How to calculate the total loss of prestress
As a general guide, it may be taken that total loss of prestress may be in the range of 20 to 25 percent of the initial prestress, the lower and higher values of this range generally applying to post-tensioned and pre-tensioned concrete members respectively. It may seem natural as prestresses in post-tensioned members are generally applied at a later stage by which time concrete is able to have gained most of its strength.