Soil Supported Slab Floor with Structural Footing.
This type
of foundation is a hybrid between a soil-supported slab and piers or strip
footing that support the main structural load. Therefore, it could be either a
deep foundation with a shallow component for the slab or a shallow foundation.
This type
of construction is frequently used for shopping centers, warehouses, and light
structures and is sometimes used for residential construction.
For
residential construction it works best for non-expansive clay sites. In expansive
soil, because the foundation elements are disconnected, a plane surface across
the floor of the house is difficult to maintain and stiffened or
uniform-thickness mat may be more appropriate.
- When to use a soil-supported slab floor with structural footing.
Used in
light commercial or industrial construction may utilize this procedure,
sometimes in expansive soils. In expansive soil the main structural load, such
as drilled piers, and the slab is soil-supported with consideration given to
the amount of differential heave that may take place because of the clay soils.
The heave
can be reduced by the upper clays being removed and replace with an inert fill
material. In some cases, we have good bearing soils, such as a weathered rock,
is available only a few meters below the surface, which may or may not have
swelling clay above it. This condition would be a good candidate for this type
of construction.
- Design considerations.
Engineer
should know the use of the structure before designing this type of foundation.
For example, in warehouse structure, the loading condition have high-pressured
came from trucks or forklifts tire, and the floor slab must be designed as if
it were a concrete pavement to avoid being damaged by wheel pressures.
The floor
slab is typically placed on a layer of compacted fill. For warehouse or
manufacturing buildings this may be 4 to 5 feet thick to provide a truck dock
heigh construction. Because this is a thin, flat slab, it must rely entirely on
the fill of support, and the fill should be well compacted and tested for density.
The fill must be not consisting of expansive clays and to be inert.
Since this
type of construction is typically used for large areas, such as shopping centres,
warehouses, or industrial complex, joint spacing or control of slab cracking
due to concrete shrinkage is important. In addition, isolation joints around
pier or footing tops or “plinths” should be supplied with expansion joint
material providing for a slight bit of movement between the more rigidly
establish footing top and the floor slab, which may tend to move a little bit.
Concrete
during shrinkage requires joint spacing ranging from 12 to 15 feet and is a
function both shrinkage characteristics of the concrete and the amount of
reinforcing steel in the slab, which must be determined by the foundation
designer.
A rule of
thump is that concrete slabs shrink due to drying about 1/8 inch per 20
horizontal feet. The contraction of the slab is resisted by subgrade friction,
setting up tensile stresses. If these stresses exceed the concrete tensile
strength aided by reinforcing steel, the concrete cracks.
- Installation.
Crack
control joints must be tooled in during finishing, formed with a plastic strip
during concrete placement, or swan with a diamond- bladed saw. Joints are formed
from 1/8 to ¼ inch wide and extend about 10 to 25 percent of the slab
thickness.
This
“control” the shrinkage cracking to straight line. Joints are typically filled
with an appropriate joint filler. If sawing is used it should commence very
quickly, within 4 to 12 hours of the concrete finishing, or as soon as the
green concrete can support the equipment and sawing will not ravel the
concrete.
The
concrete has already begun to shrink as it is losing water by evaporation, and
it is not uncommon in this type of construction to see a shrinkage crack
occurring one to two inches away from and parallel to the control joint because
the joint was swan too late. This is a tricky problem for contractors and is
frequently not done properly.
A
construction joint is needed where a large slab concrete placement is
interrupted, either by design or construction expediency. An expansion joint is
one that permits the concrete to expand or contract and is commonly used in
concrete pavement and bridge decks where temperature changes are large. Floors
inside building exist in a controlled environment where temperatures do not
fluctuate much, reducing the need for expansion joint.
Control of
cracking and joint construction is important, especially if the slab is to be
the finished floor surface, which may be stained or patterned concrete.
Unsightly random shrinkage cracks will greatly detract from the architectural
intent as well as provide a fractured area that can enlarge from wheeled
traffic. This consideration is frequently overlooked, and it is difficult to
find skilled contractors or designers who can produce this work properly.
For some
industrial or warehouse applications, a hardener may be specified over the
concrete surface to resist abrasion from high-pressure tire traffic or movement
of heavy, skid-mounted loads. The manufacture’s recommendations should be
carefully followed for the use of hardener.