Lesson 5/Learning Event 1
Asphalt Concrete
This is material that serves primarily as a wearing course and moisture barrier. It provides structural
strength to the pavement and is designed using the Marshall Test Method rather than CBR.
FACTORS AFFECTING TYPE OF TEST PROGRAM
The type of test program and the analysis of the resulting data depends on: (1) where the soil is going to
be used in the pavement; (2) whether or not the soil will be disturbed during the construction process;
(3) the level of compactive effort required to preclude settlement; and (4) the physical nature of the soil.
From Figure 33 it can be seen that the materials above the uncompacted subgrade will undergo
considerable disturbance and the laboratory test used to define strength must reflect this fact. Likewise,
a test method that gives strengths for undisturbed samples should be used for the uncompacted subgrade.
The nature of the soil will determine the amount and type of laboratory work required. For example,
coarse-grained, free-draining soils or those soils having few fines may display little strength variation
with changes in moisture content while some fine-grained soils show objectionable volumetric changes
(swelling) as the molding moisture content varies. Table 9 is a summary of the test methods which
should serve as a guide for the following discussion. However, the basic principle underlying this
categorization is simulation of the conditions to which a soil is subjected during both construction and
the life of the structure.
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