The first two-year cycle of tests at the National Center for
Asphalt Technology (NCAT) Test Track, Auburn, Ala., was completed recently. The
testing of 46 different sections was conducted by subjecting the pavements to
the equivalent of 10 to 12 years of traffic loading in two years.
Heavier-than-normal trucks punished the pavements with 10 million equivalent
single axle loads (an ESAL equals one 18,000-lb single axle load).
The results of this first series of tests were presented at
the National Transportation Symposium at NCAT held on Nov. 13-14, 2002. Most of
the 250 attendees were representatives of the state departments of
transportation that sponsored test sections at the track.
Here are some of the findings:
* A number of mixes with varying aggregate types, asphalt
grades and aggregate gradations provided good performance;
* The total average rut depth was very small--.12 in. after
10 million ESALs. The worst-performing sections had .25 in. of rutting;
* Mixes that were intentionally designed to be more likely
to rut did result in the greatest rut depths, but the magnitude of these ruts
was still small;
* Fine-graded and coarse-graded mixes provided approximately
* The higher PG grades clearly resulted in lower rut depths,
and the results indicate that potentially more asphalt binder may be added to
mixes using higher PG grades to improve durability without increasing rutting;
* Several of the tests that were evaluated have been found
to be useful in helping to identify mixes with rutting potential.
Carrying a lot of weight
Empirical laboratory tests have been used for years to test
hot-mix asphalt (HMA) to determine the potential for various mixtures to
perform satisfactorily. As the amount of traffic has increased (higher volumes,
higher loads and increased tire pressures) the ability of these laboratory
tests to evaluate potential performance has become more important.
The best way to quickly evaluate and verify the potential
for a test to be used is with
accelerated loading facilities. The test track at NCAT is one method that can
be used to rapidly test a large number of mixes simultaneously using full-scale
The sponsors of the first cycle at the track included the
Alabama Department of Transportation, which financed the overall construction
of the track, as well as the Federal Highway Administration (FHWA) and other
state DOTs, which sponsored test sections. The participating DOTs were Florida,
Georgia, Indiana, Mississippi, North Carolina, Oklahoma, South Carolina and
The 46 test sections constructed used various aggregates,
grades of asphalt and various mixture types. Some mixtures were designed with
marginal aggregates and some mixtures were designed with .5% additional asphalt.
Several mixture types were used including fine- and coarse-graded Superpave,
stone matrix asphalt, open-graded friction courses, as well as some variations
of these mixtures.
As the NCAT Test Track was constructed, 184 temperature
probes--four for each of the 46 test sections--were installed at various depths
within the pavement layer. Moisture gauges also were installed in each of the
test sections at the surface of the improved subgrade.
Four tractors pulled triple-trailer assemblies around the
track at 45 mph for 17 hours a day, six days a week, in order to apply 10
million ESALs to the pavements within two years. The seven axles of the
trailers were loaded to a total weight of 20,000 lb each (including 12,000 lb
on the steer axle), resulting in a total gross vehicle weight of each rig being
approximately 152,000 lb. Although the main focus of the research was the
accelerated performance of the test sections, data collected and observations
made in support of the trucking operations have provided valuable information
upon which future trucking operations can be refined.
Measurements and observations were made weekly to determine
the overall performance of all of the sections.
Effects of distress
The primary purpose of the track was to evaluate various
mixture types and to evaluate the ability of laboratory tests to predict
performance. The only distress expected at the test track was some type of
surface-related problem such as rutting. The pavement was designed to be strong
enough to prevent fatigue cracking. Due to the relatively short time of
evaluation, durability problems such as stripping were not expected.
All of the sections performed very well for the first 10
million ESALs. In fact, no maintenance was required on any section other than a
section in one of the curves that had a friction problem. The mixture in this
section used a limestone aggregate that was known to polish. It was selected
for use in stone matrix asphalt (SMA) to determine if the coarse surface
texture would continue to provide good skid resistance as it was subjected to
traffic. The friction was monitored on a regular basis and when the friction
fell below an acceptable point, the test section was immediately overlaid with
a maintenance course to improve friction.
The overall level of rutting was very small. The worst
section had only about .25 in. of rutting. Most state DOTs don't begin to
consider rutting to be significant until it exceeds about .5 in. Clearly, none
of the 46 sections approached this amount of rutting. For that reason, some of
the sponsors plan to continue traffic on their sections for another two years.
