Prestressed Concrete

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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 realised 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 cracks—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.

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Prestressed concrete is used ...

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In prestressed concrete steel is used...

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Plain concrete can be used for almost all kinds of …

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Silica concrete is light, acid-proof and contains no …

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Prestressed … is not a new material.

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… made of prestressed concrete is permanently under compression.

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Concrete is strong in … but weak when used for tensile stresses.

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Bricklayer's thumb and the fingers must be protected with … ... .

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One of the essential properties of concrete is its ….

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A HOUSE

Pete: Don't you know, Ann that not all buildings are houses?

Ann: Is that right? What do you call a house then, Pete?

Pete: A house is usually a building for human habitation.

Ann: I see now. And it can be intended for one family only to live in, or sometimes, if it's big enough, for several.

Pete: Certainly. ______________

Ann: What are the main sections of a house, Pete?

Pete: Do you mean the main parts of a building?

Ann: Yes. That's what I meant to ask you.

Pete: Well, any building consists of the foundation, walls and partitions, floors and the ceiling and, certainly, the roof section.

Ann: And the set of rooms on the level of one floor is called a storey, isn't it?

Pete: Yes, Ann, it is. And, accordingly, we speak of one-, two-, three- and so on storey houses. In England, the storey closest to the ground is known as the ground floor.

Ann: Most houses in our town are four- or five-storey houses.

Pete: And, I wouldn't say they are low. What are they made of?

Ann: Mostly of bricks and concrete blocks. They are prefabricated, certainly.

Pete: And what about houses of plastic sections?

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A HOUSE

Pete: And what about houses of plastic sections?

Ann: Oh, plastic houses are our future. ____________

Pete: Yes, and we shall build them. We need many good houses. Just look, there is not a single town in our country without hundreds of building cranes rising all above it, hoisting bricks and panels.

Ann: Yes, and new towns spring up everywhere like mushrooms.