While legislation such as the National Traffic and Motor Vehicle Safety
Act of 1966 helped improve vehicle safety, highway administrators across
the country began to take a hard look at safety issues of the roads themselves.
Transportation officials soon came to the conclusion that one of the most
significant factors influencing traffic safety was pavement-friction characteristics,
especially in wet weather.
Early experiments to increase road-surface friction focused on chip seals
and seal coats, but the limitations of these treatments (especially loss
of surface aggregate under high-speed conditions) soon appeared. Adjustments
were made, and, as early as 1968, a Federal Highway Administration (FHWA)
memorandum encouraged the use of plant-mix seal coats to increase roadway
durability, increase skid resistance and elevate the pavement's ability
to withstand high traffic volumes and loads.
These seal coats commonly consisted of high percentages of asphalt cement
and uniform-size chip-seal aggregate. Placed in lifts of approximately 3¦4
in., these friction coats soon lived up to their promise of increased safety
and, compared to earlier efforts, reduced aggregate loss.
Known by many names, the most common moniker for the plant-mix seal coat
was open-graded friction course (OGFC). A 1993 guideline published by the
National Asphalt Pavement Association (NAPA) defines OGFC in the following
manner: "Open-graded friction course, or porous asphalt, is a gap-graded
bituminous mixture that consists largely of single-sized crushed stone,
and that contains a relatively large percentage of air voids. The open structure
of this mix drains rainwater effectively, thus reducing splash and spray
and eliminating aquaplaning. In addition to the improved safety in all types
of weather conditions, a well-designed and constructed open-graded friction
course reduces noise generated by the rolling vehicle tires."
One look at an OGFC will reveal many of its differences from either conventional
or high-density mixes: a glossy, almost oily appearance created by the relatively
high asphalt content; a coarse, heavily voided surface created by large,
often angular, aggregate; and a dark surface, the result of the thick layer
of asphalt film surrounding aggregate and low reflectivity of the voided
surface.
In 1973, the FHWA issued a notice recommending the use of OGFC on all resurfacing
projects and new construction that called for high skid-resistance. While
problems soon began to surface, these problems have been addressed. Today
over half of the states are again using some form of this mix, and the success
of these projects is encouraging the use of the mix on additional lane miles.
Earlier this year, several industry organizations and companies pooled their
resources to sponsor an OGFC open house. Held in Atlanta and attended by
ROADS & BRIDGES, the open house consisted of educational sessions and
a trip to an OGFC project underway south of the city on Ip;75. FHWA,
Georgia DOT (GDOT) and NAPA joined forces with C.W. Matthews Contracting
Co. and APAC-Georgia Inc., both of the Atlanta area, to bring the approximately
80 attendees up to speed on the current status of the mix.
Because the meeting was held in Atlanta and included a visit to the site
of an ongoing project there, the meeting naturally included a preponderance
of information about Georgia's OGFC experience. However, the open house
also included speakers from FHWA (see sidebar, below) and a West Coast point
of view-Jim Huddleston, executive director of the Asphalt Pavement Association
of Oregon, shared his state's experience with OGFC, which differs significantly
from that of most states using this mix.
Ronald Collins, state materials and research engineer for GDOT, says "We
began using open-graded friction courses in 1970, but we had some problems
with it-[asphalt-concrete] drain-down problems, fat spots and raveling.
Moisture and air gets in and this oxidation ages the road faster."
Moisture also was a problem in that it damaged the layers beneath the OGFC,
as well as accelerating the debonding of the friction coat.
Following a moratorium on OGFC from 1981p;85, GDOT began to experiment with the material again, this time employing three significant changes:
- Using more viscous (viscosity grade 30) asphalt cements at higher percentages (currently 6%-6.3%),
- Discontinuing use of asphalt emulsion as a tack coat, and
- Using hydrated lime as an anti-stripping agent.
In 1990, the Transportation Research Record No. 1265, Porous Asphalt Pavements:
An International Perspective, 1990, published the findings of a group of
U.S. officials who investigated European OGFC research, mix design and field
experience. The European countries that frequently use OGFC-also known as
PEM, for porous European mix-are Austria, Belgium, France, Germany, Italy,
Netherlands, Spain, Switzerland and the United Kingdom.
While paving needs differ in Europe from those in the U.S.-for example,
some European countries have mandated highway-noise abatement-the Europeans
have developed stringent design, performance and monitoring systems. For
example, proof of permeability is often required at the end of construction
and closely examined over time. Because of these systems and a commitment
to porous pavements, the Europeans have had positive experiences with the
mix.
In 1991, GDOT began to take a close look at ways to adapt the European technology
to its own needs. In Europe, most countries were using polymer and/or fiber
modifiers to obtain the necessary thick and strong binder films. GDOT built
on this experience and began to incorporate the same into its modified OGFC.
"With the addition of fiber," notes Collins, "We've been
able to stabilize the binder and minimize drain down. With polymers-we are
currently using a styrene butadiene styrene additive-and fibers, we are
getting a thicker, what I would call a reinforced, film around aggregate.
