Reinforced Concrete(redirected from Armed concrete)
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reinforced concrete[¦rē·ən′fȯrst ′kän‚krēt]
a combination of concrete and steel reinforcement that are joined into one piece and work together in a structure. The term “reinforced concrete” is frequently used as a collective name for reinforced-concrete structural members and products. The idea of combining in reinforced concrete two materials that are extremely different in properties is based on the fact that the tensile strength of concrete is significantly lower (by a factor of 10–20) than its compressive strength. Therefore, the concrete in a reinforced-concrete structure is intended to take compressive stresses, and the steel, which has high ultimate tensile strength and is introduced into the concrete as reinforcement rods, is used principally to take tensile stresses. The interaction of such different materials is extremely effective: when the concrete hardens, it adheres firmly to the steel reinforcement and protects it from corrosion, since an alkaline medium is produced during the process of hydration of the cement. The monolithic nature of the concrete and reinforcement also results from the relative closeness of their coefficients of linear expansion (7.5 × 10−6 to 12 × 10−6 for concrete and 12 × 10−6 for steel reinforcement). The basic physicomechanical properties of the concrete and steel reinforcement are virtually unchanged during temperature variations within a range of –40° to 60°C, which makes possible the use of reinforced concrete in all climatic zones.
The basis of the interaction between concrete and steel reinforcement is the presence of adhesion between them. The magnitude of adhesion or resistance to displacement of the reinforcement in concrete depends on the mechanical engagement in the concrete of special protuberances or uneven areas of the reinforcement, the frictional forces from compression of the reinforcement by the concrete as a result of its shrinkage (reduction in volume upon hardening in air), and the forces of molecular interaction (agglutination) of the reinforcement with the concrete. The factor of mechanical engagement is decisive. The use of indented bar reinforcement and welded frames and nets, as well as the arrangement of hooks and anchors, increases the adhesion of the reinforcement to the concrete and improves their joint operation.
Structural damage and noticeable reduction of the strength of concrete occur at temperatures above 60°C. Short-term exposure to temperatures of 200°C reduces the strength of concrete by 30 percent, and long-term exposure reduces it by 40 percent. A temperature of 500°-600°C is the critical temperature for ordinary concrete, at which the concrete breaks up as a result of dehydration and the rupture of the cement stone skeleton. Therefore, the use of ordinary reinforced concrete at temperatures exceeding 200°C is not recommended. Heat-resistant concrete is used in thermal units operating at temperatures up to 1700°C. A protective layer of concrete 10–30 mm thick is provided in reinforced-concrete structures to protect the reinforcement from corrosion and rapid heating (for example, during a fire), as well as to ensure its reliable adhesion to the concrete. In an aggressive environment the thickness of the protective layer is increased.
The shrinkage and creep of concrete are of great importance in reinforced concrete. As a result of adhesion, the reinforcement impedes the free shrinkage of concrete, leading to the emergence of initial tensile stresses in the concrete and compressive stresses in the reinforcement. Creep in concrete causes the redistribution offerees in statically indeterminate systems, an increase in sags in components that are being bent, and the redistribution of stresses between concrete and reinforcement in compressed components. These properties of concrete are taken into account in designing reinforced-concrete structures. The shrinkage and low limiting extensibility of concrete (0.15 mm/m) cause the inevitable appearance of cracks in the expanded area of structures under service loads. Experience shows that under normal operating conditions cracks up to 0.3 mm wide do not reduce the supporting capacity and durability of reinforced concrete. However, low cracking resistance limits the possibility of further improvement of reinforced concrete and, particularly, the use of more economical high-strength steels as reinforcement. The formation of cracks in reinforced concrete may be avoided through the method of prestressing, by means of which concrete in expanded areas of the structure undergoes artificial compression through mechanical or electrothermal prestressing of the reinforcement. Self-stressed reinforced-concrete structures, in which compression of the concrete and expansion of the reinforcement are achieved as a result of the expansion of the concrete (manufactured with so-called stretching cement) during specific temperature-moisture treatment, is a further development of prestressed reinforced concrete. Because of its high technical and economic indexes (profitable use of high-strength materials, absence of cracks, and reduction of reinforcement expenditures), prestressed reinforced concrete is successfully used in supporting structures of buildings and engineering structures. A basic shortcoming of reinforced concrete, high weight per volume, is eliminated to a considerable extent by the use of lightweight concrete (with artificial and natural porous fillers) and cellular concrete.
The extensive use of reinforced concrete in modern construction has resulted from its technical and economic advantages as compared to other materials. Reinforced-concrete structures are fireproof and durable and do not require special protective measures against destructive atmospheric influences. The strength of concrete increases with time; and the reinforcement is not subject to corrosion, because it is protected by the surrounding concrete. Reinforced concrete has a high supporting capacity and bears static and dynamic loads, including seismic loads, well. Structures and structural members with extremely diverse forms and great architectural expressiveness are relatively easy to create with rein-forced concrete. The basic content of reinforced concrete consists of common materials—crushed stone, gravel, and sand. The use of precast reinforced concrete makes possible a significant rise in the level of the industrialization of construction. Structural members are manufactured in advance at well-equipped plants, and only the assembly of finished components with mechanized equipment is carried out at the construction sites. Thus, high rates of construction of buildings and structures, as well as savings in monetary and labor expenditures, are ensured.
