Concrete (redirected from Concrete (material))

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 Modern portland cement has revolutionized the production and potentialities of concrete and has superseded the natural cements, to which it is vastly superior.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, 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.

Bibliography

See A. A. Raafat, Reinforced Concrete in Architecture (1958); J. J. Waddell, Concrete Construction Handbook (1968); D. F. Orchard, Concrete Technology (1976).

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concrete

Artificial stone made of a mixture of cement, aggregate (hard material), and water. In addition to its potential for immense compressive strength and its ability, when poured, to adapt to virtually any form, concrete is fire-resistant and has become one of the most common building materials in the world. The binder usually used today is portland cement. The aggregate is usually sand and gravel. Additives called admixtures may be used to accelerate the curing (hardening) process in low temperature conditions. Other admixtures trap air in the concrete or slow shrinkage and increase strength. See also precast concrete, prestressed concrete, reinforced concrete.

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Concrete

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.