An advanced thin overlay containing highly polymer modified asphalt (HiMA) binder was recently installed by contractor Pike Industries on a Vermont highway as part of a voluntary program of nationwide field demonstrations.
Pike placed the 1-inch-thick HiMA overlay on a portion of a $1.8-million, 1112 mile pavement maintenance contract with Vermont’s Agency of Transportation (VTrans). The agency had volunteered to join the program, with Pike a willing participant, and designated a two-mile stretch of U.S. Route 7 in Danby for the demonstration.
The remaining 912 miles of the maintenance contract received VTrans’ regular Type C Surface Treatment, a thin, unmodified (by polymer) open-graded hot mix asphalt.
Hosted by the National Center for Pavement Preservation (NCPP) at Michigan State University, the demonstrations follow new regional mix design specifications developed in a joint effort of 11 state transportation agencies and the Highway Sustainability Research Center at the University of Massachusetts/Dartmouth.
Research Center Director Dr. Walaa Mogawer, P.E., coordinated the demonstration program with the agencies, which are members of the Northeast Pavement Preservation Partnership (NEPPP), one of five regional partnerships established across the U.S. to advance pavement preservation practices.
NCPP and regional partnerships cooperate with state transportation departments, the Federal Highway Administration (FHWA), the American Association of State Highway and Transportation Officials (AASHTO) and other agencies and industry groups to advance the concept of pavement preservation as the most cost-efficient way to protect the nation’s huge infrastructure investment.
FHWA regards preventive maintenance treatments such as crack sealing, chip sealing, micro surfacing and thin-lift asphalt overlays as components of pavement preservation, and provides funds for these activities.
VTrans took part in the thin-lift HiMA demonstrations to further the agency’s search for better, high-performance pavement materials that would provide longer service life at lower life-cycle costs, according to Mike Fowler, the agency’s Pavement Manager Engineer.
New specs, new polymer
The Vermont demonstration adhered to new regional specifications known as “Superpave 9.5 mm Highly Polymer-Modified Thin Overlay Specifications,” (PMTOL). PMTOL mix consists of coarse aggregate, fine aggregate, mineral filler (when needed) and a polymer modified asphalt binder with performance grade of either PG76-34 or PG82-28, depending on the condition of the pavement being overlaid.
A lift thickness of between 0.75 and 1.5 inch (19.0 mm to 37.5 mm) is called for, and up to 25% reclaimed asphalt pavement (RAP) is allowed in the mix.
A striking feature of the specifications is the high concentration of polymer allowed. Polymers are increasingly being added to asphalt binders and emulsions to improve asphalt mix durability and resistance to rutting, raveling and cracking, but there is a practical limit to the amount of traditional polymer dosage – usually around 3%. For concentrations above this, binder viscosity begins to increase, making mix thicker and stickier. Thus the mix is more difficult to produce in the plant and less workable for paving crews, boosting manufacturing and lay-down costs.
However, PMTOL specifications guiding field demonstrations allow the use of HiMA binder containing 7.5% polymer – more than double that of traditional polymers. This high dosage is possible because HiMA binder is produced with a new type of styrene-butadiene-styrene (SBS) polymer. Manufactured by Houston-based Kraton Performance Polymers, this polymer satisfies specification requirements without increasing viscosity.
According to Chris Lubbers, Kraton technical sales manager, after several years of research the company developed a new family of SBS polymers. One of these developments – Kraton D0243 polymer – is being used to produce HiMA binder. Lubbers said use of the new polymer does not require making changes at conventional mix plants or in the operations of paving crews.