Lesson 5/Learning Event 3
Learning Event 3
THE MARSHALL STABILITY METHOD: DETERMINATION OF OPTIMUM
BITUMINOUS CONTENT
The Marshall method presented here is applicable to all bituminous paving mixtures. The
Marshall method may be used for design of tar and tar-rubber mixes.
Investigation work has indicated that the "optimum" asphalt content is one of the most important
factor in the proper design of an asphaltic paving mixture. Extensive research in pavement
behavior studies has resulted in the establishment of criteria for determining the optimum asphalt
content for a given blend of aggregates.
The Marshall method consists of the determination of five Marshall properties from test
specimens that have been prepared by definite procedures and specifications. The test properties
are described below.
Stability
The stability of the test specimens is the maximum load resistance in pounds which the standard
test specimens will develop at 140F when tested as outlined hereinafter.
Unit Weight
The unit weight of the compacted specimens is expressed as pounds per cubic foot. Unit weight
is obtained by multiplying the specific gravity of the specimens by the unit weight of water (62.4
pounds per cubic foot).
Flow
The flow value is the total movement or strain, in units of 1/100 inch occur ring in the specimens
between no load and maximum.
Percent Voids Total Mix
This is the ratio of the volume of voids in the compacted specimens to the total volume of the
specimens expressed as a percentage. Experience has indicated that a compacted completed
asphalt concrete pavement must contain air voids to allow expansion space for the asphalt on hot
summer days and for anticipated future compaction by traffic. If the asphalt has no place to go
within the pavement, the pavement will "bleed," that is, free asphalt will appear on the surface
causing a slick condition and reducing the stability of the pavement. A pavement reduced to 1
percent voids may bleed. The recommended percentage is between 3 and 5 percent for surface
courses and between 4 and 6 percent for binder courses for low pressure tires and between 5 and
7 percent for binder courses for high pressure tires.
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