Better Roads Staff
There, OGFC can be placed on either asphalt or concrete pavements, and it consists of roughly 93-percent crushed stone, 7-percent modified asphalt binder and a small amount of stabilizing fibers, Clemson’s ARTS says.
“Rubber-modified open-graded friction course has all the same advantages as polymer-modified OGFC,” ARTS says. “Compared to concrete and standard asphalt dense-graded pavements, it has significantly lower noise levels, it has less surface water in wet weather, and it is generally safer in wet weather due to the better visibility and decreased hydroplaning, resulting from the lack of surface water on the pavement surface.”
In addition to sharing all the advantages of OGFC with its polymer-modified counterpart, rubber-modified OGFC also costs slightly less than conventional polymer-modified OGFC, ARTS says, adding when used in OGFC, scrap tires can be used at a rate of approximately 1,000 tires per mile of two-lane pavement.
Fighting Draindown with Fibers
While modifiers will help keep liquid asphalt binder from moving within an open-graded mix, it still may puddle at the bottom of a haul truck, or settle lower in the lift of mix just placed. This “draindown” of asphalt seriously compromises the durability of OGFCs.
The problem of draindown of liquid asphalt is solved by use of fibers – typically cellulose, but also mineral filler – which are used to hold the binder in place.
While cellulose fibers are most often used, new research indicates that polyethylene fibers may do the same thing in OGFCs. In the technical paper Characterization of OGFC Mixtures Containing Reclaimed Polyethylene Fibers written by V.S. Punith, of the Asphalt Rubber Technology Service at Clemson University, and A. Veeraragavan, Department of Civil Engineering, Indian Institute of Technology, Madras, and published last year in the Journal of Materials in Civil Engineering, it was reported that polyethylene fibers derived from recycled low-density polyethylene (LDPE) tote bags performed in OGFC mixes.
Polyethylene fibers recycled from LDPE bags collected from domestic waste improved OGFC mixes without fibers, they say. “Draindown test results indicated that OGFC mixtures with polyethylene fibers can be effectively used to retard draindown of the binder and mineral filler,” they write. “[T]est results indicated that OGFC mixtures with reclaimed PE fibers showed improvement in tensile strength and improved resistance to permanent deformation, fatigue-induced damage, and moisture susceptibility, when compared with mixtures without fibers.”
Draindown in Porous Mixes
While porous asphalt pavements are open-graded to encourage water drainage, they differ from open-graded friction or wearing courses by structure and function. Yet, they pose many of the same challenges as OGFCs, including binder draindown.
Porous asphalt pavement is an environmentally-sustainable infrastructure design that helps property owners manage stormwater effectively and inexpensively. They are specified for static traffic areas such as parking lots, and aren’t suitable for wearing courses.
Typically, a porous asphalt pavement will be composed of, from the bottom, an uncompacted soil subgrade that will optimize infiltration of water into the aquifer; a geotextile fabric that will permit water to pass but preclude movement of fines up into the structure; a stone recharge bed with same-sized aggregate and 40 percent voids; an optional stabilizing or “choker” course of single-size crushed stone smaller than that in the recharge bed; and an open-graded asphalt surface with 20-percent voids that permit stormwater to flow through the pavement into the stone recharge bed.
While OGFCs shoot for voids of 15 percent to control spray and noise, porous asphalt pavements let water drain directly into a recharge layer below, and the air-void target is 20 percent. The porous asphalt “green grinder,” or parade ground, constructed at the U.S. Marine Corps Recruit Depot at Parris Island, S.C., in May 2011 was not intended to reduce spray, hydroplaning or noise, but simply to provide the most efficient means of draining water from the paved surface and into the soil, while avoiding conventional detention ponds.
While fibers conventionally are used to stabilize porous asphalt mixes, an innovative warm-mix asphalt design for Parris Island saved money by eliminating fibers required for stability of the open-graded mix. The porous asphalt design also eliminated the expense of creating a detention pond and associated environmental requirements.
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