Relaxation of Carbon and Stainless-Steel Threaded Bars for Posttensioning Applications

An experimental program was conducted to investigate the stress relaxation properties of large-diameter bars [65 to 70 mm (2.5 to 2.75 in.) nominal] for posttensioning applications. This study was a component of a large-scale research program to develop a more corrosion-resistant, posttensioning bar. The bars were fabricated from 4142 quenched and tempered (QT) carbon steel and three stainless steel alloys: Type 450, Type 630, and Type 2507. The experimental program objective was to evaluate the stress relaxation properties of these various materials. Stress relaxation losses were determined for 24 specimens by measuring the force change in the specimens for 1,000 h under constant strain. The influence of several additional parameters on relaxation was also evaluated including initial stress level, galvanization on plain (black) steel, coupled bars, and the variability provided by different steel heats. Measured losses are compared with established relaxation theory and relaxation limits provided in standard specifications. The results indicate relaxation losses are dependent on the type of stainless steel alloy and, for the alloys tested, can vary ±50%± 50% of the relaxation of plain carbon steel bars. Furthermore, different heats of steel can result in significant variation in the amount of relaxation. While standard design approaches can be used for stainless steel bars, material-specific data are necessary to ensure appropriate posttensioning forces are applied over the life of the structure.