes down to the availability of suitable materials and budgets. It’s not a cheap solution, however it’s certainly a lower cost one compared to some alternatives,” he explains. “If you want flexible pavement, especially in an area that’s prone to flooding, it’s a great option. It’s also quite handy to recycle existing materials.”

Kovess says the practice is likely more popular in Queensland and Western Australia, because there are more areas with only remote access to an asphalt plant. This makes recycling local materials an even more attractive option.

To help encourage Victorians to use foamed bitumen on its major highways, the joint project is looking to learn more about how to improve the process, by a rather counter-intuitive method – by setting up the trials for failure.

“The trials are being under-designed so that we can asses the results of how exactly they failed,” explains Kovess. “From there, we can compare our expectations and what happens on site to our lab results, and make improvements from there.”

The first trial that was purposefully under-designed was at the Calder Highway in 2013. The two key failure modes the team was looking at were fatigue and rutting.

It took not even a week for the team to learn a quick lesson from that trial. They hadn’t brought in any additional materials. Both the shape of the road and the ride were poor, and without any additional materials, they weren’t able to fill in or shape out the road and get it level. Just a week after the treatment, the first 10 metres of the road took a huge hit.

Another lesson learnt from that trial was the need to remove existing asphalt patches. This lesson was one taken from laboratory trials following the first on-site trial. The Downer laboratory tested materials sampled from a future site with different levels of recycled asphalt, ranging from 20 to 40 per cent. During this process, they compared different mix levels used in Queensland and Victoria/NSW to see which mix would work out best. They found that the ideal mix was 3 per cent bitumen, 1.5 per cent hydrated lime, and the lowest end of recycled asphalt at 20 per cent.

With these lessons in tow, VicRoads nominated a second trial site on the Western Freeway at Ballan, approximately 78 kilometres northwest of Melbourne, the main freight route between Adelaide and Melbourne.

This time, the team brought in imported crushed rock, that they could level out at 20 to 30 millimetres over the areas. Before applying the rock, they removed existing asphalt patches, so that they could better control the asphalt mix to the ideal requirements tested in the laboratory.

The team was then ready to start the foamed bitumen treatment process. The first was to apply an initial treatment of hydrated lime, which was supplied by Sibelco and spread by Downer at 1.5 per cent, and mixed with Stabilised Pavements of Australia’s stabiliser. The lime works as a secondary binder, helping the bitumen to coat the finer particles and provide early strength. The team then lightly compacted the area to leave a level surface for the bitumen tanker.

At 190 degrees, the tanker with the bitumen and foaming agent was hooked up to the stabiliser. The stabiliser formed a train over 30 metres long, drawing the bitumen out of the tanker and into the mixing chamber, adding cold water and air to foam to over 20 times its original size.

The bitumen combines with the crushed rock and hydrated lime to create a flexible, high strength and water resistant mix. An 18-tonne pad foot roller then compacted the area, and a grader trimmed it, followed by compaction with a smooth drum. A multi tyre roller shaped and levelled the area.

The pavement cured overnight, gaining strength and drying back. A watercart from Stabilime lightly watered the pavement, and the area received a final trim to level, then the pavement was ready to seal. VicRoads inspected the site, where the team completed a final brooming and prep work, then applied a 14/7 seal onto the foamed bitumen pavement.

Although generally the project went to plan, Mr. Kovess says a few interruptions made for some long days. He says they could have planned for more time to allow on-site product sampling and visits.

The trial period is expected to be complete within three years. Mr. Kovess says they certainly consider the methodology a success, as they were able to improve the process over the Calder Highway trial. They’ll now be checking for fatigue or rutting to see how these failures occur, and use those lessons for future trials and more extended applications.

With this success under their belt, Mr. Kovess says he expects to see an increase in the use of this foamed bitumen process in the years to come.

“We’re showing that it’s suitable for patching and isolated treatments, even on major highways,” he says. “Asphalt just isn’t always readily available, and this is a cost-effective solution. We’re not seeing much more money invested into road maintenance, so we need to come up with better treatment alternatives like this one.”

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