Reinforced Concrete

( 1800 печ. зн. )

Reinforced concrete is a combination of two of the strongest structural materials, concrete and steel.

This term is applied to a construction in which steel bars or heavy steel mesh are properly embedded in concrete. The steel is put in position and concrete is poured around and over it, and then tamped in place so that the steel is completely embedded. When the concrete hardens and sets, the resulting material gains great strength. This new structural concrete came into practical application at the turn of the 19^th century. The first results of the tests of the reinforced concrete beams were published in 1887. Since that time the development of reinforced concrete work has made great progress. And the reasons of this progress are quite evident. Concrete has poor elastic and tensional properties, but it is rigid, strong in compression, durable under water and above ground and in the presence or absence of air and water, it increases its strength with age, it is fireproof.

Steel has great tensional, compressive and elastic properties, but it is not durable being exposed to moisture, it loses its strength with age or being subjected to high temperature. So what is the effect of the addition of steel reinforcement to concrete?

Steel does not undergo shrinkage or drying but concrete does and therefore the steel acts as a restraining medium in a reinforced concrete member. Shrinkage causes tensile stresses in the concrete which are balanced by compressive stresses in the steel. For getting the best from reinforced concrete the following considerations should be kept in mind:

1. For general use the most suitable proportions of cement and aggregate are: 1 part of cement, 2 parts of sand and 4 parts of gravel.

2. Only fresh water free from organic matter should be used for reinforced work. Sea water is not allowed.

3. Homogeneity of the concrete is a very important requirement.

Steel constructions with reinforced concrete have become the most important building materials invented in centuries and they have given modern architecture its peculiar features.

Reinforced Concrete (II)

( 2200 печ. зн. )

Plain concrete was used in ancient times by the Egyptians and the Romans and probably by the Mayas in Central America. Sewers, roads, aqueducts, water mains and foundations were constructed of mass concrete by the Romans who also employed it as a filling between the brick and stone ribs of their vaults and arches. The knowledge of the use of natural cements and, consequently, of concrete seems to have been lost during the Middle Ages, and it was not until the 18^th century that its value was rediscovered.

The reinforcing of concrete was first introduced in France in 1861 by Joseph Monier who constructed flower pots, tubs and tanks and Francois Coignet who published theories of reinforcing for beams, arches and large pipes. Very little was actually accomplished in building construction until 25 years later when German and Austrian engineers developed formulas for design and Hennebique in France began the use of bent-up bars and stirrups. Between 1880 and 1890 several reinforced concrete buildings were erected in the USA and since 1896 the increase in the amount of construction with this material has been remarkable.

Until recent years there was a tendency among architects to consider reinforced concrete as a method of construction suited only to heavy and massive structures, to foundations, bridges, dams, factories, warehouses and industrial buildings. This feeling was perhaps due to the apparent bulkiness of the material and to the fact that the wooden forms for plain flat surfaces, beams and columns cast less than for curves, arches and domes. The characteristics of the architecture were limited by the economical restrictions of the centering. Much study and experiment have, however, led to vast improvements in the manufacture of the concrete, in the efficiency and the simplicity of formwork and in the development of plastic molds and of self-centering reinforcement such as ribbed fabrics. Indeed, at the present time unlimited possibilities in flexibility, slenderness and aesthetic qualities of design appear to be in the hands of the creators of concrete buildings. The capacity of reinforced concrete is, in the opinion of many architects, not yet realized. The potentialities of a substance which can be poured into any form or shape from delicate ornament to huge cantilevers and parabolic arches and which is monolithic throughout its mass should indeed inspire methods of expression distinctive of its structure and quite different to those called forth by the disjointed elements of steel, wood, brick and stone.

Date: 2015-12-17; view: 1613