12. Read the sentences and add your own information.

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12. Read the sentences and add your own information.

1) Concrete is one of the most important building materials.

2) It is a mixture of cement, sand, gravel and water.

3) Concrete can be divided into two classes.

4) Reinforced concrete is a combination of concrete and steel.

5) There are two kinds of reinforced concrete.

6) The use of concrete and reinforced concrete is almost universal.

7) There is a broad division of concrete types into dense concretes and
light-weight concretes.

8) Gas concrete and foam concrete are reffered to cellular concretes.

9) Concrete may be used as the material for making prefabricated units in
a factory.

10) In alkali-slag concrete cement is replaced by a mixture of granulated,
blast-furnace slags and sodium and potassium compounds

11) Silica concrete is light, fire-resistant and acid-proof.

13. Look through the text and give it a title.

Plain concrete dates from very early days. It was used by the
Egyptians, Romans and Greeks in the construction of bridges, roads, town
walls. Romans used it even in under-water structures, some of which have
survived till our time. A large part of the Great Chinese Wall (the 3^rdcentury before our era) was also built of concrete.

The concrete remains of the foundations of buildings built several thousand years ago have been found in Mexico. As cement was not known in those times, concrete was made of clay and later of gypsum and lime. Nowadays concrete is made in up-to-date machinery with very careful regulation of the proportion of the mix.

Concrete is made by binding together particles of sand and gravel,
stone or broken brick. The binding agent used is a paste of cement and
water, in suitable proportions. When water is added to the cement,
hydration takes place. This causes the whole mixture to set and harden,
forming a solid mass.

The idea of strengthening concrete by a network of small iron rods was developed in the 19^th century, and reinforced concrete was introduced into engineering practice. The first results of the tests of the reinforced concrete beams were published in 1887. The first reinforced concrete skyscraper in the world was built in 1902-03 in Cincinnati, Ohio. The 16-storey structure demonstrated for the first time the safety and economy of reinforced concrete frames for high-rise construction, and was a vital stimulus for using reinforced concrete in fireproof construction. Since that time the development of reinforced concrete work has made great progress.

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