When an engineer talks about "loss of prestress" does he mean there has been an accident or is this something that always happens?
He's probably talking about a reduction in prestress, something that is to be expected. The American Concrete Institute publication SP-19, Cement and Concrete Terminology, defines "loss of prestress" as "The reduction of the prestressing force which results from the combined effects of creep in the steel and creep and shrinkage of the concrete; does not normally include friction losses but may include the effect of elastic deformation of the concrete." Generally the designer takes account of a certain amount of loss of prestress so that even after the loss takes place the amount remaining is adequate.