Role of Residual Stresses in Stress Relaxation of Prestressed Concrete Wires

In prestressed concrete elements, the stress relaxation losses in steel tendons are of paramount importance to structural design. In this work, experimental and numerical results show a clear relationship between stress relaxation and the presence of residual stresses in steel wires for prestressing concrete. Stress relaxation tests were performed at different initial loads, from 50 to 98% of the rupture load, with three types of wire differing only in their distribution of residual stresses. Cold-drawn wires having the same mechanical properties required by standards (ASTM E421, ISO15630/3) but with different residual stresses will exhibit different stress relaxation losses; such losses increase with increasing surface tensile residual stresses (as a consequence of cold drawing) or increasing inner residual tensile stresses (as a consequence of mechanical postdrawing treatments). Actual expressions for prediction of stress relaxation losses do not take into account residual stresses. Better performance of the wires can be achieved by changing the profile of residual stresses according to the requirements of the structures. This note shows how to deal with this challenge.