also been developed and successfully applied to prestressed concrete, 

 resulting in considerable economy at times. 



The prestressing wires now in the market are mostly high tensile wires 

 obtained by cold-drawing high tensile steel bars through a series of dies. 

 The process of cold drawing tends to realine the crystals, and the strength 

 of the wires is increased by each drawing so that, the smaller the diameter 

 of the wires, the higher their ultimate unit strength. The ductility of 

 wires, however, is somewhat decreased as a result of cold drawing. It 

 must be recognized that the actual strength will vary with the composition 

 and manufacture of the wire as well as with its diameter. 



The "as-drawn" wire, although possessing a high ultimate strength, has a 

 relatively low proportional limit, e.g., about 414 to 552 megapascals (60 000 

 to 80 000 pounds per square inch), above which the stress-strain curve flat- 

 tens at an increasing rate. This is objectionable, since the deformation 

 characteristics are relatively uncertain and the amount of elongation during 

 prestress cannot be easily determined. Hence various methods commercially 

 known as the "stress-relieving" process have been used to increase the pro- 

 portional limit of the "as-drawn" wire. Two common stress-relieving methods 

 are as follows. 



(1) Time-Stress Treatment . This treatment consists of stretching 

 the wire to a stress level higher than that to be used in the final applica- 

 tion. This increases the proportional limit to about 60 or 70 percent of 

 the ultimate strength while the ultimate strength itself remains about the 

 same. After this process of stretching, the wire will still have slight 

 creep at an eventual stress of 50 percent of the ultimate, but, when 

 stressed up to 70 percent, the creep will not be much more than 5 percent. 



(2) Time-Temperature Treatment . This consists of heating the wire 



to 399° to 427° Celsius (750° to 800° Farenheit) for a period of 30 to 40 

 seconds. The heating is accomplished by drawing the wire through a molten 

 lead bath, or through a hot-air tunnel such as a ceramic tube with heat 

 applied on the outside. This treatment will have an effect on the propor- 

 tional limit and ultimate strength of the wire similar to that of the 

 previous process. But this "time-temperature treated" wire has practically 

 no creep when subjected to 50 percent of the ultimate strength. At 60 

 percent of the ultimate strength it shows slightly more creep than the 

 "stretched" wire, and at 70 percent and above the creep becomes excessive. 



It is usually left to the engineer to specify the physical properties 

 and sometimes the chemical composition desired. The chemical composition 

 of prestressing wire may vary with the manufacturer. Some manufacturers may 

 use a certain amount of silicon in the steel, although this is not included 

 in the following sample analysis: 



Carbon 0.60 to 0.85 percent 



Manganese 0.70 to 1.Q0 percent 



Phosphorus Q.Q50 percent maximum 



Sulfur 0.055 percent maximum 



114 



