VII. Insert the words from the box in the sentences

resistance alloys corrosion concrete bars crack  fatigue    load

 

1. Modern bridges should be built with materials that will meet the requirements of the increasing_________ from traffic on them.

2. Good concrete attains high compressive strength and__________ against most natural attacks.

3. For special elements of the bridge aluminum and its________ and some types of plastics are used.

4. Prestressed concrete also has a high ________strength under the heaviest traffic loads.

5. All materials have different qualities of strength, workability, durability and resistance against________. 

6. The steel _______only really come into play after the concrete cracks under tensile stresses.

7. If the reinforcing bars are correctly designed and placed, then these ________remain harmless.

8. Almost every building contains some_________.

 

VIII. Make a presentation about modern materials used in bridge building. The following ideas will help you:

1. Introduce yourself.

2. Give the main aim / goal/purpose/idea of the presentation. For example, the main idea is to describe the new material/ to revise modern materials/ to compare advantages and disadvantages of materials/ to show the development of different materials;

3. In the body of the presentation give examples of materials and describe them. For example: it is necessary to mention/ to note/ to emphasize that the properties of this material/materials is/are….;

4. Give examples of using these materials in practice. For example: The material is widely used in urban bridge construction/ as facing of piers, etc./for details of the structure;

5. Describe visual information.

For example: The slide/description/photo/picture/graph/drawing contains valuable information on properties/characteristics/features of the material.

6. Sum all you have said in conclusion. For example: In conclusion/at the end/ finally it is important to say that…

7. Invite your colleagues to ask questions. For example: I’ll be very glad to answer your questions/Do you have any questions?

Unit 2. PRESENTING MODERN MATERIALS IN BRIDGES

Part II

I. Read the text paying attention to the words in bold.

Among bridge materials steel has the highest and most favourable strength qualities, and it is therefore suitable for the most bridges with the longest spans. Normal building steel has compressive and tensile strengths of 370 N/mm², about ten times the compressive strength of a medium concrete and a hundred times its tensile strength. A special merit of steel is its ductility due to which it deforms considerably before it breaks, because it begins to yield above a certain stress level. This yield strength is used as the first term in standard quality terms.

For bridges high strength steel is often preferred. For members under tension only, like ropes or cables, there are special steels, processed in different ways which allow us to build suspension or cable-stayed bridges.

The high strengths of steel allow small cross-sections of beams or girders and therefore a low dead load of the structure. It was thus possible to develop the light-weight " orthotropic plate" steel decks for roadways.

So plate girder construction now prevails, in which large thin steel plates must be stiffened against buckling. Previously, vertical stiffeners were placed by preference on the outer faces; longitudinal stiffeners were then arranged on the inside surface.

Today all stiffeners are placed on the inside surface so as to achieve a smooth outer surface allowing no accumulation of dust or dirt deposits that retain humidity and promote corrosion - the "Achilles heel" of steel structures.

Aluminum was occasionally used for bridges and the same form was used as for steel girders. Aluminum profiles are fabricated by the extrusion process which allows many varied hollow shapes to be formed, so that aluminum structures can be more elegant than those of steel. Aluminum profiles are popular for bridge parapets because they need no protective paint.

Timber.

Timber has favourable qualities of strength for resisting compression, tension and bending. Rough tree trunks or sawn timber beams have been used since primitive times for beam bridges; raking frames and arches soon allowed larger spans. Timber should be protected against rain and therefore covered bridges with a roof and sidewalls with windows evolved, and many of these are rightly preserved in the Alpine countries. Now timber is used essentially for temporary structures.

Smart materials.

    Smart materials or shape memory materials are an invention that has changed the world of engineering. There are two types: metal alloys and plastic polymers. Smart materials are called so because they react to changes in their environment. For example:

- plastics that return to their original shape when the temperature changes;

- metal alloys that have a ‘memory’ and can return to their original shape;

- solids that darken in sunlight like the lenses in some sunglasses;

- liquid crystals that change shape and colour. These have been used in climbing ropes that change colour if there is too much strain and weight on them.