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Superpave Method
WSDOT is transitioning from the Hveem mix design
method to the Superpave mix design method. 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.
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| Figure 1: Placing the Sample in the
Gyratory Compactor |
Figure 2: Running the Gyratory Compactor |
Figure 3: Extruding the Sample from the
Mold |
As done by WSDOT, 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
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. WSDOT uses the
Superpave PG system
for asphalt binder specification. 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, determined based on
standard PG
binders that WSDOT has specified for each area of the State.
Sometimes, these standard binder grades can be adjusted based on the
anticipated traffic level, type or speed.
- Optimum asphalt binder content determination. In the
Superpave method, this step can be broken up into 4 substeps:
- Prepare 6 initial samples, 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.
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