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System Concept

Product Overview

One of the major advancements in construction in the united  states in the second half of the twentieth century  was  the development & use of pre-stressed concrete.

Pre-stressed Concrete Offers a broad range of engineering solutions & variety of aesthetic opportunities.

The invention of pre-stressed concrete is often accredited to Eugene Freyssinet who developed the first practical post-tensioning system in 1939 .Systems were developed around the use of multi-wire tendons located in large ducts cast into the concrete section, and fixed at each end by anchorages.

They were stressed by jacking from either one or both ends, and then the tendons were grouted within the duct.

The tensile strength of concrete is only about 10% of its compressive strength. As a result Plain concrete members are likely to crack when loaded .

In order to resist tensile stresses which plain concrete cannot resist , it can be reinforced with steel reinforcing bars .

Reinforcing is selected assuming that the tensile zone of the concrete carries no load & that tensile stresses are resisted only by tensile forces in the reinforcing bars. The resulting reinforced concrete member may crack, but it can effectively carry the design loads.

Although cracks occur in reinforced concrete are normally very small and uniformly distributed.

 However, cracks in reinforced concrete can reduce long-term durability.

Introducing a means of pre-compressing the tensile zones of concrete members to offset anticipated tensile stresses reduces or eliminates cracking to produce more durable concrete structure.

The function of pre-stressing is to place the concrete structure under compression in those regions where load causes tensile stress. Tension caused by the load will first have to cancel the compression induced by the pre-stressing before it can crack the concrete.

(a) Reinforced concrete cracked under load.

(Figure a) shows a plainly reinforced concrete simple-span beam & fixed cantilever beam cracked under applied load .

(b) Post Tensioning concrete before loading.

(Figure b) Shows the same unloaded beams with pre-stressing forces applied by stressing high strength tendons. By placing pre-stressing low in the simple-span beam & high in the cantilever beam, compression is induced in the tension zones, creating upward Camber

(c) Post Tensioning concrete after loading.

( Figure c ) shows the two pre-stressed beams after loads after loads have been applied. The loads causes both the simple-span beam & cantilever beam to deflect down, creating tensile stresses in the bottom of the simple-span beam & top of the cantilever beam.

 

One of the simplest examples of pre-stressing is trying to lift a row of books. To lift the book it’s necessary to push them together  , to apply a pre-compression to the row , this increases the resistance to slip between the books so that they can be lifted. Where there is no grip between the books but the books can withstand compression loads which can be applied.

Simple definition of pre-stressing is ( Act of applying forces to a structure, other the loads the structure is designed to carry, in order to enhance the structure’s ability to carry those loads).

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