stress for a period of time. The two methods give about the same results 

 when the creep is not excessive, but the constant strain method is more 

 often employed as a basis for measurement, because of its similarity to the 

 actual conditions in prestressed concrete. Creep varies with steel of 

 different compositions and treatments; hence exact values can be determined 

 only by test for each individual case if previous data are not available. 



Approximate creep characteristics, however, are known for most of the 

 prestressing steels now in the market. Speaking in general, the percentage 

 of creep increases with increasing stress, and when a steel is under low 

 stress, the creep is negligible. The following summarizes the creep 

 characteristics of different steels. Compared to stress-relieved wires, 

 the "as-drawn" wires have somewhat higher creep. Prestretched wires will 

 have about 2 to 3 percent creep when subject to 0.50 f' (f' = ultimate 

 strength) but when stressed to 0.70 f, the creep will still be no more 

 than 5 percent. Time -temperature treated wires have practically no creep 

 when subject to 0.50 f. At 0.60 f, it has slightly more creep than 

 "prestretched" wires, and at 0.70 to 0.80 f the creep becomes excessive. 

 Galvanized wires have about the same creep characteristics as the time- 

 temperature treated wires, and should preferably not be subjected to any 

 stress above 0.60 f without carefully considering the effect of creep. 

 For high tensile bars, some limited tests seemed to show that, for stress 

 up to about 0.55 f, creep is not more than 5 percent. 



While creep in steel is a function of time, there is evidence to show 

 that under the ordinary working stress for high tensile steel, creep takes 

 place mostly during the first few days. Under constant strain, creep 

 ceases entirely after about 2 weeks. If the steel is stressed to a few 

 percent above its initial prestress and overstress is maintained for a few 

 minutes, the eventual creep can be greatly lessened; it practically stops 

 in about 3 days . 



In order to prevent corrosion in unbonded prestressed concrete, wires 

 are sometimes galvanized. When galvanized, the tensile strength is slightly 

 reduced, but its other characteristics are similar to those of the time- 

 temperature treated wire. It has practically no creep when used within 55 

 percent of its ultimate strength, but its tendency to creep at stresses 

 above 55 percent cannot be well controlled. 



c. Embedded Hardware . Embedments are common to all concrete construc- 

 tion. The materials are usually steel and generally consist of fabricated 

 structural steel materials such as plates, angles, bars and sleeves; rein- 

 forcing steel of varying grades and strengths; and prestressing tendons 

 such as wires, strands, and bars. Occasionally, other materials such as 

 stainless steel, copper pipe, bronze plates, and Teflon are employed. 

 Structural connections usually require the greatest care during the con- 

 struction process. 



For all embedments, care must be taken to ensure no disintegration of 

 the concrete. Metal hardware must be protected from corroding either by 

 sufficient concrete cover, alloying, or coatings. Common forms of coating 

 are galvanizing or use of epoxy. All metal embedments should be protected 

 from chlorides or sulfides or possibly other negative ions occurring in a 



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