Reinforced-concrete Structural Members and Products

components of buildings and installations made from reinforced concrete; also combinations of these components. The high technical and economic indicators of reinforced-concrete structural members and products and the relative ease of imparting to them the required shape and size while maintaining a predetermined strength have resulted in their extensive use in virtually all branches of construction. Modern reinforced-concrete structural members and products are classified according to several attributes: the method of production (cast, precast, or a combination of the two), the type of concrete used in their manufacture (heavyweight, lightweight, cellular, and heat-resistant types), and the type of stressed condition (ordinary and prestressed).

Cast reinforced-concrete structural members. Cast reinforced-concrete structural members manufactured directly on construction sites are generally used for buildings and installations that are difficult to divide, in cases of nonstandard quality and low degree of replication of their components, and in cases of particularly large loads (foundations, frames, and ceilings of multistory industrial buildings; hydraulic-engineering, reclamation, and transportation installations). In a number of cases they are expedient for implementation of operations by means of industrial methods using standard timbering, which may be sliding, adjustable (towers, cooling towers, silos, smokestacks, and multistory buildings), or removable (certain thin-walled roof shells). The erection of cast structural members has been technically well developed. Significant achievements have also been made in the use of the prestressing method in production of cast structural members. A large number of unique installations, such as television towers, very tall industrial stacks, and the reactors of atomic power plants, have been built using cast rein-forced concrete. Cast reinforced-concrete structural members are widespread in the contemporary building practice of a number of capitalist countries, including the United States, Great Britain, and France; this is explained basically by the absence in these countries of a state system for standardization of parameters and classification of building and installation structural members by type. In the USSR, cast structural members predominated in construction until the 1930’s. The introduction of more industrial precast structures at that time was retarded by the insufficient level of mechanization of construction and by the absence of special equipment for their mass production and of high-productivity assembly cranes. Cast reinforced-concrete structural members account for about 35 percent of the total volume of reinforced-concrete production in the USSR (1970).

Precast reinforced-concrete structural members and products. Precast reinforced-concrete structural members and products are the main type of structural members and products used in various branches of construction (civil and housing, indus-trial, agricultural, and so on). Built-up structural members have essential advantages over cast structures, and they create extensive opportunities for industrialization of construction. The use of large reinforced-concrete components makes possible the transfer of basic operations from construction sites to plants with highly organized industrial production processes, thus considerably reducing construction time and ensuring higher quality of the products at lower cost and with lower labor expenditures. The use of built-up reinforced-concrete structural members permits extensive use of new, efficient materials, such as lightweight and cellular concrete and plastics, and lowers expenditures of lumber and steel, which are needed in other sectors of the economy. Built-up structural members and products must be technologically effective and easy to transport. They are especially well suited for a small number of component types that are used repeatedly.

The large-scale manufacture of precast reinforced concrete began in the USSR after the decree of the Central Committee of the CPSU and the Council of Ministers of Aug. 19, 1954, On the Development of the Production of Precast Reinforced-concrete Structural Members and Components for Construction. In recent years in the Soviet Union a large number of mechanized plants for reinforced-concrete structural members and products have been built in large cities and centers of concentrated construction. From 1954 through 1970 the manufacture of built-up reinforced concrete increased by a factor of 30; in 1970 it totaled 84 million cu m. The USSR has surpassed the most developed capitalist countries in the volume of use of precast reinforced-concrete structures. The production of reinforced-concrete structural members and products has been transformed into an independent sector of the building-materials industry. Increases in the production and use of precast reinforced concrete for construction have been accompanied by improvements in its manufacturing technology. Standardization of the main parameters for various types of buildings and installations has also been carried out. Types of structural members and products have been developed on the basis of such standardization.

The following most common precast reinforced-concrete structural members and products are distinguished, depending on their function in the construction of apartment, public, industrial, and agricultural buildings and installations: structures used for foundations and underground units of buildings and installations (foundation blocks and slabs; panels and blocks of basement walls), for building frames (columns, spandrel beams, girders, crane beams, rafter and subrafter beams, and trusses), for outer and inner walls (wall and divider panels and blocks), for floors between stories and for roofs of buildings (panels, slabs, and flagging), for staircases (stair risers and landings), and for sanitary-engineering equipment (heating panels, ventilation units and waste conduits, and toilet compartments).

Precast reinforced-concrete structural members and products are manufactured principally in mechanized enterprises and partly in casting yards. The process of production of reinforced-concrete products consists of a number of operations that are performed in sequence: preparation of the concrete mix, production of the reinforcement (reinforcing frames, grids, bent rods, and so on), reinforcement of the products, shaping of the products (pouring and thickening of the concrete mix), heat and moisture treatment to provide the required strength of the concrete, and finishing of the front surface of the products.

Three basic methods of organization of the production process may be distinguished in the modern technology of pre-cast reinforced concrete: the unit-flow method of producing articles in movable molds, the conveyor method of production, and the stand method in stationary molds.

