Civil Engineering Reference
In-Depth Information
ditions on the setting time of the concrete and the size of
the placement to be completed.
Subgrade Preparation
Cracks, slab settlement, and structural failure can often be
traced to an inadequately prepared and poorly compacted
subgrade. The subgrade on which a slab on ground is to
be placed should be well drained, of uniform bearing
capacity, level or properly sloped, and free of sod, organic
matter, and frost. The three major causes of nonuniform
support are: (1) the presence of soft unstable saturated
soils or hard rocky soils, (2) backfilling without adequate
compaction, and (3) expansive soils. Uniform support
cannot be achieved by merely dumping granular material
on a soft area. To prevent bridging and settlement
cracking, soft or saturated soil areas and hard spots (rocks)
should be dug out and filled with soil similar to the sur-
rounding subgrade or if a similar soil is not available, with
granular material such as sand, gravel, or crushed stone.
All fill materials must be compacted to provide the same
uniform support as the rest of the subgrade. Proof rolling
the subgrade using a fully-loaded dump truck or similar
heavy equipment is commonly used to identify areas of
unstable soils that need additional attention.
During subgrade preparation, it should be remem-
bered that undisturbed soil is generally superior to com-
pacted material for supporting concrete slabs. Expansive,
compressible, and potentially troublesome soils should be
evaluated by a geotechnical engineer; a special slab design
may be required.
The subgrade should be moistened with water in
advance of placing concrete, but should not contain pud-
dles or wet, soft, muddy spots when the concrete is placed.
Fig. 11-15. Nuclear gauges containing radioactive sources
used to measure soil density and moisture can determine if
a subbase has been adequately compacted. (69932)
Vapor Retarders and
Moisture-Problem Prevention
Many of the moisture problems associated with enclosed
slabs on ground (floors) can be minimized or eliminated
by (1) sloping the landscape away from buildings, (2) us-
ing a 100-mm (4-in.) thick granular subbase to form a
capillary break between the soil and the slab, (3) pro-
viding drainage for the granular subbase to prevent
water from collecting under the slab, (4) installing foun-
dation drain tile, and (5) installing a vapor retarder, often
polyethylene sheeting.
For years vapor retarders have been mistakenly called
vapor barriers. A vapor retarder slows the movement of
water vapor by use of a 0.15 to 0.25 mm (6 to 10 mil) poly-
ethylene film that is overlapped approximately 150 mm
(6 in.) at the edges. A vapor retarder does not stop 100% of
vapor migration; a vapor barrier does. Vapor barriers are
thick, rugged multiple-ply-reinforced membranes that are
sealed at the edges. Vapor retarders are discussed in this
text because they are more commonly used; but many of
the following principles apply to vapor barriers as well.
A vapor retarder should be placed under all concrete
floors on ground that are likely to receive an impermeable
floor covering such as sheet vinyl tile or be used for any
purpose where the passage of water vapor through the
floor might damage moisture-sensitive equipment or
materials in contact with the floor. However, a few project
sites with deep groundwater tables and sandy soils con-
taining very low silt or clay contents may not require the
use of a vapor retarder under concrete slabs.
Vapor retarders placed directly under concrete slabs
may increase the time delay before final finishing due to
longer bleeding times, particularly in cold weather. To
minimize this effect, a minimum 75-mm (3-in.) thick layer
Subbase
A satisfactory slab on ground can be built without a sub-
base. However, a subbase is frequently placed on the sub-
grade as a leveling course to equalize minor surface
irregularities, enhance uniformity of support, bring the
site to the desired grade, and serve as a capillary break
between the slab and the subgrade.
Where a subbase is used, the contractor should place
and compact to near maximum density a 100-mm (4-in.)
thick layer of granular material such as sand, gravel,
crushed stone, or slag. If a thicker subbase is needed for
achieving the desired grade, the material should be com-
pacted in thin layers about 100 mm (4 in.) deep unless tests
determine compaction of thicker a lift is possible (Fig. 11-15).
Subgrades and subbases can be compacted with small plate
vibrators, vibratory rollers, or hand tampers. Unless the
subbase is well compacted, it is better not to use a subbase;
simply leave the subgrade uncovered and undisturbed.
