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|Federal Highway Administration > Publications > Public Roads > Vol. 68 · No. 2 > Rustic Pavements|
by Michael P. Dallaire and Scott A. Saunders
Synthetic binders could help designers create more aesthetically pleasing roadways.
Selecting a pavement color for a highway project may someday be as easy as shopping for paint. With the growing interest in context-sensitive solutions for roadways and transportation facilities, Federal and State agencies are looking for more and more opportunities to create transportation systems that respect and protect local environmental, cultural, and historic resources.
When developing pavement mixes, designers traditionally focus on choosing materials that meet stringent standards for quality, durability, smoothness, and cost. "Typically we don't worry about color," says Laurin Lineman, technical services engineer with the Eastern Federal Lands Highway Division (EFLHD) of the Federal Highway Administration (FHWA). "We use a local source aggregate that does the job cost effectively." Except for nuances particular to the chosen aggregate, the range of colors for most asphalt and concrete pavements spans from black to various shades of gray.
But recently FHWA began performance testing on a new synthetic binder that could add "color" to the list of key criteria that pavement engineers consider as they develop their designs. Researchers at the Turner-Fairbank Highway Research Center in McLean, VA, are in the process of studying a clear polymer binder (or resin) to assess its performance characteristics. When combined with an architecturally aesthetic aggregate (gravel), the binder could help designers create "rustic pavements" that look like old dirt roads or historic pavements but have the structural capacity necessary to carry modern traffic loads. Unlike the liquid binder in hot-mix asphalt, which lends a black color to whatever aggregate it covers, a more translucent, synthetic binder allows the natural color of the aggregate to show through.
"Rustic pavement represents one approach to satisfying a number of issues for our partner agencies," Lineman says, "including aesthetics, preservation of historic character, and lying gently on the land." EFLHD, which provides highway assistance to the U.S. Department of Agriculture's Forest Service and the U.S. Department of the Interior's National Park Service (NPS) and U.S. Fish and Wildlife Service, requested the work to support its search for context-sensitive solutions. "Our goal is to add another ‘tool' to the context-sensitive solutions toolbox," says Lineman.
EFLHD recently completed a successful field test at the Richmond National Battlefield Park in Virginia. With ongoing testing at Turner-Fairbank and a high-profile project planned for a section of Pennsylvania Avenue in front of the White House in Washington, DC, rustic pavements are poised to roll out in new applications across the United States.
According to Lou Benvenuti, director of marketing, strategy, and development with the Neville Chemical Company, which developed the material, synthetic binders have been around for several decades. "We did many jobs using a similar binder back in the 1960s and early 1970s," he says. "We initiated work to improve upon the original invention and characterized the binder according to the testing protocols developed from the Strategic Highway Research Program."
The synthetic binder offers two distinct ways to add color to a pavement. In the first, the binder replaces the black liquid binder in hot-mix asphalt, so the natural color of the aggregate becomes the dominant pavement color. A second application involves adding a pigment to the mixer during hot-mix production to create red, yellow, blue, green, white, or other colored pavements.
In Europe, pavement designers use synthetic binders with pigments to delineate intersections, bicycle paths, historic landmarks, busways, and roundabouts. "Adding a pigment to the pavement offers safety benefits, as well, from improved visibility," Benvenuti adds.
Testing at Turner-Fairbank
To assess the performance of the rustic pavement concept, researchers at Turner-Fairbank are conducting a series of laboratory tests to study properties such as durability, weathering, moisture susceptibility, and resistance to rutting.
According to Tom Harman, pavement materials and construction team leader at Turner-Fairbank, his team is comparing the performance of various rustic pavement mixes to a standard hot-mix asphalt that is known to perform well. "Basically we are subjecting samples of both materials to a series of extreme torture tests," he says. "The question we are trying to answer is, ‘Will rustic pavement provide equal or better performance when compared with standard hot-mix asphalt?'"
The tests include laboratory rut testing using the Hamburg Wheel-Tracking Device and the SuperpaveTM shear tester. "More important," Harman says, "we're looking at the aging susceptibility of the clear binder using a weatherometer and assessing the material's infrared spectrum. This tells us the aging properties of the clear binder. We need to make sure the binder will not oxidize too quickly and become brittle."
Once the testing of the clear binder with standard aggregates is complete, Harman and his team will repeat the experiments using the architectural aggregates planned for use on a project on Pennsylvania Avenue in Washington, DC.
