Bitumen technology: three ways to more sustainable roads

Strong performance for Cargill’s bio-based rejuvenator

A parallel testing programme, with pavements in the North and South of the US, aims to encourage more widespread use of RAP. Cargill, which produces bio-based rejuvenator Anova, wants to provide DOTs countrywide with the data and confidence they need to increase recycled content in their pavements.

Two sections of test pavement containing 45% RAP and the Anova rejuvenator were laid in 2018, one on the test track at NCAT, the National Centre for Asphalt Technology at Auburn University for Virginia DOT, the other on the Minnesota Department of Transportation test track (MnRoad) for Minnesota DOT.

“It has become apparent that the agencies need some infrastructure to allow them to depart from their recipe-based specifications,” said Hassan Tabatabaee, global technical manager for asphalt at Cargill. “We looked around the county and saw that different agencies in different regions are trying in local pockets to work some of this out. We found there was certainly a need to provide unified, credible information that can tie this stuff together for those agencies to be able to support their work.”

Initial results from NCAT after around 18 months and 5.5 million equivalent single axle loads (ESALS) show that the section surfaced with asphalt containing Anova and 45% RAP have shown 0percent cracking, lower loss of roughness compared to the control section and the same level of rutting as the control section.

Anova is based on chemically modified vegetable oil. Cargill developed it around five years ago and has brought it to the market in North America, Europe, and South America. According to Tabatabaee, the rejuvenator has been used in millions of tonnes of hot mix asphalt. However, in the US, its use has been limited to smaller roads, private roads and parking lots where specifications are more flexible.

“There is a big distinction between what is being used in the private market and what is allowed and used by agencies and public roads,” said Tabatabaee. “The volumes have really come about from being able to provide technology and reliability and savings to contractors, but when it comes to the public sector, the process for change is a lot slower, and the requirements are quite different.”

Cargill estimates that the savings from being able to use 45% RAP equate to between 50 Cents and $1.50/ton, which amounts to $750,000/year for a medium-sized contractor. Lower capital cost attractive is an important part of the sustainability jigsaw, said Tabatabaee: “Beyond the fact that we are recycling, we are enabling that recycling with an additive that comes from a renewable agricultural source, we are enhancing performance and we are saving cost. It’s one of those rare instances where there’s not a lot of compromise.”

The use of the rejuvenator also improves workability and compactability. “Our contractor on the MnRoad project, even with a 45-minute haul at zero degrees, managed to get his density bonus,” says Tabatabaee.

In Minnesota, the test section includes the 45% RAP and the Anova rejuvenator in both the base course and the wearing course. Laid in 2018, the section will be monitored for three years. “After the monitoring period, the pavement will stay in place as long as it is performing,” said Susan Listberger, global product manager for asphalt at Cargill.

At the NCAT track, only the wearing course is under test. This also contains 45% RAP, with the section to be tested for three years, in line with NCAT’s testing cycles.

A huge array of tests will be carried out on the mixes used on both test facilities. “NCAT is running a full battery of tests, all the specification tests we know of,” said Tabatabaee. “It’s going to give a unique set of data which will tie different regional specifications together.”

Cargill chose to work with 45% RAP because this is the maximum proportion that can be used with a conventional continuous batching plant without making modifications. At this level, the RAP can be added cold with the virgin aggregate superheated.

At a continuous plant – the type most commonly used in the US – the Anova rejuvenator would be injected into the bitumen at the plant. For batch plants, which are more widely used in Europe, it can be added to the RAP on the belts, added to the virgin bitumen or injected into the mill itself.

Cargill works with the contractor and asphalt supplier to getting the dosing of the Anova right, taking samples of the RAP that will be used and conducting tests in its laboratory. Because RAP is, by its nature, variable, it is important to keep monitoring to ensure homogeneity of mixes, according to Tabatabaee:

“Contractors should not focus only on performance data and comparisons,” he said. “Performance is important, but you should not just be looking at buying the chemical, you should find a partner that can allow you to bring reliability to the system.”