Lesson 1/Learning Event 2
Where the construction is to be made on surface deposits, undesirable material is removed by stripping
operations.
Tuff. Tuff and other cementaceous materials of volcanic origin may be used for base courses. Tuff
bases are constructed the same as other base courses except that after the tuff is dumped and spread, the
oversize pieces are broken and the base compacted with sheepsfoot rollers. The surface is then graded,
and final compaction and finishing are done.
Rubble. In many cases it may be advantageous to use the debris or rubble of destroyed buildings in
constructing base courses, but care must be exercised to see that jagged pieces of metal and similar
objects are removed. Before removing any rubble, check for mines or boobytraps.
Bituminous base. Bituminous mixtures frequently are used as base courses beneath high type
bituminous pavements, particularly for rear area type airfields which carry heavy traffic. Such base
courses may be used to advantage when aggregates locally available are relatively soft and otherwise of
relatively poor quality, when mixing plant and bituminous materials are readily available, and when a
relatively thick structure is required for the traffic. In general, a bituminous base course may be
considered equal on an inch-for-inch basis to other types of high-quality base courses. When a
bituminous base course is used, it will be placed in lifts not exceeding 3 inches in thickness. If a
bituminous base is used, you may omit the binder course and lay the surface course directly on the base
course.
Selection of Type of Base Course
Selection of the type of base construction depends principally upon materials available at the particular
site, but equipment available and prevailing weather conditions during construction also are important
factors. A complete investigation should be made to determine the location and characteristics of all
natural materials suitable for base-course construction. Construction of untreated base courses with
natural materials is affected less by adverse weather than other types and requires less technical control.
Untreated bases are relatively easy and fast to build and are recommended in preference to bituminous
or cement-stabilized types, except where suitable materials for such construction are more readily
available. If they are not locally available, the transportation of bituminous material or cement for base
stabilization is a major supply problem in forward areas.
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