Special kinds of heavy concrete.

Hydraulic engineering concrete is a variety of heavy concrete that is characterized by high water resistance, water impermeability, frost resistance, low heat release and an adequate resistance to chemical attack in aggressive media. Hydraulic engineering concrete is used for installations or parts of constructions periodically or continuously washed by water.

The classification of hydraulic engineering concretes takes into account the zones where concrete is used:

-underwater concrete, permanently located in water;

-concrete in the zone of fluctuating water level;

-concrete located above the fluctuating water level.

Underwater concrete, as well as that for fluctuating water level service and that exposed to the action of subsoil water should be resistant to corrosion by water of a given composition.

The quality of hydraulic engineering concrete is favourably affected by active mineral admixtures to portland cement which interact with calcium oxide hydrate and consolidate concrete, thus improving its water-resisting property and reducing heat release.

Natural aggregates (sand and gravel) for hydraulic engineering concrete should satisfy the more stringent specifications than common concretes. Content of clay, slit and fine particles should not exceed 1-2%. Coarse and medium-size sands may be used; fine sands should be used only if there are sufficient engineering and economical grounds. As it hardens, concrete should be given thorough care; it should be supplied with adequate humidity and maintained at a constant temperature to avoid volumetric deformations of constructions.

Acid-resistant concrete is obtained from acid-resistant cement and aggregates. Concrete is mixed with water glass in amounts ensuring its adequate mobility. Acid-resistant concrete is used for various constructions and for lining apparatus in the chemical industry. Concrete resistant to the action of non-organic acids (except hydrofluoric acid) is manufactured from a mixture of water glass (sodium silicate) and 15% of sodium fluosilicate. The aggregates for acid-resistant concretes are quartz sand, crushed stone from beschtaunite, andesite or quartzite and from acid-resistant materials in powder form (finer than 0.15 mm). Acid-resistant concrete should be hardened in a warm air-dry atmosphere (in contrast to common concrete). Acid-resistant concrete disintegrates gradually when acted upon by water and weak acids; concrete withstands well the action of concentrated acids, but decays rapidly when attacked by alkali liquors.

Refractory concrete is capable of retaining (within specified limits) its physical and mechanical properties under prolonged exposure to high temperatures. By the kind of binding material, heat-resistant concretes fall into the following categories:

-concretes from portland cement (or slag portland cement);

-concretes from high-alumina cement;

-concretes from periclase cement;

-concretes from water glass.

High-alumina cement is a hydraulic binding material containing not less than 75% of aluminium oxide and not more than 1 % of ferric oxide. Periclase cement is a binding material obtained by fine grinding of high-burned magnesite containing no less than 85% of magnesium oxide.

By degree of refractoriness, refractory concretes are divided into:

-highly-refractory concrete (refractoriness above 1770°C);

-refractory concrete (refractoriness from 1580 to 1770cC);

-heat-resistant concrete (refractoriness below 1580°C).

Highly-refractory concrete is prepared in the following compositions: from portland cement with phosphorous anhydrite and finely-ground admixture, sand and crushed stone from chromite; from high-alumina cement, sand and crushed stone from high-alumina brick waste. Concrete suffers deformation at a temperature of 1500°C, and disintegrates at a temperature above 1600°C.

Refractory concrete is prepared from: aluminous cement, sand and crushed stone from chromite.

Heat-resistant concrete. Cementing materials used for heat-resistant concretes include aluminous cement, portland cement, slag portland cement and soluble glass with sodium fluosilicate. Fire-clay, common clay brick waste, fuel cinder, blast-furnace slag, basalt, diabase, andesite, tuff, all may be used as substitutes for sand and crushed stone. With properly chosen binding materials and aggregates, concretes may withstand prolonged exposure to temperatures up to 1200°C and are suitable for building stacks and foundations for blast furnaces, open hearth and other types of industrial furnaces. Heat-resistant concrete from portland cement has sufficiently high physical and mechanical properties.

Coloured concretes are obtained by introducing alkali- and light-resistant pigments into the mix in amounts ranging from 8 to 10% of the weight of cement (ochre, mummy, minium and others) or by using coloured cements. Sometimes recourse is made to aggregates possessing the required colour, e.g. tuffs, red quartzites, marble and other coloured rocks. Coloured concretes are used for ornamental purposes in construction of buildings and installations, underground pedestrian crossings, separating lines on traffic lanes, park lanes and also for the manufacture of items for public welfare.

Road concrete. Road concrete is subdivided according to its application into concrete for single-layer roads, for surface courses of two-layer concrete roads, for bed courses of two-layer roads and super-highways.

Service conditions of road concrete are very severe. It is exposed to repeated wetting and drying, freezing and thawing and wear by vehicle wheels, and, therefore, it must satisfy stringent requirements as regards strength, wear and frost resistances, as well as resistance to atmospheric erosion. Road concrete is prepared from road portland cement and its varieties, such as plasticized and hydrophobic portland cements. Hardening accelerants, particularly effective in freezing weather, are admixtures of calcium chloride and sodium chloride in amounts up to 3% for non-reinforced, and 2% for reinforced surface courses. No such admixtures are allowed in prestressed surface courses.

Concrete for biological shielding. The use of atomic energy for peaceful purposes has necessitated means for protecting the personnel against the radiation hazard caused by nuclear reactors, atomic power stations, plants for manufacturing and processing isotopes, etc. Particularly hazardous to living organisms are γ-radiation and neutrons. The degree of protection against the latter is determined by the thickness of the shield and its bulk density. It has been established that substances with a considerable amount of hydrogen, water in the first place, offer effective protection against neutron radiation. Water co-uld have been a suitable material, if it were not of low density. Protection against both neutrons and γ -rays requires a very thick water shield, which is complicated and expensive. The material that possesses both properties required for a biological shield is concrete. Aggregates used for shielding concretes are heavy materials, such as barite, magnetite, limonite and metal scrap. Binding materials for preparing superheavy shielding concretes are portland cement, slag portland and aluminous cements. In special kinds of concrete, the most effective binding material may be a substance which binds the greatest amount of water as it hardens, since the purpose is to obtain concrete of maximum hydrogen content. Spontaneous disintegration of cement is prevented by introducing hydraulic admixtures. Along with portland cement, use is made of alumina, expanding and shrinkage-free cements, the latter being quite expensive.

LESSON 9