AASHTO Mechanistic/Empirical Design Procedures
Dan A. Dawood, P.E., Chief, Pavement Design and Analysis
Section, Bureau of Maintenance & Operations, PENNDOT
In the 40 years since the empirical based pavement design procedures
of AASHO Road Test were developed, very little change has come
about on how we design pavement structures. Traffic volumes
and pavement design requirements have outgrown the empirical
procedures. Furthermore, the AASHO Road Test was based on one
environmental region, geological formation and one specific
vehicle type to formulate a system of pavement design for use
nationally.
Pavements today are being designed well off the charts and
graphs provided by the AASHO Road Test. Traffic volumes and
truck axle weights are so dramatically different today that
it becomes quite questionable to many if the designs seen today
on major interstates and expressways are realistic. It is for
this and many other reasons such as the wealth of accumulated
knowledge since the Road Test, that the AASHTO Joint task Force
on Pavements initiated the effort to develop an improved Guide
by 2002.
National Cooperative Highway Research Project (NCHRP) 1-37A
was the initial step to develop the 2002 Guide. NCHRP awarded
the project to ERES Consultants, a division of Applied Research
Associates, Inc., to develop the new guide using existing models
and practical and realistic design procedures for the design
of new and rehabilitated pavement structures. The consultant
was encouraged to use large national databases, such as the
Long Term Pavement Performance (LTPP) database, to validate
and calibrate their models and designs.
The new guide will use a hierarchical approach for determining
design inputs, which means that the level of input should be
consistent with importance of the project. There will be three
levels of input, with Level One being the highest quality data.
Of the three levels, most state highway agencies (SHA) will
be using Level Two inputs, which are more readily obtainable
than Level One. Level One requires specific traffic data such
as weigh-in-motion data for the specific site, resilient modulus
for subgrade testing, advance materials and Dynamic Modulus
Testing for asphalt materials. Level Two allows the use of regional
factors and available test procedures with correlation equations.
This is not to say that Level Two is easy, since SHAs
will still be required to do a considerable amount of testing
in order to have good and reliable data to develop state wide
or regional factors. It should also be noted that the analysis
process will remain consistent and will be independent of the
level of input.
Although the NCHRP 1-37A project was scheduled to be completed
in 2002, unforeseen problems have delayed the completion of
the project to later this year. The final product of the NCHRP
1-37A effort will be a Paper Guide and rudimentary
software, which will allow users to use the new guide. The new
design procedure will be significantly more complex than the
current AASHTO 93 Guide and therefore cannot be done manually.
The software being provided at the completion of the NCHRP 1-37A
project will be sufficient to perform most pavement designs
for new and rehabilitated pavements. However, it will only provide
users with models calibrated for regional areas from national
data.
It is anticipated that SHAs will receive the NCHRP 1-37A
product late this year or early next year after the AASHTO Joint
Task Force on Pavements has approved it. Furthermore, to become
an AASHTO product the NCHRP 1-37A product must undergo a balloting
procedure with all the states and not just the states represented
on the Joint Task Force on Pavements.
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