thickness of the sandwich panels varies from 5 to 8
inches, and the face slabs are tied together with wire,
small rods, or in some other manner. Welded or bolted
matching plates are also used to connect the wall
panels to the building frame, top and bottom. Caulking
on the outside and grouting on the inside should be
used to make the points between the wall panels
Precast Concrete Joists, Beams,
Girders, and Columns
Small closely spaced beams used in floor
construction are usually called JOISTS; however,
these same beams when used in roof construction are
called PURLINS. The cross sections of these beams
are shaped like a T or an I. The ones with the
inverted T-sections are usually used in composite
construction where they support cast-in-place floor or
BEAMS and GIRDERS are terms usually applied
to the same members, but the one with the longer span
should be referred to as the girder. Beams and girders
may be conventional precast design or prestressed.
Most of the beams will be I-shaped unless the ends are
rectangular. The T-shaped ones can also be used.
Precast concrete COLUMNS may be solid or
hollow. If the hollow type is desired, heavy cardboard
tubing should be used to form the core. A looped rod
is cast in the column footing and projects upward into
the hollow core to help hold the column upright. An
opening should be left in the side of the column so that
the column core can be filled with grout. This causes
the looped rod to become embedded to form an anchor.
The opening is dry-packed.
Advantages of Precast Concrete
Precast concrete has the greatest advantage when
identical members are to be cast because the same
forms can be used several times. Some other
advantages are listed below.
Control of the quality of concrete.
Smoother surfaces and plastering are not necessary.
Less storage space is needed.
Concrete member can be cast under all weather
Better protection for curing.
Weather conditions do not affect erection.
Faster erection time.
A prestressed concrete unit is one in which
engineered stresses have been placed before it has
been subjected to a load. When PRETENSIONING 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 feet 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
pretensioning 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. 3-20). 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