what is precast building?

Answer 1

It is the alternative to cast in place. When you see a truck going down the road with a large beam or piece of concrete deck, that is precast. When you seen a building with lots of scaffolding holding up wooden or metal flooring form and they are pumping concrete or lifting and dumping concrete in buckets, then that is cast in place.
The advantage of precast is that it can be tensioned (pull on the cables inside for reinforcement) more easily (not having the jacks to do it hanging over the street) and with the concrete batch plant right there, be made accurately. The problems with precast are moving the beams through traffic to the site and fitting the parts together. Some buildings that might be precast would require so much additional maneuvering and work to fit the precast inside the poured concrete frame that it is not worth any savings from precast.
Precast is also used whenever it would be a real problem to build the scaffolding and support the forms such as a bridge over an open highway. With precast the supports on either side are built and the highway is closed for an hour or two while the prepared beams are put in place.