Prestressed Concrete - Design of Reinforced Concrete - page 572

relaxing the cables. The result is some reduction in cable tension with a result-
ing loss in prestressing forces. Shrinkage and creep of the concrete add to this
effect.
3. Additional stress conditions must be checked in design, such as the stresses
occurring when prestress forces are first applied and then after prestress
losses have taken place, as well as the stresses occurring for different loading
conditions.
4. Cost of end anchorage devices and end-beam plates that may be required.

19.3 PRETENSIONING AND POSTTENSIONING

The two general methods of prestressing are pretensioning and posttensioning.
Pretensioning was illustrated in Section 19.1, where the prestress tendons were ten-
sioned before the concrete was placed. After the concrete had hardened sufficiently,
the tendons were cut and the prestress force was transmitted to the concrete by bond.
This method is particularly well suited for mass production because the casting beds
can be constructed several hundred feet long. The tendons can be run for the entire bed
lengths and used for casting several beams in a line at the same time, as shown in Fig-
ure 19.3.
In posttensioned construction (see Figure 19.4), the tendons are stressed after the
concrete is placed and has gained the desired strength. Plastic or metal tubes, conduits,
sleeves, or similar devices with unstressed tendons inside (or later inserted) are located in
the form and the concrete is placed. After the concrete has sufficiently hardened, the ten-
dons are stretched and mechanically attached to end anchorage devices to keep the ten-
dons in their stretched positions. Thus by posttensioning, the prestress forces are
transferred to the concrete not by bond, but by end bearing.
It is actually possible in posttensioning to have either bonded or unbonded tendons. If
bonded, the conduits are often made of aluminum, steel, or other metal sheathing. In addi-
tion, it is possible to use steel tubing or rods or rubber cores that are cast in the concrete
and removed a few hours after the concrete is placed. After the steel is tensioned, cement
grout is injected into the duct for bonding. The grout is also useful in protecting the steel
from corrosion. If the tendons are to be unbonded, they should be greased to facilitate ten-
sioning and to protect them from corrosion. 1

Figure 19.3 Prestress bed.

1 Nawy, E. G., 2000, Prestressed Concrete , 3rd ed. (Upper Saddle River, NJ: Prentice-Hall), pp. 64-69.