The beginning of the use of reinforced concrete is generally associated with the Parisian gardener J. Maunier, who obtained a number of patents in France and other countries for inventions using reinforced concrete. His first patent, for a flower tub made of a wire grid covered with cement mortar, dates to 1867. Actually, concrete structures with steel reinforcement were built even earlier. Reinforced concrete began to play a noticeable part in the building technology of Russia, Western Europe, and America only at the end of the 19th century. A great deal of credit for the development of rein-forced concrete in Russia is due professor N. A. Beleliubskii, under whose direction a number of structures were built and tests were conducted of various reinforced-concrete structural members. In the early 20th century prominent Russian scientists—Professors I. G. Maliuga, N. A. Zhitkevich, S. I. Druzhinin, and N. K. Lakhtin—worked on questions of the technology of concrete, of concrete and reinforced-concrete operations, and of the design of structures using reinforced concrete. Original designs proposed by the engineers N. M. Abramov and A. F. Loleit appeared. The Volkhov Hydro-electric Power Plant was the first large structure in the Soviet Union to be made with concrete and reinforced concrete; it served as an important practical school for Soviet specialists on reinforced concrete. In subsequent years rein-forced concrete was used in ever-increasing amounts. Significant achievements in developing the theory of structural design using this new building material contributed to the expanded production of reinforced concrete. The progressive method of structural design of reinforced concrete in terms of stage of collapse, which was developed by the Soviet scientists A. A. Gvozdev, la. V. Stoliarov, V. I. Murashev, and others based on the proposals of A. F. Loleit, began to be used in the USSR in 1938. This method was comprehensively developed in designing reinforeed-concrete structures for limiting states. The attainments of the Soviet school of the theory of reinforced concrete have received universal recognition and are used in most foreign countries. The further improvement of reinforced concrete and the expansion of the spheres of its application are related to the conduct of a broad range of scientific research operations. Significant increases in the technical level of reinforced concrete are anticipated through reduction of its weight per volume, the use of high-strength concrete and reinforcement, the development of methods of structural design of reinforced concrete for complex external influences, and an increase in the durability of reinforced concrete under the influence of a corrosive medium.
REFERENCESStoliarov, la. V. Vvedenie v teoriiu zhelezobetona. Moscow-Leningrad, 1941.
Gvozdev, A. A. Raschet nesushchei sposobnosti konstruktsii po metodu predel’nogo ravnovesiia, fasc. 1. Moscow, 1949.
Murashev, V. I. Treshchinoustoichivost’, zhestkost’ i prochnost’ zhelezobetona. Moscow, 1950.
Berg, O. la. Fizicheskie osnovy teorii prochnosti betona i zhelezobetona. Moscow, 1961.
Razvitie betona i zhelezobetona v SSSR. Edited by K. V. Mikhailov. Moscow, 1969.
Cent ans de beton armé: 1849–1949. Paris, 1949.
K. V. MIKHAILOV
Portland cement concrete containing higher-strength, solid materials to improve its structural properties. Generally, steel wires or bars are used for such reinforcement, but for some purposes glass fibers or chopped wires have provided desired results.
Unreinforced concrete cracks under relatively small loads or temperature changes because of low tensile strength. The cracks are unsightly and can cause structural failures. To prevent cracking or to control the size of crack openings, reinforcement is incorporated in the concrete. Reinforcement may also be used to help resist compressive forces or to improve dynamic properties.
Steel usually is used in concrete. It is elastic, yet has considerable reserve strength beyond its elastic limit. Under a specific axial load, it changes in length only about one-tenth as much as concrete. In compression, steel is more than 10 times stronger than concrete, and in tension, more than 100 times stronger.
During construction, the bars are placed in a form and then concrete from a mixer is cast to embed them. After the concrete has hardened, deformation is resisted and stresses are transferred from concrete to reinforcement by friction and adhesion along the surface of the reinforcement. Individual wires or bars resist stretching and tensile stress in the concrete only in the direction in which such reinforcement extends. Tensile stresses and deformations, however, may occur simultaneously in other directions. Therefore reinforcement must usually be placed in more than one direction. For this purpose, reinforcement sometimes is assembled as a rectangular grid. Bars, grids, and fabric have the disadvantage that the principal effect of reinforcement occurs primarily in the plane of the layer in which they are placed. Consequently, the reinforcement often must be set in several layers or formed into cages. Under some conditions, fiber-reinforced concrete is an alternative to such arrangements. See Composite beam, Concrete, Concrete beam, Concrete column, Concrete slab, Prestressed concrete