Concrete is an artificial engineering material made from a mixture of Portland cement, water, fine and coarse aggregates together with a small amount of air. It is among the most widely used construction materials in the world. Concrete is the only major building material that can be delivered to the job site in a plastic state (that is, capable of being moulded). This unique quality makes concrete desirable as a building material because it can be moulded to virtually any form or shape. Concrete provides a wide latitude in surface textures and colors and can be used to construct a wide variety of structures, such as highways and streets, bridges, dams, large buildings, airport runways, irrigation structures, breakwaters, piers and docks, pavements, silos and farm buildings, homes, and even barges. Other desirable qualities of concrete as a building material are its strength, economy, and durability. The tensile strength of concrete is much lower, but by using properly designed steel reinforcing structural members can be made that are as strong in tension as they are in compression. The two major components of concrete are a cement paste and inert materials. The cement paste consists of Portland cement, water, and some air either in the form of naturally entrapped air voids or minute, intentionally entrained air bubbles.
The inert materials are usually composed of fine aggregate (which is a material such as sand) and coarse aggregate, such as gravel, crushed stone, or slag. In general, fine aggregate particles are smaller than 6.4 mm in size, and coarse aggregate particles are larger. Depending on the thickness of the structure to be built, the size of coarse aggregate particles used can vary widely. In building relatively thin sections, a small size of coarse aggregate, with particles about 6.4 mm in size, is used. At the other extreme, aggregates up to 15 cm or more in diameter are used in large dams. In general the maximum size of coarse aggregates should not be larger than one fifth of the narrowest dimensions of the concrete member in which it is used. When Portland cement is mixed with water the compounds of the cement react to form a cementing medium. In properly mixed concrete each particle of sand and coarse aggregate is completely surrounded and coated by this paste, and all spaces between the particles are filled with it. As the cement paste sets and hardens, it binds the aggregates into a solid mass. Under normal conditions concrete grows stronger as it grows older. The chemical reactions between cement and water that cause the paste to harden and bind the aggregates together require time. The reactions take place very rapidly at first and then more slowly over a long period. In the presence of moisture concrete continues to gain strength for years. Concrete mixtures are usually specified in terms of the dry-volume ratios of cement, sand, and coarse aggregates used. A 1:2:3 mixtures, for instance, consists of 1 part by volume of cement, 2 parts of sand, and 3 parts of coarse aggregate. Depending on the applications the proportions of the ingredients in the concrete can be altered to produce specific changes in its properties, particularly strength and durability. The ratios can vary from 1:2:3 to 1:2:4 and 1:3:5. The amount of water added to these mixtures is about 1 to 1.5 times the volume of the cement.
For high-strength concrete, the water content is kept low, with just enough water added to wet the entire mixture. In general, the more water in a concrete mix, the easier it is to work with, but the weaker the hardened concrete becomes. Concrete can be made to have any degree of water tightness. It can be made to hold water and resist the penetration of wind-driven rains. On the other hand, for purposes such as constructing filter beds, concrete can be made porous and highly permeable. Concrete can also be given a polished surface that is as smooth as glass. By using heavy aggregates, including steel fragments, dense concrete mixtures can be made that weigh 4,005 or more kg/cu m (250 or more lb/cu ft). Concrete that weighs only 480 kg/cu m (30 lb/cu ft) can be made by using special lightweight aggregates and foaming techniques. Forms consisting of such lightweight aggregates can be floated on water, sawed into pieces, or nailed to another surface. For small jobs and minor repairs, concrete can be mixed by hand, but machine mixing ensures more uniform batches and, therefore, superior performance. For most home repairs and improvements, for example, floors, paths, driveways, patios, and garden pools, the recommended proportion is a 1:2:3 mix.
