From the history of concrete

( 3500 печ. зн. )

Mass or plain concrete dates from very early days. It was employed by the Egyptians, Romans and Greeks in the construction of aqueducts and bridges, in the construction of roads and town walls. Romans used it even in under-water structures some of which have survived till our time. A large part of the Great Chinese Wall (the 3d century B.C.) was also built of concrete.

The concrete remains of the foundations of buildings built several thousand years ago have been found in Mexico. As cement was not known at those times, concrete was made of clay and later of gypsum and lime. Nowadays concrete is made in up-to-date machinery with very careful regulation of the proportion of the mix.

The idea of strengthening concrete by a network of small iron rods was developed in the 19^th century, and reinforced concrete was introduced into engineering practice.

Concrete (I)

It is difficult to imagine modern structure without concrete. Concrete is the very building material which led to great structural innovations. The most important quality of concrete is its property to be formed into large and strong monolithic units. The basic materials for making concrete are cement, aggregate and water. Cement is the most essential material and the most important one for making concrete of high quality. Cement is made of limestone and clay. It is burnt at high temperature and ground up into powder. Depending on the kind and composition of the raw materials different types of cement are obtained. Portland cement, blast furnace cement are suitable for putting up marine structures.

Concrete is made by mixing cement, water, sand and gravel in the right amount. As soon as it is thoroughly mixed it is poured into forms that hold it in place until it hardens. The crystals forming in the process of making concrete stick together in a very hard artificial stone. Cement starts hardening one hour after the water has been added and the process of hardening lasts for about 28 days. The process is called concrete curing.

The characteristics of concrete depend on the quality of the materials used, grading of the aggregates, proportioning and amount of water. The most important requirements for concrete are: it should be hard, strong, durable, fire-resistant and economical. Concrete can be divided into two classes: mass or plain concrete and reinforced concrete where it is necessary to produce steel. Plain or mass concrete can be used for almost all building purposes. Ferro-concrete is used in building bridges and arches, dams and dock walls, for structures under water, for foundations, columns, girders, beams. The use of concrete and ferro-concrete is almost universal.

Builders now produce two types of new building materials: alkali-slag concrete and silica concrete. In alkali-slag concrete cement is replaced by a mixture of granulated blast-furnace slag and sodium and potassium compounds. The fillers can be sand or sandy loams containing various amounts of clay which usually cannot be used with conventional cement. The new material has been tested successfully and is now being used for irrigation systems, roads, pavements and other structures. Silica concrete is widely used in aviation and in underwater constructions.

Concrete (II)

The term “concrete” is used to describe a dense material composed of cement and aggregate mixed with water. The density of such a material and therefore many of its properties depend upon the density of the aggregate. Therefore there is a broad division of concrete types into:

1. Dense concretes – composed of heavy aggregates;

2. Light-weight concretes – composed of light aggregates.

The aggregates are graded in size from fine to coarse in order to reduce the amount of void spaces to be filled by cement.

There are “cellular” concretes made by using materials which foam or form gas during the mixing of the concrete. These give a product of very light weight because after setting it contains a large number of small voids.

The reduction in weight is accompanied by a considerable decrease in strength. Another type of light-weight concrete is made by “entraining”

air bubbles in the mix to which a substance has been added to keep the bubbles stable during setting.

Gas Concrete

( 1500 печ. зн. )

Lime and silica are ground together to very fine limits. The silicious material can vary considerably in its composition. Much use is made of such waste materials as fly ash from power-stations, blast furnace slag, as well as natural pozzolanas, pumice, etc. The degree of foaming in the gas concrete and thus its specific gravity is determined by the amount of aluminium powder or other agent added. The practical limits of the final density are between 13 and 90 lb. per cu. ft. If the gas concrete is allowed to harden on its own, it usually takes about three weeks before the final strength is achieved. It is more customary to accelerate the setting of the gas concrete by steam hardening it in autoclaves with superheated steam at about 140 lb. per sq. in. The steam hardening process takes about 15-20 hr. Air-cured gas concrete can be used for the manufacture of special components for the refrigeration industry. Such blocks are cast to special dimensions.

Gas concrete can be cast horizontally to form room sized outer wall units.

It is possible to incorporate electric conduit pipes, piping for the cold and hot water systems and also drainage pipes. The units usually include windows and doors and are reinforced by embedding steel mesh in the mix.

Gas concrete can be used as thermally insulating floor screeds or as an additional thermally insulating layer on top of a concrete roof.

Cast gas concrete is often used as the thermally insulating layer in “sandwich” wall units.

Gas concrete is often used as a thermally insulating layer when casting buildings by a continuous technique.

Date: 2015-12-17; view: 1513