| Road Science
NCAT Pavement Track Puts Superpave to
Test
What industry has learned from the
first two years of the NCAT test facility.
by Tom
Kuennen, Contributing Editor
Real-world load testing of Superpave asphalt mix designs at a test
track near Opelika, Alabama already is bearing fruit for specifiers and
owners of hot-mix asphalt pavement.
After just two years of use, at the end of the application of 10
million equivalent single axle loads in December, researchers at the
National Center for Asphalt Technology Pavement Test Track at Auburn
University have drawn preliminary conclusions that promise benefits to
asphalt-mix designers, the industry at-large, and for taxpaying road
users.
These include:
- What little rutting took place in test sections occurred only during
summer, and there was less during the second summer even though the
average temperature was hotter than the first summer. Rutting stopped
altogether after the seven-day average high air temperature dipped below
82 degrees F.
- The small amount of rutting observed probably was related to “densification”,
or long-term compaction of mix under traffic.
- Under this truck traffic torture, mixes using PG-67 asphalt binder “densified”
more than mixes using the stiffer PG-76 asphalt binder.
- Adding an additional 0.5% asphalt binder increased the rutting in the
PG-67 mixes by a whopping 50%, but had negligible effect on PG-76 mixes.
“It may be possible to design mixes, with higher PG grades, at slightly
higher asphalt contents to improve durability,” NCAT reported.
- The amount of rutting was more than 60% less in sections with the
stiffer PG-76 binder than with PG-67.
- Performance of coarse-graded and fine-graded mixes was approximately
equal from a rutting standpoint.
These initial findings were released at a National Transportation
Symposium at NCAT November 13 and 14, which brought nearly 200 luminaries
— from both public and private sectors — to Auburn. Speakers described
what they’d learned and the early data were gathered into a proceedings.
“The purpose of the symposium was to deliver the results of the first
cycle of the test track,” said NCAT director Dr. Ray Brown, P.E. “We
had intended to go for two years with 10 million ESALs of truck traffic,
and evaluate the sections at the end of two years. The symposium brought
those results forward.”
Validating Superpave lab tests
Evaluations during the first two-year cycle included performance of
fine-graded verses coarse-graded mixes, the effect of asphalt grade on
performance, the effect of aggregate type on performance, and performance
of several mixture types, including Superpave, stone matrix asphalt, and
open-graded friction courses.
Other studies included the effect of grinding transverse joints on
performance, effect of traffic on friction, permeability of various HMA
mixtures, densification (post-placement compaction of mix by traffic over
time) of HMA, and the effect of pavement smoothness on fuel consumption.
But most significantly, the NCAT Pavement Test Track is essential to
the future use of HMA in the United States because it is providing field
validation for Superpave test procedures and results gained in the lab.
Superpave — an acronym of Superior Performing Asphalt Pavements —
requires a performance-based binder specification. Because the engineering
attributes of these binders are known in advance, a pavement mix engineer
may expect them to behave in a certain way under traffic loads, speed of
traffic, and in their ambient environment, including average maximum and
minimum temperatures (each a factor of the Superpave specification). This
is a big leap forward from old-fashioned viscosity-based grading because
viscosity could not be related to performance of hot-mix asphalt.
The ability to quantitatively measure performance in the field over
time — as undertaken at the NCAT Pavement Test Track — is a major tool
for making sure Superpave mix designs work the way they’re supposed to,
beyond the testing that takes place in the lab.
“We have identified several tests with potential to predict rutting,”
Brown told Better Roads. “Some are being looked at by a National
Cooperative Highway Research Program study [NCHRP 9-19] to develop a test
to predict performance. We looked at those tests — along with other
tests — and verified that at least one looks promising.”
Three lab tests for rutting have been field-evaluated at NCAT:
- Early NCAT results indicate that the “dynamic modulus test”, a
means to measure the stiffness of a mix under a repeated load, did not
correlate well in the NCAT performance test results.
- The “repeated load test”, which applies repeated loads to a
confined sample, looks promising, based on field experience at NCAT.
- And a so-called “creep test” — in which a specimen deforms over
time, under a static load — is still being studied.
Superpave: Fine verses coarse mixes
The debate over the performance of fine- versus coarse-graded Superpave
mixes was studied at the NCAT test track, on behalf of its state DOT
sponsors.
“With Superpave, we have fine-graded and coarse-graded mixes,”
Brown said. “There has been a question over which performed better.
WesTrack (a private test track facility near Reno) showed that fine-graded
mixes did better, even as most states were recommending coarse-graded
mixes, so there was a real question as to which performed best.
“We found the coarse-graded performed slightly better, but for
practical purposes there was no difference,” Brown said. “These were
test sections that were side by side. So we feel strongly that we can do
either mix and get good performance from a rutting standpoint.”
