Lesson 1/Learning Event 2
essed materials. Do not use materials which slake on exposure to air or water, nor use processed
materials if materials such as gravel or sand-clay are available, except when studies show that the use of
processed materials will save time and effort or when they are made necessary by project requirements.
Bases made from processed materials can be divided into three general types: stabilized, coarse graded,
and macadam.
Stabilized base material. A stabilized type base course is one in which all the material ranging
from coarse to fine is intimately mixed either before or as the material is laid into place. If practicable,
materials for this type of base should meet the requirements given in Table 5. Because the aggregates
produced in crushing operations or obtained from deposits are often deficient fines, it may be necessary
to blend in selected fines to obtain a suitable gradation. Screenings, crusher-run fines, or natural soil
containing no clay may be added and mixed either in the processing plant or during the placing
operation.
TABLE 5. DESIRABLE GRADATION FOR CRUSHED ROCK, GRAVEL, OR SLAG, AND
UNCRUSHED SAND AND GRAVEL AGGREGATES FOR BASE COURSES
Coarse-graded type base material. A coarse-graded type base course is composed of crushed
rock, gravel, or slag. If practicable, material for this type of base should meet the gradation
requirements given in Table 5. This base may be used to advantage when it is necessary to produce
crushed rock, gravel, or slag on site or when commercial aggregates are available. When gravel is used,
it is desirable that 90 percent of the material by weight have two or more freshly fractured faces, with
the area of each face being equal to at least 75 percent of the smallest midsectional area of the piece.
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