The Richmond Site
The first field application of rustic pavement was at the Richmond National Battlefield Park near Richmond, VA. Established in 1936, the park protects 309 hectares (763 acres) of historic ground, commemorating 11 different sites associated with the Civil War.
When a section of Picnic Road in the park began to fail, especially in the wheel path, engineers from EFLHD suggested replacing the existing chip seal with a rustic pavement. The National Park Service agreed, and in September 2003, EFLHD paved a 1.6-kilometer (1-mile) strip of roadway and a parking area using the synthetic binder and brown and tan aggregates to achieve a brown-colored pavement.
"We had a light-brown pavement to start with," says Dan Hodgson, facility manager at the park. "The pavement was placed many years ago and had a brownish aggregate. When we couldn't find a good replacement for the aggregate, we started looking at other options. Our goal was to find a pavement that would look like a dirt road."
According to EFLHD's Lineman, gravel roads do not hold up well to traffic and are maintenance intensive. "We needed to use a pavement," he says, "and the rustic pavement option enabled the park to have a more aesthetically pleasing road that, from the engineering standpoint, can carry traffic."
No special equipment is necessary to place rustic pavement. "You transport the materials in the same trucks, run the materials through the equipment just like you do for hot-mix pavement, and compact and roll the same way," says Thomas Scott, construction operations engineer with EFLHD. "As with typical asphalt pavements, the pavement crew makes the standard adjustments for the mix-specific combination of binder and aggregate. Once it cools, the rustic pavement acts just like a regular pavement."
The Richmond job involved placing approximately 450 metric tons (500 tons) of hot-mix synthetic binder concrete pavement on the parking lot and a section of roadway. EFLHD specified two mix formulas; one was 9.5 millimeters (0.4 inch) and the other 12.5 millimeters (0.5 inch). The project called for two sources of crushed, coarse aggregate.
The contractor paved the parking lot using the 12.5 millimeter (mm) mix, which contained a darker brown aggregate. After three passes with a steel drum roller in vibratory mode and one pass in the static mode, the contractor achieved an in-place density of 92 percent or greater. Compaction occurred immediately after laydown at temperatures near 143 degrees Celsius (290 degrees Fahrenheit). The contractor used a release agent on the steel drums to prevent pickup of the mat during compaction.
Paving on Picnic Road featured the 9.5 mm mix and a lighter, tan-colored aggregate. The contractor needed to hold off on compaction until the mat temperature dropped below 110°C (230°F) because the freshly laid mat of 9.5 mm mix was tearing and picking up in patches when the compaction roller followed closely behind the paver. To avoid tearing, workers in the field need to determine the appropriate temperature for compaction based on the characteristics of the mix.
Results from the Richmond Test
Based on the Richmond project, researchers learned a number of lessons about how to improve future applications of rustic pavement. Most important, with the new goal of producing an aesthetically pleasing color on the final product, the team realized the need for even tighter quality control measures. For example, a portion of the second travel lane on Picnic Road was darker in color than the first lane. According to Scott, the most probable cause was an increase in the amount of fine material, which acted as a pigment to cause the color change. "We're not used to watching out for color during a paving operation, so this is something we'll have to be more conscious of on future applications," he says.
Similarly, the field test highlighted the need for attention to smaller details, such as the tack coat and cleanliness of tools and equipment. The tack coat, for example, which helps bind the new pavement to the old, is black. "Usually that's not an issue because you're placing black on black," says Nelson Clark, project engineer for EFLHD, "But when workers got the tack coat on their shoes or on the equipment, they tracked it back onto the fresh pavement." To avoid this problem in the future, the binder manufacturer is in the process of developing a synthetic tack coat to match the binder color.
Having clean tools and equipment also is essential. "If the equipment still has leftover asphalt in it from the previous job, black streaks will show up in the new pavement," Clark says.
To date, the NPS staff at Richmond National Battlefield Park has been pleased with the performance of the pavement. "The road looks great," Hodgson says. "We've received a number of positive comments from the park's neighbors who live along the border of the park and drive on the road every day."
In fact, Hodgson adds, the park plans to continue using the rustic pavement application when it repaves two entrances and parking lots later this year. "To me, the rustic pavement appears to be a better product than regular asphalt," he says. "We've had trucks and cars on it, and it looks like the rollers just came off it."
