“Counties, for example, have the same range of treatments available to them that you would see states using,” Varnedoe points out. “There is a lot of opportunity to do preservation on secondary roads. Chip seals are very common on the secondary system, and generally are well accepted by the public, while being a cost-effective way of preserving a pavement. In North Carolina, probably more than half the secondary system was originally constructed with chip seal over base as the original driving surface.”
In 2009, a double-chip seal preservation treatment in Cherokee County, Ala., was made possible by use of a rapid setting asphalt emulsion that saved time between applications, and expedited the return of traffic to the county road.
Gadsen, Ala.-based paving contractor Charles E. Watts Inc., received permission from the county highway department to use CHFRS-2P, a cationic, high-float rapid-setting emulsion modified with latex, on its contract to double chip seal a section of County Route 29. This was the first-time use of the product in an Alabama county.
A double-chip seal treatment consists of spraying a pavement surface with asphalt emulsion, covering this with a layer of stone, and repeating the process but using the same emulsion at a different application rate, and smaller stone. Compaction by rollers forces the smaller stone to interlock with the larger.
Throughout the years, this process has proven to be the best surface maintenance treatment for their roads, according to county engineer Corey Chambers, who heads up the department’s staff of 30 personnel. “We get more roads done for the dollar with double chip seal than we do with 1 1/2- to 2-inch hot-mix asphalt, about three times the length,” Chambers said, adding the treatment adds 10 years or more of pavement life.
New materials mean help for counties
Like the technique of double-chip seals, innovative new materials can help counties solve secondary road problems. For example, in 2009, a Minnesota county was using warm-mix asphalt to fight destructive asphalt thermal, or low-temperature, cracking caused by fierce winter weather.
In bituminous pavements, thermal or low-temperature cracking is non-load associated and appears as transverse shrinkage cracks in the asphalt layer. As frigid ambient temperatures chill the surface, they contribute to tensile stresses in the pavement due to material shrinkage. If these stresses exceed the fracture strength of the asphalt pavement layer, transverse cracking develops.
To fight thermal cracking, the county hopes Evotherm warm-mix asphalt (WMA) additive from MWV Specialty Chemicals will get from a lower-cost PG 58-28 binder the same cold weather performance of the more expensive polymer modified PG 58-34 binder specified in Minnesota for newly constructed low-volume bituminous roadways.
But it also hopes to use the WMA additive to improve binder durability by reducing the premature or “artificial” aging to binder caused by exposure to the scorching heat of the asphalt plant burner.
Last summer, Crow Wing County, Minn., was in the second year of testing of warm-mix asphalt to ameliorate thermal cracking and improve binder durability. Following a test project in late 2008, testing in 2009 included placement of two layers of bituminous mix containing a warm-mix additive, and 30 percent reclaimed asphalt pavement, on County Road 2.
“One of the hoped-for benefits of warm-mix asphalt is greater durability over time,” says Wayne Dosh, senior engineering technician, Crow Wing County Highway Department. “One of the issues we struggle with in Minnesota is thermal cracking, It’s usually the biggest killer of our roads. One of the ways to get around it is by using ‘softer’ grades of binder. The performance-grade binders used are based on our ambient temperature extremes, and with that kind of spread, it usually implies polymer modified asphalt. By using a warm-mix additive we hope to be able to get the same performance with a non-modified oil as with a modified.”
But premature aging of binder also is an issue that the county hopes to minimize with warm-mix asphalt. “One of the benefits of warm-mix over hot mix is that in the hot-mix plant, the heat contributes to premature or artificial aging of the binder, which breaks down the oils,” Dosh says. “This gets even worse as the mix ages in-place over time. One of the benefits we’re hoping from the warm-mix additive for is for us to be able to use cheaper binder — PG 58-28, that doesn’t require a polymer modifier — with the added benefit of less premature aging in addition to less thermal cracking.”