A prestressed concrete unit is one in which
engineered stresses have been placed before it has
been subjected to a load. When PRETENSION-
ING is used, the reinforcement (high-tensile-
strength steel strands) is stretched through the
form between the two end abutments or anchors.
A predetermined amount of stress is applied to
the steel strands. The concrete is then poured,
encasing the reinforcement. As the concrete sets,
it bonds to the pretensioned steel. When it has
reached a specified strength, the tension on the
reinforcement is released. This prestresses the
concrete, putting it under compression, thus
creating a built-in tensile strength.
POST-TENSIONING involves a precast
member that contains normal reinforcing in
addition to a number of channels through which
the prestressing cables or rods maybe passed. The
channels are usually formed by suspending
inflated tubes through the form and casting the
concrete around them. When the concrete has set,
the tubes are deflated and removed. Once the
concrete has reached a specified strength,
prestressing steel strands or TENDONS are pulled
into the channels and secured at one end. They
are then stressed from the opposite end with a
portable hydraulic jack and anchored by one of
several automatic gripping devices.
Post-tensioning may be done where the
member is poured or at the jobsite. Each member
may be tensioned, or two or more members may
be tensioned together after erection. In general,
post-tensioning is used if the unit is over 45 ft long
or over 7 tons in weight. However, some types
of pretensioned roof slabs will be considerably
longer and heavier than this.
When a beam is prestressed, either by pre-
tensioning or post-tensioning, the tensioned steel
produces a high compression in the lower part of
the beam. This compression creates an upward
bow or camber in the beam (fig. 7-19). When a
load is placed on the beam, the camber is forced
out, creating a level beam with no deflection.
Those members that are relatively small or that
can be readily precast are normally pretensioned.
These include precast roof slabs, T-slabs, floor
slabs, and roof joists.
SPECIAL TYPES OF CONCRETE
Special types of concrete are essentially those
with unique physical properties or those produced
with unusual techniques and/or reproduction
processes. Many special types of concrete are
made with portland cement as a binding medium;
some use binders other than portland cement.
Conventional concrete weighs approximately
150 lb per cubic foot. Lightweight concrete weighs
20 to 130 lb per cubic foot, depending on its
intended use. Lightweight concrete can be made
by using either gas-generating chemicals or
Figure 7-19.-Comparison of plain and prestressed concrete beams.
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