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.

General Properties of Cement

( 1000 печ. зн. )

All types of cement shrink during setting. In a normal concrete the amount of this shrinkage will depend both on the proportion of cement in the mix and the quantity of mixing water employed. Provided enough water is present to enable the chemical action of setting to take place, then the smaller the amount of water the less shrinkage there will be. The type of aggregate used has an appreciable effect upon both the amount of water and the amount of aggregate that can be mixed with given quantity of cement. Strength and durability of concrete are linked properties in that they are both associated with the low water-cement ration. In addition to the proportion of cement and the water cement ratio of a cement product, the method of curing will also affect the amount of shrinkage. Normally, the slower the drying the less shrinkage there will be. All cement products are liable to a considerable shrinkage during setting and hardening.

Strength

The important thing is the strength of the final cement product rather than the strength of the cement itself. The strength of the cement, however, gives some indication of the possible variation in the former, although the strength of the product will also depend upon the type and grading of the aggregate used the proportion of aggregate and other factors such as water cement ratio and quality of workmanship.

ФАКУЛЬТЕТ ДИСТАНЦИОННОГО ОБУЧЕНИЯ

СПЕЦИАЛЬНОСТИ ПГС, ГСХ.

ТЕКСТЫ ДЛЯ ЧТЕНИЯ.