Figure 3-20.Comparison of plain and prestressed concrete beams.
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
pounds per cubic foot. Lightweight concrete weighs
90 to 120 pounds per cubic foot, depending on its
intended use. Lightweight concrete can be made by
using either gas-generating chemicals or lightweight
aggregates, such as expanded shale, clay, or slag.
Concrete, containing aggregates like perlite or
vermiculite, is very light in weight and is primarily
used as insulating material. Lightweight concrete is
usually classified according to its weight per cubic
Semi-lightweight concrete has a unit weight of
115 to 130 pounds per cubic foot and an ultimate
compressive strength comparable to normal concrete.
Sand of normal weight is substituted partially or
completely for the lightweight fine aggregate.
Insulating lightweight concrete has a unit weight
ranging from 15 to 90 pounds per cubic foot, and its
compressive strength seldom exceeds 1,000 psi. This
type of concrete is generally used for insulating
applications, such as fireproofing.
Structural lightweight concrete has a unit weight
of 85 to 115 pounds per cubic foot and a 28-day
compressive strength in excess of 2,500 psi. This type
is used primarily to reduce the dead-load weight in
concrete structural members, such as floors, walls, and
roof sections in high-rise structures.
Heavyweight concrete is produced with special
heavy aggregates and has a density of up to 400 pounds
per cubic foot. This type is used principally for
radiation shielding, for counterweights, and for other
applications where higher density is desired. Except
for density, the physical properties of heavyweight
concrete are similar to those of normal or conventional
Tilt-up concrete construction is a special form of
precast concrete building. This method consists
basically of jobsite prefabrication in which the walls
are cast in a horizontal position, tilted to a vertical
position, and then secured in place. Tilt-up
construction is best suited for large one-story
buildings, but it can be used in multistory structures.
Usually, multistory structures are built by setting the
walls for the first story, placing the floor above, then
repeating the procedure for each succeeding floor. An
alternate method is to cast two- to four-story panels.
The wall panels are usually cast on the floor slab
of the structure. Care must be exercised to ensure that
the floor slab is smooth and level and that all openings
for pipes and other utilities are temporarily plugged.
The casting surface is treated with a good
bond-breaking agent to ensure the panel does not
adhere when it is lifted.
Reinforcement of Tilt-Up Panels
The steel in a tilt-up panel is set in the same
manner as it is in a floor slab. Mats of reinforcement