Most private laboratories use the Marshall method because it is a proven method and requires relatively light, portable and inexpensive equipment. Like the Hveem and Superpave methods, the Marshall method has been proven to produce quality HMA from which long-lasting pavements can be constructed. This section briefly discusses the Marshall mix design method.
The basic concepts of the Marshall mix design method were originally developed by Bruce Marshall of the Mississippi Highway Department around 1939 and then refined by the U.S. Army.
The Marshall method is very popular because of its relative simplicity, economical equipment and proven record.
Typically, the Marshall mix design method consists of three basic steps:
- Aggregate selection. Different agencies/owners specify different methods of aggregate acceptance. Private labs may or may not run periodic aggregate physical tests on a particular aggregate source. For each mix design, gradation and size requirements are checked. Often, aggregate from more than one source is required to meet gradation requirements.
- Asphalt binder selection.
- Optimum asphalt binder content determination. In the Marshall method, this step can be broken up into 5 substeps:
- Prepare a series of initial samples, each at a different asphalt binder content. For instance, two to three samples each might be made at 4.5, 5.0, 5.5, 6.0 and 6.5 percent asphalt by dry weight for a total of 10 to 15 samples. There should be at least two samples above and two below the estimated optimum asphalt content.
- Compact these trial mixes using the Marshall drop hammer (see Figure 1). This hammer is specific to the Marshall mix design method.
- Test the samples in the Marshall testing machine (see Figure 2) for stability and flow. This testing machine is specific to the Marshall mix design method. Passing values of stability and flow depend upon the mix class being evaluated.
- 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 is selected as long as this binder content passes stability and flow requirements.