New building materials

When a horizontal beam is supported at each end (1)but not in the middle, it bends. The bending is caused by the beam's own weight and by the weight of anything that it has to support. When it bends, the bottom surface is normally in tension; (2) it is pulled in the direction of its length. If the total load is too heavy, the beam will break, and if it is a concrete beam, its chief weakness is in the lower surface, which is in tension.

Steel rods may be placed inside concrete beams while the mixture is still wet, and if these rods have hooked ends, they will grip the concrete (3) and tend to prevent stretching. The steel and concrete together make a good combination to resist compression and tension, and such material is known as reinforced concrete or ferro-concrete.

If the concrete is in the form of a beam which is to be used in a horizontal position, these rods can be placed inside it, but they need not go through the middle. Most of the tension will be along the lower surface, and therefore that is the best place for the rods.

Because concrete does not resist tension well,(4) a system is now in use in which it is not stretched. This is not as difficult to arrange as it seems to be. The concrete beanris compressed along its length by means of steel rods inside it; these rods remain in the beam and therefore the concrete is always compressed unless some greater force tries to pull it apart. As long as the beam is not lengthened more than it is already compressed, the concrete will never be in tension.

Suppose, for example, that the compression in the concrete (caused by the steel rods) is 1,000 pounds per square inch and that the force pulling the beam lengthways (5)when it is in position is 700 pounds per square inch. In that case the concrete is still compressed by a force equal to 300 pounds per square inch, and so it is not in tension at all.(6)

Such material is called prestressed concrete. Holes are often left in the beam when it is made; then the steel rods are placed in the holes and fixed at one end. Nuts on screws at the other end are turned to compress the concrete, or the beam may be compressed by powerful machinery.

There is another method of preventing tension from arising in a beam. If it is arched, and if its ends press against a firm abutment, any pressure on its top surface will tend to compress it instead of putting the lower edge in tension. This is therefore a good system to use with a concrete beam.

The steel rods which pass through an arch may themselves be straight, so that in the middle of the arch they are near the lower surface. If the beam is straight, the same kind of result may be obtained by allowing the rods to be curved or bent, so that they pass along the lower surfaces in the middle of the beam.

Prestressed concrete can be used where ordinary concrete would fail.(7) High buildings are now made with this material and its use has greatly increased. Smaller houses in the city gradually give place to big blocks owing to the great demand for land. In northern countries, where the weather is a constant nuisance to the citizens, even the streets may be roofed over.(8)

Concrete is a bad conductor of heat; moreover, it does not catch fire.(9) These are two great advantages in a building, and especially in a high building. Even if the furniture or the wooden floors are set on fire, and the surfaces of the concrete walls are heated, the interior parts of the walls do not become very hot for the concrete does not conduct heat. The danger of fire in dwelling places is therefore decreasing, and the use of electricity instead of gas and oil for lighting and heating helps in this matter.

ПРИМЕЧАНИЯ:

(1) is supported at each end – имеет опоры на концах

(2) in tension – в состоянии растяжения

(3) will grip the concrete – схватят бетон

(4) does not resist tension well – довольно плохо сопротивляется растяжению

(5) pulling the beam lengthways – растягивающая балку в продольном направлении

(6) it is not in tension at all – совсем без растяжения

(7) where ordinary concrete would fail – где обычный бетон разрушится

(8) even the streets may be roofed over – даже целые улицы могут быть перекрыты

(9) to catch fire – загораться

III. Ответьте на следующие вопросы:

1. Why is it called reinforced concrete? 2. What is prestressed concrete? 3. What is concrete? 4. Why does concrete not catch fire?