Leaders within the National Asphalt Pavement Association (NAPA) in the mid-80s envisioned a research center that would advance pavement technologies and improve asphalt pavement performance under increasing traffic loads. They also saw this center providing the necessary training for pavement engineers and technicians to help move new technologies from the laboratory into every-day practice.
The NAPA Research and Education Foundation made this vision a reality in 1986, forming a partnership with Auburn University to establish the National Center for Asphalt Technology (NCAT). Twenty-five years later, NCAT celebrates this milestone anniversary with the numerous research and outreach accomplishments NAPA anticipated and with many more possibilities to help the industry grow.
"All our asphalt paving programs – research, outreach, educational – there may be some that as good, but I find it hard to believe there are many better than Auburn's and NCAT's," said Dr. Paul Parks, an original member of the NCAT Board of Directors.
This article highlights the NCAT's most influential research and outreach projects since the center opened 25 years ago, with just three employees and 5,300 square feet of lab space in its original location.
The NCAT Pavement Test Track, research laboratory and field equipment have allowed researchers to develop practical and cost-effective solutions to problems facing the asphalt paving industry. And there are still many great opportunities ahead to improve pavement performance, said Dr. Randy West, NCAT director.
"As pavement engineers, we now better understand the stress states and temperatures in pavement structures, and with better lab tests to assess asphalt mix properties under those conditions, we will be able to better explore innovative paving materials," West says.
Mix designs
NCAT has led the way in the asphalt paving industry in developing and refining innovative mix designs. For instance, test track results indicate that even high reclaimed asphalt pavement (RAP) content mixes – up to 45 percent RAP – can provide excellent rutting performance and durability. Similarly, the track has shown that warm-mix asphalt (WMA) can be as durable and rut-resistant as hot-mix asphalt for at least 10.5 million equivalent single-axle loads (ESALS).
NCAT has also helped pavement engineers create stronger, more durable mixes by modifying stone-matrix asphalt (SMA) to align with standard U.S. mix design practices. SMA was originally developed in Europe. "Usually, with strength and durability, to get one you sacrifice the other," says Dr. Ray Brown, NCAT director from 1991-2007. "The technique used to design SMA gives it both qualities."
NCAT researchers have also improved the mix design procedure for open-graded friction course (OGFC) and refined Superpave mix design tests and specifications to correct some early problems that occurred with Superpave.
New test methods
The ignition method is one of NCAT's most notable contributions to the asphalt paving industry, providing a safer, less expensive alternative to solvent extraction for determining asphalt content of plant mixes. With the ignition method, an HMA sample is subjected to extremely high temperatures in a furnace, burning off the asphalt binder. The difference in weight of the sample before and after the ignition indicates the asphalt content of the HMA mixture. "The NCAT furnace revolutionized that whole process," said Charlie Potts, an original member of the NCAT board of directors.
NCAT has also provided the industry with fast, efficient ways to test pavement characteristics such as surface friction and permeability. The Three-Wheel Polishing Device uses rotating pneumatic rubber tires to simulate wear caused by traffic on actual pavements, preparing slabs for testing surface friction using the Dynamic Friction Tester (DFT) and surface texture using the Circular Texture Meter (CTM). NCAT devised a field permeability test that is not only fast, but easy to use and transport. This method measures the drop in water level in a standpipe during a specified amount of time to determine pavement permeability.
Bond-strength testing was developed at NCAT as well. This destructive-testing procedure involves placing cores in a bond-strength loading frame and shearing apart layers using the Marshall load frame apparatus Interface bond strength is then calculated by dividing the maximum shear load by the cross-sectional area of the core.
Pavement structural design
One way NCAT has impacted pavement structural design is through its test track validation of mechanistic-based design models. The stress and strain measurements at the bottom of asphalt layers in track sections have correlated well with predicted values, using these models for a wide range of pavement materials and thicknesses at high speeds.
NCAT also used results of the NCAT Test Track to revise the asphalt layer coefficient used in pavement design, potentially saving highway agencies millions per year in construction costs. Funded by the Alabama Department of Transportation, this study examined structural test section information from past track cycles and resulted in an increase of the asphalt layer coefficient by 18.5 percent. This translates to an 18.5 percent reduction in design thickness for new pavements.
The industry also benefits from the results of a strain threshold study on the test track, which showed that pavements can withstand higher levels of strain without accumulating fatigue damage. This allows perpetual pavements to be designed with thinner cross-sections and, thus, makes HMA more competitive against other pavement types.
The test track has become NCAT's research centerpiece, representing about half of the center's annual research program. Brown calls the test track "our proving ground," as it unites real-world pavement construction with live heavy trafficking for rapid testing and analysis of asphalt pavements. "We can run tests and predict performance in the lab, but we have to wait on results. The test track does that for us," Brown says.
The 1.7-mile track is funded and managed as a cooperative project among highway agencies and industry sponsors, who sponsor 200-ft test sections on three-year cycles. Heavily loaded tractor-trailers circle the track five days per week for 16 hours per day, applying 10 million equivalent single-axle loads (ESALs) to the pavements during each three-year research cycle. Each sponsor has specific research objectives for their section(s) and shared objectives for the track as a whole. Three cycles have been completed since the test track opened in 2000, and a fourth cycle will end in 2011.
"When we started the track, we were the only track in the country that could do what we're doing – where in two years, you get 15 years of wear (on the asphalt pavement)," says Mike McCartney, an original member of the NCAT Board of Directors.
NCAT recently highlighted major research from the track in a video and accompanying brochure, both available at www.ncat.us.
NCAT's education and training efforts have rounded out its mission to be a world-wide technical leader in asphalt pavement community. A particular outreach project that is cited by many people in the pavement industry is the NCAT textbook, Hot Mix Asphalt Materials, Mixture Design and Construction. This was the first-ever comprehensive textbook on HMA technology when initially published in 1991, and Potts says there was a pressing need for it. "If you are going to try and get students more informed in respect to HMA, you need a reference book," says Potts. "There were technical papers, but not a reference book."
A source of information for practicing engineers as well as students, the textbook has undergone two revisions since first published. The most recent version, published in 2009, contains topics such as manufacturing and evaluation of asphalt materials and aggregates; HMA mixture design; and characterization of HMA in terms of engineering properties, construction, performance, and maintenance and rehabilitation of aging HMA surfaces. "What NCAT did was not only write a good textbook, but keeps it up to date," Parks said. More than 15,000 copies of the textbook are in circulation.
Almost 1,000 people attend NCAT's training classes each year, including the annual NCAT Professor Training Course. Conducted for the first time in 1988, this course is so important because you have to thoroughly and properly train asphalt technology instructors before you can train students, Potts says. And you have to get these professors interested in making this area of civil engineering a larger part of their curriculum. "Before the Professor Training Course, the time many universities spent on HMA and asphalt pavement construction was maybe a week," Potts added. The eight-day summer course consists of intensive lectures, laboratory exercises, and discussions to investigate all phases of asphalt technology.
As a way to disseminate NCAT's research findings and training opportunities, Asphalt Technology News has been arriving in practicing engineers' mailboxes – and, now, e-mail inboxes – since January 1989. McCartney said that outreach efforts such as the newsletter are critical to garnering more publicity for the asphalt paving industry. "This is a very competitive market today," he added. Last year, NCAT engineers also relayed research findings and updates through the 85 technical presentations they gave at national conferences, association meetings and regional meetings.
Not all of NCAT's successful outreach efforts come in the form of a glossy publication, presentation or training course, however. Some of NCAT's and Auburn University's biggest accomplishments are the revered leaders they've turned out to the industry, say both Dr. Brown and Dr. Parks. "We have so many great people (from Auburn) working for contractors, DOTs, and the FHWA instead of just for the university," said Dr. Brown.