I Прочтите и устно переведите с 1 по 7 абзацы текста. Перепишите

Письменно переведите 2, 3, 4 и 7 абзацы текста.

PRESTRESSED CONCRETE

1. It was not until the beginning of the twentieth century and the beginning of the present century that reinforced concrete really came into its own as a structural material with a great variety of uses. Great advances have been made in the knowledge of its behaviour in structures and much experience has been gained in its use so that today it is deservedly a material of importance.

2. The outstanding characteristic of concrete is that it can be manufactured readily and cast in a mould to any desired shape. It can be said to combine strength with plasticity. The addition of reinforcement or the application of the more recent technique of prestressing adds to its strength and does not decrease its plasticity.

3. Concrete has a much higher strength when it is under compression, than it has when it is subjected to tension. In fact its tensile strength is only one-tenth of its compressive strength. Its ability to sustain shearing forces is also fre­quently inadequate. The purpose of reinforcing is to provide a material with a high tensile strength which can be cast in the concrete in such a way that it is able to take the tensile and shear stresses which would otherwise have to be taken by the concrete, and at the same time allows full advantage to be taken of the concrete's high compressive strength.

4. Prestressed concrete has captured the imagination of many designers for several reasons. It enables lighter construc­tion than ordinary reinforced work. This means that not only is less material required in prestressed members, but that reduced weight of the members makes for lighter columns and foundations. Less steel is required than in ordinary rein­forced concrete. The lightness of prestressed concrete also makes it particularly useful for precast work. It is not sub­ject to cracking, so that there is little danger of corrosion of prestressing wires, and the concrete will not weather easily.

5.Prestressed concrete depends for its reliability on first-class control of the concrete aggregates and mix proportions, the placing of the concrete, the quality of the prestressing wires or bars and the application of the precise degree of pre-stress to the wires. These requirements make it particularly suitable for factory production.

6. The idea of prestressing is an extremely simple one. If one has a material that has no tensile strength it will fracture immediately it is called on to take tension. If, however, the material is given an initial compression, then, when the tension force is applied the material will be able to withstand the tension provided it does not exceed the initial compres­sion.

7. The great merit of prestressed concrete is that, in giving to the concrete itself an effective tensile strength, it enables all the concrete to contribute to the strength of the member, unlike reinforced work where only the concrete in compres­sion is contributing.

 

II Прочитайте 5 абзац текста и письменно ответьте на следующий вопрос:

On what does prestressed concrete depend for its reliability?

 

Контрольная работа № 5

(для специальностей ЛХФ)

Вариант 1

I Прочтите и устно переведите на русский язык с 1-го по 4-й абзацы

Текста. Перепишите и письменно переведите 2,3 и 4-й абзацы.

What a Forest Is?

1. Forests to a non-specialist seem to be nothing but a collection of trees. The closer they are exam­ined, the more they differ. Furthermore, their difference may be expressed in a number of ways. Some scientists write: "A plant association mainly of trees or other woody vegetation occupying an area of land is a forest." Oth­ers say that a collection of trees becomes a forest only when it has sufficient density and covers a large enough area to develop local climatic and ecological conditions "that are different from those outside. There must be some changes in temperature, moisture, light and wind as well as in the character of upper soil layer. With these changes comes a different vegetation under the trees and different animal life in the forest. In technical language a specialist says that a forest "biocenoze" (life community) has been set up. Originally "forest" meant simply wild or unculti­vated land. Today the term "forest" is sometimes applied to an economic unit of operating area. A forest of trees of similar age and composition is called a "stand". Every stand has a more or less regular upper layer of green crowns, called "the forest canopy" under which there may be an open space except for the stems of the forest trees or more or less occupied with lower canopies. They arc-called understories and are typical of tropical forests but may be in the forests of the temperate zone too.

2. Very often a single understorey consists of saplings-which are coming in to replace the falling stand of veter­ans above. Where the forest is open or thin there may be ground cover of grass. The nature of this cover is very different. In dense forests the ground under the trees —the forest floor — may have no living vegetation and be cov­ered with dead leaves and branches.

3. The species composition of the forest is one of the most important features. The stand may be composed of a single species making a pure forest or several species are associated to form a mixed forest. Perfectly poor for­ests over large areas occur not often however. It was ob­served that species do not do well when planted in pure stands, especially upon usual forest soils. The reason is that they make very heavy demands upon soil plant food (nutrients). Such pure stands may do well in youth, but as they become older, their growth becomes very slow and the trees die.

4. Stands are classified according to age classes of which they are composed. Even-aged stand is one in which all the trees are of one age. Uneven-age stand, on the other hand, theoretically has trees of every age, from seedlings to old veterans.

II Прочтите 1-ый абзац текста и письменно ответьте на следующий вопрос:

What did “forest” mean originally?

 

Контрольная работа № 5

(для специальностей ЛХФ)

Вариант 2