709

8.3.2 Prestressing Reinforcement

Because of the high creep and shrinkage losses in concrete, effective prestressing can be achieved by using very high strength steels in the range of 270,000 psi or more (1,862 MPa or higher). Such highly stressed steels are able to counterbalance these losses in the surrounding concrete and have adequate leftover stress levels to sustain in the long term the required prestressing force (see Table 8.2). A typical reinforcement stress-strain plot is shown in Figure 8.1.

TABLE 8.2 Seven-Wire Standard Strand for Prestressed Concrete

Nominal diameter of strand (in.)

Breaking strength of strand (min. lb)

Nominal steel area of strand (sq in.)

Nominal weight of strands (lb per 1000 ft)a

Minimum load at 1% extension (lb)

Grade 250

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