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concrete, structural masonry material made by mixing broken stone or gravel with sand, cement, and water and allowing the mixture to harden into a solid mass. The cement is the chemically active element, or matrix; the sand and stone are the inert elements, or aggregate. Concrete is adaptable to widely varied structural needs, is available practically anywhere, is fire resistant, and can be used by semiskilled workers.
The use of artificial masonry similar to modern concrete dates from a remote period but did not become a standard technique of construction until the Romans adopted it (after the 2d cent. B.C.) for roads, immense buildings, and engineering works. The concrete of the Romans, formed by combining pozzuolana (a volcanic earth) with lime, broken stones, bricks, and tuff, was easily produced and had great durability (the Pantheon of Rome and the Baths of Caracalla were built with it). Enormous spaces could be roofed without lateral thrusts by vaults cast in the rigid homogeneous material.
Scientifically proportioned concrete formed with cement is an invention of modern times; the name did not appear until c.1830. Modern portland cement has revolutionized the production and potentialities of concrete and has superseded the natural cements, to which it is vastly superior. The component materials of concrete are mixed in varying proportions, according to the strength required and the function to be fulfilled; the proportions were first worked out by Duff Abrams in 1918. The ideal mixture is that which solidifies with the minimum of voids, the mortar and small particles of aggregate filling all interstices. A typical proportioning is 1:2:5, i.e., one part of cement, two parts of sand, and five parts of broken stone or gravel, with the proper amount of water for a pouring consistency. A simple test called a “slump test” is used to confirm the proportions and consistency of the mixture, and it is then poured into wood or steel molds, called forms. Concrete usually takes about five days to cure, or reach acceptable hardness, but a technique called steam saturation can shorten that curing time to less than 18 hours. A wide variety of additives allow the concrete to harden faster or slower, resist scaling, have increased strength, or adopt the final shape more easily.
Concrete used without strengthening is termed mass, or plain, concrete and has the structural properties of stone—great strength under compressive forces and almost none under tensile ones. F. Joseph Monier, a French inventor, found that the tensile weakness could be overcome if steel rods were embedded in a concrete member. The new composite material was called reinforced concrete, or ferroconcrete. It was patented in 1857, and a private house in Port Chester, N.Y., first demonstrated (1857) its use in the United States. It is now rivaled in popularity as a structural material only by steel. Concrete reinforced with polypropylene fibers instead of steel yields equivalent strength with a fraction of the thickness. Reinforced concrete was improved by the development of prestressed concrete—that is, concrete containing cables that are placed under tension opposite to the expected compression load before or after the concrete hardens. Another improvement, thin-shell construction, takes advantage of the inherent structural strength of certain geometric shapes, such as hemispherical and elliptical domes; in thin-shell construction great distances are spanned with very little material. The perfecting of reinforced concrete has profoundly influenced structural building techniques and architectural forms.
See A. A. Raafat, Reinforced Concrete in Architecture (1958); J. J. Waddell, Concrete Construction Handbook (1968); D. F. Orchard, Concrete Technology (1976).
an artificial stone material obtained by a rational selection of a mixture of the binding material (with water, more rarely without water), aggregates, and special additives (in certain cases) and by its subsequent forming and hardening; one of the principal structural materials. Before its forming this mixture is called a concrete mix.
Historical survey. In erecting massive buildings and such structures as vaults, domes, and triumphal arches, the ancient Romans used concrete, and as cementing materials they utilized clay, gypsum, lime, and asphalt. With the fall of the Roman Empire the use of concrete ceased and was revived only during the 18th century in the countries of Western Europe.
The development and improvement of the technology of concrete was related to the production of cement, which appeared in Russia at the beginning of the 18th century. According to archival records, cement was used in building the Ladoga Canal during the years 1728–29; it was made at a cement plant that existed in Konorsk District, St. Petersburg Province. In 1824, J. Aspdin obtained a patent in England for a method of making hydraulic cement. The first cement plant in France was opened in 1840; in Germany, in 1855; and in the USA, in 1871. The widespread development of concrete was facilitated by the invention of reinforced concrete in the 19th century.