After exposed surfaces of concrete have hardened sufficiently to resist marring, they should be cured by sprinkling or ponding (covering) with water or by using moisture-retaining materials such as waterproof paper, plastic sheets, wet burlap, or sand. Special curing sprays are available. The longer concrete is kept moist, the stronger and more durable it will become. In hot weather, it should be kept moist for at least three days. In cold weather drying concrete must not be allowed to freeze. This can be accomplished by covering the cement with a tarpaulin or some other material that helps trap the heat generated by the chemical reactions within the concrete that cause it to harden. Concrete is poured into place in a number of ways. For the footings of small buildings the wet concrete is poured directly into trenches dug into the earth below frost level. Concrete for foundations and certain types of walls is placed between supporting wood or metal “forms” that are removed after the concrete has hardened. In lift-slab construction, floors and roof slabs are cast at ground level and then raised by hydraulic jacks and fastened to columns at the desired elevation. Slip forms are used to produce vertical shafts for silos and the cores of buildings. They are moved upward at a rate of 15 to 38 cm (6 to 15 in) per hour while concrete and reinforcements are put in place. The tilt-up method of construction is frequently used for one- and two-storeyed buildings. Walls are cast in place on the ground or on the previously laid concrete floor and tilted into position by cranes. The walls are joined at the corners or between panels with cast-in-place concrete columns. To pave a highway or road with concrete, a slip-form paver is used. Two metal side forms are connected to a slip-form paver. A layer of concrete is poured between the side forms as the paver slowly moves forward on its treads; the side forms keep the concrete in position as it dries. Slip-form pavers can lay continuous strips of one or two lanes of concrete pavement.
For certain applications, such as the construction of swimming pools, canal linings, and curved surfaces, concrete may be applied by the shot Crete method. In shotcreting, concrete is sprayed under pneumatic pressure rather than placed between forms. Often the use of shotcrete eliminates the need for formwork and permits placement of concrete in confined areas where conventional forms would be difficult or impossible to construct. Air-entrained concrete is concrete in which minute air bubbles are intentionally trapped by the addition of an admixture to the cement, either during its manufacture or during the batching and mixing of the concrete. The presence of a properly distributed amount of these bubbles imparts desirable properties to both freshly mixed and hardened concrete. In freshly mixed concrete, entrained air acts as a lubricant, improving the workability of the mix, thereby reducing the amount of water that needs to be added. Entrained air also reduces the need for fine material (sand). Entrained air in hardened concrete dramatically reduces the scaling that might otherwise result from the use of chemicals to melt ice on roads and streets. It also prevents damage to pavements caused by freezing and thawing. The air bubbles function as minute safety valves by providing room for the free water in concrete to expand harmlessly as freezing occurs. Concrete masonry consists of block- and brick-building units moulded out of concrete and used in all types of masonry construction.
Concrete masonry is used for load-bearing and non-load-bearing walls; piers; partitions; fire walls; back-up for walls of brick, stone, and stucco facing materials; fireproofing over steel structural members; fire-safe walls around stairwells, lifts, and other enclosures; retaining walls and garden walls; chimneys and fireplaces; concrete floors; and many other purposes. About 60 per cent of all concrete masonry units, such as cinder blocks, are made with lightweight aggregates. Processed clays, blast-furnace slag, shales, natural volcanic aggregates, and cinders are the lightweight aggregates most commonly used. The size of the masonry unit most commonly used for walls, both below and above ground, is 20 by 20 by 40 cm (8 by 8 by 16 in). Masonry units are laid horizontally, and are cored to reduce weight and to provide an insulating air space within the block. New types of concrete masonry, such as split and slump block, are being used as facing in homes, commercial buildings, schools, churches, and municipal facilities. Basic block types are fairly well standardized today. Specific types can usually be supplied for any construction without cutting or fitting. Special moulds are available for the production of patterned shadow effects on exterior and interior block walls. It is possible to supply virtually any color or type of texture. Concrete used in most construction work is reinforced with steel. When concrete structural members must resist extreme tensile stresses, steel supplies the necessary strength.
Steel is embedded in the concrete in the form of a mesh, or roughened or twisted bars. A bond forms between the steel and the concrete, and stresses can be transferred between both components. Prestressing concrete has removed many limitations on the spans and loads for which a concrete structure can be economically designed. The basic function of Prestressing is to greatly reduce the tensile stresses to which crucial areas of concrete structures are subjected. Prestressing is accomplished by stretching high-strength steel to induce compressive stresses in concrete. The strengthening effect of compression in concrete acts like horizontally squeezing a row of books: when sufficient pressure is applied to the books at each end, compressive stresses are induced throughout the entire row; thus, although the centre volumes are unsupported, the books can be lifted, and carried horizontally. Compressive stresses are induced in prestressed concrete by either pre-tensioning or post-tensioning the steel reinforcement. In the pre-tensioning process, the steel is stretched before the concrete is placed. After the concrete has hardened around the tensioned reinforcement, the stretching forces are released. The steel shortens somewhat, and because of the bond between the steel and concrete, the compressive stress in the concrete increases. In post-tensioning, the concrete is cast around, but not in contact with, unstretched steel. The steel is stretched after the concrete has hardened by anchoring one end against the concrete and using hydraulic jacks to pull the other end. After stretching, the second end is also anchored, compressing the concrete.
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