Lesson 1/Learning Event 2
Macadam type base material. The term macadam is usually applied to construction where a
coarse, crushed aggregate is placed in a relatively thin layer and rolled into place; then fine aggregate or
screenings are placed on the surface of the coarse aggregate layer and rolled and broomed into the
coarse rock until it is thoroughly keyed in place. Water may be used in the compacting and keying
process. When water is used, the base is termed a waterbound macadam. The crushed rock used for
macadam base courses should consist of clean, angular, durable particles free of clay, organic matter,
and other objectionable material or coating. Because of the method of construction, it is necessary to
maintain the coarse and fine aggregates separately. Aggregates for macadam type construction should
meet the gradation requirements given in Table 5. Any hard, durable crushed aggregate can be used,
provided the coarse aggregate is primarily one size and the fine aggregate will key into the coarse
aggregate.
Other materials. In many areas in a theater of operations, deposits of natural sand and gravel and
sources of crushed rock are not available. This has led to the development of base courses from
materials that normally would not be considered. These include caliche, limerock, shells, cinders, coral,
iron ore, rubble, and other select materials. Some of these are primarily soft rock, and crush or degrade
under construction traffic to produce composite base materials similar to those previously described.
Others develop a cementing action which results in a satisfactory base. These materials cannot be
judged on the basis of the gradation limits used for other materials, but must be judged on the basis of
service behavior. Strength tests on laboratory samples are not satisfactory because the method of
preparing a sample seldom duplicates the material in place. The plasticity index is a reasonably good
criterion, and as a general rule, a low plasticity index is a necessity. Observation of the performance of
these types of base materials in existing roads and pavements is the most reliable clue as to whether or
not they will be satisfactory.
Coral. Coral is commonly found in the Pacific and Caribbean areas. Since uncompacted coral has a
high capillarity, uncompacted and poorly drained coral often results in an excessive moisture content
and loss of stability. The bonding properties of coral, which are its greatest asset as a construction
material, vary with the amount of volcanic impurities, the proportion of fine and coarse material, age,
length of exposure to the elements, climate, traffic, sprinkling; and method of compaction. Proper
moisture control, drainage, and compaction are essential to obtain satisfactory results. Variations over 1
percent from OMC should be avoided. Sprinkling with sea water or sodium chloride in solution has a
beneficial effect on bonding when rollers are used. However, because of the possible corrosive effects
on aircraft engines, sprinkling with sea water on airfields should be undertaken only with specific
approval of higher authority.
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