The widespread use of concrete in the USSR was prepared for by the studies of the Russian scientists N. A. Beleliub-skii, A. R. Suliachenko, and I. G. Maliuga, who in 1881 jointly worked out the first standard specifications for port-land cement. In 1890, I. Samovich published the results of tests on the strength of mixtures with varying amounts of cement, and he proposed compositions of concrete mix that would provide maximum density. In 1895, Professor I. G. Maliuga established a qualitative ratio between the strength of concrete and the percentage content of water in the mass of cement and aggregates. In the work of the American scientist D. Abrams, published in the USA in 1918, detailed graphical representations were given of the dependence of the strength of concrete on the water-cement ratio and the workability of the concrete mix on the composition of the concrete, on the fineness of the aggregates, and on the water-cement ratio. The scientific foundations for selecting the composition of the concrete, including the calculation of its strength and the workability of the concrete mix, were developed by the Soviet scientist N. M. Beliaev. The ideas concerning the dependence of concrete strength on the water-cement ratio did not basically change for a long time. The Swiss scientist Bolome simplified the practical use of this complex (hyperbolic) ratio by changing it into the linear dependence of concrete strength on the reverse dimension—the cement-water ratio. During the course of a number of years this dependence was applied in practice. In 1965 the Soviet scientist Professor B. G. Skramaev, together with other researchers, established that the linear dependence is valid only within a fixed range of the cement-water ratio.
Classification and areas of application of concrete. Concretes are classified according to the type of binding agents that are employed; there are concretes based on inorganic binders (cement concretes, gypsum concretes, silicate concretes, acid-resistant concretes, heat-resistant concretes, and other special concretes) and concretes based on organic binders (asphalt concretes and plastic concretes).
Depending upon the volumetric weight, concretes are divided into extra-heavy (more than 2,500 kg/cu m), heavy (from 1,800 to 2,500 kg/cu m), light (from 500 to 1,800 kg/cu m), and extra-light (less than 500 kg/cu m).
Extra-heavy concretes are designed for special protective structures (providing safety from radioactive substances); they are made primarily with various types of portland cement and natural or artificial aggregates (magnetite, limo-nite, barite, cast-iron scrap, and fragments of hardware). In order to improve the protective properties against neutron radiations, extra-heavy concretes usually include an additive of boron carbide or other additives containing lightweight elements, such as hydrogen, lithium, or cadmium.
Most widespread are the heavy concretes, which are used in reinforced and nonreinforced concrete structural elements of industrial and public buildings, as well as in hydraulic engineering installations and in the construction of canals and transportation and other kinds of structures. Of particular importance in hydraulic engineering construction is the durability of the concrete, which is subjected to the action of sea water, freshwater, and the atmosphere. The aggregates for such heavy concretes have to meet special requirements of granulometric composition and purity. Harsh climatic conditions in a number of regions in the Soviet Union have led to the need for developing and implementing methods of working with concrete in winter. In regions with a moderate climate, processes for hastening the hardening of the concrete are very important; this is achieved by the use of rapid-hardening cements, by thermal processing (electric heating, steaming, and autoclave treatment), and by the introduction of chemical additives and other means. Also belonging in the heavy-concrete classification is silicate concrete, in which the binding agent is soda lime. An intermediate stage between heavy and light concrete is coarsegrained (sandless) concrete, produced with dense, coarse aggregate and made porous by means of air-entraining or foam-producing cement stone.
Light concretes are made with hydraulic binding agents and porous artificial or natural aggregates. There are many varieties of lightweight concrete; they are classified according to the type of aggregate used—for example, miculite concrete, keramzit concrete, pumice concrete, perlite concrete, and tuff concrete.
