Better Roads Staff
But what’s not happening today is an increase in deeper-lift OGFCs. While most states that use them place OGFCs as thin asphalt lifts at 3/4-inch depth or less, the Oregon DOT has been placing 3/4-inch open-graded mixes in structural layers of 2 inches or more for about 30 years, reports Dr. Steve Muench, et. al., at the University of Washington-Seattle, in a June 2011 report for the Oregon DOT. He adds the Washington State DOT has used similar mixes since the early 1990s, although more sparingly. Due in part to the damage done to OGFCs by studded tires used in Oregon, the report recommends that use of 3/4-inch OGFCs be discontinued in the state (see below).
OGFCs for Cold Weather?
Typically, OGFCs tend to be confined to states without severe winters, as it’s perceived that water trapped within the drainable layer can expand and cause the open-graded layer to ravel, or create dangerous icing, leading to accidents.
One cold weather state, Wisconsin, recently looked at OGFCs as used and discontinued by northern-tier states and provinces, and received a recommendation that it not proceed.
“OGFC has historically not been used in Wisconsin due to concerns about its performance in a climate with a large number of freeze-thaw cycles,” says Richard E. Root, P.E., Root Pavement Technology, in his 2009 Wisconsin Highway Research Program report, Investigation of the use of Open-Graded Friction Courses in Wisconsin. “Questions also exist about the cost/benefit of these mixtures.”
The study’s primary objectives were to determine if the OGFC mixture could be successfully used in the Wisconsin climate. After a literature review that listed many northern-tier states that had started “new generation” OGFCs, but had since discontinued them, Root concluded that Wisconsin should avoid them.
“While the use of OGFC mixtures in warm southern climates has been successful, this pavement has not proven to have the same successes in the northern freeze/thaw environment,” Root says. “None of the states or Canadian provinces with climates that duplicate Wisconsin’s use OGFC mixtures. On a routine basis, it is recommended that Wisconsin should not currently build pavements with an OGFC surface.”
This summer, Oregon had reason to abandon OGFCs altogether. In June, a technical report – Open-Graded Wearing Courses in the Pacific Northwest: Final Report by Stephen T. Muench, Ph.D.; Craig Weiland; Joshua Hatfield and Logan K. Wallace of the Department of Civil and Environmental Engineering, University of Washington – suggested that use of the 3/4-inch open-graded hot-mix asphalt (previously referred to as “F-Mix”) be curtailed.
“The best estimated service life of [Oregon] DOT 3/4-inch open-graded HMA ranges from 14 years (< 5,000 ADT) down to seven years (> 100,000 ADT), which is less than comparable dense-graded mixes,” Muench and researchers wrote in June. “The primary mode of distress is raveling and studded tire wear. Reduced service life, along with uncertain and unquantified safety benefits and a possible greater risk of early failure lead to a recommendation to discontinue use of 3/4-inch open-graded HMA in Oregon as a standard surface mix.”
Open-graded wearing courses used elsewhere in this country are not likely suited for use by Oregon DOT, due to their susceptibility to studded tire wear, they say, and the writers don’t recommend their adoption. “If 3/4-inch open-graded HMA does continue in use,” the writers say, “recommendations are [to] quantify its benefits, restrict its use to low-traffic routes (< 30,000 ADT), recalibrate the state’s pavement management system’s expected life to be more in line with observed historical life, and require the use of a windrow pick-up machine or end-dump transfer machine when paving OGFCs.”
Download the new research at http://ntl.bts.gov/lib/41000/41300/41329/SPR680.pdf
Binder Modification Key
Open-graded friction courses have been used since 1950 in the United States to improve the frictional resistance of asphalt pavements, promote drainage of water from pavement and thus reduce tire spray, and reduce noise from the tire/pavement interface.
Spaces within the “open-graded” or “gap-graded” mix — and amounting to as much as 20 percent of the mix or more in some European mixes — help drain water and attenuate tire noise.
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