Open-graded Friction Courses: A View from the Wet Coast

Dec. 28, 2000
Oregon comprises a wide variety of climatic regions, but one of the constants in its heavily populated areas-the western third of the state-is rain. Parts of the state receive this form of precipitation nearly 50% of the year.

Given such conditions, one would expect Oregon DOT (ODOT) to have a high interest in any form of paving that promises high efficacy in rainy conditions, and-not surprisingly-Oregon has been one of the leaders in porous-pavement use.

Oregon comprises a wide variety of climatic regions, but one of the constants in its heavily populated areas-the western third of the state-is rain. Parts of the state receive this form of precipitation nearly 50% of the year.

Given such conditions, one would expect Oregon DOT (ODOT) to have a high interest in any form of paving that promises high efficacy in rainy conditions, and-not surprisingly-Oregon has been one of the leaders in porous-pavement use. Since 1979, Oregon has placed over 1,400 center-lane miles of the mix type and is continuing to add to that figure this year.

"Oregon really got into open-graded friction courses in the 1970s," says Jim Huddleston, executive director of the Asphalt Pavement Association of Oregon. "Some of the roads placed then are still around. The state never really quit using them, but slowed the mix's usage in the late 1970s and early '80s due to problems."

The difficulties encountered mirrored the experiences of other states using OGFCs, including durabiltiy, drain-down and fat spot problems. The state never really gave up on the material, however, and continued a somewhat trial-and-error effort to fine-tune its open-graded friction course mix design.

In 1989, the state began to accelerate its use of the material, and it has yet to slow this process. Unlike most other states, Oregon does not use the material as a thin friction course, such as the modified OGFC used in Georgia.

Instead, Oregon places the material much like the Europeans do, in thicker lifts-typically, 2-in.-thick lifts. Air-void percentages are similarly higher than other states', approximating percentages used in Europe. Oregon uses larger aggregate than most states, as well-up to 1 in. in diameter.

"Oregon's OGFC mix-we call it our F mix-uses aggregate that is 100% crushed, 90% of which has 2 fractured faces; we use no sand in the mix," Huddleston says. "We have not had any significant stripping problems like some other states have, and I think this is due to the porosity-our air voids are typically 14%-18%-and thick film coatings we use. In addition, we require lime treatment of aggregate on all heavy-duty and Eastern projects."

The eastern portion of the state, far from the modulating influence of the Pacific Ocean, experiences more freeze-thaw cycles than the western half, making lime treatments a must.

"As far as performance history goes, we have found these pavements reduce spray, glare and noise; they provide excellent rut-resistance; and they reduce design and construction risk.

"There is a 50% decrease in pavement spray and splash reduction over the first two to three years, after which the pavement stays at that level for its life. We have found this to be much better than our dense-graded mixes in this regard."

Huddleston says the friction-increasing abilities of the pavement are not as across-the-board remarkable as some might claim. "Friction is lowest right after construction because of the thick layer of asphalt around the aggregate. However, it is still well within safe levels. It begins to rise as vehicles are in the process of wearing that film off, and it soon levels off to approximately the same as-or slightly better than-our dense-graded mixes. However, the open-graded friction courses are much better in really wet conditions. For a state with Oregon's unique climatic conditions, this advantage may greatly reduce potential accidents."

In addition to its significantly thicker lifts of OGFC, Oregon has experimented with full-depth OGFC paving. The state has found the mix provides a sound structural capacity, and this depth of paving has enabled the mix to live up to its European promise of lowered sound levels.

"For a single lift, we get about a 3 db reduction in noise, and about an 8 db reduction for full-depth OGFC pavements. One other advantage we have noted is that this mix especially helps reduce high-frequency noise-the 'tire whine' people really hear."

Huddleston says the state has yet to rehabilitate an F-mix road, so he doesn't have any information about that, but it does point out the longevity of Oregon's open-graded friction course pavements: Some of those roads were placed as early as 1983.

Kirk Randolph, APAC-Georgia Inc.'s division president, has stated that Georgia's modified OGFC is a difficult mix to produce in that it is very sensitive to production variables and difficult to assay in the field. Huddleston, on the other hand, views Oregon's F mix as a very forgiving one: "We have put this in places where we really shouldn't have, and it has done well. [This mix] eliminates compaction concerns, and you can add asphalt to it after laydown. It's hard to screw up this mix."

While a stanch supporter of the mix, Huddleston is forthright about its drawbacks and limitations. For example, the relatively high asphalt content and high air-void percentages of the mix can still lead to fat spots and drain-down problems. (While asphalt-cement content is only marginally higher in Oregon's F mix than in its dense-graded mixes, F-mix air-void percentages range from double to quadruple those of the dense-graded mixes.)

On a more humorous note, Huddleston says the state has also discovered an open-graded friction course problem he has not seen documented elsewhere: what he euphemistically refers to as the "possum pothole phenomena." He says roadkill ground into the surface of Oregon's F mixes seem to "eat away" at the pavement, resulting in a pothole. The reason why is unclear; the only solution is to make certain the roadkill is cleared away before the damage occurs.

Huddleston does not tout the F mix as a universal solution. He recommends states not use the mix if any of the following conditions occur:

  • Haul distance is over 50 miles. While the state has had success with hauls as long as 70 miles, the current policy is to truck the mix only 35 miles, with 50 miles as the absolute upper limit.
  • An inlay is needed. Again, the state has had luck with the procedure, but Huddleston views the success as just that. He cautions against it, at least until testing can prove/disprove its performance for this use.
  • The site is a heavy-snow zone (such as mountain passes). Plow blades can catch on the coarse aggregate and "pick it" from the mat. In these areas, a chip seal can be placed over the open-graded friction course, but this nullifies one of OGFC's most beneficial properties: high drainage capacity.
Oregon has experimented with the F mix on logging roads, and these pavements seem to tolerate the heavy loads with few problems. However, logging areas are usually areas that receive considerable snow, so this violates Huddleston's OGFC guideline No. 3: Do not place this mix in a heavy-snow zone.

In addition to plowing problems, voids in these coarse pavements tend to create conditions that lead to faster icing than conventional or dense-graded mixes. On the other hand, OGFCs thaw more quickly, as well.

The bottom line for Oregon is that its open-graded friction course, the F mix, works-and works well. The deep lifts the state places have created pavements that show increased rut-resistance over dense-graded mixes, provide better friction and crack-resistance, and help decrease road spray and roadway noise.

The mix is not a "magic bullet," but it is another powerful tool to help increase safety and quiet in this often wet western state.