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

Lee Geistlinger / December 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. 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.

About the Author

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