Technical Services
Marshall Mix Design
  • Overview
  • Basic Steps
Marshall mix design

The Marshall mix design method consists of 6 basic steps:

  • Aggregate selection.
  • Bitumen binder selection.
  • Sample preparation (including compaction).
  • Stability determination.
  • Density and voids calculations.
  • Optimum bitumen binder content selection.

Aggregate Selection

Aggregate Selection

A typical aggregate evaluation includes three basic steps
Determine aggregate physical properties. This consists of running various tests to determine properties such as:

  • Toughness and abrasion
  • Durability and soundness
  • Cleanliness and deleterious materials
  • Particle shape and surface texture

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:

  • Gradation and size
  • Specific gravity and absorption

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.

Bitumen Binder selection

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

Sample Preparation

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.

Stability Determination

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.

Density and Voids Calculation

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:

  • Air voids (Va),
  • Voids in the mineral aggregate (VMA)
  • Voids filled with Bitumen (VFB)

Optimum Binder content selection

The optimum bitumen binder content is selected based on the combined results of Marshall stability and flow, density analysis and void analysis

  • After plotting graphs, determine the bitumen binder content that corresponds to the specifications median air void content (typically this is 4 percent)
  • Determine properties at this optimum bitumen binder content by referring to the plots.  Compare each of these values against specification values and if all are within specification, then the preceding optimum bitumen binder content is satisfactory.  Otherwise, if any of these properties is outside the specification range the mixture should be redesigned.