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Text B. Prestressed Concrete

Prestressed concrete is not a new material. We know it to be an improved form of reinforced concrete. It can be defined as concrete in which highly stressed tendons are used instead of reinforcing bars. These tendons consist of high-tensile steel while reinforcing bars are usually made of ordinary steel.

There are two ways of achieving prestress in concrete: pretensioning and posttensioning. The word “pretensioning” means that the reinforcing steel is stretched and anchored before the concrete is cast around it. When the concrete has hardened the anchorage of the tendons is released. The bond which has formed between the steel and the concrete prevents the steel from contracting to its original length, and in the way a compressive stress is induced. The compression in the concrete must be great enough to annul any future tension that loads applied may develop. Since the bond is the principal means of transfer of stress from the steel to the concrete, wires are preferred to bars because of their relatively larger surface area. Pretensioning is usually employed for prefabricated elements.

Post-tensioning can be defined as a method of prestressing reinforced concrete in which the reinforcing steel is tensioned after the concrete has hardened. The tendons are usually inserted through holes in the concrete, and after the stretching the space around the tendons may be filled with cement grout under pressure. The post-tensioning of steel is more complicated and less suitable to mass production than the pretensioning process, but the specialists consider it to have some advantages over the other: the elements can be prefabricated in a plant, but also can be produced on the site; post-tensioned concrete is less influenced by early creep and shrinkage, because the stretching can be delayed until a large part of shrinkage has taken place; the reinforcing steel may be bent up to achieve better stress distribution. There are several different systems of post-tensioning, but the procedure itself is almost the same and variations are only in the details of the tendons and their anchorages. The main use of prestressed concrete is for long span structures such as bridges and large tanks and high-pressure pipes.

3. Listen to the text again and answer the following questions:

1. What is the difference between reinforced concrete and prestressed concrete?

2. Haw many ways of achieving prestress in concrete are there?

3. What does the word pretensioning mean?

4. What is pretensioning usually employed for?

5. How can post-tensioning be defined?

6. Is the post-tensioning of steel more complicated?

7. Where is prestressed concrete used?

4. Annotate the text either in English or in Ukrainian. Use the following phrases:

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It/the text informs the reader about…

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Further…

The author points out/stresses/informs/considers…

The text is useful and interesting for…

Читайте також:

1. Text 10. Prestressed Concrete
2. Text 8. Properties And Manufacture of Concrete
3. Text A. Concrete

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