A New Look for Pennsylvania Avenue
Building on the success in Richmond, the National Capital Planning Commission and FHWA are planning to use rustic pavement on a restoration project on a section of Pennsylvania Avenue between 15th and 17th Streets in front of the White House in Washington, DC. The project involves creating a new pedestrian plaza that will improve access for the public, provide generous open space, and enhance views of the White House and other landmarks in the Nation's Capital. Other components include granite sidewalks and new site furnishings such as granite benches, streetlamp installations, and landscaping.
"The crux of the project is determining how to transform a roadway into something that has a different identity," says Gullivar Shepard, a senior associate at Michael Van Valkenburgh Associates, the landscape architects working on the project. "The road is closed to vehicular traffic, but still it needs to meet the standards for a traffic-ready roadway," he says.
To respect the historic context, the landscape architect wanted to have a warm-colored, granular surface for the roadway area. "Our original concept for the roadway material in front of the White House was to create a landscape-material connection between Lafayette Park and the White House grounds to make the area more park-like," Shepard says.
But this presented several problems: loose granular pavement would be difficult to maintain, it would represent a security concern in front of the White House, and functionally it would not carry traffic as well. Says Shepard, "We quickly learned that the soft paving system we proposed in our competition entry—decomposed granite—would not work with vehicles such as the presidential limousine, which has the weight of a small tank on four small wheels—it would be like high heels in grass."
The other avenue of exploration was a clear synthetic binder, suggested by FHWA. "In redefining the basic components of asphalt—a binder and stone aggregate—we were presented with the opportunity to elevate the aesthetic quality of the pavement system," Shepard says. The selected paving materials will accommodate pedestrians, skaters, cyclists, and horses, and withstand the loading requirements of delivery vehicles, emergency equipment, motorcades, and inaugural parade traffic.
In August 2004, before beginning work on the Pennsylvania Avenue project, FHWA researchers installed a test strip on another NPS site in Rock Creek Park in Washington, DC, using the synthetic binder and chosen aggregates. The purpose of the test was to help the contractor gain experience handling the materials and matching the colors from load to load.
Construction of the Pennsylvania Avenue project is scheduled for completion by October 2004, just in time for the presidential inauguration in January 2005.
Weighing the Costs
Because synthetic binders are a new product on the market, they do cost more. According to Lineman, the binder itself costs seven or eight times the cost for a standard asphalt binder. "Because of the cost, rustic pavements are only an option for selected applications where the historical and cultural significance justifies the expense," he says. "And you wouldn't do a base course, only a surface course."
Using local materials can help keep the costs down. In Richmond, EFLHD and NPS used brown gravel from a local source, keeping transportation costs to a minimum. With the Pennsylvania Avenue job, however, the landscape architect selected three different aggregates from Maine, South Dakota, and Texas, which drove up the costs.
State departments of transportation and municipalities might consider using rustic pavements selectively for projects in historic districts or for bike paths or trails. "The Richmond project is not that well known," says Gary Brown, technology coordinator for EFLHD, "but once Pennsylvania Avenue is finished, we expect to have more people inquiring about the technology."
A Bright Future?
For most highway projects, designers are concerned with selecting paving materials that will handle traffic loading and fit the budget. But the growing interest in context-sensitive solutions for transportation facilities opens up new opportunities for product innovations like synthetic binders that offer functionality and aesthetic benefits.
"Synthetic binders are one approach, but FHWA continues to work with other partners and manufacturers to identify and test other surface treatments," Lineman says. "The goal is to come up with cost-effective, aesthetically pleasing alternatives that meet the requirements for skid resistance, durability, and traffic loading."
Michael P. Dallaire is a materials engineer at EFLHD.
He has more than 18 years of civil and materials engineering experience focused on the performance and durability properties of constructed systems, and materials quality control and assurance. Dallaire has a B.S. in civil engineering and an M.S. in civil engineering, both from the University of New Hampshire. He is registered as a professional engineer in New Hampshire.
Scott A. Saunders is the division materials engineer at EFLHD. He has more than 10 years experience in geotechnical engineering, pavement design, and materials quality control and assurance. Saunders has a B.S. in civil engineering and an M.S. in geotechnical engineering, both received from Virginia Polytechnic Institute and State University. He is registered as a professional engineer in Virginia.
For more information, contact Michael Dallaire at 571–434–1573 or email@example.com.
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