The Marshall mix design method consists of 6 basic steps:
A typical aggregate evaluation includes three basic steps
Determine aggregate physical properties. This consists of running various tests to determine properties such as:
Determine other aggregate descriptive physical properties. If the aggregate is acceptable according to step #1, additional tests are run to fully characterize the aggregate. These tests determine:
Perform blending calculations to achieve the mix design aggregate gradation. Often, aggregates from more than one source or stockpile are used to obtain the final aggregate gradation used in a mix design. Trial blends of these different gradations are usually calculated until an acceptable final mix design gradation is achieved.
The Marshall test does not have a common generic bitumen binder selection and evaluation procedure. Binder selection can be based on local experience, previous performance or specification. Once the binder is selected, several preliminary tests are run
The Marshall method uses several trial aggregate-bitumen binder blends (typically 5 blends with 3 samples each for a total of 15 specimens), each with a different bitumen binder content. Then, by evaluating each trial blend’s performance, an optimum bitumen binder content can be selected. Typically, the trial blends must contain a range of bitumen contents both above and below the optimum binder content.
The Marshall stability and flow test provides the performance prediction measure for the Marshall mix design method. The stability portion of the test measures the maximum load supported by the test specimen at a loading rate of 50.8 mm/minute (2 inches/minute). Basically, the load is increased until it reaches a maximum then when the load just begins to decrease, the loading is stopped and the maximum load is recorded.
During the loading, an attached dial gauge measures the specimen’s plastic flow as a result of the loading. The flow value is recorded in 0.25 mm (0.01 inch) increments at the same time the maximum load is recorded.
All mix design methods use density and voids to determine basic HMA physical characteristics. Two different measures of densities are typically taken:
Bulk specific gravity (Gmb).
Theoretical maximum specific gravity ( Gmm).
These densities are then used to calculate the volumetric parameters of the HMA. Measured void expressions are usually:
The optimum bitumen binder content is selected based on the combined results of Marshall stability and flow, density analysis and void analysis