The Federal Highway Administration (FHWA) has contracted with
the University of Maryland at College Park for the management
and refinement of the Superpave performance models, including
supplemental efforts to support State highway agencies (SHA's)
implementation of the Superpave system. The contract, which is
divided into two phases, provides for continued improvement of
all aspects of the Superpave system with emphasis on evaluation
and further development of the pavement performance prediction
models.
A main effort of Phase I of this contract, was for the Models
Contract Team to evaluate the Superpave performance prediction
models that resulted from the SHRP research. A draft evaluation
report on their findings was submitted to FHWA on September 30,
1996. The report documents the Team=s evaluation of the existing
Superpave pavement performance prediction models and makes recommendations
for corrections and enhancements to these models. The evaluation
concludes that for the load associated distress models (fatigue
and permanent deformation) substantial corrections and enhancements
are considered mandatory to make the performance prediction models
and analysis software reliable and suitable for general use by
the industry. In addition to errors in the programming code,
technical constraints, fundamental errors and restrictive assumptions
were used in developing the load associated distress models.
The existing nonload related models (thermal fracture) have been
found to be in reasonably good shape. Revising and enhancing
the load associated distress Superpave performance prediction
models would require an additional effort to recalibrate and revalidate
the resulting models, in order to provide the highway community
with a credible product.
The Superpave Models ETG met on October 22-23, 1996, in Washington,
D.C. The ETG consists of representatives from States, industry,
academia, and FHWA. The purpose of the meeting was to obtain
the feedback of the ETG on the Draft Superpave Models Evaluation
Report that has been written by the Models Contract Team. At
the meeting, an overview of the report and followup questions
were addressed by the Models Contract Team.
Overall, the ETG was supportive of the many changes that are being proposed. Many members expressed the view that if the models are not enhanced to their fullest potential now, they never will be. There was also the consensus that mix design and structural design are interrelated and that taking this approach would give the Superpave models the broadest applicability within the highway community. They were in general agreement that the calibration and validation of the models is vital and that this should be performed by the existing Team, but there was some concern as to how this should be performed. Some issues are the proposed approach of using reconstituted specimens and whether GPS sites or specially constructed sites should be used. Some of the ETG members are concerned about the traffic data in the LTPP database and its effect on the calibration/validation.
In the near term (3-5 years) most owner agencies and the industry
will be fully involved in adapting to the Superpave volumetric
mix design using the gyratory compactor. It was felt that the
models would be used to analyze mixes on projects where performance
is critical, but that these projects make up a relatively small
percentage of the total hot mix asphalt produced. Considering
the scope of the work proposed, the ETG advised FHWA not to hold
to the current 5 year time line. Extending the time by a couple
of years will not diminish the support for this effort.
The ETG also agreed that review of the models work was important
to avoid some of the problems discovered with the original models.
They were supportive of frequent presentations to the ETG, and
suggested the possibility of review by outside experts.
The ETG members will be submitting their written comments to the
COTR within the next week or two. A meeting is being coordinated
whereby the original SHRP principal researchers will have an opportunity
to comment on and discuss the Models Evaluation Report. This
is to take place by the end of November. Once the input from
the Models ETG and the SHRP researchers has been received, a written
technical decision on the activities for Phase II of the Models
Contract will be prepared. The technical decision will be reviewed
by the Superpave Technology Delivery Team who will then forward
a consensus position to the Pavement Management Coordinating Group
(PMCG) and other management groups. It is anticipated that the
final decision can be provided to the contractor by mid-January.
You may wish to provide this update on the Models Contract to
the State highway agency and others working to implement Superpave.
When the technical decision for the Phase II work of the Models
Contract has been concurred in, we will advise you of the scope
of the future action and relative time line for further models
refinement. Should you have questions, please feel free to direct
them to Ms. Katherine Petros at 202-366-1340 or to any member
of the Superpave Team.
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