Superpave Mix Design
Superpave mix design is a structured approach consisting of the following four steps:
- selection of materials,
- selection of design aggregate structure,
- selection of design asphalt binder content, and
- evaluation of moisture susceptibility.
Selection of Materials
This step is accomplished by first selecting a Performance Grade asphalt binder (PDF file) for the project climate and traffic conditions. Superpave binders are designated with a high and low temperature grade, such as PG 64-22. For this binder, "64" is the high temperature grade and is the 7-day maximum pavement design temperature in degrees centigrade for the project. The low temperature grade, "-22," is the minimum pavement design temperature in degrees centigrade. Both high and low temperature grades are established in 6-degree increments. Thus, the binder grade is an indication of the project-specific temperature extremes for which the asphalt mixture is being designed.
In addition to climate, traffic speed and traffic level may also influence Superpave binder selection. A project with slow moving or stationary traffic would require a binder with one or two higher temperature grades than would otherwise be selected on the basis of climate alone. Projects with very high traffic levels in excess of 30 million 80 kN equivalent single axle loads would also require an increase in high temperature binder grade.
Five asphalt mixture types are specified in Superpave according to nominal maximum aggregate size: 9.5 mm, 12.5 mm, 19 mm, 25 mm, and 37.5 mm. To specify mineral aggregate, Superpave uses two approaches. First, it places restrictions on aggregrate gradation by means of broad control points and a restricted zone. Second, it places consensus requirements on coarse and fine aggregate angularity, flat and elongated particles, and clay content.
Selection of Design Aggregate Structure
Once binder and aggregate materials have been selected, various combinations of these materials are evaluated using the Superpave gyratory compactor. Three, and sometimes more, trial blends are evaluated. For example, assume that four aggregate stockpiles have been selected for use. The following table shows possible blend percentages for each of three trial blends.
|Stockpile||Trial Blend 1||Trial Blend 2||
Trial Blend 3
|Coarse Aggr 1||20||30||
||Course Aggr 2||50||40||
Once the trial blends are established, a trial asphalt binder content is selected for each blend. The trial asphalt binder content is selected using an estimation procedure contained in Superpave or on the basis of the designer's experience.
Two specimens of each trial blend are batched and compacted in the Superpave gyratory compactor. In addition, two loose specimens of each trial blend are produced and used to measure maximum theoretical specific gravity. The volumetric and densification characteristics of the trial blends are analyzed and compared with Superpave mix design criteria. Any trial blend that meets these criteria can be selected as the design aggregate structure.
Selection of Design Asphalt Binder Content
The next step involves selection of the design asphalt binder content for the design aggregate structure. This step is necessary to verify the approximate binder content used in the preceding step. The Superpave gyratory compactor is used to fabricate test specimens composed of the selected design aggregate structure, but at four different asphalt contents. The asphalt content that results in 4 percent air voids at the design number of gyrations is the design asphalt binder content. The design aggregate structure containing the design asphalt binder content becomes the design asphalt mixture.
Evaluation of Moisture Susceptibility
This final step requires that the design asphalt mixture be evaluated using a test procedure called, AASHTO T283, "Resistance of Compacted Bituminous Mixture to Moisture Induced Damage." This test method was already in wide use prior to the development of Superpave. Six test specimens are fabricated using the Superpave gyratory compactor. Three of the six are moisture conditioned. The remaining three specimens remain unconditioned. All of the test specimens are evaluated for their indirect tensile strength. The ratio of conditioned to unconditioned tensile strength is called tensile strength ratio or TSR. The design asphalt mixture is judged to be non-moisture susceptible if it has a TSR greater than 80 percent.