Relaxation of Steel in Prestressed Concrete

where fpx , = initial stress immediately after stretching (at time tt) = strength of prestressed steel When X is smaller than 0.5 the intrinsic relaxation is negligible, but its value increases rapidly as A approaches 1. A tendon in a prestressed concrete member loses a part of its initial tension due to the combined effect of creep and shrinkage of concrete and steel relaxation; the loss in tension is associated with shortening of the tendon. The reduction in tension caused by creep and shrinkage has the same effect on the magnitude of the relaxation as if the initial stress were smaller. Thus, the value of relaxation to be used in predicting prestress loss and the associated deformations of prestressed concrete members should be smaller than the intrinsic relaxation obtained from a constant length relaxation test. Hence, one can write: