By: Gary Henderson
The Superpave system. We hear it mentioned at meetings, we read
about it in magazines and newsletters, and we discuss it in the
office. Yet what's the bottom line? Where are we right now in
the transition to this performance-based asphalt mix design
system? This month's column will provide a brief overview on
where we are, and where we are heading.
Several years ago, a
national group of representatives from industry, highway
agencies and academia set forth two goals: implementation of the
Superpave binder specification by 1997 and of the Superpave
volumetric mix design procedures by 2000. Today, the first goal
has been largely met: More than two-thirds of the states are
already using the Superpave binder specification, and most of
the remaining states will make the switch this year. States and
industry are well on their way to meeting the second goal: The
Superpave volumetric mix design procedures, which employ the
Superpave gyratory compactor to prepare trial mixes, were used
on highway projects in two-thirds of the states in the 1996
construction season. Highway agencies at the local level are
following suit.
Of age, but still growing
The improved
binder specifications and mix design procedures, which are the
key components of the Superpave system, are clearly ready to be
used now. The system will, however, continue to evolve and
expand over the next decade. Here's a preview of what's ahead.
A beta release of Version 2.0 of the Windows 95-based
Superpave software will be available in June. It will include
step-by-step procedures for the Superpave volumetric mix design
process. The final release of Version 2.0, available this fall,
will include both a field management module, which can be used
to verify that the asphalt mix delivered to a job site matches
the job mix specifications, and a module for analyzing reclaimed
asphalt pavement.
A prototype (Version 1.0) of the Superpave
software, which was originally conceived under the Strategic
Highway Research Program by researchers from the Texas
Transportation Institute, University of California-Berkeley,
Pennsylvania State University, and University of Florida, had
been released in 1996. Since then, under an FHWA contract, a
team headed by Matt Witczak at the University of Maryland (UM)
has refined, enhanced, and improved the software to make it
easier to use and to take advantage of the computer processing
power and operating systems now readily available.
The UM
team will concentrate on developing methods for accurately
characterizing the materials properties of asphalt concrete and
building the framework for the performance models, which predict
how different mixes will perform under various environmental and
traffic conditions. The models will primarily be used to design
asphalt pavements that carry very heavy traffic loads and large
volumes, such as interstate highways. Once the models have been
completed and rigorously field validated under a follow-on
contract, they will be incorporated into the Superpave software.
At a recent meeting between FHWA top management and
representatives from the National Asphalt Pavement Association
(NAPA), Asphalt Institute, American Association of State Highway
& Transportation Officials, and Transportation Research Board,
Dick Morgan, NAPA vice-president said, "When the performance
models are ready for implementation, industry will be ready to
use them. In the meanwhile, the Superpave mix design procedures
give us a mix design system that should be much more reliable
than anything we've had before."
The UM team and the
University of California-Berkeley also will work on development
of a fundamental strength test to be used in conjunction with
the Superpave volumetric mix design procedures. The test, which
can determine a mix's susceptibility to rutting, is on a fast
track and should be available for widespread use by early 1999.
Useful developments on the horizon
Several projects now
in the pipeline will allow us to complete the Superpave system
(the expected completion dates are shown in parentheses):
--
Development of an integrated environmental effects and aging
model (2000),
-- Completion and verification of an
integrated performance prediction model for both new and
rehabilitated pavements (2005), and
-- Validation and
calibration of the asphalt performance prediction model (2006).
If you haven't yet had any training in the Superpave system,
there's no shortage of places to turn. Various levels of
training are offered by numerous organizations, including the
National Highway Institute, the Superpave regional centers, and
others. The upcoming Fifth Annual U.S. Hot-Mix Asphalt
Conference and National Superpave Workshop (to be held Oct.
29-31 in Phoenix) will feature a full day of presentations and
discussions on the Superpave system.
About The Author:
For more information,contact Gary Henderson at (202) 366-1549 or fax: (202) 366-9981; e-mail: [email protected].
