Concrete 117aggregates, and strength specified must also be noted. Any
inserts, anchors, or special requirements must be noted as well.
Estimating. The precast slabs are generally quoted by the
square foot or in a lump sum. Solid panels and channel slabs
can often be purchased from the manufacturer and installed
by the general contractor. The manufacturers of cored units
generally furnish and install the planks themselves. When
calculating the square footage, deduct all openings. Also
determine who will cut the holes, do any welding and special
finishing, and provide anchors and inserts. Concrete topping
is commonly used over the cored units, the most commonly
specified thickness being 2 inches. Determine the cubic
yards of the concrete, square feet of the reinforcing mesh,
and square feet of the surface to be finished.
Check the drawings and specifications to determine
how the planks will be held in place, special anchorage
details, inserts, and any other items that may present a cost
or problem on the job. Mechanical and electrical require-
ments should also be checked.
10–17 PRECAST BEAMS
AND COLUMNS
Precast beams and columns are available in square, rectan-
gular, T- and I-shaped sections. They are available simply
reinforced, with reinforcing bars, or prestressed with high-
strength wire. The sizes and spans depend on the engineer-
ing requirements of each project, and the beams and
columns are not poured in any particular size or shape. Spe-
cial forms can easily be made out of wood to form the size
and shape required for a particular project.
Specifications. The manufacturers’ specified strengths
and materials, reinforcing, connection devices, anchors,
inserts, and finishes required should all be noted. The differ-
ent shapes needed throughout and any other special require-
ments should also be specified.
Estimating. If the contractor intends to precast the con-
crete, the costs involved are indicated in Section 10–19.
Manufacturers who bid this item will bid it per linear foot or
in a lump sum. In doing a takeoff, keep the various sizes sep-
arate. Take special note of the connection devices required.
Labor. Precast beams (lintels) for door and window
openings are generally installed by the mason. If the weight
of an individual lintel exceeds about 300 pounds, it may be
necessary to have a small lift or crane to put it in place.
10–18 Miscellaneous Precast
Precast panels for the exterior walls of homes, warehouses,
apartment, and office buildings are available. Their sizes, thick-
nesses, shapes, designs, and finishes vary considerably. Each
individual system must be analyzed carefully to determine the
cost in place. Particular attention should be paid to the attach-
ment details at the base, top, and midpoints of each panel, how
attachments will be handled at the job site, how much space is
required for erection, how much bracing is required for all pan-
els to be securely attached, and how many men are required.
Some of the various methods involve the use of panels
4 feet wide, panels the entire length and width of the house,
and precast boxes that are completely furnished before they
are installed on the job. A tremendous amount of research
goes into precast modules. The higher the cost of labor in the
field, the more research there will be to arrive at more eco-
nomical building methods.
The estimator must carefully analyze each system, consider
fabrication costs and time, space requirements, how mechani-
cals will relate and be installed, and try to determine any hidden
costs. New systems require considerable thought and study.10–19 Precast Costs
If the specifications allow the contractor to precast the con-
crete shapes required for a project, or if the contractor
decides to at least estimate the cost for precasting and com-
pare it with the proposals received, the following considera-
tions will figure into the cost:- Precasting takes a lot of space. Is it available on the job
site, or will the material be precast off the site and trans-
ported to the job site? If it is precast off the site, what-
ever facilities are used must be charged off against the
items being made. - Determine whether the types of forms will be steel,
wood, fiberglass, or a combination of materials. Who
will make the forms? How long will the manufacture
take? How much will it cost? The cost of forms must be
charged to the precast items being made. - Will a specialist be required to supervise the manufac-
ture of the items? Someone will have to coordinate the
work and the preparation of shop drawings. This cost
must also be included. - Materials required for the manufacture must be pur-
chased:
(a) Reinforcing
(b) Coarse and fine aggregates
(c) Cement
(d) Wa t e r
(e) Anchors and inserts - Allowance must be made for the actual cost of labor
required to
(a) Clean the forms.
(b) Apply oil or retarders to the forms.
(c) Place the reinforcing (including pretensioning if
required).
(d) Mix and pour the concrete (including troweling the
top off ).
(e) Cover the concrete and apply curing method.
(f ) Uncover the concrete after the initial curing.