On March 25 and 26, 1997 representatives of the Superpave Lead States, the FHWA Office of Technology Applications, and selected guests met in Baltimore, Maryland to discuss a variety of technical issues associated with Superpave implementation.
During discussions at this meeting it became apparent there were steps that states could take to insure the successful implementation of Superpave. If taken, these steps could minimize the potential risks associated with aspects of Superpave not fully validated before the conclusion of SHRP or that have not yet been tried in the field. In developing the guidance provided, the Lead States felt it was vitally important to maintain the uniform implementation of Superpave in accordance with AASHTO standards until valid documented research is available indicating the need for a change. Therefore, all of the guidance provided is intended to enhance the success of Superpave implementation by providing advice based on experience and caution where indicated.
Calculation of an exact ESAL for individual projects is subjective and may lead to extremes in corresponding N-values. Thus, ESAL estimates should be reviewed and evaluated to determine whether the values are reasonable.
For very high traffic volumes (higher than 30 million ESAL's) preliminary mixture designs should be developed and evaluated in terms of practicality and costs. Based on this evaluation the agency may elect to reduce the N-design to the value corresponding to 10-30 million ESAL's. For very low values (less than 300,000 ESAL's) care should be taken to ensure a minimum asphalt content for durability. This may require a series of experimental designs by the agency to establish an estimated minimum value as a guide.
This guidance does not preclude the use of the existing N-design table. States should be encouraged to construct projects on the table's extremes so that data needed to refine the N-design table can be obtained. NCHRP research is currently underway to possibly refine the N-design table. Until this research can be concluded, the above guidance should be considered.
Aggregate Depth From Surface Criteria - If less than 25% of a layer is within 100 mm of the surface, the layer should be considered to be below 100 mm for mixture design purposes. Therefore, the less than 100 mm aggregate consensus properties would apply. This will have the effect of eliminating the more stringent aggregate consensus properties for most 37.5 mm base course mixes.
Aggregate Consensus Properties - If Superpave criteria allows the use of aggregates with lesser quality than previously used in a state, consideration should be given to maintaining the states' more stringent requirements until all Superpave validation work is complete. With respect to specific aggregate consensus properties the following is offered:
Coarse Aggregate Angularity - Previous references in SHRP reports and elsewhere to the Pennsylvania Department of Transportation Test Method No. 621 for determining Coarse Aggregate Angularity has been revised in AASHTO MP2, "Standard Specification for Superpave Volumetric Mix Design" to reference ASTM D5821, "Standard Test Method for Determining the Percentage of Fractured Particles in Coarse Aggregate," to more critically discriminate between aggregates.
Flat and Elongated Particles - The Lead States recommend that the criteria for Flat and Elongated Particles be changed from 5:1 to 3:1. Construction problems have been reported when materials meeting the 5:1 criteria were used, the same materials would not meet the 3:1 criteria.
Fine Aggregate Angularity - The Lead States recommend a firm stance on the specified requirement of 45 at greater than 3 million ESALs.
Reclaimed Asphalt Pavement - The Lead States support the use of the Mixture Expert Task Group's recently completed "Guidelines for the Design of Superpave Mixtures Containing Reclaimed Asphalt Pavement." A copy of this guideline is attached for your use.
Field Construction Concerns - Compaction difficulties have occurred on some Superpave projects. It is the opinion of the Lead States that appropriate in-place densities on Superpave projects will only be achieved with good paving practices (i.e., proper rolling equipment, rolling practices, placement temperatures, lift thickness, non-segregated mixes, etc.) and that the AASHTO QC/QA specification for checking in-place densities should be used. (Quality Control/Quality Assurance; Specification and Implementation Guide; AASHTO Highway Subcommittee on Construction; August 1993). Additionally, the mixing and placement temperatures used during mix design preparation should be taken into consideration during production and placement in the field.
Use of Modified Binders - States which have previously
specified a viscosity graded binder with a specific type of modification
may consider specifying Performance Graded Binders with a similar
type of modification. The binder specification is under continuous
review and recently added requirements which may make this practice
unnecessary in the future.
N-design - Evaluate N-design and N-max in the areas of the extrapolated data to determine their appropriateness, determine if the current compaction levels are appropriate for all pavement layers, and review the current N-design table to determine if the mixes designed in adjacent cells are similar in a "practical nature," thereby, indicating the potential for a reduction in the number of compaction levels.
Voids in the Mineral Aggregate - Evaluate if the VMA requirement should be different for fine and coarse graded mixes of the same nominal maximum aggregate size, determine if the VMA requirement should be increased for low volume mixes, and determine if the states are correctly determining VMA at the production facility.
Field Construction Concerns - Determine what is an appropriate in-place density to specify for fine and coarse graded Superpave mixes, develop an accurate means to determine the bulk specific gravity of potentially permeable samples, evaluate the effect of lift thickness in developing proper in-place density, and develop recommended lift thicknesses for each nominal maximum aggregate size mix.
This information is provided for use at your discretion. When
contemplating a revision to Superpave specifications, please
keep in mind the need for uniform implementation of Superpave
nationwide. We will keep you informed as additional information
is obtained on these and other issues.
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