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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. Currently, the Marshall method is used in some capacity by about 38 states. The Marshall method seeks to select the asphalt binder content at a desired density that satisfies minimum stability and range of flow values (White, 1985).
This section consists of a brief history of the Marshall mix design method followed by a general outline of the actual method. This outline emphasizes general concepts and rationale over specific procedures. Detailed procedures vary from state-to-state but typical procedures are available in the following documents:
During World War II, the U.S. Army Corps of Engineers (USCOE) began evaluating various HMA mix design methods for use in airfield pavement design. Motivation for this search came from the ever-increasing wheel loads and tire pressures produced by larger and larger military aircraft. Early work at the U.S. Army Waterways Experiment Station (WES) in 1943 had the objective of developing:
"...a simple apparatus suitable for use with the present California Bearing Ratio (CBR) equipment to design and control asphalt paving mixtures..."
The most promising method eventually proved to be the Marshall Stability Method developed by Bruce G. Marshall at the Mississippi Highway Department in 1939. WES took the original Marshall Stability Test and added a deformation measurement (using a flow meter) that was reasoned to assist in detecting excessively high asphalt contents. This appended test was eventually recommended for adoption by the U.S. Army because:
WES continued to refine the Marshall method through the 1950s with various tests on materials, traffic loading and weather variables. Today the Marshall method, despite its shortcomings, is probably the most widely used mix design method in the world. It has probably become so widely used because (1) it was adopted and used by the U.S. military all over the world during and after WWII and (2) it is simple, compact and inexpensive.
The Marshall mix design method consists of 6 basic steps:
Although neither Marshall nor WES specifically developed an aggregate evaluation and selection procedure, one is included here because it is integral to any mix design. A typical aggregate evaluation for use with either the Hveem or Marshall mix design methods includes three basic steps (Roberts et al., 1996):
The Marshall test does not have a common generic asphalt binder selection and evaluation procedure. Each specifying entity uses their own method with modifications to determine the appropriate binder and, if any, modifiers. Binder evaluation can be based on local experience, previous performance or a set procedure. Perhaps the most common set procedure now in use is based on the Superpave PG binder system. However, before this system there was no nationally recognized standard for binder evaluation and selection. Once the binder is selected, several preliminary tests are run to determine the asphalt binder's temperature-viscosity relationship.
The Marshall method, like other mix design methods, uses several trial aggregate-asphalt binder blends (typically 5 blends with 3 samples each for a total of 15 specimens), each with a different asphalt binder content. Then, by evaluating each trial blend's performance, an optimum asphalt binder content can be selected. In order for this concept to work, the trial blends must contain a range of asphalt contents both above and below the optimum asphalt content. Therefore, the first step in sample preparation is to estimate an optimum asphalt content. Trial blend asphalt contents are then determined from this estimate.
The Marshall mix design method can use any suitable method for estimating optimum asphalt content and usually relies on local procedures or experience.
Based on the results of the optimum asphalt binder content estimate, samples are typically prepared at 0.5 percent by weight of mix increments, with at least two samples above the estimated asphalt binder content and two below.
Each sample is then heated to the anticipated compaction temperature and compacted with a Marshall hammer, a device that applies pressure to a sample through a tamper foot (see Figure 5.8). Some hammers are automatic and some are hand operated. Key parameters of the compactor are:
The standard Marshall method sample preparation procedure is contained in:
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 (see Figure 5.9). The flow value is recorded in 0.25 mm (0.01 inch) increments at the same time the maximum load is recorded.
Typical Marshall design stability and flow criteria are shown in Table 5.3.
|Mix Criteria||Light Traffic
(< 104 ESALs)
(104 - 106 ESALs)
(> 106 ESALs)
|Stability (minimum)||2224 N
|Flow (0.25 mm (0.01 inch))||8||20||8||18||8||16|
|Percent Air Voids||3||5||3||5||3||5|
All mix design methods use density and voids to determine basic HMA physical characteristics. Two different measures of densities are typically taken:
These densities are then used to calculate the volumetric parameters of the HMA. Measured void expressions are usually:
Generally, these values must meet local or State criteria.
|Minimum VMA (percent)|
|4.75||No. 4 sieve||18|
|2.36||No. 8 sieve||21|
|1.18||No. 16 sieve||23.5|
The optimum asphalt binder content is finally selected based on the combined results of Marshall stability and flow, density analysis and void analysis (see Figure 5.10). Optimum asphalt binder content can be arrived at in the following procedure (Roberts et al., 1996):
The Marshall mix design method was developed to address specific mix design issues confronting the USCOE during World War II. Therefore, it was developed to be simple, light, quick, and reasonably accurate for the wheel loading of the time. Since then it has been modified and supplemented to address new concerns but the basic testing apparatus and selection criteria remain the same.
The biggest differentiating aspects of the Marshall method are the Marshall hammer and the Marshall stability and flow apparatus. Both are probably overly simplistic for high-end or high-load pavements but they are simple, light, portable and inexpensive.