Better Roads Digital Magazine
Issue link: https://read.dmtmag.com/i/85930
Applications & Innovations 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 specifi - cation 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 lim- ited use on state routes with a specifi c drainage problem," Wu says. 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 poly- mer modifi ed asphalt cement. Typically cellulose fi bers at 0.3 percent by weight are specifi ed, and if mineral fi bers are used, their weight is 0.4 percent. Wu says in Georgia crumb rubber is under consideration in lieu of polymer modifi ed asphalt. European Experience After OGFC was developed in the United States in the early 1970s, it has been used extensively in many Euro- pean 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 fre- quently in Germany, the Netherlands, France, Italy, United Kingdom, Belgium, Spain, Switzerland and Austria. However, unlike the OGFC mixes that didn't work very Write 114 on Reader Service Card or visit www.betterroads.com/info. 26 September 2012 Better Roads