How Well Do You Know Your Ashpalt Mix Design?
You've chosen an asphalt mix design and you're ready to pave. But
how will the mix perform under the traffic and climate conditions
at the pavement site? According to a canvass of State highway agencies
conducted earlier this year by the Federal Highway Administration
(FHWA), 29 agencies now use some sort of a test prior to paving to
indicate how their mix design will hold up. Of those highway agencies
not currently conducting tests, 13 are gathering information and considering
implementing a test. Research is being conducted nationally to develop
a simple performance test to be used with the Superpave volumetric
mix design procedures as a way of addressing States' need for a test
tied to field performance (see sidebar).
As many agencies await the results of this national research into
the simple performance test, they continue to use a variety of other
test methods. Some of the evaluation methods used include the Hveem
Stabilometer, Asphalt Pavement Analyzer (APA), and Marshall Criteria
tests. Some of the other tests either being used or evaluated include
the Static Creep, Nottingham Asphalt Tester, French Rut Test, and
the Hamburg Wheel-Track Test.
Seventeen highway agencies report they have altered their decision
to place a mix because of test results. Oklahoma, for example, says
that "Hveem stability tests have been used on as-produced mixtures
as a quality measurement. Production is stopped if tests on field
mixtures consistently fail." And Washington State reports that
it "runs stability tests on the first 5 days of production and
then once a week thereafter. We use this information to adjust the
mixture or to halt production or to be able to use this design for
future projects out of the same aggregate source and the same asphalt
source and grade."
Only four States currently compare the actual field performance of
their mixes with the testing results. Georgia notes that it "did
a limited study but was unable to get a direct correlation."
The State has seen, however, "that mixtures that meet our APA
test requirements do not cause rutting problems in the field."
Thirteen States indicated they are working to set up systems for comparing
results to field performance.
The review was conducted at the request of the Pacific Coast Conference
on Asphalt Specifications, which serves as the asphalt user/producer
group for the West Coast. "The Conference's Task Force on Performance
Testing realized the need for using a performance test and wanted
to see the direction agencies around the country are taking,"
says Katherine Petros of FHWA, who conducted the review. As a result
of the review, the conference is now looking more closely at the simple
Superpave performance tests being evaluated under National Cooperative
Highway Research Program (NCHRP) Project 9-19 and gathering more information
on the APA test.
For more information on the review or the individual State responses,
contact Katherine Petros at FHWA, 202-493-3154 (fax: 202-493-3161;
email: katherine.petros@fhwa.dot.gov).
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One of the tasks of NCHRP Project 9-19, "Superpave Support
and Performance Models Management," which is the continuation
of an FHWA-sponsored project of the same name originally begun
in 1994, is to identify simple performance tests for permanent
deformation and fatigue cracking for incorporation in the Superpave
volumetric mix design method. In the initial phase of the task,
the project team selected three candidate tests (dynamic modulus,
static creep, and triaxial repeated load permanent deformation)
from among 33 test/parameter combinations. The three candidates
were selected on the basis of good statistical correlation with
pavement distress, as measured in tests at the Mn/ROAD test
road in Minnesota, WesTrack pavement test track in Nevada, and
FHWA's accelerated loading facility at the Turner-Fairbank Highway
Research Center in McLean, Virginia. In the current phase of
the project, which is expected to be completed in February 2002,
the ability of these tests to provide an accurate measure of
potential field performance is being validated through testing
of hot-mix asphalt materials from the Long Term Pavement Performance
Specific Pavement Study -1, -5, and -9 experiments, as well
as from several State field experiments. For more information,
contact Ed Harrigan at NCHRP, 202-334-3232 (email: eharriga@nas.edu). |
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