Most highway agencies have indicated that they intend to implement
the Superpave asphalt binder specification in 1997. The Federal
Highway Administration (FHWA) fully endorses this implementation.
The final development of the binder specification and equipment
specifications are still underway. This development work should
not impede the implementation process. As the Superpave specification
is evaluated minor changes and refinements will be made to correct
errors and omissions. This will be a continuing process. The
Superpave system offers a major improvement in asphalt materials
evaluation and mix design. Volumetric mix design using the gyratory
compactor better replicates the in-service conditions and allows
the designer to better select the binder for the environment and
traffic conditions.
In the evaluation of the Superpave binder equipment it was determined
that there is a difference in the results reported by the Cannon
and ATS bending beam rheometers. The bending beam rheometer (BBR)
measures the low temperature properties of the asphalt binder,
the stiffness (S) and the slope of the stiffness curve (m). There
is a 0.01 difference in the m value as reported by the rheometers.
This difference is considered significant and may effect proper
binder selection. The Cannon rheometer reports m values which
are typically 0.01 higher than ATS. These differences were determined
through a detailed evaluation completed for the FHWA by the Asphalt
Institute and verified by evaluation of AMRL round robin test
data.
The FHWA Expert Task Group (ETG) on Superpave Binders is currently
evaluating both pieces of equipment to determine why there is
a difference. Both pieces of equipment are being completely evaluated
to determine if there is any difference in how the test is being
performed and the data collected. Once this evaluation is completed
a revised specification will be developed to eliminate the potential
for future discrepancies.
Until the basis for this difference is determined and modifications
are made to the software or the test devices, the ETG recommends
that the results of the m value reported by the ATS BBR be increased
by 0.010. This recommendation is based on the fact that many
of the industry labs have the Cannon BBR and they have formulated
their binders based on those results. To give the suppliers the
benefit of the doubt, it was determined that the ATS results should
be increased. A value of 0.010 in the m value typically represents
about 1 C.
Please provide this information to your State Highway materials
lab as soon as possible. As soon as any definitive results are
developed and a resolution to this is determined they will be
reported.
If you have any questions please contact John D'Angelo, Binder
ETG Chairman at 202-366-0121 or John.DAngelo@FHWA.DOT.GOV.
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