According to the structure and the degree to which the spaces between the grains are filled with cement stone, light concretes are subdivided into ordinary light concretes (with complete filling of the spaces between the grains), slightly sandy light concretes (with partial filling of the spaces between the grains), coarsely porous light concretes made without fine aggregate, and light concretes made porous with the help of air-entraining or foaming agents. According to the type of binding agent used, light concretes made with porous aggregates are subdivided into cement, cement-lime, lime-slag, and silicate concretes. The areas in which the use of light concretes is expedient include the exterior walls and roofs of buildings, where low thermal conductivity and light weight are required. High-strength light concrete is also used in the bearing structural elements of industrial and public buildings (in order to reduce the empty weight of these buildings). Included as well among the light concretes are stiuctural thermal-insulation and structural cellular concretes with a volumetric weight ranging from 500 to 1,200 kg/cu m. According to the method of forming the porous structural element, cellular concretes are divided into air-entrained concretes and foam concretes; according to the binding agent employed, they may be also classified as air-entrained and foam concretes made with either portland cement or mixed binders, air-entrained and foam silicates made with a lime base, or air-entrained and foam slag concretes using pulverized blast-furnace slags. When ash is used instead of quartz sand, cellular concretes are called air-entrained and foam ash concrete, air-entrained and foam ash silicate concrete, or air-entrained and foam slag concrete.
Extra-light concretes are used, for the most part, for thermal insulation.
The areas of application of concrete in construction are constantly expanding. There are prospective plans for the use of high-strength concretes (heavy and light), as well as concretes with specific physical and engineering properties, such as slight shrinkage and creep, frost resistance, durability, resistance to cracking, thermal conductivity, heat resistance, and properties that afford protection from radioactive effects. In order to achieve this goal, a wide range of research must be conducted with an aim to provide solutions to the most important theoretical problems in the technology of heavy, light, and cellular concretes, such as the macrostructural and microstructural theories of concrete strength with the calculation of internal stresses and micro-crack formation and the theories of short-term and long-term concrete deformation.
Physical and engineering properties of concrete. The principal properties of concrete are consistency, the content of bonded water (for extra-heavy concretes), strength in resisting compression and expansion, frost resistance, thermal conductivity, and technical viscosity (stiffness of mix). The strength of concretes is characterized by their specifications (temporary resistance to compression, axial expansion, or flexural expansion). High-compression strength specifications of heavy-cement, extra-heavy, light, and coarse-grained concretes are determined by compression testing of concrete cubes with equal sides of 200 mm made from a working mixture and tested after a fixed period of curing. For test samples of monolithic concrete intended for use in industrial and public buildings and structures, under normal conditions of hardening (at a temperature of 20° C and a relative humidity no lower than 90 percent), the period of curing is 28 days. The strength, or resistance, of concrete at an age of 28 days (R28) of normal hardening may be determined by the following formula:
R28 = aRc (C/W – b),
where Rc is the activity (strength) of the cement, C/W is the cement-water ratio, a is a coefficient ranging from 0.4 to 0.5, and b is a coefficient ranging from 0.45 to 0.50, depending on the type of cement and aggregates. In order to establish the specifications of concrete to be used in massive, hydraulic engineering structures, the period of curing the test samples is set at 180 days. The curing period and hardening conditions for test samples of concrete intended for use in precast products are indicated in the appropriate GOST (All-Union State Standard). For the characterization of silicate and cellular concretes the temporary resistance is taken in kilograms-force (kgf.) /cm2 to compression of test samples of the same dimensions but which had been previously subjected to autoclaving at the same time with the products themselves (1 kgf/cm2 ≈ 0.1 meganewtons [MN]/m2). Extra-heavy concretes have characteristics ranging from 100 to 300 (~ 10–30 MN/m2), and heavy concretes have characteristics ranging from 100 to 600 (~ 10–60 MN/m2). High-strength types of concrete have a range of from 800 to 1,000 (~ 80–100 MN/m2). High-strength concretes are used most efficiently in centrally compressed columns or those that are compressed with slight eccentricity in multistoried industrial and public buildings, as well as in long-span lattice girders and arches. Light concretes with porous aggregates have characteristics ranging from 25 to 200 (~ 2.5–20 MN/m2), and high-strength characteristics ranging up to 400 (~ 40 MN/m2); coarse-grained concretes have characteristics ranging from 15 to 100 (~ 1.5–10 MN/m2) and cellular concretes, from 25 to 200 (~ 2.5–20 MN/m2). The tensile strength of concrete is approximately one-tenth of its compressive strength.
