Concrete 115most common types of structural precast concrete are dou-
ble and single tees, floor planks, columns, beams, and wall
panels (Figure 10.34).
Depending on the requirements of the project, precast
concrete is available reinforced or prestressed. Reinforced
concrete utilizes reinforcing bars encased in concrete; it is
limited in its spans, with 40 feet being about the maximum
for a roof. When longer spans are required, prestressed con-
crete is used, either pretensioned or posttensioned. Prestress-
ing generally utilizes high-strength steel or wire or wire
strands as the reinforcing. In pretensioning, the longitudinal
reinforcing is put in tension before the concrete is cast. The
reinforcing is stretched between anchors and held in this
state as the concrete is poured around the steel and cured.
With posttensioning, the longitudinal reinforcement is not
bonded with the concrete. The reinforcement may be
greased or wrapped to avoid bonding with the concrete, or
conduits of some type (tubes, hoses) may be cast in the con-
crete and the reinforcement added later. The reinforcement
is then stretched and anchored at the ends; when released,
the stretched wires tend to contract and in this manner com-
press the concrete.
Most precast concrete is priced by the square foot, linear
foot, or in a lump sum. It is important to determine exactly
what is included in the price. Most suppliers of precast con-
crete items price them delivered to the job site and installed,
especially if they are structural items. When the specifica-
tions permit, some contractors will precast and install the
pieces required themselves. Unless experienced personnel
are available, doing your own precasting may cost more than
subcontracting the work to others.
The aggregates used for precast concrete may be heavy-
weight or lightweight; however, it should be noted that some
types of lightweight concrete are not recommended by some
consultant engineers for use on posttensioning work, and care
should be taken regarding all materials used in the concrete.
Hole-cutting in tees is subject to the distance between
the structural tee portions of the members. The lengths of
holes are not as rigidly controlled, but should be approved
by the structural engineer.10–13 Specifications
The specifications must be checked to determine whether a
particular manufacturer is specified. All manufacturers who
can supply materials and meet the specifications should be
encouraged to bid the project. Often there is only limited
competition in bidding on precast concrete items. The esti-
mator must also determine the strength, reinforcing, and
inserts as well as any special accessories required.
If the project entails any special finish such as sandblast-
ing, filling air holes, colored concrete, special aggregates, or
sand finish, this should be noted on the workup sheets. Be
certain that there is a clear understanding of exactly what the
manufacturer is proposing to furnish. For example, it should
be understood who will supply any required anchor bolts,
welding, cutting of required holes, and filling of joints.
Always check to see whether the manufacturer (or subcon-
tractor) will install the precast. If not, the estimator will have
to calculate the cost of the installation.10–14 Estimating
Floor, wall, and ceiling precast concrete are most commonly
taken off by the square foot with the thickness noted. Also be
certain to note special requirements such as insulation cast
in the concrete, anchorage details, and installation problems.FIGURE 10.34.Precast Shapes.