3.8—Special slab-on-ground support problem
Placement of slabs on topsoil should generally be avoided
because of their low shear strength and high compressibility.
Project specifications generally require that the building site
be stripped of all topsoil.
Expansive soils are defined as fine-grained soils, as shown
in
Tables 3.1
and
3.2
. Soils with a plasticity index of 20 or
higher have a potential for significant volume change. A
geotechnical engineer should examine the soil data and
recommend appropriate options. Potential problems can be
minimized by proper slab designs, stabilization of the soil, or
by preventing moisture migration through the slab. Failure to
manage the problem can, and often will, result in slab failure.
Frost action may be critical to silts, clays, and some fine
sands. These soils can experience large changes of volume,
and consequently heave due to the growth of ice lenses when
subjected to freezing cycles and loss of support due to
saturation upon thawing. Three conditions must be present
for this problem to occur:
• Freezing temperature in the soil;
• Water table close to the frost level to provide water for
the formation of ice lenses; and
• A soil that will transmit water from the water table into
the frost zone by capillary action.
Possible remedies include lowering the water table,
providing a barrier, or using a subbase/subgrade soil that is
not frost-susceptible. Properly designed insulation can be
beneficial. Volume changes due to frost action occur at
building perimeters, under freezer areas, and under ice-
skating rink floors (NCHRP 1974).
CHAPTER 4—LOADS
4.1—Introduction
This chapter describes loadings, the variables that
control load effects, and provides guidance for factors of
safety for concrete slabs-on-ground. Concrete slabs are
typically subjected to some combination of the following
loads and effects:
• Vehicle wheel loads;
• Concentrated loads;
• Line and strip loads;
• Distributed loads;
• Construction loads;
• Environmental effects; and
• Unusual loads.
Slabs should be designed for the most critical combination
of these loadings, considering such variables that produce
the maximum stress. The PCA guide for selecting the most
critical or controlling design considerations for various loads
(Packard 1976) is presented in
Fig. 4.1
. Because a number of
factors, such as slab thickness, concrete strength, subgrade
stiffness, and loadings, are relevant, cases where several
design considerations may control should be investigated
thoroughly.
Other potential problems, such as loadings that change
during the life of the structure and those encountered during
construction (Wray 1986), should also be considered. For
example, material-handling systems today make improved
use of the building’s volume. Stacked pallets that were once
considered uniform loads may now be stored in narrow-aisle
pallet racks that produce concentrated loads. The environmental
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