We have begun to see the interlocking of fibers in the AC that surrounds
each stone, so we are getting more strength."
Inclusion of the SBS polymer modifier also increased the binder stiffness
8-10 times that of neat asphalt cement, elevated the softening point of
the asphalt cement approximately 40 F and produced an asphalt film more
ductile and flexible than that of unmodified asphalt cement.
"Polymers and fibers almost eliminate drain down," Collins says,
referring to the problem of settling asphalt cement, "but it is a very
stiff system." To help combat this, higher mixing temperatures are
employed, and contractors have to be careful to not allow the mix to cool
before reaching the site, or cold clumping may occur. Tarped trucks and
close attention to the continuity of the paving operation is especially
important in OGFC mixes; use of a materials-transfer vehicle can help avoid
cold clumps by remixing materials before they reach the paver hopper.
Larger aggregate, such as that used in Europe, can intensify the "thick"
nature of the mix. Thus far, Oregon is the only state routinely placing
truly European-style OGFC mixes, but Collins does not rule out its use in
Georgia: "We have done some European-style OGFC, with a coarser mix
that gives better drainage. We will probably go more this way in the future,
but we want to do our homework before we do so."
From all indications, GDOT's European-style test sections have been a success.
A 1-mile-long PEM test section was placed on Ip;85 just north of Atlanta
in 1991. The road, which today carries 2 million ESALs (equivalent single-axle
loads) annually, has shown that PEM is more than twice as permeable as GDOT's
modified OGFC mix.
However, economics also are a concern: The coarse gradation of PEM requires
a thickness (11¦4 in.) twice that of the state's modified OGFC mix
(5¦8 in.). This increase would approximately double the tonnage needed
to surface a project. The thicker lifts placed in Europe also have a greater
ability to decrease tire noise, but this is less of a concern in the U.S.,
so few states are using true PEM-type mixes.
Economics also are a concern with U.S.-style modified OGFC mixes: The addition
of polymers and fibers can significantly raise the initial cost of building
any given stretch of road. Collins, however, says the cost is worth it.
"The modified OGFC is approximately 34% more expensive than a conventional
OGFC mix," he says. "But if you do a life-cycle analysis on the
material-which we have done-you can see that the annualized cost is only
$37,000 vs. $50,000 for conventional OGFC. This has convinced our department
to spend more up front."
Overall, Collins says GDOT is extremely pleased with the performance of
its OGFC projects; he says the benefits are many but the following predominate:
reduced glare, smooth finish (Mays readings of 3-4 in. per mile) and the
lack of hydroplaning.
The use of modifiers in OGFC mixes is a thorny issue: Getting just the correct
balance of the right materials is a difficult task. Ray Brown, director
of the National Center for Asphalt Technology (NCAT), Auburn, Ala., has
taken an in-depth look at fibers and modifiers.
"Why use polymers and fibers in OGFC mixes?" he asks rhetorically.
"Use polymers to improve asphalt-cement properties and prevent drain
down of the AC. Polymers also help hold stripping to a minimum. Fibers are
used to help prevent drain down, but-contrary to what you may believe-you
don't use fibers so more AC can be added to the mix.
"There are two type of fibers used to modify OGFCs: Cellulose and mineral
fiber, such as slag wool or basalt-based materials. Some of the potential
problems that come with fibers are getting an even distribution of the material
in the mix, the higher mix temperatures required for their use and the reduced
workability of the resulting mix." As Collins notes, the fibers have
a tendency to "interlock" with each other, which lends strength
to the final product but makes it more difficult to place and perform handwork
on.
Costs aside, Brown says the polymer additives commonly used with OGFC mixes
come with their own set of caveats. "Polymers can separate during storage,
so that is a concern; in the same manner, you have to pay close attention
to the blending of the polymer additives at the HMA plant. As with fibers,
polymers tend to reduce the workability of the mix-making it stiffer-and
require an elevated mixing temperature, approximately 330 F, vs. about 290
F for conventional HMA. And with polymer-modified mixes, you have to work
it when it is hot-the compactors have to be right behind the paver."
Looking down the road, Brown says he sees four areas that need to be fully
addressed before fiber and polymer additives are well accepted:
- Cost: Contractors and agencies will, like GDOT, have to do life-cycle cost analyses. This will help them overcome higher initial-cost concerns.
- Environmental issues: While there has been no extensive testing of recycled OGFC pavements, fibers and polymers may create reclaimed-asphalt pavement (RAP) problems. In addition, the higher mixing temperatures required for modified mixes create more fumes and increase fuel expenditures. While too soon to tell what the long-term impact of this trait will be, it bears careful examination.
- Construction concerns: Contractors may have to buy new equipment to handle this dense, stiff mix. In the same vein, the workability of the mix itself can create problems on any given job.
- Performance: Modified OGFC pavements are a relatively new breed, and projects must be monitored to identify any weaknesses or strengths in the mix's long-term performance.
