required is greatly reduced. The labor involved in
slab form concrete casting is much less than that
involved in filling a high wall form. One side of a
precast unit cast as a slab may be finished by hand
to any desired quality of finishing. The placement of
reinforcing steel is much easier in slab forms, and it
is easier to attain thorough filling and vibrating.
Precasting of wall panels as slabs may be expedited
by mass production methods not available when
casting in place.
Generally, structural members, including standard
highway girders, poles, electric poles, masts, and
building members, are precast by factory methods
unless the difficulty or impracticability of
transportation makes jobsite casting more desirable.
On the other hand, concrete that is cast in the position
that it is to occupy in the finished structure is called
Precast Concrete Floors, Roof Slabs,
Walls, and Partitions
The most commonly used precast slabs or panels
for FLOOR and ROOF DECKS are the channel and
double-T types (fig. 3-17, views A and B).
The channel slabs vary in size with a depth ranging
from 9 to 12 inches, width 2 to 5 feet, and a thickness
of 1 to 2 inches. They have been used in spans up
Figure 3-17.Typical precast panels.
50 feet. If desired or needed, the legs of the channels
may be extended across the ends and, if used in
combination with the top slabs, may be stiffened with
occasional cross ribs. Wire mesh may be used in the
top slabs for reinforcement. The longitudinal grooves,
located along the top of the channel legs, may be
grouted to form keys between adjacent slabs.
The double-T slabs vary in size from 4 to 6 feet in
width and 9 to 16 feet in depth. They have been used
in spans as long as 50 feet. When the top-slab size
ranges from 1 1/2 to 2 inches in thickness, it should be
reinforced with wire mesh.
The tongue-and-groove panel (fig. 3-17, view C)
could vary extensively in size according to the design
requirement. They are placed in position much like
tongue-and-groove lumber; that is, the tongue of one
panel is placed inside the groove of an adjacent panel.
They are often used as decking panels in large pier
Matching plates are ordinarily welded and used to
connect the supporting members to the floor and roof
Panels precast in a horizontal position, in a casting
yard or on the floor of the building, are ordinarily used
in the makeup of bearing and nonbearing WALLS and
PARTITIONS. These panels are placed in their
vertical positions by cranes or by the tilt-up procedure,
as shown in figures 3-18 and 3-19.
Usually, these panels are solid, reinforced slabs,
5 to 8 inches in thickness, with the length varying
according to the distances between columns or other
supporting members. When windows and door
openings are cast in the slabs, extra reinforcements
should be installed around the openings.
A concrete floor slab with a smooth, regular
surface can be used as a casting surface. When this
smooth surface is used for casting, it should be covered
with some form of liquid or sheet material to prevent
bonding between the surface and the wall panel. The
upper surface of the panel may be finished as regular
concrete is finished by troweling, floating, or
SANDWICH PANELS are panels that consist of
two thin, dense, reinforced concrete-face slabs
separated by a core of insulating material, such as
lightweight concrete, cellular glass, plastic foam, or
some rigid insulating material.
These panels are sometimes used for exterior
walls to provide additional heat insulation. The
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