Prestressed Concrete - Design of Reinforced Concrete - page 573

Figure 19.4 Posttensioned beam.

19.4 MATERIALS USED FOR PRESTRESSED CONCRETE

The materials ordinarily used for prestressed concrete are concrete and high-strength
steels. The concrete used is probably of a higher strength than that used for reinforced
concrete members, for several reasons, including the following:

1. The modulus of elasticity of such concretes is higher, with the result that the elas-
tic strains in the concrete are smaller when the tendons are cut. Thus the relax-
ations or losses in the tendon stresses are smaller.
2. In prestressed concrete, the entire members are kept in compression, and thus all
the concrete is effective in resisting forces. Hence it is reasonable to pay for a
more expensive but stronger concrete if all of it is going to be used. (In ordinary
reinforced concrete beams, more than half of the cross sections are in tension and
thus assumed to be cracked. As a result, more than half of a higher-strength con-
crete used there would be wasted.)
3. Most prestressed work in the United States is of the precast, pretensioned type
done at the prestress yard where the work can be carefully controlled; conse-
quently, dependable higher-strength concrete can readily be obtained.
4. For pretensioned work, the higher-strength concretes permit the use of higher
bond stresses between the cables and the concrete.

Prestressed concrete segmental bridge over the River Trent near Scunthorpe, Lincolnshire,
England. (Courtesy of Cement and Concrete Association.)