Applications & Innovations
Better Roads Staff
The control section was SMA-9.5 with PG 76-22 laid at 1 1⁄2 inches. The first experimental section used a porous friction course (PFC) with a 9.5-millimeter nominal maximum aggregate size (PG 70-28 polymer modified binder) as one section (1-inch application).
For another section, VDOT specified a PFC with a 12.5-millimeter maximum nominal aggregate size and PG 70-28 binder. The larger aggregate size and an application thickness of two inches were selected to improve drainage characteristics and promote a void structure that might support longer-term noise reduction, Clark says. “The best way to make a pavement quiet is the surface texture and porosity.”
They used a PFC-9.5 with a rubber-modified binder for the last experimental section to explore the noise-reducing attributes of rubber. This binder met the requirements of a PG 82-22 with a minimum of 10-percent ground-tire rubber by weight of asphalt binder.
While the polymer-modified binder and aggregate were provided from different sources, all of the rubber-modified binder came from Blacklidge Emulsions in Gulfport, Miss. This binder was introduced to the mix through specialized tanker equipped with an agitator.
“We had to make some very minor adjustments to the plant to accommodate this procedure [pumping from an agitator tank], and it worked great,” says David White, general manager, Superior Paving.
Depending on the project, the crews used either a new surface (one placed in the last year or two) or placed a binder layer of asphalt prior to laying the final surface for the quiet pavement sections. For two of the projects, the existing surface was removed through milling to maintain or improve cross sections.
Both companies used a trackless tack-coat material and a standard paver, and the paving procedures were slightly modified based on the application thickness. For their rolling operations, they did four passes with a 10-ton static roller with no density requirement.
After all three projects were completed, VDOT performed noise, ride-quality and friction testing. Their initial tests showed both Superior Paving and Branscome were able to provide a smooth, quiet and skid-resistant surface. The rubber-modified PFC-9.5 and the PFC-12.5 were the quietest surfaces, and both had an average decibel reading of approximately 98 to 98.5.
“For the sections installed with rubber, the final ride quality was as good as the sections without rubber,” Clark says. “For a few sections, the ride quality was better.” However, he doubts there’s a statistical difference between the two and says the road condition under the pavement materials may have also played a factor.
The PFC-9.5 with PG 70-28 was slightly higher with an average reading of 99.6 decibels. The ride quality and wet-skid resistance on all of these sections were excellent, he says.
“Given this surprising outcome and the expectation of lower noise levels using rubber, VDOT has installed rubber-modified PFC-12.5 on a section of the Fairfax County Parkway in 2012, as well as a section on the NCAT Test Track,” Clark says.
But every project and choice of materials and mixes boils down to a dollar amount, and Clarks says the use of rubber or other asphalt mixes should be based on the best economic value and benefit for the owner.
“When rubber can be used to reduce the overall cost of the asphalt mix and provide equal or superior performance, then rubber should be used,” Clark says. “The biggest bang for the buck will be in dense graded and gap graded (i.e., stone matrix asphalt) mixes. However, unless the use of rubber-modified binders is permitted by an owner/transportation agency or specified in the contract, then the overall use will be minimal.”
For 2012, VDOT plans to construct additional experimental sections in Virginia and at the NCAT test track. These mixes, which will include a PFC-9.5, a SMA-9.5 and a SMA-12.5, will also use the rubber-modified asphalt binder. Future testing will monitor changes in noise, ride, skid resistance and crack resistance, Clark says.
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