Even though many states use fine-graded hot mix asphalt for their Interstate paving projects. Illinois defers.
For 10 years Timothy Murphy, president of Murphy Pavement Technology, has pounded the –er, pavement — trying to chip away at that reluctance. He believes Illinois and its municipalities stand to improve road construction and lower costs by allowing fine-graded HMA into their bid specifications.
A number of northern municipal entities have acceded, including the Village of Cary, Kendall County, Skokie, Wilmette, Oak Brook and Coal City.
“Overall resistance to it seems to be moving that direction,” noted Paul Yerkes, lab manager at Chicago Testing Laboratory.
What might tip the scales is a U.S. Department of Transportation case study on fine-graded thin asphalt overlays that was authorized in 2009. The findings of that study are scheduled to be released later this year.
The U.S. DOT’s accelerated loading facility in Virginia is investigating fine-graded thin asphalt overlays with specifications for Superpave 4.75 mm in nominal maximum aggregate-size mixes for fatigue and rutting. The expectation is that state agencies will be able to specify and select more durable asphalt binders.
Fine-graded asphalt has a lower permeability, greater workability and durability, smoother texture, a nice dark appearance and allows thinner lifts than coarse-graded mix, which conversely increases macro texture, allows thicker lifts and tends to exhibit a greater resistance to rutting, according to the Asphalt Institute. Yerkes notes that fine graded also allows center lifts, which can address cost issues when working on a one-inch overlay.
The number of severely deteriorated Illinois interstate roads prompted the creation of a Bituminous Task Force on Stability and Durability in 1984. That task force quickly recommended the use of washed, manufactured F20 with 4 percent voids, void and mineral aggregate and 75-blow. To save money, however, some cities and state contracts eventually switched to F21 Stone Matrix, which is washed.
Yerkes noted that SMA mixtures are used primarily in high traffic areas, including the Illinois Tollway mainline pavements. In a community setting, he continued, SMA can be used for high-traffic intersections or in area that have a large volume of truck traffic. Because it can be an expensive mix, he recommends that the application warrant such a mixture.
Dense fine-graded asphalt is a cost effective choice for streets with moderate to high traffic loading and allows a one inch top size mix that lasts 15-20 years in Interstate applications.
Recycled asphalt products are naturally fine and can be added to the menu with this mixture. That’s important to green cities. Half of the states in the country increased their use of RAP between 2007 and 2009, and research led by the FHWA has shown that up to 20 percent RAP can be used in HMA mixtures without having to change the asphalt binder.
For Interstate applications of finegraded HMA, Murphy recommends the following specs: N50 gyrations, optimum asphalt content at air voids of 35 percent, maximize RAP and PG528-28 oil to avoid cracking, use a binder of 40 percent minimum on a No. 4 sieve and a surface mix of 40 percent minimum on a No. 8 sieve. This setup will require chips and F20. Additionally, he said, consider closing longitudinal joints nightly, require the use of auger extensions and enforce the longitudinal joint density, quality control or quality assurance and the field VMA — the “leader of the band,” as he refers to it.
To address concerns about friction and to meet friction standards, Yerkes notes that many different types of sand can be used. Dirty, crushed sand doesn’t do as well as other aggregates, though, Murphy said. He predicts that in the near future quarries will start washing F21 to turn it into F20.