Precast Construction. Precasting is the fabrication of a member 
in a location other than its final position in a structure. Usually 
precasting is done at a specialized factory from which the pieces are 
shipped to the assembly site. However, job-site precasting is not 
unusual; tilt-up slabs used for constructing buildings is an example of 
the latter. 
A majority of the precast items are mass-produced with each manu- 
facturer supplying well-defined specifications. Piles, girders, roof 
members, and other standard products form the great bulk of the products 
fabricated at the factories. The acceptance of off-the-shelf precast 
items by engineers has increased the quality and decreased the unit: 
costs of the end product. This is due primarily to the cyclic nature 
of the precast technique, which can be performed daily, thereby increasing 
the efficiency of the labor. Another added cost-reducing factor is 
the manufacturers' willingness to invest in better quality equipment to 
produce the standardized items, knowing that they can amotorize the 
initial investment over a longer period of time. In short, all the 
benefits which accrue in mass production in general can be had in a pre- 
cast concrete operation. 
The equipment used in a precasting operation does not differ from 
that found in any concrete project. Conventional mixers, vibrators, 
and associated equipment are suitable for mixing and placing the concrete 
into reusable forms. Transporting and handling of precast concrete 
items are carefully controlled operations. Pickup points are designated 
and used to lift and place the units. Indiscriminate attachment of lift- 
ing lines will likely overstress the member, as will tilting or placing 
in a way other than the position designated. None of these provisions 
and restrictions on handling is unduly troublesome, as the widespread 
use of precast concrete elements indicates. 
In addition to providing items of high quality, precasting also 
offers the advantage of easy prestressing. Either pretensioning or post- 
tensioning can be done quickly and economically, under the best of 
conditions, at the casting site. 
A disadvantage of precasting, particularly with respect to MOBS, is 
the limitation on the size of the units that can be cast. The size 
constraints are not inherent in the technique itself, but are imposed by 
the availability and capacities of heavy-lift equipment. Heavy and large 
concrete sections are not practical to transport and difficult to place 
and position. Consequently, the weight of a precast section is foremost 
in the design. For MOBS elements, or in a platform of moderate size, 
this is not a drawback. But if a large platform were to be assembled 
from easily-managed components, it appears certain there would be too 
many elements and connections to be practical. 
The cost of a precast concrete item is usually determined on the 
basis of area or length. Precast concrete panels 7 inches thick and 
300 to 500 square feet in area will cost about $1.80 per square foot, 
including erection and alignment. This is approximately $60 per cubic 
yard (using 3,000 psi concrete). Standard precast bridge beams (3 feet 
5-8 
