When it comes to testing hot-mix asphalt (HMA), one can scarcely top the usefulness of real-world performance results.
That is the basic principle behind the construction by the National Center for Asphalt Technology (NCAT) of the new $8-million NCAT Pavement Test Track in Auburn, Ala. When coupled with the new $3.5-million NCAT Research Center, the facilities make a unique worldwide combination of HMA testing, training and research.
Construction of both the test track and the 40,000-sq-ft research center were finished in July. And following completion of some instrumentation, NCAT has scheduled truck traffic to start up on the test track this month.
"It’s a perfect combination of having a world-class research facility that will combine laboratory research, classroom activities and the construction and performance of real-world pavements," said Mike Acott, president of the National Asphalt Pavement Association (NAPA). The NAPA Research and Education Foundation (NAPAREF) funded construction of the NCAT Research Center. On behalf of NCAT, Auburn University will lease the building from NAPAREF.
The cost of test track construction, meanwhile, was largely paid by the Alabama Department of Transportation. As for operating costs, state departments of transportation are sponsoring various test sections on the track. The original sponsors are nine state DOTs and the Federal Highway Administration (FHWA). For two years of testing on one 200-ft-long, 12-ft-wide section of pavement, the cost runs about $250,000. Much of that goes for the shared cost of the truck traffic that will provide accelerated load testing.
The combination of the new research center and test track represent "a unique partnership of industry, academia and the highway departments—and it has been that way from the start," said Acott. "This is not NAPA’s lab. The board of directors of NCAT consists of this partnership—industry, academia and our customers—including FHWA, AASHTO and TRB. The research facility will directly benefit all the partners. For example, NCAT’s professor training program will take place at the new facility."
Acott said more than 200 university professors already have gone through NCAT’s professor training program in its existing facilities.
NCAT meets the needs
The industry’s need for training stems from a variety of sources, said Ray Brown, director of NCAT. Most states are going to a certification program for inspectors and technicians, so they need to be trained to become certified. Secondly, turnover in the industry creates a constant volume of new people to be trained. "The industry is so large that if even 1% leave each year, it’s still a large number of people," said Brown.
Because few universities offer formal courses in hot-mix asphalt, even graduate engineers may have little or no training in HMA, Brown pointed out. "Also, we quite often have new technology coming online—Superpave is a major example—and people need to be trained in these new technologies," he said. Plus, people experienced in HMA often go back to school for refresher courses.
Not only did the industry’s needs for research and training spur forward construction of the new facilities, but NCAT’s recent, rapid growth played a role as well. "NCAT has grown by leaps and bounds, both in terms of the research and training it does and in the number of employees," said Acott. "It was outgrowing the facility it had, and the NCAT board decided that to get the full benefit of the organization, it needed a world-class facility to be housed in one building."
Test the tests
A primary near-term goal at the new NCAT Research Center and NCAT Pavement Test Track is to discover which test or tests best predicts HMA performance in the field. "We want to develop ways to test hot-mix asphalt so that the tests will predict how the HMA will perform in the field," said Brown.
"One of the top goals is to use the test track to validate and calibrate a simple performance test that is the major missing piece of our Superpave design procedures. These facilities will allow NCAT to determine if various performance tests can predict how the pavement will perform under traffic. That is the over-arching goal," added Acott.
Brown said this process of "testing the tests" takes both the laboratory and the test track working together. "We do the testing in the research center, and the track is used to validate these tests. And the results from the test track can be used to determine the effectiveness of various laboratory tests in predicting performance."
The NCAT Research Center is designed as a state-of-the-art facility for teaching, training and research. The 40,000-sq-ft building will house classrooms, teaching laboratories, office space and an HMA research laboratory. "In the classroom area we have an auditorium and three laboratory areas to teach students," said Brown. "The teaching laboratories are separate from the HMA laboratory. That way we can teach students without disturbing the research laboratory."
In the research laboratory, there are separate areas for binder testing, aggregate testing and HMA testing. "And with the equipment we have, we will have the capability to run nearly any test there is to characterize hot-mix asphalt," said Brown. The lab will have about $1.5 million of testing equipment, including a full set of binder testing equipment, a set of aggregate testing equipment and a set of testing equipment for HMA itself.
Among its various research studies, NCAT currently has three research projects under way for the National Cooperative Highway Research Program (NCHRP). Research on those studies moved into the new research center in late July.
The NCAT Pavement Test Track is a 1.7-mile-long oval. The pavement is 32-ft-wide, but only the 12-ft-wide outside lane will carry traffic. There are 46 test sections on the track. The states running tests are: Alabama, Florida, Georgia, Indiana, Mississippi, North Carolina, Oklahoma, South Carolina and Tennessee. FHWA has taken two test sections. About two-thirds of the mixtures being tested are Superpave mixes, Brown estimated.
The goal of the test track is to accelerate truck traffic loading to perform 10 million equivalent single axle loads over a two-year period. That is equivalent to 10 to 12 years of traffic on a fairly high-volume interstate highway. At least four, and up to six, tractor-trailer rigs at a time will each pull three trailers to apply the loading. The design speed of the track is 45 mph.
The mixes being tested are 4-in. thick, and many states are placing two mixes to make up the 4 in. The pavement base is designed to be strong enough to be permanent. After two years of testing, the top test layers of pavement will be milled off and replaced with new HMA mixtures for testing. The pavement base starts with a 1-ft layer of improved subgrade soil, followed by 6 in. of crushed granite and 5 in. of permeable asphalt-treated base stone. All that is topped by a 9-in. layer of Superpave lower binder course mix with 1 1/2 in. top-size aggregate and a 6-in. layer of Superpave upper binder course mix, also with 11/2 in. top-size aggregates. The contractor for the test track earthwork was W.S. Newell; for test track paving the contractor is APAC Inc., Couch Construction Division.
