Where the Rubber Quiets the Road
Creating mixtures for smoothness and low noise
By Lauren Heartsill Dowdle
Grinding, loud, uneven roads can leave drivers and their vehicles shaken up after a long ride. So it’s no surprise that road conditions are the public’s No. 1 criterion for satisfaction, according to a 2002 Federal Highway Administration (FHWA) survey.
“Noise can affect the passengers in a vehicle, as well as the people living and working along a road,” says Trenton Clark, director of engineering, Virginia Asphalt Association (VAA), in Richmond, Va. “The owner should keep that in mind when specifying a treatment for a roadway – either new construction or resurfacing. It is a balance between managing costs and benefits.”
And although a road project’s checklist might not include the end-user’s approval, smoothness standards and specifications continue to be an important part of many road builders’ contracts and planning stages.
“Several factors go into achieving a smooth ride: mix design, mix delivery to the project, proper paver operation and compaction,” Clark says.
But eliminating bumps is not the only, or main, benefit of laying an even surface – a National Cooperative Highway Research Program analysis shows improved smoothness extends pavement performance life by up to 50 percent.
“Transportation agencies have a goal to provide the lowest lifecycle cost for road treatment,” says Jim Barnat, vice president of innovation, Road Science, a division of ArrMaz. “As secondary considerations, these agencies look at aesthetics, ride quality and quietness characteristics. They generally focus on durable pavement structures and crack-resistant surfaces, providing the best return on their investment.”
On the road to create lasting, smooth surfaces, some have stopped to address noise concerns, as well. These departments of transportation, including those in Arizona, Florida, Minnesota and Kansas, have tested pavement alternatives by using rubberized-asphalt mixtures near residential and sound-sensitive areas to decrease the traffic noise.
“Road noise is generated from two main sources – a vehicle’s engine/drive train and the tire-pavement interaction,” Clark says.
Virginia roadbuilders conducted an experiment with various surface mixes and aggregate sizes with the goal of discovering which materials created quieter, yet smooth, roads.
When a bill was introduced in Virgina in 2011 to develop quiet-pavement technologies to aid in sound mitigation, the Virginia Department of Transportation (VDOT) constructed road demonstrations to test various surface mixtures.
VDOT and its asphalt industry partner, the VAA, created noise-reducing surface mixes to be used on the projects, with each site including a control section and three experimental sections.
“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.” Trenton Clark, director of engineering, Virginia Asphalt Association (VAA)
The quiet pavement project on State Route 7 went to Superior Paving, and Branscome won the remaining two sites – one west of Williamsburg on State Road 199 and another on State Road 288.
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.