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Applications & Innovations: OGFCs

Applications & Innovations Whispering Asphalt OGFCs cut spray and can be very, very quiet. F or 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, lowspeed 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, 24 September 2012 Better Roads 24 September 2012 Better Roads 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.

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