The Superpave design process is a product of the Strategic Highway Research
Program (SHRP); the goal of the system is to overcome many of the problems
associated with asphalt pavements-in particular, rutting, low-temperature
cracking and fatigue cracking.
Since local conditions-such as temperature ranges, moisture levels and aggregate
type- can greatly influence HMA performance, SHRP focused on developing
a system that would provide performance-based specifications for asphalt
binders and asphalt-aggregate mixes. These specifications allow asphalt-pavement
developers to tailor mixes to their unique needs. In this way, contractors
in Florida can follow the same mix guidelines as contractors in Alaska and
both can obtain optimal results. The actually properties of the mixes will,
of course, vary greatly, but each mix will be best for the given conditions.
Mike Anderson, associate director of research at the Asphalt Institute,
Lexington, Ky., was involved with Superpave research during SHRP. He says
the design process for a Superpave mix is a three-part operation: volumetric
mix design, Superpave mix analysis and field control. Each of these processes
contains a formatted-yet flexible-quality control/quality assurance program
designed to help produce a superior mix.
"Within volumetric mix design, materials selection and development
of aggregate structure are vital," Anderson explains. "Once these
are set, the determination of binder content and moisture-sensitivity just
flow from the other conditions."
Aggregate selection is critical: Aggregate should be clean and angular,
and the gradation Superpave employs is designed to control the amount of
rounded, natural sand in a mix that may create instability. For the most
part, a Superpave mix is coarser than most conventional mixes, not unlike
stone-matrix asphalts.
Binder selection is based on a number of factors, according to Anderson.
"We use local weather data, physical-property tests, AASHTO MP-1 guidelines
and Superpave guidelines."
Once the materials have been selected, the design of the aggregate structure
begins. "We evaluate multiple blends using the Superpave gyratory compactor,"
Anderson says. "There are no strength tests, such as the Marshall stability
and flow test, with Superpave mixes. We then do a short-term aging of the
mix; the final mix choice will-by this point-come down to necessary properties
and economics."
A mix designer should be "very close" to the actual asphalt content
by this point, Anderson says, adding that Superpave uses AASHTO T-283 for
moisture-sensitivity determination.
At this point, a Superpave Level 1 mix should be ready to go. SHRP has divided
highway traffic into three levels, with Level 1 designated as "light
to moderate" traffic; levels 2 and 3 are for increasingly heavy traffic
loads. All Superpave mixes undergo the Level 1 analysis outlined by Anderson;
the magnitude of compaction in the gyratory compactor depends on the anticipated
traffic and environmental conditions at the site.
For pavements that will carry heavier traffic loads, additional analysis-using
the Superpave shear tester and the indirect tensile tester-is performed.
Level 3 pavements, defined by SHRP as those carrying over 10 million equivalent
single-axle loads (ESALs), are subjected to the same tests as Level 2 loads
(1 to 10 million ESALs), but over a greater range of temperatures and pressures.
While SHRP has published guidelines for Superpave mixes, there may be some
adjustments in the near future. Gerry Eller, chief of the FHWA's construction
and maintenance division, comments, "As we are implementing our [Superpave]
knowledge, we are still testing." For example, SHRP expects to publish
guidelines for incorporating reclaimed asphalt pavement (RAP) into Superpave
mixes sometime early this year.
Although there may be some subtle changes to mix specifications, don't expect any radical departures from current norms. Both laboratory analysis and field tests (see accompanying story) have demonstrated the quality of the process that creates Superpave mixes; as Eller notes, "Superpave is the future of asphalt pavement."
