"Crow Wing County showed a lot of leadership on this application," says Jonathan MacIver, business development manager, MeadWestvaco Asphalt Innovations.
"The county came to us and wanted more information about the technology," MacIver says. "We focused on the reduced binder oxidation properties of warm mix asphalt, determining whether Evotherm-treated binder without polymer modification can give similar performance to polymer modified mixes for thermal cracking control. We've given the county the support it needs to move forward, but the work has been done almost entirely at the county level."
Benefit in life cycle costs
The county did extensive research into how premature aging develops, and anticipates that over the life cycle of the pavement, the field tests will show the benefit of lower production temperatures.
"The initial rate of oxidation is less for WMA pavements than for HMA, which should slow down the binder aging process and extend the life of the pavement, so long as it's maintained properly," Dosh says. "Based on our analysis, the life cycle cost of a low-volume roadway like C.R. 2 constructed with warm mix PG 58-28 will be lower than a similar road constructed with hot mix PG 58-34, assuming the rate of aging is slower for WMA than HMA."
For the C.R. 2 project, initial cost of the WMA PG 58-28 was $3 per ton less than the HMA PG 58-34 binder, so the test already was ahead in initial cost. More data and time are required for long-term performance answers, but the initial indicators are positive.
Projections of life cycle costs for the WMA C.R. 108 project in 2008 yielded very favorable numbers. The life cycle analysis showed that WMA alternatives would cost less per mile than HMA, the main reason being the extended time to the first rehabilitative overlay. As Crow Wing County is responsible for 613 miles of roadway, the cost savings over the life of the roads using WMA would be some $1 million per year.
Lower fuel costs notwithstanding, the real financial benefit of WMA will come as the life cycle of a pavement plays out, notes the contractor and mix supplier on both the C.R. 2 project in 2009, and the C.R. 108 project in 2008.
"The workability of WMA is fine," says Terry McFarlin, senior vice president of operations, Anderson Brothers Construction Co., Brainerd, MN. "For the crews, with the reduced heat and odor, it's a better climate to work in. There is less fuel consumption at the plant, and warm mix gives the crews a bigger window to work in for rolling patterns. Otherwise, it's basically the same placement procedure as HMA, once you've found your rolling pattern."
At the plant, a small metering device pumps the Evotherm additive directly into the liquid AC line, so it's added as any other liquid additive. "The biggest difference is in the consumption of burner fuel," McFarlin says. "Last year we found a 20% to 23% reduction in fuel oil consumption, depending on the day's particular run. In cost, it's not enough to justify the price of the additive alone, but in the long term, with the lower temperatures of mixing, thus not burning off the light ends of the binder and causing premature aging, it will be a much better product than conventional HMA and that's where you will see the savings."
Increasing RAP content
The WMA test also permitted the county to boost the percentage of RAP used in the mix. RAP that had been removed from the pavement the year earlier was screened and crushed, and stockpiled during the winter prior to reuse in the warm mix.
"In Minnesota the type of binder determines the maximum RAP that can be used, and because we are using PG 58-28 oil - non-polymer modified - we can use up to 30% RAP," Dosh says. "If it were polymer modified we'd be restricted to 20% RAP. That was one of the benefits of using the warm mix additive, as we were able to bump up the RAP another 10%, with further savings."
Warm mix was being delivered by bottom-dump trailers from a portable plant minutes away, and placed by an Ingersoll-Rand/Blaw-Knox PF-3200 paver.