Superpave Lead State Recommendations

By Paul J. Mack, P.E.
Deputy Chief Engineer
New York Department of Transportation

1998 AASHTO Superpave Lead State Guidance

In 1996, the AASHTO Task Force on SHRP Implementation developed the concept of Lead States for the various SHRP technologies, including Superpave. The Lead State Program was implemented to ensure that practical, real-world experience is shared among all states. The Superpave Lead State Team includes Florida, Indiana, Maryland, New York, Texas and Utah.

On March 12, 1998, representatives of the Superpave Lead States met in Orlando, Florida, with representatives from the states of Arizona, Kentucky, Virginia, and Washington, the FHWA Office of Technology Applications, and selected guests to discuss a variety of technical issues associated with Superpave implementation. During this meeting it became apparent that states could take additional actions to insure the successful implementation of Superpave. The following guidance document was developed to outline these actions and provide a rational basis for their recommendation. If taken, these actions could minimize 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. Therefore, all of the guidance provided is intended to ensure the success of Superpave implementation by providing advice based on experience and caution where indicated.

In developing the guidance provided, the Lead States felt it was vitally important to maintain the uniform implementation of Superpave. The Lead States will propose specification revisions to AASHTO standards based on the recommendations contained in the guidance document where appropriate.

In addition to the guidance developed at the March 1998 meeting, this document includes guidance previously issued by the Lead States in May 1997. The previous guidance has been revised and updated to reflect current knowledge and experiences.

N-design Table -The data used to develop the N-design table were primarily centered around the central N-values and extrapolated at the table’s extremes (less-than 0.3 and greater-than 30 million ESALs). To date, there is limited performance data available on mixes designed at the table’s extremes. Therefore, states should be cautious in these areas.

Calculation of an exact ESAL for individual projects is subjective and may result in an extreme N-value if incorrect or inappropriate assumptions are made. Thus, ESAL estimates should be reviewed and evaluated to determine whether the estimated values are reasonable.

For very high design ESALs (greater-than 30 million), preliminary mixture designs should be developed and evaluated in terms of practicality and costs. The state may elect to reduce N-design to the value corresponding to 10-30 million ESALs based on this evaluation. For very low design ESALs (less-than 300,000), care should be taken to ensure a minimum binder content for durability. This may require a series of experimental designs by the state to establish an estimated minimum binder content 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 9-9, "Refinement of the Superpave Gyratory Compaction Procedure" research is currently underway to possibly refine the N-design table. Until this research can be concluded and considered for adoption by AASHTO, the above guidance should be considered.

Additionally, the design ESALs shown in AASHTO PP-28, "Standard Practice for Superpave Volumetric Design for HMA" Table 2, "Gyratory Compaction Criteria" represent the pavement’s cumulative ESALs for a 20-year design life. Accordingly, regardless of a project’s intended life the specified mixture design compaction level must be selected based on an estimate of the pavement’s cumulative ESALs over 20-years. Experience has shown rutting damage occurs in the first few years of a pavement’s life. Therefore, the rate of loading must be considered. Estimating ESALs over a 20-year design life may affect the mixture design compaction level, performance graded binder selection, and the aggregate consensus properties specified for the project. For example, a project with a 5-year intended life may have a 5-year cumulative ESAL count of 2.9 million. This relates to a 20-year cumulative ESAL count of 11.6 million (neglecting traffic growth compounding).

Recommended standard change: AASHTO PP-28, "Standard Practice for Superpave Volumetric Design for HMA" - Define the ESAL levels shown in Table 2, "Gyratory Compaction Criteria" as being based on a 20-year design life.

Mixtures Located 100 mm Below the Pavement Surface - The NCHRP 9-9 report (to be available in the summer of 1998) will propose revisions to the N-design table as well as develop compaction criteria for mixtures located more than 100 mm below the pavement surface. Until the NCHRP 9-9 report is completed, the Lead States recommend consideration be given to lowering the required design compaction level by one ESAL level when the specified mixtures are located more than 100 mm below the pavement surface. This practice will result in mixtures with higher binder contents, thus improving mixture durability and moisture resistance. The mixture’s structural performance should not be impacted by this change since mixtures located more than 100 mm below the pavement surface do not experience the same stresses as surface mixtures. However, the construction schedule must be considered since mixtures designed at a lower-than-required compaction level may prematurely deteriorate if the mixture is exposed to traffic for an extended period prior to being overlaid.

From a practical viewpoint, the Lead States also recommend 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 Superpave greater-than 100 mm aggregate consensus properties would apply.

Aggregate Consensus Properties - If Superpave criteria allow the use of aggregates with lower 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:

Reclaimed Asphalt Pavement - The Lead States recommend the use of the Superpave Mixture Expert Task Group’s "Guidelines for the Design of Superpave Mixtures Containing Reclaimed Asphalt Pavement."