The design of the test track fits with the concept of the "perpetual pavement" being developed by the HMA industry. Perpetual pavements use multiple layers of durable HMA. When the surface layer needs maintenance, it can be milled off for recycling, then replaced in a timely and cost-effective manner—but the road structure never needs to be entirely removed and replaced.
All states except Indiana have delivered their own local aggregates and binders to the test track for use in building the test sections. Indiana is using a local Alabama aggregate that is similar to its own. Tennessee, for example, is testing four mixes on two test sections, said Buzz Powell, test track manager. Georgia has two test sections, one topped with stone matrix asphalt (SMA) and one topped with a Superpave mix. The same binder course is used in both sections. "We’ll be able to tell Georgia whether the difference in performance between their SMA and their Superpave is worth the extra upfront cost of SMA," said Powell. For another example, he said Oklahoma is comparing a traditional Hveem mixture with a Superpave mix.
FHWA is enthusiastic about the potential results from the NCAT Pavement Test Track. "FHWA believes that a resource of this nature is very important to the nation," said Byron Lord, deputy director at FHWA’s Office of Pavement Technology. "By accelerating the loadings, we can see how these pavements are going to perform in the future. If we come up with a good idea that we think is going to improve performance, and we find it works at the test track, then we can begin to use the technology sooner, rather than waiting years to evaluate the full-scale pavements in place."
The two FHWA sections will test an HMA mixture with the same limestone and slag aggregates, but with different asphalt contents, said Stuart Malley, a graduate research assistant who is helping with track construction. FHWA will combine its sections with Alabama and Indiana to look at the effect of binder grade, modifier type and asphalt content upon the performance of the mixes.
From each HMA mixture used at the test track, technicians have taken 70 gyratory compactor samples, or "pills," and 20 vibratory beam samples (the vibratory beam is a sample measuring 300 mm long x 75 mm x 150 mm). The samples were taken from the HMA immediately after it was mixed, while it was still hot. Reheating samples was intentionally avoided to ensure that lab sample properties would match roadway mix properties. Other samples will be taken from the pavements after they’re compacted in the field.
NCAT plans to compare test results from samples that have differing air void contents, to see which samples and tests produce the most accurate predictions of performance. Samples compacted in the lab, for example, typically have about 4% air voids, while roller-compacted HMA samples from the field have about 7% air voids.
"We’re going to evaluate both beams and pills at different levels of compaction to see which ones do the best job of predicting performance," said Powell. "There’s a difference of opinion about which samples are best to run the tests on. We can also examine the air voids issue because we will have both lab-compacted samples and field-compacted samples."
Powell said if the samples compacted with design effort turn out to produce the best predictions, it will save having to take, compact and test a separate set of samples from the HMA just out of the plant. Specimens compacted with design effort are often called quality-control (QC) samples. "We’re making a large set of QC-type specimens in addition to a large set of specimens compacted to approximate mat densities," said Powell.
"We can run tests on the samples and predict the rutting susceptibility of those specimens and then compare those results to actual measured deformation results from the field over the two-year life of the study. We’re basically applying a pavement life’s worth of truck traffic in a two-year period of time."
Various rut testers
One of the first orders of business at the new NCAT Research Center, said Brown, will be to characterize test track samples with a battery of different tests. Such tests will include the resilient modulus test, the indirect tensile strength test, various pavement deformation tests, extractions of the asphalt cement from the aggregate and various tests on the asphalt cement binders and aggregates.
Powell said the primary goal of the test track research is to find which test or tests can best predict rutting. To that end, NCAT has lined up several testing devices whose results will be evaluated in light of actual pavement performance. Such rutting tests include three loaded wheel testers, three types of creep testing and tests involving properties of gyratory compactor samples.
In addition to rut testing, Powell said, first-round test track research will include "all types of surface distress," including cracking, friction testing, smoothness testing and tests with the falling weight deflectometer. "Every week on Monday we’ll shut down the truck traffic and dedicate the day to collecting data," said Powell.
The test track pavement is being heavily instrumented to monitor temperature and moisture conditions. "For each test section we’ll monitor temperature at four depths," said Malley. And moisture detectors will measure moisture in the subgrade. "Temperature and moisture levels will all be detected and are hard-wired back to our computer," said Powell. There also is an on-site weather station at the test track.
Results on the web
"We hope to build a window to the project through our webpages," said Powell. "That way all our sponsors can log onto the web and track the progress of their test sections.
"I envision a weather-type box on a webpage; we would put up air temperature information and rainfall for the past 24 hours. Plus, we hope to put the latest test results on the website so that any sponsor can go to the webpage and read test results on his section."
Are the NCAT Pavement Test Track and the NCAT Research Center being set up to further refine and develop Superpave as a mix design system? "We’ll do that, but it’s not the primary purpose of the facilities," said Brown. "To me our goal is to improve hot-mix asphalt. I don’t think we need to put any special emphasis on Superpave."
Brown said he estimates that some 20% of the work at the new laboratory will stem from the test track, and the remainder will be research and training from other sources, such as NCHRP. NCAT already has a 15-person staff for the new research center. "Within a short period of time we’ll get to 20 full-time people, and we’d like to have 15 graduate students. We’ll use about 10 undergraduate students," said Brown.
NCAT’s most popular course these days is the Asphalt Technology Course, which covers all aspects of mix design, materials, mix production, laydown and compaction. The course takes a week and costs $900.
Work at the NCAT Pavement Test Track is summed up perfectly by FHWA’s Byron Lord: "We’ve put all the ingredients together and put it in the oven. Now we’ve got to see how it comes out."