Saturday, April 12, 2014

Sources of Prestress force

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.

No comments:

Post a Comment