Requirements for great compressive and tensile strength may be specified, for example, in concretes to be used for road and airport pavements. For concretes to be used in hydraulic engineering and special structures (television towers, cooling towers, etc), in addition to indicators of strength, stipulations are made for frost resistance. This property is evaluated by testing the samples by means of freezing and thawing them (alternately) in a water-saturated state for 50 to 500 cycles. For structures that operate under pressure, requirements are specified for water impermeability, whereas for structures that are subjected to the effect of seawater or other corrosive liquids and gases, requirements must be met for resistance against corrosion. In planning the composition of heavy cement concrete, calculations are made concerning requirements for compressive strength and the workability of the concrete mix and its stiffness (technical viscosity); in planning the composition of light and extra-heavy concretes, the density requirement is also taken into consideration. The maintenance of a predetermined degree of workability is especially important for modern industrial methods of production; extreme workability leads to a waste of cement, whereas insufficient workability makes it difficult to place the concrete mix by existing means and leads often to defects in production. The workability of the concrete mix is determined by the size (in cm) of the slump of a standard concrete cone (a truncated cone 30 cm high, with a base diameter of 20 cm and top diameter of 10 cm). The stiffness is determined by a simplified method developed by Professor B. G. Skramtaev or with the help of an engineering viscometer, and it is expressed by the time (in seconds) that is required to turn the cone of concrete mix into an equal-sized prism or cylinder. These tests are carried out on a standard laboratory vibration platform equipped with an automatic switch; this apparatus is also used to make the control samples. The gradations of workability of a concrete mix are provided in Table 1.
|Table 1. Gradations of workability of a concrete mix|
|Concrete mix||Stiffness by engineering viscometer (in seconds)||Slump of cone (in cm)|
|Stiff||More than 60||0|
The selection of a concrete mix by the degree of workability or stiffness is made depending upon the type of structure to be concreted and the means of transporting and placing the concrete. In addition to its valuable structural properties, concrete also possesses decorative qualities. By the choice of the components in a concrete mix and by the preparation of the decking or forms, it is possible to change the concrete’s color, texture, and surface finish; the finish also depends on the methods of mechanical and chemical treatment of the concrete surface. The plastic expressiveness of structures and sculpture made of concrete is enhanced by the concrete’s surface, which absorbs light, whereas the rich gradation of concrete’s decorative properties is employed in finishing interiors and in decorative art.
REFERENCESMaliuga, I. G. Sostav i sposob prigotovleniia tsementnogo ra-stvora (betona) dlia polucheniia naibol’shei kreposti. St. Petersburg, 1895.
Samovich, I. “Sostavlenie proportsii tsementnykh rastvorov i betonov.” Inzhenernyi zhurnal, 1890, nos. 7–8, 9.
Beliaev, N. M. Metod podbora sostava betona. Leningrad, 1927.
Skramtaev, B. G. Issledovanie prochnosti betona i plastichnosti betonnoi smesi. Moscow, 1936. (Dissertation.)
Moskvin, V. M. Beton dlia morskikh gidrotekhnicheskikh sooru-zhenii. Moscow, 1949.
Shestoperov, S. V. Dolgovechnost’ betona transportnykh sooru-zhenii, 3rd ed. Moscow, 1966.
Mironov, S. A., and L. A. Malinina. Uskorenie tverdeniia betona, 2nd ed. Moscow, 1964.
SNiP, part 1, section V. chapter 3: “Betony na neorganicheskikh viazhushchikh i zapolniteliakh.” Moscow, 1963.
Desov, A. E. Tiazhelye i gidratnye betony. (Dlia zashchity ot radioaktivnykh vozdeistvii.) Moscow, 1956.
Nekrasov, K. D. Zharoupornyi beton. Moscow, 1957.
Suzdal’tseva, A. Ia. Beton ν sovremennoi arkhitekture. Moscow, 1968.
Taylor, W. H. Concrete Technology and Practice, 2nd ed. New York, 1967.