Another issue is modified asphalt binder for Superpave, now being
promoted under the moniker “Superpave Plus”.
“We looked at the effect of modifiers,” Brown said. “Most states
are using PG-64 as a binder. What would happen if we bumped the grade up
to a PG-76? We know if we bump it up to a PG-76 it should be stiffer and
perform better, but this hasn’t been quantified.
“We found that if we bump the grade up, the amount of rutting will be
about 60% less,” Brown told Better Roads. “And we also showed that if
you use the higher grade of PG-76, you can put a little bit more asphalt
in and still get good performance, which will improve durability without
increasing rutting.”
Yet another issue the NCAT test track is shedding light on is type of
aggregate used in Superpave. “We also looked at the effects of using
different types of aggregate,” Brown said. “We know different
aggregates perform differently. One question is of the type of aggregate.
Another is, does crushed gravel perform as well as crushed stone.
“We looked at a number of gravel sources and got very good
performance with them,” he said. “That doesn’t indicate all crushed
gravels are good; just that the ones we looked at performed well. We put
down SMA mixes that are performing excellently with crushed gravel.”
In service since 2000
In mid-December the NCAT Test Track ended a two-year cycle in which
10-million equivalent single axle loads were applied to the track, equal
to 1.6-million miles having been driven on the track.
Operation of the 1.7-mile test track began in September 2000, when four
fully loaded trucks with three trailers per tractor began their work.
Each tractor pulled a load of approximately 152,000 pounds, 20,000 for
each of seven loaded axles, and approximately 12,000 pounds for the
front-steer axle, NCAT said. The triple-trailer units were driven by
professional drivers at 45 miles per hour for 17 hours a day (six days a
week) in order to apply the required 10-million ESALs of traffic to the
track within two years.
The use of actual trucks in this accelerated, closed-loop format allows
multiple test sections to be evaluated simultaneously, in a loading
situation very similar to that on highways.
Under conventional conditions a government agency would have to wait
for at least 10 years for such an accumulation of real world data. But
instead of unpredictable combinations of truck and auto traffic on public
roads over many years, the track is an isolated pavement in which the
accelerated axle loadings are perfectly observed under consistent
environmental conditions, and data gathered, compiled, and analyzed.
“The test track approach is less disruptive to traffic, safer for
workers, and the amount of traffic applied to the test sections is better
controlled,” NCAT said.
Traffic will continue on many of the sections for another two years to
better identify mixes that will provide better performance, and to
determine lab tests that will correctly quantify the performance, NCAT
said.
Sections comprise track
As constructed by NCAT, the first phase of track testing incorporated
46 sections, averaging 200-feet long, each of a different permutation of
asphalt mix or lift design, as specified by one of the co-sponsoring state
DOTs, or by the Federal Highway Administration.
Through a competitively bid contract administered by the Alabama DOT,
APAC-Alabama, Inc.’s, Couch Division, Dothan, Alabama was selected to
build the track.
As a condition of the work, Couch had to supply an on-site plant, a
material transfer vehicle, a rubber-tired roller and other compactors, and
other types of equipment to meet particular specification requirements.
The track consisted of 26 sections in the tangents (straightaways) and
another 20 sections in the curves. “Initially, only the tangents were
going to be evaluated,” NCAT reported, “but it was decided that the
curves should not be completely wasted, hence test sections were also
constructed in the curves.”
The pavement was designed with sufficient thickness to ensure that no
structural damage would occur during testing, and that failures should be
limited to the surface or near surface layers of the HMA, NCAT said.
Because so many varieties of mix were to be studied, aggregates were
brought in from different states for the test sections, including
limestone, granite, marine limestone, gravel, and slag. Reclaimed asphalt
pavement also was used in a few sections.
Over 60 different stockpiles were selected to be used to construct the
test sections. Generally, each state DOT sponsor provided funds to pay the
expenses of constructing, testing, and analyzing two sections on the
track.
Weekly monitoring
Each section’s condition was monitored weekly to evaluate rutting,
cracking, and other surface problems. On a monthly basis, testing was
conducted to evaluate friction, roughness, falling weight deflectometer
measurements, and densification.
Also, the pavement structure was instrumented during construction to
determine moisture in the subgrade, and temperature at four elevations in
the pavement structure. These data were collected on a continuous basis
throughout the life of the project, NCAT said.
“At the end of the project the measured performance was correlated to
various mixture and material properties to help identify those properties
that could be used to ensure good performance,” NCAT said.
Direct comparisons of HMA mixtures containing PG 76-22 and PG 67-22
performance-graded binders were undertaken, with all other mix properties
constant. This allowed a direct comparison of the performance of mixes
containing the two grades of asphalt binder.
