Modern building materials. Part П.

Concrete is perhaps the most widely spread building material used nowadays. Concrete is an artificial stone, made by thoroughly mixing such natural ingredients or aggregates as cement, sand and gravel or broken stone together with sufficient water to produce a mixture of the proper consistency. It has many valuable properties. It sets under water, can be poured into moulds so as to get almost any

desirable form, and together with steel in reinforced concrete it has very high strength, and also resists fire. Prestressed concrete is most widely used at present while prefabricated blocks are employed on vast scale for skeleton structures.

AGGREGATES FOR CONCRETE

By the simple definition from the dictionary "aggregates are the materials, such as sand and small stones, that are mixed with cement to form concrete". In other words aggregates (or cushioning materials) can be defined as a mass of practically inert mineral materials, which, when surrounded and bonded together by an active

binder, form the rock. This rock is denoted by the general term concrete.

Aggregates have three principal functions in the concrete: they provide a relatively cheap filler for the concreting material, or binder; they provide a mass of particles which are suitable for resisting the action of applied loads, of abrasion, of percolation of moisture through the mass, and of climate factors; they reduce volume changes resulting from the action of the setting and hardening of the concrete mass. All aggregates, both natural and artificial, which have sufficient

strength and resistance to weathering, and which do not contain harmful impurities may be used for making concrete. As aggregates such natural materials as sand, pebbles, broken stone, broken brick, gravel, slag, cinder, pumice and others can be

used.

PRESTRESSED CONCRETE

Prestressed concrete is not a new material. Its successful use has been developed rapidly during the last two decades, chiefly because steel of a more suitable character has been produced. Concrete is strong in compression but weak when used for tensile stresses. If, therefore, we consider a beam made of plain concrete, and spanning a certain distance, it will at once be realized that the beam's

own weight will cause the beam to "sag" or bend. This sagging at once puts the lower edge of the beam in tension, and if the cross-sectional area is small, causes it to break, especially if the span is relatively large. If, *on the other hand', we use a beam of similar cross-section, but incorporate steel bars in the lower portion, the steel will resist the tensile stress derived from the sag of the beam, and thus assist in preventing it from breaking. In prestressed concrete steel is not used as reinforcement, but as a means of producing a suitable compressive stress in the concrete. Therefore any beam (or member) made of prestressed concrete is permanently under compression, and is consequently devoid of crack under normal loading, or so long as the "elastic limit" is not exceeded. Prestressed concrete is not only used for beams but is now employed extensively for columns, pipes, and cylindrical water towers, storage tanks, etc.

 

Questions

Choose the right answer

1. What influences the choice of building materials?

a) The choice of building materials is governed by the type and the function of a building;

b) Availability, cost and physical properties are the main considerations for an engineer in selecting materials for construction;

c) The techniques and methods of construction are the main factors influencing the choice of building materials.

2. What are lime, gypsum and cement most widely used for?

a) These three materials are not widely used for the pur­pose of binding together masonry units;

b) They are used as components to produce concrete;

c) With the large-scale con­struction, lime, gypsum and cement may be considered to be the most important-binding-materials.

 

 

Tasks

1.Match the English equivalents to the following words

to select 1. особый

effective 2. обломочная горная порода

to vary 3. различаться

uniform 4. главный (основной)

to modify 5. выбирать

conglomerate 6. действенный

fundamental 7. однородный

requisite 8. необходимый

selection 9. выбор

accurate 10. точный

variations 11. изменять

specific 12. изменения

2. Match the Russian equivalents to the following words

high rate of strength resistance to high-alumina cement crushed stone mortar aggregate fine aggregate workability compressive load lime gypsum moisture content masonry   3.Translate into Russian rapid-hardening rocklike high-early strength water tightness high-alumina light-weight purpose-made large-size elements blast-furnace semi-rigid

1. раствор 2. гипс 3. заполнитель 4. содержание влаги 5.нагрузка на сжатие 6. цемент с высоким со­держанием глинозема 7. высокая прочность 8. щебень 9. прочность на 10.обрабатываемость, 11.мелкий заполнитель 12.каменная (кирпичая) кладка 13. известь

 

4.Answer the following questions. Work in pairs.

1.For what types of construction are reinforced concrete structures and elements used?

2. What new materials have come into use both in industry and building?

3. What made possible the mass production of prefabricated large-size con­crete elements?

4. In what industrial fields are plastics used?

5. How does steel introduced into concrete change its properties?

 

5.Choose the right variant.

1.... is now in operation in our country and abroad.

a) A vast amount of research work intended to raise the standards...

b) A great number of plants producing precast r.-c. elements...

c) A well-designed plant of sufficient capa­city...

2....above the general grade of the yard.

a) To assure adequate drainage the plant floor level should be not less than 6 inches...

b) The yard should slope away from the main building in all directions...

c) The following general aspects should be...

3....aggregates and cement should be stored as close to the mixers as possible.

a) For obvious reasons it is not advisable that...

b) In large plants producing steel structures...

c) Regardless of the general lay-out of the plant....

 

 

Before you start

Work in groups and discuss the questions.

What is the most impressive building you have ever seen?

Do you generally prefer modern or traditional architecture? Give examples.