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 crosssectional 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 crosssection, 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 if from breaking.

In prestressed concrete steel is not used as reinforce-ment, 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 cracs-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, ets.

EXERCISES

I. Choose the correct variant and complete the sentences. Memorise them:

1.Prestressed concrete is... (a) a completely new building material, b) not really a new material. 2. The successful use prestressed concrete has been developed rapidly... (a) long ago, b) during the last two decades). 3. Plaint concrete is... (a) strong in compression, b) weak in compression). 4. Plaint concrete is... a) weak when used for tensile stress, b) strong when used for tensile stress). 5. In prestressed concrete steel is used... (a)As reinforcement, b) as a means of producing a suitable compressive stress). 6. Pressed concrete is used... (a) only for beams, b) for beams, columns, pipes, ets).

ІІ. Complete the following sentences:

1.Plaint concrete is strong in....2. The sagging of a beam made of plaint concrete may cause it to.... 3. Incorporated steel bars in the lower portion of a beam prevent. 4. A beam made of prestressed concrete is permanently under.... 5. Prestressed concrete is now employed extensively for....

III. Answer the following questions:

1.Is prestressed concrete a new building material? 2. How long has prestressed concrete been used in construction? 3. What disadvantages has plaint concrete for? 5. What will happen if elastic limit of a beam is exceeded? 6. What is prestressed concrete used for?

PLASTICS ALL OVER THE WORLD

EXERCISES

I. Choose the correct variant and complete the sentences. Memories them:

1. Plastics can be applied... (a) only in radio engineering, b) to almost every branch of building}. 2.Decorative plastics has brought about... (a) some advantages, b) a revolution in interior and exterior design). 3. Plastics are used... (a) only for decoration, b) are sufficiently rigid to stand on their own without any support). 4. Laminate has been developed for... (a) only inside use, b) only outside use, c) both inside and outside use). 5. Laminate is impregnated with... (a) thermosetting resins, b) rubber). 6. The sandwich is pressed and subjected... (a) to cold, b) to heat). 7. The laminate gives... (a) a mild surface, b) tough surface).

II. Answer the following questions:

1.Where can plastics be applied? 2. What advantages do plastics offer? 3. What can plastics be used for except decoration? 4. What does plastic material consist of? 5. What for is plastics recommended as a structural material?

FOAMED GLASS

Foamed glass is a high-porosity heat insulating material, available in blocks made of fine-ground glass and a frothing agent.

Foamed glass is widely used in prefabricated house building, to ensure heat insulation of exterior wall panels, and in industrial construction.

Foamed glass has a high mechanical strength, is distinguished by moisture, vapour and gas impermeability. It is non-inflammable, offers resistance to frost, possesses a high sound adsorption, and it is easily sewn and nailed.

Structural foamed-glass blocks designed to fill ceilings, and for making interior partitions in buildings and rooms, to ensure heat and sound insulation:

Volume mass, kg/m.......................... 150-250

Heat conductivity at 20±5° C, kcal/m-hr° C up to 0.1

Block thickness, mm............................. 120

Heat-and-moisture resistant foamed glass used for heat insulation of surfaces at a service temperature up to 190° C:

Volume mass, kg/m¹..... 130-JSOu

Heat conductivity, kcal/m-hr °C 0.045-0.055

Overall dimensions, mm 300x300x80

Structural elements and goods of decorative-acoustic foamed glass designed to ensure interior sound proofing and decorative finish:

Volume mass., kg/m..... 150-300

Sound absorption coefficient 0.4-0.55

Overall dimensions, mm 300x300x80

EXERCISES

I. Answer the following questions:

I. What is foamed glass? 2. Where is foamed glass used? 3. What are structural foamed-glass blocks used for? 4. What is the thickness of a structural block? 5. What are overall dimensions of decorative-acoustic foam^A4 glass? 6. What is the heat conductivity of structural foamed-glass blocks?

II. Read and translate the following word combinations:

insulating material; prefabricated house-building; resistance to frost; a high sound absorption; to fill ceilings; to ensure heat insulation

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