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 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 previously specifying 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 this practice may become unnecessary as the specified criteria are refined in the future.

Dust-to-Binder Ratio - Prior to Superpave, the dust-to-binder ratio was specified by some states as part of their volumetric mixture design criteria. The usually accepted range for this property was 0.6 to 1.2, calculated using the total binder content. Superpave includes dust-to-binder ratio criteria with the same range of 0.6 to 1.2. However, Superpave calculates this ratio using the effective binder content. Using the effective binder content, rather than the total binder content, will normally result in a higher dust-to-binder ratio due to binder absorption into the aggregate.

High dust-to-binder ratios, up to 1.6, have been encountered in Superpave mixtures, particularly those with coarse-graded blends (passing below the restricted zone). In these cases, all other mixture criteria, including VMA, were satisfied. Experience with these mixtures has been positive. Therefore, the Lead States recommend the dust-to-binder ratio range be specified as 0.6 to 1.6.

Calculating VMA - It is important to have representative aggregate bulk specific gravity values in order to accurately calculate the mixture’s Voids in the Mineral Aggregate (VMA) during design and production. For this reason, aggregate bulk specific gravities should be determined on a frequency appropriate for the variability of the source. VMA is calculated using the following equation:

The variability of the mixture bulk specific gravity and aggregate bulk specific gravity test results used in this calculation must be considered when establishing production tolerances on VMA.

During production, it may be more expedient to determine an "effective" VMA based on the aggregate’s effective specific gravity, since the test method used to determine the aggregate’s bulk specific gravity is more time consuming and variable than calculating aggregate’s effective specific gravity based on the mixture’s maximum specific gravity. An "effective" VMA will be higher than the "actual" VMA since the absorbed asphalt is not considered. "Effective" VMA is calculated using the following equation:

If an "effective" VMA is calculated, one of the following must be done:

• Increase the VMA criteria to account for the calculated increase in VMA due to the use of G_se instead of G_sb.

or

• Determine a correction factor to adjust the "effective" VMA to the "actual" VMA.

Determining VMA with G_sb is more conservative and better guarantees that the "actual" VMA is sufficiently high to insure long term durability.

Field Aging of Asphalt Mixtures - The purpose of the short-term oven aging (STOA) procedure (AASHTO PP-2, "Short and Long Term Aging of Hot Mix Asphalt") included in Superpave was to account for mixture aging and binder absorption during production and the first years of the pavement’s life. No additional STOA is necessary, or recommended, except to bring the mixture to the appropriate compaction temperature.

N-initial for Low ESAL Roads - It is difficult to design Superpave mixtures, with gradations passing above the restricted zone, that satisfy the currently specified N-initial criteria. Fine-graded mixtures or mixtures with less manufactured aggregate typically have high densities at N-initial, usually exceeding the current criteria of 89.0% maximum. The Superpave system should not eliminate the use of fine-graded mixtures, particularly on low ESAL pavements. Mixtures placed on low ESAL pavements do not require the same structural strength as mixtures placed on high ESAL pavements. Further, fine-graded Superpave mixtures have surface textures similar to mixtures previously used and may be easier to compact than coarse-graded Superpave mixtures of the same design level. Therefore, the Lead States recommend the density at N-initial be specified as shown in Table 1 - N-initial Criteria:

VFA for 9.5 mm Mixtures - The Voids Filled with Asphalt (VFA) criteria for mixtures designed for greater-than 3 million ESALs is limited to 75.0% by Superpave. This requires 9.5 mm mixtures to be designed within the narrow VMA range of 15.0 to 16.0 %. To ease mixture design development, the Lead States recommend the upper VFA limit for 9.5 mm mixtures designed for greater-than 3 million ESALs be specified at 76.0%. This change effectively increases the design VMA range to 15.0 to 16.7 % for these mixtures.

Use of SMA Within the Superpave Design System - The Lead States recommend the use of the "Guidelines for Materials, Production and Placement of Stone Matrix Asphalt (SMA)", NAPA Information Series 118 issued September 1994, when Superpave specifications are used to design SMA mixtures.

The following modifications are recommended:

• Binders selected in accordance with AASHTO MP-2 should have the high temperature grade increased by at least one grade. A PG 70-XX should be the minimum high grade specified. While there is no maximum high temperature grade recommended, consideration should be given to a PG 82-XX as the practical upper limit.

• A fiber modifier should be used to facilitate placement regardless of the binder grade selected. This recommendation can be eliminated based on acceptable drain-down test results.

• Voids in the Total Mixture (VTM) at an N-design gyration level of 100 should be 4.0%. There are no recommended N-initial or N-maximum values.
• Use 150 mm diameter Superpave specimens in mixture design development.

The following contributed to the development of this document: