Lesson 5/Learning Event 2
Learning Event 2
PROPER BLENDING METHODS
TRIAL AND ERROR METHOD OF AGGREGATE BLENDING
The features of the trial and error method are very simple but it may require-several trials to meet
the specified gradation selected. The information required is:
Sieve analysis of each stockpile.
Specified gradation of blend (gradation selected).
Quantity of materials available.
The example problem below illustrates this method.
EXAMPLE
Requirements
Determine an acceptable aggregate blend for a hot-plant asphalt concrete mix, surface course for
an airfield, low pressure tires.
Situation
Materials for the mix are in stockpiles and are in ample quantities. The materials consist of
coarse aggregate, fine aggregate, fine river bar sand (FRB), and limestone dust (LSD). The
washed sieve analyses of stockpiled materials are shown on lines 1, 2, 3, and 4 of Figure 12.
Selection of Proper Aggregate Gradation
The first step in the trial and error method is to select the proper aggregate gradation from the
gradation tables. The selection of the aggregate specification is based on the requirements of
asphalt cement binder for a surface course and for low pressure tires. The sieve analysis of
materials in the stock piles indicates that the maximum size aggregate is 3/4-inch (100 percent
passes the 3/4-inch sieve). With all of this information a study is made of the aggregate
specifications listed in the table. The gradation meeting the requirements is gradation #3. Next
it is necessary to select either gradation 3a, 3b, or 3c. Gradation A has more coarse AGG;
Gradation C has more fine AGG, and Gradation B has a good mixture of both.
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