Q.: What do people mean by "loss of prestress"? Does this mean that the prestressing force is all going to be lost eventually, and if so, how long can you expect prestressed concrete to last, or to be safe? Or does it mean prestressed concrete isn't really dependable because it may lose some of its prestress? Does anybody know how to prevent this loss?

A.: The industry pretty well understands how both concrete and steel behave, and how concrete, steel and prestressing forces interact with one another. This makes it possible to predict approximately how much prestress will be lost in a given kind of service. In designing prestressed concrete the designer takes all of this into account--the slipping of the anchorage; the frictional loss from curvature in the tendons; and the losses caused by elastic shortening, drying shrinkage and creep of the concrete. The result is a concrete that has the required strength even after loss of prestress.