III. Read and translate the words and say what the difference between various kinds of concrete is

Concrete, reinforced concrete, prestressed concrete, painted concrete, precast concrete, underwater concrete, foam concrete, gypsum concrete, homogeneous concrete, polymer concrete, in-situ concrete, high-strength concrete, asphaltic concrete.

 

IV. Read and translate the first part of the text about evolution of building materials and answer the questions below.

Part I

The traditional building materials for bridges and other man-made facilities are stone, timber and steel, and more recently reinforced and prestressed concrete. For special elements aluminum and its alloys and some types of plastics are used. These materials have different qualities of strength, workability, durability and resistance against corrosion. They differ also in their structure, texture and colour.

For man-made facilities, bridges, tunnels and dams one should use that material which results in the best shape, technical quality, economics and compatibility with the environment.

Natural Stone

The great old bridges of the Romans of the Middle Ages were built with stone masonry. With stone one can build bridges which are both beautiful, durable and of large span (up to 150 m). Over a long period stone bridges, which are well designed might be the cheapest, because they need almost no maintenance. Because of long spans and large loads nowadays stone is usually used only as facing for abutments, piers or arches.

Artificial Stones, Clinker and Brick

Among the artificial stones clinker and hard-burned brick are used in bridges and other facilities both as liners and for bearing vaults. The warm colours of clinker or brick blend happily into the landscape. Also in an urban environment, they are preferable to plain concrete, if brick is the regional construction material.

Reinforced and Prestressed Concrete

Concrete is an all-round construction material. Almost every building contains some concrete. Its dull grey colour has contributed to the fact that the word concrete has become a synonym for ugly. In the field of bridges, concrete deserves a more favourable judgement.

Not all concrete bridges have turned out to be beauties, but pleasing bridges can be built with concrete if one knows the art. Concrete is poured into forms as a stiff but workable mix, and it can be given any shape; this is an advantage and a danger. The construction of good durable concrete requires special knowledge which the bridge engineer is to have.

Good concrete attains high compressive strength and resistance against most natural attacks. However, its tensile strength is low, and the use of concrete alone is therefore limited to structures which are only subject to compressive stresses. But tensile stresses also occur in abutments and piers due to earth pressure, wind, breaking forces and to internal temperature gradients.

To resist these tensile forces, steel bars must be embedded in the concrete, the so-called reinforcing bars, and this has lead to the development of reinforced concrete. The steel bars only really come into play after the concrete cracks under tensile stresses. If the reinforcing bars are correctly designed and placed, then these cracks remain harmless. A second method of resisting tensile forces in concrete structures is by prestressing.

Prestressed concrete – if correctly designed – also has a high fatigue strength under the heaviest traffic loads. Prestressed concrete bridges soon became much cheaper than steel bridges.

 

V. Answer the following questions.

1. What material is used for longer bridge spans?

a) stone b) reinforced concrete c) concrete

2. What material was the earliest in bridge building?

a) timber b) stone c) aluminum

3. Which material better resists natural attacks?

a) stone b) reinforced concrete c) prestressed concrete

4. Which constructions look slender?

a) made from stone  b) made from reinforced concrete c) made from prestressed concrete.

5. What forces do reinforcing bars resist?

a) compression b) tension        c) bending

 

VI. Is it True or False?

Ex. To use natural stone in bridge building is very expensive today. T

1. Prestressed concrete was used in bridges much earlier than simple concrete.

2. Natural stone is not widely used in bridge construction today.

3. Artificial stone like brick is more preferable than concrete in urban environment if it is a regional construction material.

4. Some people consider concrete bridges to be plain and ugly.

5. Concrete attains high tensile strength and resistance against most natural attacks.

6. Reinforced concrete is the type of concrete when steel bars are embedded to resist compressive forces.