Generally, the rutting was higher in the right wheel path
than in the left path. There were probably at least two reasons for this. First
of all, there was a 2% transverse slope on each of the pavement sections in the
tangents. This slope likely resulted in a slightly heavier load on the right
side of the traffic lane than on the left side. Secondly, the material adjacent
to the right side of the lane likely did not provide as much confinement as the
well-compacted mix on the left side of the traffic lane. Hence, more rutting
would be expected in the right wheel path.
It also is interesting to note that the variability of the
rutting within a section was very low. Even though the overall rutting was low,
it appeared to be consistent between tests within each section. Hence, the
measured rutting appeared to be actual rutting and not some random variability.
Some of the sections appeared to have statistically
significantly higher rutting values than others. Some were designed so they
would be more likely to rut.
For example, the sections with the most rutting were
designed with .5% additional asphalt binder so they might experience rutting
when exposed to this high level of traffic. These sections also included
asphalt binders that were not bumped to the next higher PG (performance graded)
level. Also, some of the sections used aggregates that were marginal, and it
was expected this could cause some rutting problems.
Another item of interest was the observed rutting rates. The
rutting rate was relatively higher during the first 800,000 ESALs even though
much of the traffic was applied during the winter months. It appears this first
significant rate of rutting was caused more by initial seating of the aggregate
and initial compaction. For example, if the average density in the top 4 in.
increased by 1% this alone should result in an average rut depth of
approximately .04 in.
After this initial densification and seating of the
aggregate, the rutting rate was reduced to near zero until the average
seven-day maximum daily temperature reached approximately 28°C (82°F),
at which time the rutting rate again began to climb. However, the rate appeared
to be a little less than the initial rate. After the seven-day maximum daily
temperature dropped below approximately 28°C, the rate of rutting almost
went to zero again. The rate of rutting stayed near zero until the temperature
exceeded approximately 28°C, at which time the rate began to increase a
The rate was much lower during the second summer than it was
during the first summer, even though the temperature was higher during the
Based on this observation, it appears that a mix that is
properly designed will stabilize within a couple of years due to aging and
compaction. The data also shows that the initial seating and densification
resulted in an overall average rut depth of approximately .03 in. The first
summer resulted in an additional .07 in. of rutting and the second summer
resulted in an additional .02 in. of rutting on average. As stated earlier, the
total average rut depth was very small--.12 in.
A mini experiment involving 10 sections was set up to look
at the effect of PG grade, asphalt content and fine-graded vs. coarse-graded
mixes. One observation from this evaluation was that the mixes with modified
asphalts (PG-76) had significantly lower rutting (66% lower) than the mixes
without modified asphalts. This indicates the importance of bumping the PG
grade on high-volume roads, as specified by Superpave.
Another observation was that binders modified with SBS and
SBR gave very similar results. The fine-graded and coarse-graded mixes provided
approximately equal performance.
Increasing the asphalt content by .5% resulted in an
increase of 54% in the rutting of the unmodified mixes. When the mixes were
modified, the increase in rutting as a result of the increased asphalt content
was very small (less than .04 in.). This indicates that one may be able to use
slightly higher asphalt content with modified asphalts to improve durability
without causing a loss in performance due to rutting.
There were three mini experiments to look at fine-graded vs.
coarse-graded mixes. The data from these three mini experiments clearly shows
that there was very little difference in the amount of rutting of fine-graded
and coarse-graded mixes. Hence, from a rutting standpoint, good performance can
be obtained with fine-graded as well as with coarse-graded mixes.
Of course, one of the keys to ensuring good performance is
to have a test accurately related to performance. Tests have been used over the
years to predict performance, and new tests are being evaluated. Several
laboratory tests were evaluated for the purpose of predicting performance,
including wheel tracking tests, Superpave simple shear, dynamic modulus and
confined repeated load test.
Keep in mind that the rutting observed at the track was very
small, so it is difficult for these tests to accurately predict the rutting. If
the rutting numbers were higher, then a better evaluation could be made
concerning the potential for each of these tests to predict performance.
The rutting results are very preliminary; hence, one must be
careful in making too many conclusions. However, it is worth noting that there
appeared to be no relation between rutting and dynamic modulus. There was a
reasonable relationship between the confined repeated load test and the
Superpave shear test. The rut testers also indicated a correlation with
In summary, the first series of tests showed that a number
of mixes with varying aggregate types, asphalt grades and aggregate gradations
provided good performance when subjected to 10 million ESALs of traffic. Mixes
intentionally designed to be more likely to rut did result in the greatest rut
depths, but the magnitude of these ruts was still small. Fine-graded and
coarse-graded mixes provided approximately equal performance. The higher PG
grades clearly resulted in lower rut depths, and the results indicate that
potentially more asphalt binder may be added to mixes using higher PG grades to
improve durability without increasing rutting.