Safety First: Open-Graded Friction Courses

Dec. 28, 2000
In 1961, only 7% of all vehicle travel in the U.S. was on interstate highways; by 1989, that figure had more than tripled. Accompanying this rise in limited-access road usage was a steady increase in highway injuries and fatalities.
In 1961, only 7% of all vehicle travel in the U.S. was on interstate highways; by 1989, that figure had more than tripled. Accompanying this rise in limited-access road usage was a steady increase in highway injuries and fatalities.

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.
Collins says these measures solved most of the problems encountered, but there were still concerns about the very characteristic that made the mix desirable: its permeability. While the voids help reduce tire noise and aid in drainage, surface voids tended to clog with dust and other debris. This not only reduced the surface's drainage capacity, but moisture retained by clogged voids created the same problems encountered in the past: increased oxidation leading to raveling and damage to layers beneath the surface coat.

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.
While modified OGFC pavements are a new breed, Georgia has embraced them. In 1995, Atlanta-host to this year's Summer Olympics-made the decision to resurface over 300 lane miles of two major roadways that are among the busiest in this burgeoning southern metropolis. Called the Atlanta Express Lane Project, the project turned four lanes of traffic on I­p;75 and I­p;85 into five by converting part of the shoulders to high-occupancy vehicle (HOV) lanes.

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.