Classification of Concretes

Concretes are classified according to the following main characteristics: bulk density, type of binding material, strength, frost resistance, application.

By their bulk density concretes are divided into superheavy concretes with a bulk density over 2500 kg/m3, heavy concretes with a bulk density of 1800-2500 kg/m3, lightweight concretes with a bulk density between 500 and 1800 kg/m3 and extra-lightweight concretes (heat-insulation) with a bulk density below 500 kg/m3.

By the maximum size of aggregate, concretes are subdivided into fine-grained varieties with aggregates of up to 10 mm in size and coarse-grained varieties, the maximum size of aggregates ranging from 10 to 150 mm.

Strength and durability are major characteristics of the quality of concretes. By compressive strength, concretes fall into a number of grades. Heavy concretes have strength grades between 100 and 600, lightweight ones, between 25 and 300, and extra-heavy ones, from 100 to 20. Durability of concretes is evaluated in terms of their frost resistance. By this characteristic, concretes are subdivided into grades: heavy concretes, Мрз from 50 to 300; lightweight concretes, from 10 to 200.

According to the kind of binding material, concretes are available in the following varieties:

– cement concretes manufactured from hydraulic binding materials, such as portland cement and its varieties;

– lime-sand concretes, from lime binders in combination with silicate or aluminate compounds;

– gypsum concretes, from gypsum-anhydrite binding materials; concretes from organic binding materials.

Heavy concrete is manufactured from cement and common dense aggregates, and lightweight concrete, from cement and natural or artificial porous aggregate. A variety of lightweight concrete is cellular concrete which is a hardened mixture of a binding material, water, fine silicate component and pore-forming agent. It is distinguished by a high porosity (up to 90%) with uniformly distributed pores, up to 3 mm in size.

Lime-sand concretes are obtained by mixing lime and quartz sand and subsequently hardening moulded items in autoclaves at pressures between 8 and 12 atm and temperatures from 170 to 200° C.

By application, concretes are divided into the following kinds:

– common, for concrete and reinforced concrete supporting elements of buildings and installations (columns, beams, plates);

• hydraulic engineering for dams, sluices, facing of channels, etc.; for walls of buildings and light floors;

– for floors and road surfaces and bases;

– for special purposes, such as acid- and heat-resistant superheavy concretes, the latter being for biological shielding; extra-heavy concretes are manufactured from cement and special aggregates of high bulk density.

Materials for Heavy Concrete

Heavy concrete for the manufacture of foundations, columns, beams, bridge spans and other supporting elements and constructions of industrial and residential buildings and engineering installations should acquire a predetermined strength after a certain period of hardening, whereas the concrete mix should be readily placeable and economical. When used in locations exposed to weather conditions, concrete should have a high density and adequate frost and corrosion resistances. Depending on application and working conditions, the ingredients of concrete must satisfy various requirements.

Cement. Heavy concretes are prepared from portland cement, plasticized portland cement, portland cement with hydraulic admixtures, slag portland cement, etc.

The type of cement is chosen according to the demands placed upon concrete (strength, frost resistance, chemical stability, water impermeability, etc.).

Water. Water for concrete mixtures should be free from harmful impurities which may hinder the normal hardening process of concrete. Classed with these impurities are acids, sulphates, grease, vegetable oil, sugar, etc. No swamp or drainage water or water contaminated by harmful impurities (with hydrogen-ion concentration pH less than 4 or containing more than 0.27% of sulphates in terms of S0₃) should be used.

Sea and other types of water, containing mineral salts, may be used if their amount is limited to 2%. The suitability of a certain type of water for concretes is attested by chemical analysis and comparative tests of the strength of concrete specimens made with this water and with pure drinking water and tested after 28 days of hardening under normal conditions. Water is considered suitable, if specimens prepared on it are not inferior in strength to specimens from pure drinking water.

S and is a loose assembly of particles, 0.14 to 5 mm in size, formed as a result of natural disintegration of massive rocks (natural sands). Along with natural sands, use is made of artificial ones obtained by crushing or granulating metallurgical slags or fuel cinders, or of specially prepared materials, such as ceramsite, agloporite, etc.