BIBLIOGRAPHYBibliograficheskii spravochnik literatury po tekhnologii betona za 1895–1940. Edited by B. G. Skramtaev. Moscow, 1941.
A. E. DESOV
Any of several manufactured, stonelike materials composed of particles, called aggregates, that are selected and graded into specified sizes for construction purposes and that are bonded together by one or more cementitious materials into a solid mass.
The term concrete, when used without a modifying adjective, ordinarily is intended to indicate the product formed from a mix of portland cement, sand, gravel or crushed stone, and water. There are, however, many different types of concrete. The names of some are distinguished by the types, sizes, and densities of aggregates—for example, wood-fiber, lightweight, normal-weight, or heavyweight concrete. The names of others may indicate the type of binder used—for example, blended-hydraulic cement, natural-cement, polymer, or bituminous (asphaltic) concrete.
Concretes are similar in composition to mortars, which are used to bond unit masonry. Mortars, however, are normally made with sand as the sole aggregate, whereas concretes contain much larger aggregates and thus usually have greater strength. As a result, concretes have a much wider range of structural applications, including pavements, footings, pipes, unit masonry, floor slabs, beams, columns, walls, dams, and tanks. See Concrete beam, Concrete column, Concrete slab, Masonry; Mortar
Because ordinary concrete is much weaker in tension than in compression, it is usually reinforced or prestressed with a much stronger material, such as steel, to resist tension. Use of plain, or unreinforced, concrete is restricted to structures in which tensile stresses will be small, such as massive dams, heavy foundations, and unit-masonry walls. For reinforcement of other types of structures, steel bars or structural-steel shapes may be incorporated in the concrete. Prestress to offset tensile stresses may be applied at specific locations by permanently installed compressing jacks, high-strength steel bars, or steel strands. Alternatively, prestress may be distributed throughout a concrete component by embedded pretensioned steel elements. Another option is use of a cement that tends to expand concrete while enclosures prevent that action, thus imposing compression on the concrete. See Prestressed concrete, Reinforced concrete
There are various methods employed for casting ordinary concrete. For very small projects, sacks of prepared mixes may be purchased and mixed on the site with water, usually in a drum-type, portable, mechanical mixer. For large projects, mix ingredients are weighed separately and deposited in a stationary batch mixer, a truck mixer, or a continuous mixer. Concrete mixed or agitated in a truck is called ready-mixed concrete. In general, concrete is placed and consolidated in forms by hand tamping or puddling around reinforcing steel or by spading at or near vertical surfaces. Another technique, vibration or mechanical puddling, is the most satisfactory one for achieving proper consolidation.
Finishes for exposed concrete surfaces are obtained in a number of ways. Surfaces cast against forms can be given textures by using patterned form liners or by treating the surface after forms are removed, for instance, by brushing, scrubbing, floating, rubbing, or plastering. After the surface is thoroughly hardened, other textures can be achieved by grinding, chipping, bush-hammering, or sandblasting. Unformed surfaces, such as the top of pavement slabs or floor slabs, may be either broomed or smoothed with a trowel. Brooming or dragging burlap over the surface produces scoring, which reduces skidding when the pavement is wet.
Adequate curing is essential to bring the concrete to required strength and quality. The aim of curing is to promote the hydration of the cementing material. This is accomplished by preventing moisture loss and, when necessary, by controlling temperature. Moisture is a necessary ingredient in the curing process, since hydration is a chemical reaction between the water and the cementing material. Unformed surfaces are protected against moisture loss immediately after final finishing by means of wet burlap, soaked cotton mats, wet earth or sand, sprayed-on sealing compounds, waterproof paper, or waterproof plastic sheets. Formed surfaces, particularly vertical surfaces, may be protected against moisture loss by leaving the forms on as long as possible, covering with wet canvas or burlap, spraying a small stream of water over the surface, or applying sprayed-on sealing compounds. The length of the curing period depends upon the properties desired and upon atmospheric conditions, such as temperature, humidity, and wind velocity, during this period. Short curing periods are used in fabricating concrete products such as block or precast structural elements. Curing time is shortened by the use of elevated temperatures.