TM 5-809-1/AFM 88-3, Chap. 15
5-4. Base courses.
e. Nonuniformity. Where it is not possible to
create uniform subgrade conditions by the methods
a. Requirements. Base courses may be required
described herein, the slab design can be varied
under rigid floor slabs to provide protection against
throughout the project to maximize economy. Con-
detrimental frost action, drainage, a suitable
crete flexural strength, percent reinforcing steel, and
working platform for the construction operation
slab thickness can all be adjusted to provide a design
during adverse weather conditions, and additional
which is balanced in terms of service life. The
support to the floor slab. In any of the above-
specific combinations to be used will depend upon
mentioned applications for base courses, an
local conditions and costs, and selection of design
economic study is required to determine base course
alternatives is left to the discretion of the design
requirements in floor-slab design. The economic
engineer.
study will typically include costs of base course
f. Special soils. Although compaction increases
materials such as hauling and required floor-slab
the stability and strength of most soils, some soil
thickness with and without base course.
types show a marked decrease in stability when
Consideration should also be given to the use of the
scarified, worked, and rolled. Also, there are some
floor slab, i.e., what material is to be stored and
soils that shrink excessively during dry periods and
what operations are likely to occur on the floor slab.
expand excessively when allowed to absorb
These considerations will also have an impact on
moisture. In general, these are inorganic clays of
whether to include a base course.
relatively high plasticity usually classified as CH
b. Compaction. Where base courses are used, the
soils. Special types of soils are discussed in TM 5-
base-course materials should be compacted in
825-2/AFM 88-6, Chap. 2, TM 5-818-1/AFM 88-3,
accordance with the criteria given above. With this
Chap. 7, and TM 5-818-7.
in mind, note that compaction of thin base courses
g. Back filling. Special care should be exercised
placed on yielding subgrades to high densities is
in backfill areas around walls and columns to ensure
difficult.
compliance with compaction requirements outlined
c. Drainage. Adverse moisture conditions result-
in the above paragraphs. Backfilling around walls
ing from high water table and subsoils subject to
and columns should be performed with pneumatic
capillary action may cause damage to floor covering
tampers, gasoline-powered tampers, and other
and stored material. If the subgrade soils provide for
mechanized hand-operated devices. Soil moisture
movement of water by capillary flow (CH, CL, MH,
content and lift thickness should be carefully con-
and ML types) and the ground-water table is less
trolled to ensure that compaction requirements are
than 5 feet from the final grade, a minimum
met through the full depth of the backfill.
thickness of 6 inches of free-draining base course
h. Treatment of unsuitable materials. Soils desig-
will be required. Base courses for drainage will not
nated as unsatisfactory for subgrade use by MIL-.
be required under conditions of deep ground-water
STD-619 should be removed and replaced. The
table. Positive drainage is to be provided to ensure
depth to which such undesirable soils should be
against water being trapped beneath the pavement.
removed depends on the soil type, drainage
conditions, type of material stored, magnitude of
The floor should be protected against the migration
tolerable differential settlement, and depth of
of water vapor through the slab and joints. Water
freezing-temperature penetration. The depth of
vapor damage is to be prevented by an impermeable
removal and replacement should be determined by
membrane placed on the subgrade prior to concrete
the engineer on the basis of judgement and previous
placement. Such vapor barriers shall be installed
experience and with due consideration of the traffic
even in conjunction with base courses if moisture-
to be served as well as the costs involved. In some
susceptible floor coverings or conduits are present.
instances, unsatisfactory or undesirable soils may be
See TM 5-809-2/AFM 88-3, Chap. 2 for
improved economically by stabilization with such
embedment of conduits.
materials as cement, fly ash, lime, or certain
d. Materials. If conditions indicate that a base
chemical additives whereby the characteristics of the
course is desirable, a thorough investigation should
composite material become suitable for use as
be made to determine the source, quantity, and
subgrade. Criteria for soil stabilization are given in
characteristics of the available materials. A study
TM 5-822-4. Subgrade stabilization, however,
should be made to determine the most economical
should not be attempted unless the cost reflects
thickness of material for a base course that will meet
corresponding savings in base course, floor slabs, or
the requirements. The base course may consist of
drainage facilities construction and is approved by
natural materials, processed materials, or stabilized
HQDA (DAEN-ECE-G) Washington, DC 2031 4-1
materials as defined in TM 5-882-4. The material
000 or Headquarters, Air Force Engineering
selected should be the one that best accomplishes
Services Center (DEMP), Tyndall AFB, Fla. 32403.
5-5
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