Better Roads Staff
Because of early problems, Georgia put a moratorium on OGFCs in 1982. Likewise, Washington State DOT tried smaller stone-sized (3/8-inch) mixes as thin (0.15-foot) wearing courses in the 1980s and early 1990s, but discontinued their use because of excessive studded tire wear problems.
But in the 1990s, Georgia developed a mix that incorporates a high degree of single-sized coarse aggregate, polymer-modified asphalt binder, stabilizing fibers and hydrated lime.
“This mix has been used extensively statewide since 1993,” the DOT reports. Now Georgia, pleased with its success, is urging other state DOTs to reassess OGFCs. “[Agencies] should reconsider the possibility of using this modified OGFC on high-volume-traffic facilities,” they say. “It is now Georgia DOT policy to use modified OGFC as the final ride surface on all Interstates and on state route projects that have daily traffic volumes exceeding 20,000 and are not in a reduced speed zone area.”
Modified vs. Standard Binders
After Georgia, use of polymer-modified — and more recently, rubber-modified — binders for OGFCs has become common. But the use of polymer or crumb rubber modifiers in OGFC binder may come at a cost: the ability of the pavement to drain water, at least in 4.75-mm nominal maximum aggregate size (NMAS) mixes.
That’s what Qing Lu, assistant professor, Department of Civil and Environmental Engineering, University of South Florida, and John T. Harvey, professor, Department of Civil and Environmental Engineering, University of California-Davis, say in their 2011 Transportation Research Board paper, Laboratory Evaluation of Open-Graded Asphalt Mixes with Small Aggregates and Various Binders and Additives.
Their paper is part of ongoing research to develop alternative asphalt surface mixtures that are quieter and durable without much sacrifice of safety, they say.
Five binder types (PG 64-16, PG 58-34PM, PG 76-22PM, asphalt rubber and PG 76-22TR) and two additives (hydrated lime and cellulose fiber) were selected for a 4.75-mm NMAS gradation, they write. A series of laboratory tests were conducted to evaluate their pavement surface performance-related properties, including acoustic absorption, texture, resistance to raveling, moisture sensitivity, permeability, friction, resistance to permanent deformation and resistance to reflective cracking.
“Results show that using polymer-modified or rubberized binders instead of unmodified binder in the 4.75-mm NMAS open-graded mixture reduces permeability,” they write, “but increases acoustic absorption, with the mixture containing asphalt rubber binder showing the most acoustic absorption improvement.”
Using asphalt rubber also can enhance the mix’s resistance to moisture damage or premature failure, raveling, rutting and potential resistance to reflective cracking. “There are also preliminary indications of friction improvement by replacement of conventional binder with asphalt rubber binder in the small-size aggregate open-graded asphalt mix,” Lu and Harvey say.
Rubber-Modified OGFC Binders
Rubber-modified binders for OGFCs now are seen on the west coast (California) and the east coast (South Carolina), as well as in the epicenter in Arizona.
Modified asphalt cement binder is required in OGFCs to prevent draindown of the binder and achieve the necessary level of adhesion and mix stability, reports the Asphalt Rubber Technology Service (ARTS) at Clemson University. “Although typically a polymer is used as the modifying additive to the asphalt cement binder, crumb rubber made from scrap tires may also be used as an alternative modifier,” according to the university.
The South Carolina rubber-modified open-graded friction course is placed 3/4- to 1-inch deep, with a crumb rubber content of 12 percent by weight of the liquid asphalt, or 0.85 percent by weight of the mix.
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