“The basic components are always the same,” says Hansen. “We just add or subtract equipment.”
- The MARS process begins with two or more custom-made, propane-fueled preheaters.
- The next heater in the train is equipped with grade-controlled milling drums which windrow the top 1 inch of material. The milling heads are capable of milling 15 feet wide and 1 inch deep.
- The tunnel heaters begin heating the underlying pavement while maintaining the temperature of the windrow.
- The surface is then milled and heated by up to three more milling heaters followed by tunnel heaters depending on material and depth.
- The last milling heater in the process has an oil metering system that injects and mixes rejuvenating agent, which is a water-based emulsion that replaces the chemical constituents of the asphalt that has oxidized. The rejuvenating agent contains a polymer-modified asphalt which further improves elasticity and coating. This unit performs the last phase in the MARS process.
- The windrow – after having rejuvenating agent added and mixed – is picked up with a conventional elevator.
- The paving process is performed with a conventional electronic grade control, electric-screed paver.
- After paving, the material is rolled using conventional rollers.
The finished surface can be opened to traffic after a cool-off period similar to an asphalt paving operation, says Hansen.
Mix designs & surface layers
The type of mix designed for the project depends on the climate, traffic, environment and what type of material is already on the road, says Hansen.
“If there is a good, quality material on the road already, then that material can be recycled and made into good material again,” he says. “If the road has some rutting because the initial mix was too fine, then we can add a coarse aggregate to stabilize the road.
“We do mix designs on every single project based on the situation,” says Hansen. “The mix itself can be modified by the addition of new hot mix or aggregates.”
Dustrol recommends some sort of riding surface be placed on top of the HIR surface. “Most states place some sort of finished riding surface, such as a chip seal, micro surfacing, thin hot mix overlay, UBAS or an open-graded friction course. There’s a multitude of thin surface treatments that can be placed on top of the HIR when we’re done.”
HIR gaining in popularity through education
Hansen says he is seeing a slight uptick in agencies using HIR as a viable preservation method, but the process is slow.
“There’s a lot of educating going on,” he explains. “We work in multiple states, and it’s a long process to educate the engineers on what we’re doing and to get them to try a project. Usually it’s a multi-year process. They don’t just come out and look at a project once and decide they’re going to bid a dozen projects.
“We like to show them what we’re doing,” he continues. “When we show them, it helps them to see they’re going to get a good, quality material on the road and we can save them some money. Many engineers are leery about spending money on a new, unknown process.
“So it takes time and effort to convince these engineers that we have a good, quality product,” Hansen says. “We spend a lot of time and money marketing to different state agencies to get them to take a look at what we’re doing.”
Hansen says the quality work the MARS process creates is the primary reason agencies usually decide to try the process. He also notes the high cost of liquid asphalt makes it more economic for agencies to look at options other than just placing hot mix.
“The combination of being able to do quality work along with the continued high price of asphalt is making more agencies take a look at the MARS option,” says Hansen.