Track not only NCAT activity
There’s more to NCAT than its renowned test track. Current NCAT
research beyond the test track include:
- Studies of moisture susceptibility in asphalt pavements.
- Investigation of an automated way to run a fine aggregate specific
gravity test.
- Investigation of a potentially better way to measure density of
coarse mixes, the Corelok method.
- Improvement of measurement of asphalt content with the NCAT-developed
Ignition Oven for aggregates with high weight loss.
- Noise-attenuation by HMA pavements.
- NCAT’s sponsored Professor Training Course, which has graduated
some 300 educators in engineering since 1988.
NCAT itself was created in 1986 through a partnership between the
National Asphalt Pavement Association’s Research & Education
Foundation, and Auburn University. Members of NAPA gave generously to the
foundation and the fund was able to provide construction money, as well as
continuous financing and support.
“NCAT was founded with a goal of developing technologies that would
improve the performance of hot-mix asphalt pavements while reducing life
cycle costs of HMA,” said NAPA president Mike Acott at the NCAT
symposium held in November.
An important element of NCAT is that it is functionally independent of
NAPA, Acott said. “We set up NCAT to be at arm’s length from NAPA,”
he said. “We wanted objective, credible, and practical research that
would improve our product and let the chips fall where they may.”
Now NCAT stands as a strong example of public/private sector partnering
to solve common problems. “One reason NCAT works so well is the
partnership between academia, contractors, FHWA, state DOTs, and the
Transportation Research Board,” Acott said, “not only in identifying
the right projects, but with the implementation of results.”
The sponsors of the first, 10-million ESAL-cycle at the track included
the state departments of transportation of Alabama, Florida, Georgia,
Indiana, Mississippi, North Carolina, Oklahoma, South Carolina, and
Tennessee, as well as the FHWA. Other support was provided from the
private sector in the HMA industry, including APAC, Astec
Industries, Caterpillar,
Compaction America, Vulcan Materials, Ergon, and
Koch Materials.
NCAT to Host Symposium on Long-Lasting Pavements
An international symposium on Design and Construction of Long-Lasting
Asphalt will be held under the auspices of the National Center for Asphalt
Technology at Auburn University, Alabama, June 7-9, 2004.
Sponsored by the International Society for Asphalt Pavements and
co-sponsored by the National Asphalt Pavement Association and the Federal
Highway Administration, the objective of this conference will be to
address mix design and construction procedures to ensure that
long-lasting, “Perpetual Pavements” are built.
Emphasis will be on materials, mix design and construction, and
procedures used for pavement design for long lasting pavements (perpetual
pavements).
Topics will include materials and mix design, construction issues,
quality control/quality assurance, contracting methods, design for
Perpetual Pavements, and more.
The conference will begin with a reception on Sunday, June 6, 2004 and
conclude on Wednesday, June 9. An optional tour of the NCAT laboratories
and test track will be provided on Thursday morning. A spouses’ program
will also be available.
Call for papers
Abstracts of potential papers must be received by NCAT by June 1, 2003.
The abstract length should be 300 words or less and should be provided by
e-mail to NCAT director Dr. Ray Brown, P.E.
Abstracts will be reviewed and the potential authors will be notified
by August 1, 2003 concerning the acceptance of their abstract. Full papers
will be due by January 1, 2004. The authors will be notified by April 1,
2004 about the acceptance of their papers and concerning any modifications
that may be necessary. Final papers will be due by May 1, 2004.
For more information, contact NCAT’s Carol Tapley at 334-844-6228, or
e-mail her at ctapley@eng.auburn.edu.
NCAT — A Powerful Roadbuilder Resource
The National Center for Asphalt Technology is a public resource for
users and specifiers of hot-mix asphalt. Its Web site, http://www.eng.auburn.edu/center/ncat, provides dozens of new and recent technical papers on
asphalt, downloadable at no charge (hard copies are available for a fee).
An archive of NCAT’s valuable newsletter, Asphalt Technology News —
downloadable in .pdf files — also is available. And capsule descriptions
of current “Research in Progress” in HMA in all 50 states is available
off the Web site as well: click on Information on the home page, and then
Research in Progress.
NCAT was to post the proceedings of the November National
Transportation Symposium to its Web site. Until then you may obtain the
proceedings on CD-ROM by contacting NCAT at 334-844-NCAT.
Then, visit NCAT’s Pavement Test Track Web site at http://www.pavetrack.com/.
Not only will you receive current information on the track, you can watch
real-time video shots of the test track. There also are links to other
test tracks in Europe and around the world.
Reprinted from Better Roads Magazine
February 2003 |
