Like the Hveem and Marshall methods, the Superpave method has been proven to produce quality HMA from which long-lasting pavements can be constructed. This section briefly discusses the Superpave mix design method.
The Superpave mix design method consists of three basic steps:
- Aggregate selection. Aggregate is specified in three ways. First, restrictions on aggregate gradation are specified by using gradation specifications. Second, there are physical property requirements on aggregate angularity, flat and elongated particles, and clay content. Third, aggregate criteria, which the Asphalt Institute (2001) calls “source properties” (because they are considered to be source specific), such as durability and soundness are specified.
- Asphalt binder selection. Superpave PG asphalt binders are selected based on the expected pavement temperature extremes in the area of their intended use. These extremes can be calculated using software (such as LTPPBind) or, more commonly, a standard grade of PG 64-16 is used statewide.
- Optimum asphalt binder content determination. In the Superpave method, this step can be broken up into 4 substeps:
- Prepare several initial samples, usually two at the proposed design asphalt content, two at 0.5 percent below the design asphalt content and two at 0.5 percent above the design asphalt content.
- Compact these trial mixes in the Superpave Gyratory Compactor (see Figures 1, 2 and 3). This compactor is specific to the Superpave mix design method.
- Determine the density and other volumetric properties of the samples.
- Select the optimum asphalt binder content. The asphalt binder content corresponding to 4 percent air voids.
In the Superpave mix design process there is are no accepted standard performance tests so nothing analogous to the Hveem stability and cohesion tests is used. Research into creating a standard performance test is ongoing.