Prestress can be applied to a concrete member in many ways.
Perhaps the most obvious method of precompressing is to use jacks reacting
against abutments, as shown in Fig.1a. Such a scheme has been employed for
large projects. Many variations are possible, including replacing the jacks
with compression struts after the desired stress in the concrete is obtained or
using inexpensive jacks that remain in place in the structure, in some cases
with a cement grout used as the hydraulic fluid. The principal difficulty
associated with such a system is that even a slight movement of the abutments
will drastically reduce the prestress force.
In most cases, the same result is more conveniently obtained
by tying the jack bases together with wires or cables, as shown in Fig. 1b.
These wires or cables may be external, located on each side of the beam; more
usually, one end of the prestressing tendon is anchored, and all of the force
is applied at the other end. After reaching the desired prestress force, the
tendon is wedged against the concrete and jacking equipment is removed for
reuse. In this type of prestressing, the entire system is self-contained and is
independent of relative displacement of the supports.
Another method of prestressing that is widely used in
illustrated by Fig. 1c. The prestressing strands are tensioned between massive
abutments in a casting yard prior to placing the concrete in the beam forms.
The concrete is placed around the tensioned strands, and after the concrete has
attained sufficient strength, the jacking pressure is released. This transfers
the prestressing force to the concrete by bond and friction along the strands,
chiefly at the outer ends.
it is essential, in all three cases shown in Fig. 1, that
the beam be supported in such a way as to permit the member to shorten axially
without restraint so that the prestressing force can be transferred to the
concrete.
Other means for introducing the desired prestressing force
have been attempted on an experimental basis. Thermal prestressing can be
achieved by preheating the steel by electrical or other means. Anchored against
the ends of the concrete beam while in the extended state, the steel cools and
tends to concrete. The use of expanding cement in concrete members has been
tried with varying success. The volumetric expansion, restrained by steel
strands or by fixed abutments, produces the prestress force.
Most of the patented
system for applying prestress in current use are variations of those shown in
Fig. 1b and c. Such systems can generally be classified as pretensioning or
post-tensioning systems. In the case of pretensioning, the tendons are stressed
before the concrete is placed, as in Fig. c. This system is well suited for
mass production, since casting beds can be made several hundred feet long, the
entire length cast at once, and individual beams can be fabricated to the
desired length in a single casting. Fig 2 show workers using a hydraulic jack
to tension strands at the anchorage of a long pretensioning bed. Although each
tendon is individually stressed in this case, large capacity jacks are often
used to tension all strands simultaneously.
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