Applications & Innovations
Better Roads Staff
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.
Georgia’s regular OGFC is a 12.5-mm mix with a target asphalt content of 6 percent. The PEM, with higher air voids, is designed to drain water more quickly than the regular OGFC. Georgia applies the PEM at 135 pounds per square yard. “The higher air voids give it more room to drain water off,” says Wu. The 135 pounds is equivalent to 1.25 inches.
Both Georgia’s PEM and the OGFC use PG 76-22 polymer modified asphalt cement. Typically cellulose fibers at 0.3 percent by weight are specified, and if mineral fibers are used, their weight is 0.4 percent. Wu says in Georgia crumb rubber is under consideration in lieu of polymer modified asphalt.
After OGFC was developed in the United States in the early 1970s, it has been used extensively in many European countries, according to IS 115, a technical document known as Design, Construction, and Maintenance of Open-Graded Friction Courses and published by the National Asphalt Pavement Association (NAPA). The document says porous asphalt, as it is called in Europe, has been used frequently in Germany, the Netherlands, France, Italy, United Kingdom, Belgium, Spain, Switzerland and Austria.
However, unlike the OGFC mixes that didn’t work very well in the United States in the 1970s, porous asphalt in Europe evolved with a coarser gradation of aggregates, and higher in-place air voids – 17 to 22 percent. That gave the mixes higher permeability, and European countries generally place them in thicknesses up to 2 inches. Nominal maximum aggregate sizes ranged from 11 mm, or 7/16 inch, up to 16 mm, or 5/8 inch.
Polymer-modified binders or fibers, or sometimes both, are used in the European mixes to obtain thick and strong binder films for maximum resistance to aging and raveling. OGFC is used in Europe in a full range of climates, from hot and dry to cold and wet.
Tires rolling on the road force air away in front of the tire, and suck air in behind the area of contact between the tire and the road. This air pumping generates high-frequency noise, according to the NAPA document. But with OGFC, the pumping, and therefore the noise, is reduced because air is pumped down into the porous pavement. Porous OGFC also reduces noise by absorbing some of the noise emitted by vehicles. On dense surfaces, the noise emitted towards the pavement is reflected to the surroundings.
Because of its noise-reducing ability, OGFC is known in Germany as “Fliisterasphalt,” or whispering asphalt. Full-scale trial sections of OGFC on German motorways and trunk roads between 1986 and 1990 measured reductions in rolling noise of up to 6 decibels(dBA), and this value has since been adopted as the basis for calculating the noise-reducing properties of OGFC.
NAPA does not recommend applying a dense-graded asphalt to an OGFC at the end of its service life. Studies have found that the overlay will trap water in the OGFC and cause the pavement to deteriorate. Generally, it is recommended to mill off the existing OGFC prior to replacing it with a new OGFC or any other wearing course.
Fixing ‘Dead Man’s Curve’
The Texas Department of Transportation (TxDOT) has a notable success story to tell about its permeable friction courses (PFCs). In the late 1990s, a 2-mile section of RM 1431 near Jonestown, Texas, in the Austin District, came to be called “Dead Man’s Curve” by the media and local residents. For years, the Austin District took proactive measures to reduce the number of accidents along RM 1431, but the rate of wet weather accidents remained unchanged.