Precasting is often thought of as a factory operation but many contractors today have staffs with sufficient ability and experience to do good precasting work themselves. Successful operation does require technically oriented staff capable of performing the complex technical operations responsibly. Two of the reasons for precasting are to achieve high dimensional accuracy of building elements and to get a cost advantage by using repetitive, automated procedures with fixed forming units that do not have to be dismantled.
Sometimes it is more economical for the contractor to purchase units than to produce them himself. The next question is whether the builder is capable of setting up to produce the elements that are needed. The key consideration, however, may be the needs of the project itself. There may be a big advantage in being able to control the timing of the production and in having the precast units always readily available at the site. It is also advantageous to be able to produce exactly the shape and size of precast element needed for efficiency of the structure without being limited to commercially available units.
ONE EFFICIENT SYSTEM
A combination of slipforming, precasting and casting-in-place has worked very well for our company in constructing a variety of buildings. The construction sequence includes the following steps: slipforming the central core, installing precast beams, installing precast floor planks, setting precast wall units, casting exterior columns formed by the wall units, and placing 2-inch-thick structural lightweight aggregate concrete floor topping. The slipforming operation proceeds about 3 stories ahead of the rest of the operation. Precasting of walls, beams and planks is carried out at the same time that erection is going on.