Sands used for heavy concretes are generally natural sands; according to their mineralogical composition, they fall into quartz, feldspar, limestone and dolomite varieties. Of natural sands, mostly used for heavy concrete are the quartz sands.

Crushed sand is made from unweathered eruptive, metamorphic or solid carbonate sedimentary rocks with a compressive strength in water-saturated state not less than 400 kg/cm². The shape of crushed sand particles should closely approximate cubes.

Chemical admixtures are materials in the form of powder or fluids that are added to the concrete to give it certain characteristics not obtainable with plain concrete mixes. In normal use, admixture dosages are less than 5% by mass of cement and are added to the concrete at the time of mixing.

Concrete additives have been used since Roman and Egyptian times, when it was discovered that adding volcanic ash to the mix allowed it to set under water. Similarly, the Romans knew that adding horse hair made concrete less liable to crack while it hardened and adding blood made it more frost-resistant.

Recently the use of recycled materials as concrete ingredients has been gaining popularity because of increasingly stringent environmental legislation. The most conspicuous of these is fly ash, a by-product of coal-fired power plants. This use reduces the amount of quarrying and landfill space required as the ash acts as a cement replacement thus reducing the amount of cement required.

In modern times, researchers have experimented with the addition of other materials to create concrete with improved properties, such as higher strength or electrical conductivity. Marconite is one example.

The common types of admixtures are as follows.

Accelerators speed up the hydration (hardening) of the concrete. Typical materials used are CaCl₂, Ca(NO₃)₂ and NaNO₃. However, use of chlorides may cause corrosion in steel reinforcing and is prohibited in some countries, so that nitrates may be favored.

Retarders slow the hydration of concrete. Typical retarders are sugar, sucrose, sodium gluconate, glucose, citric acid, and tartaric acid.

Air entrainments add and entrain tiny air bubbles in the concrete, which will reduce damage during freeze-thaw cycles, thereby increasing the concrete's durability. However, air entails a trade off with strength, as each 1% of air may result in 5% decrease in compressive strength.

Plasticizers increase the workability of plastic or "fresh" concrete, allowing it be placed more easily, with less consolidating effort. A typical plasticizer is lignosulfonate. Plasticizers can be used to reduce the water content of a concrete while maintaining workability and are sometimes called water-reducers due to this use. Such treatment improves its strength and durability characteristics. Superplasticizers (also called high-range water-reducers) are a class of plasticizers that have fewer deleterious effects and can be used to increase workability more than is practical with traditional plasticizers. Compounds used as superplasticizers include sulfonated naphthalene formaldehyde condensate, sulfonated melamine formaldehyde condensate, acetone formaldehyde condensate and polycarboxylate ethers.

Pigments can be used to change the color of concrete, for aesthetics.

Corrosion inhibitors are used to minimize the corrosion of steel and steel bars in concrete.

Bonding agents are used to create a bond between old and new concrete (typically a type of polymer) .

Pumping aids improve pumpability, thicken the paste and reduce separation and bleeding.

Mineral admixtures and blended cements. There are inorganic materials that also have pozzolanic or latent hydraulic properties. These very fine-grained materials are added to the concrete mix to improve the properties of concrete (mineral admixtures), or as a replacement for Portland cement (blended cements).

Fly ash: A by-product of coal-fired electric generating plants, it is used to partially replace Portland cement (by up to 60% by mass). The properties of fly ash depend on the type of coal burnt. In general, siliceous fly ash is pozzolanic, while calcareous fly ash has latent hydraulic properties.

Ground granulated blast furnace slag (GGBFS or GGBS): A by-product of steel production is used to partially replace Portland cement (by up to 80% by mass). It has latent hydraulic properties.

Silica fume: A by-product of the production of silicon and ferrosilicon alloys. Silica fume is similar to fly ash, but has a particle size 100 times smaller. This results in a higher surface to volume ratio and a much faster pozzolanic reaction. Silica fume is used to increase strength and durability of concrete, but generally requires the use of superplasticizers for workability.

High reactivity Metakaolin (HRM): Metakaolin produces concrete with strength and durability similar to concrete made with silica fume. While silica fume is usually dark gray or black in color, high-reactivity metakaolin is usually bright white in color, making it the preferred choice for architectural concrete where appearance is important.