In the United States, there are two kinds of high tensile wire strands 

 available—one for pretensioning and another for posttensioning. Pre- 

 tensioning strands are made of seven or more small uncoated wires as drawn. 

 The strands are then drawn through a lead hath for stress-relieving and 

 also to improve their bond characteristics. For posttensioning and unbonded 

 work, strands consisting of 7 to 61 galvanized wires are produced. These 

 strands are machine fabricated and stress-relieved to increase their 

 proportional limit and to minimize creep. When the strands are to be 

 bonded to the concrete, the wires should preferably be ungalvanized. 



High strength bars up to 1 034 megapascals (150 000 pounds per square 

 inch) or more are made by cold-working special alloy steels. By alloying 

 high carbon steel with proper agents such as silicon and manganese, high 

 strength is obtained. Then the proportional limit is raised by cold 

 working. The chemical contents of these bars again may differ. A sample 

 composition of high strength steel bars is: 



Carbon 0.6 percent 



Silicon 2.0 to 2.5 percent 



Manganese 0.7 to 1.0 percent 



Phosphorus 0.2 percent 



Sulfur 0.2 percent 



To get a better bond between steel and concrete, various forms of 

 surface indentation afford direct mechanical keys with the surrounding 

 concrete. It is assumed that the corrugations now commercially used will 

 not alter the stress-strain properties of the wires, although some question 

 has been raised as to their fatigue strength in comparison with the straight 

 ones. Some pretensioning factories pass their wires through a small 

 machine, forming permanent waves which are believed to increase the bond 

 resistance of the wire. 



The ultimate strength of steel wires, strands, or bars varies with 

 their manufacture, so that it is frequently necessary to obtain sample 

 tests for each lot of products. However, the general range of values is 

 listed in Table 17. While the ultimate strength of high strength steel can 

 be easily determined by testing, its elastic or proportional limit, or its 

 yield point, cannot be so simply ascertained. First, there is no yield 

 point for high strength steel as there is for ordinary low carbon steel. 

 Second, the gradual curving of the stress-strain curve makes it difficult 

 to fix a point for the proportional limit. Consequently, different methods 

 for defining the yield point of high tensile steel have been adopted. 



Yield point and proportional limit must be obtained by testing the 

 particular steel. But as a rough approximation Table 18 gives the usual 

 values for high tensile steels expressed in terms of the respective ultimate 

 strength. Approximate average values for the secant modulus at the pro- 

 portional limit are shown in Table 19. 



In order to avoid brittle failures in the prestressed concrete, a 

 certain amount of ductility in the steel is desirable. This is measured by 

 the amount of elongation in a certain gage length, generally a 254-milli- 

 meters (10-inch) gage in this country. The average ultimate elongation is 

 about 5 percent for wires and 5 percent for bars. For evident reasons, it 



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