The ambitious project, which began in mid-1995 and finished up just before
the Olympics opened in July, involved milling off approximately 11¦2
in. of the existing roadway and replacing it with the same depth of Georgia's
stone-matrix asphalt (SMA) mix. On top of the SMA went a 3¦4-in.-thick
layer of GDOT's modified OGFC.
Kirk Randolph, division president of APAC-Georgia Inc., shared a contractor's
impressions of the current project and OGFC in general with open house attendees.
"There are some OGFC production measures that you should be aware of,"
Randolph says. "Fibers are introduced during the mixing process-they
are weighed as they are mixed and blown into the mix about 10 in. ahead
of the asphalt cement." The fibers, a Fiberand Corp. product that resembles
blown-insulation material, does cause skin and eye irritation, and Randolph
says workers "suit up" for this part of the mix process.
"The mix we used-GDOT's modified open-graded friction course-requires
an increased mixing time and temperature," he continues. "The
increase in time is about 40 to 60 seconds, and this reduces production
about 30%. The higher temperatures also mean higher fuel costs.
"We mix our OGFC at a constant temperature of 340 F; you have to keep
it constant because a change in mix temperature will cause changes in screed
depth on the paver, and that is something you don't want to do. To keep
the mix temperature as constant as possible, it is important to get the
asphalt to the site fast." FHWA guidelines recommend a hauling limit
of 40 miles or 1 hour, close to figures offered by Oregon's Jim Huddleston.
The trucking procedure used by APAC modifies normal hauling processes slightly
to maintain the high mix temperature. "Do a continuous operation, and
space the trucks well," Randolph says.
"Drain release agents from beds before loading, since the agents can
cool the mix. We tarp our trucks-this is a Georgia law, but it is especially
important for this mix to keep crusting down."
Randolph says APAC's use of Roadtec Inc.'s SB-2500 Shuttle Buggy as a mix-transfer
device was "very important on this job. It remixes the mix and prevents
any bumping of the paver by trucks loading the paver hopper." As Collins
notes, a materials-transfer vehicle is particularly important on OGFC jobs,
as the remixing the transfer vehicle performs re-establishes a uniform temperature
to the mix. It helps eliminate cold clumping and generally aids the placement
of a smooth, uniform mat.
Heat in general is important on OGFC jobs. "A hot screed is very important,"
Randolph explains. "We use a propane torch to heat it up before each
use."
To prepare the road surface for the friction course, APAC applied a 0.06-0.08
gal/sq yd tack coat layer on the SMA (Randolph says to shoot for 0.07 gal/sq
yd layer). This tack coat is spread to extend several inches beyond the
expected edge of the OGFC mat to make certain the entire layer bonds.
With the paver-for this portion of the project, a Roadtec RP-180 rubber-wheeled
paver-moving at approximately 60 fpm, the consistent stream of trucks fed
the material-transfer vehicle that, in turn, fed mix to the paver.
"It is critical to have the roller right up to the paver, where the
mix is hot and workable," Randolph says, and on site the Caterpillar
CB-634 double-drum vibratory compactor (operating in static mode) stayed
within 50 ft of the paver train, providing breakdown and keying the mix
into the tack coat. Further behind, a smaller Ingersoll-Rand compactor provided
finish rolling.
Joints are handled a bit differently with OGFC mixes than with conventional
mix. "To handle longitudinal joints, don't overlap adjacent lanes after
compaction," Randolph emphasizes. Because of the high temperature needed
for workability, crews can't just lay an adjacent lane next to the existing
friction course and expect to "hot roll" the one into the other-the
first layer will have cooled past workability.
"Instead," he continues, "tack the joints heavily and leave
the new OGFC mat about 3¦16 in. above the adjacent lane, and then overlap
the joint with the breakdown roller." For taper cuts, he says APAC
just lays a full width and a motor grader makes the cut in the warm mix;
a front-end loader then removes the excess.
The finished product has a somewhat incongruous look: For those used to
full-width paving projects, the sight of the relatively thin, large-stoned
OGFC sitting on top of the SMA-and not extending the same width as the SMA-gives
it an almost "unfinished" look. However, the OGFC mat must be
placed in such a way so that water draining though the porous mix can move
laterally to the edges and drain off. Just days after the Ip;75 lanes
were placed, Atlanta was hit with a major rain. Those who traveled the partially
finished section of roadway were pleased to see vehicles traveling on the
OGFC kicking up little spray, while the cars and trucks on the SMA-only
lanes were engulfed in a fog of "recycled rain."
Open-graded friction courses are not magic bullets that will solve all highway-safety
problems. GDOT's Collins says the department has overlaid OGFC, but the
results have been almost uniformly poor. GDOT now mills off all OGFC before
resurfacing.
Collins also thinks that the mix holds the potential to rut under heavy loads (Oregon's Huddleston disagrees), but it is too early to tell. Byron Lord, chief of the Office of Technology Applications for FHWA, says Europeans have not experienced rutting with their porous pavements, but they are using thicker lifts of the mix, so it is difficult to directly compare.
