Whispering Asphalt

OGFCs cut spray and can be very, very quiet.

For several years, a number of states have placed open-graded friction courses (OGFCs) as a wearing course on their high-volume, high-speed roadways.

An OGFC is a thin, permeable mix with high air voids and a relatively high asphalt content. These mixes are safer than dense-graded asphalt because they reduce splash and spray in wet weather, and they drastically lower the potential for hydroplaning because water drains into them and out to the side. Driver visibility is improved, and headlight glare is reduced.

But OGFCs are not recommended for low-volume, low-speed pavements. That’s because high-speed traffic actually helps maintain the benefits of OGFCs. The action of moving traffic cleans dust and other materials that can clog the mix and reduce its permeability and noise-reducing ability.

In the early 1970s, several Western states began placing plant mix seals in response to the Federal Highway Administration’s (FHWA) program to improve the frictional resistance of U.S. road surfaces. The aggregates typically had a top size of 9.5 mm to 12.5 mm (3/8 to 1/2 inch) and were mixed with a relatively high percentage of asphalt cement in a conventional asphalt plant. Mixes were placed at 15- to 20-mm thick (5/8- to ¾-inch).

Durability of these OGFC mixes, however, became a problem. They wore out quickly and raveled. The underlying asphalt in some pavements suffered from stripping. So in the 1980s, some states placed a moratorium on the use of OGFCs. But a handful of states – Georgia, Oregon and Texas among them – saw the potential of OGFCs and set about improving the mix.

FHWA encouraged the improvement efforts. States began to add both polymer-modified asphalts and fibers to the mix to prevent drain-down. The combination of polymers and fibers stabilized the mixtures. It became easier to produce OGFCs because the mixes were less sensitive to mixing plant temperature variations and could be produced at more conventional operating temperatures. State agencies boosted the asphalt content and the air voids in OGFCs, and they specified a high percentage of polish-resistant, crushed aggregate.

The modified asphalts created a thicker film on the OGFC aggregates, so raveling and oxidation were reduced. Durability increased because of the increased asphalt content and polymer modification.

“We made a trip to Georgia to see what they [Georgia DOT] were doing right,” Dale Rand, flexible pavements branch director for the Texas Department of Transportation (TxDOT), tells Better Roads. “[Georgia had] added cellulose fibers and mineral fibers and polymers to make the mixes more durable. Georgia had started placing [its] improved mixes in the early 1990s and they were seven to eight years old by the time we saw them. So we came back and tweaked our design somewhat and came up with our version, which is very similar to the Georgia version. We just call it Permeable Friction Course (PFC).”

For the PFCs, Rand says Texas now uses PG 76-22 asphalt cement – polymer modified – and typically 0.3 percent of cellulose fibers. Some 80 to 100 percent of the aggregate passes the 1/2-inch sieve and 35 to 60 percent passes the 3/8-inch sieve. A minimum of 6 percent asphalt is required in Texas PFCs, and they are designed to have about 20-percent air voids, Rand says.

“We typically specify them on roadways that have a history of wet weather accidents,” Rand points out. “In my opinion, they are most beneficial on undivided highways where you have a history of wet weather accidents. Our TxDOT official policy is that we target pavements in areas to prevent wet weather accidents. We also use a number of PFCs routinely on our Interstate system. We use them on high-speed facilities, if the posted speed limit is above 45 mph.”

Rand says PFC mixes cost about 20 percent more per ton than conventional dense-graded mixes. But because the PFCs have 20 percent air voids in them vs. 7 percent in dense-graded mixes, the cost per square yard of the PFCs is roughly equivalent to the dense-graded mixes. Typically, Rand says, TxDOT applies the PFCs at 1-1/4 inches thick to 1-1/2 inches thick, with 1-1/4 inches as the typical thickness.

Georgia’s Experience

The Georgia DOT uses not one, but three versions of open-graded surface mixtures. One is a conventional OGFC, designed with 18 percent air voids. Another is a porous European mix (PEM), featuring even higher air voids, at 24 percent, and a 12.5-mm top size aggregate. “Our specification calls for PEM on all Interstates that have an asphalt surface,” says Peter Wu, assistant state materials engineer.

The third open-graded mix is a 9.5 mm (3/8 inch) nominal maximum aggregate size OGFC. “That is for limited use on state routes with a specific drainage problem,” Wu says.