Better Roads Staff
The elimination of fibers from the Parris Island mix was achieved by use of a warm-mix asphalt additive, Evotherm 3G from MWV Asphalt Innovations, which adds lubricity to individual microscopic asphalt particles, permitting production of asphalt at significantly lower temperatures than conventional mixes.
Evotherm stops the draindown of liquid asphalt by virtue of the lower mixing temperature it enables. “We were able to make the mix at 285 degrees F, instead of 350 degrees, which completely eliminates the problem of draindown,” says Dean Frailey, business development manager, MWV Asphalt Innovations. The mix was emerging from the truck at approximately 275 degrees and from the screed at about 245 degrees. No liquid asphalt was visible in the truck bed as mix was fed to a material transfer vehicle ahead of the paver.
CRM Binder in California
Crumb rubber-modified binder was used recently for an ultra-thin, open-graded wearing course in California’s Napa Valley. In the town of Napa, a 10-year-old asphalt overlay on the Silverado Trail was showing signs of wear over a 2.9-mile section. There was minor cracking, some raveling and a few areas required full-depth asphalt patching. But for the most part, the road was in good shape; it was structurally sound.
To preserve the road and extend its life, Caltrans turned to an ultra-thin bonded wearing course. In this process, a Roadtec SP-200 paver sprays a tack coat down just in front of the spreading augers, and the screed levels off a 3/4- to 1-inch-thick layer of open-graded hot mix.
On California Highway 50 between Placerville and South Lake Tahoe, an ultra-thin bonded wearing course lasted seven years, says Brian D. Toepfer, maintenance engineer, Caltrans. “I think it [performs] better than a mill-and-fill, and it is a lot less expensive.”
On the Silverado Trail, the Construction Division of Telfer Oil, Martinez, Calif., used the specially-equipped paver to spray down a heavily polymer-modified emulsion at a rate of 0.17 to 0.20 gallons per square yard. “The emulsion is similar to a PMCRS-2H emulsion, which is a standard chip sealing emulsion,” says Karl Meyers, general manager of Telfer’s Construction Division.
The paver immediately followed the emulsion with a 7/8-inch-thick layer of open-graded hot mix made with PG 64-16 liquid asphalt that was modified with crumb rubber. The target value for binder content was 8.5 percent, and the top-size aggregate in the mix was 3/8 inch. It also contained a small amount of sand.
Static compaction with two double-drum rollers followed the paver. “You have to run two rollers because you need to hit the temperature range on compaction, which is 180 to 280 degrees,” Meyers says. “That thin lift behind the screed is cooling fast, and you want to release the road to traffic quickly. Plus, the paver is moving at 70 to 100 feet per minute, so you need to run two rollers behind.”
The process has a number of advantages, Meyers says. “You are not getting any tack coat dragged around the city, you get an outstanding bond with the hot mix, you are forming a waterproof membrane, there is no water splash, and you can release the road quickly to traffic,” he says.
In the meantime, a specialized paver has given a Las Vegas asphalt contractor entry into the growing market of ultra-thin asphalt concrete surfacings in Nevada and throughout the Southwest.
However, to correctly place UTACS or other bonded wearing courses, the right kind of paver is needed: One that has the ability to spray asphalt emulsion onto a pavement, and then immediately place a thin overlay on top. Las Vegas Paving found the Super 1800-2 with optional SprayJet module from Vögele fit the bill.
With its new Super 1800-2 SprayJet paver, Las Vegas Paving now is able to undertake pavement preservation contracts as agencies like Clark County, Nev., use available funds to prolong the life of pavement structures in its desert locale.
Las Vegas Paving acquired its Super 1800-2 SprayJet in early 2010, and has been using it for UTACS ever since. “We were the new kids on the block with this process,” says Clark Webster, general superintendent. “2010 was our first year paving UTACS, and we were concerned that interest would not last, but agencies are still interested.”
Late in 2010, Las Vegas Paving was applying a UTACS to busy Jones Avenue between Tropicana Avenue and Russell Road for the Clark County Department of Public Works. This 2,700-ton job involved three lanes each way, including shoulders and turn pockets. “Jones Avenue is a piece of a larger contract we have with the county, with each piece being anywhere from 15,000 to 20,000 square yards of UTACS,” Webster says. “Jones was a 1-inch deep UTACS, with almost 100-percent passing 1/2-inch with some fines.”
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