In the unit-flow method, all technological operations—cleaning and lubrication of molds, reinforcement, shaping, hardening, and striking—are performed at specialized stations equipped with machines and units, forming a flow production line. The molds and products move from station to station along the operational line at arbitrary time intervals, which depend on the length of an operation at a given station and vary from several minutes (for example, the lubrication of molds) to several hours (the hardening of products in steaming compartments). This method is used profitably at medium-output plants, particularly in the manufacture of a wide variety of products.

The conveyor method is used at high-output plants that manufacture a limited variety of products of the same type. Using this method, the operational line works according to the principle of a pulsing conveyor—that is, the molds and products move from station to station over a strictly defined time interval required for performance of the longest operation. A variation of this technology is the method of vibration-rolled concrete, which is used in manufacturing flat and ribbed slabs; in this case all production operations are performed on a single moving steel belt. In the stand method, the articles remain in place (in a stationary mold) during the process of production and until the concrete has hardened, whereas the production equipment for performing separate operations is moved from one mold to another. This method is used in manufacturing items with large dimensions (trusses, beams, and so on). Matrices—reinforced-concrete or steel molds that reproduce the impression of a product’s ribbed surface—are used to shape products with complex configuration, such as stair risers and ribbed slabs. The caisson method is a variety of the stand method in which the items are made in vertical molds (caissons) that consist of a series of compartments formed by steel walls. The products are molded and hardened in the caisson unit, which has equipment for heating the products with steam or electric current, thus considerably accelerating the setting of the concrete. The caisson method is generally used for mass production of thin-walled products.

The finished products must satisfy the requirements of current standards or technical conditions. The surfaces of the products are usually produced with a degree of factory preparation that does not require additional finishing at the construction site. During assembly, precast components of buildings and installations are joined to each other by casting or welding of laid elements designed to take certain force effects. Considerable attention is devoted to reduction of the metal content of welded joints and to their standardization. Precast structural members and products have become most widespread in housing and civil construction, where large-component apartment building (large-panel, large-block, and volumetric) is seen as the most promising. Mass production of articles from built-up reinforced concrete has also been organized for engineering structures (so-called special reinforced concrete), including bridge spans, supports, piles, water-conducting pipes, flumes, blocks and tubing for lining tunnels, slabs for road and airfield surfaces, ties, supports for traction power-supply systems and power-transmission lines, components of barriers, and pressurized and nonpressurized pipes. A large proportion of these products is produced from prestressed reinforced concrete by the stand or unit-flow methods. Extremely effective methods are used to mold and thicken concrete: vibration molding (pressurized pipes), centrifugation (pipes and supports), and vibration stamping (piles and flumes).

The tendency toward larger products and an increase in the degree of their factory preparation is characteristic of the development of precast reinforced concrete. For example, multilayer panels delivered to construction sites complete with heaters and layers of waterproofing, as well as blocks 3 × 18 m and 3 × 24 m, which act as both supporting and protective structures, are used for the roofs of buildings. Joggled roofing slabs made of lightweight and cellular concrete have been developed and are being used successfully. Prestressed reinforced-concrete columns several stories high are being used in multistory buildings. Panels one to two rooms in size, with various outer finishes and equipped with window or door (balcony) units, are being manufactured for the walls of apartment buildings. The method of erecting buildings from modular construction units has considerable potential for further industrialization of housing construction. Such units for one and two rooms or for an entire apartment, with full interior finishing and equipment, are produced at plants. The assembly of apartment buildings from the components takes only a few days.

Precast-cast reinforced-concrete. Precast-cast reinforced-concrete structural members are combinations of built-up components (reinforced-concrete columns, crossbars, and slabs) and cast concrete that provide reliable joint operation of all integral units. They are used principally for the roofs of multistory buildings, in bridges and overpasses, and during the erection of certain types of casings. They are less industrial (in relation to erection and assembly) than built-up structures. Their use is particularly advantageous if there are large dynamic loads, including seismic loads, or if the division of large structures into component parts is necessary because of transport and assembly conditions. The chief virtues of precast-cast structures are reduced expenditure of steel and higher spatial rigidity as compared to precast structures.

Most reinforced-concrete structural members and products are prepared from heavyweight concrete with a volumetric mass of 2,400 kg/m³. However, the proportion of products made from thermal-insulation and lightweight structural concrete with porous fillers, as well as from cellular concrete of all kinds, is continually increasing. Such products are used primarily as protective structures (walls and roofs) of apartment and production buildings. Supporting structures made from high-strength heavyweight concrete of grades 600–800 and from lightweight concrete of grades 300–500 have considerable potential. Essential economic effects are achieved through the use of structures made from heat-resistant concrete (instead of piece refractory substances) for thermal units of metallurgical, petroleum-refining, and other industrial sectors. The use of stretching concrete for a number of products (for example, pressurized pipes) is promising.

Reinforced-concrete structural members and products are generally made with flexible reinforcement in the form of separate rods, welded grids, and flat frames. The use of pres-sure contact welding, which assures a high degree of industrialization of reinforcing operations, is expedient in producing nonstretching reinforcement. Structures with supporting (rigid) reinforcement are used rather infrequently, primarily in cast reinforced concrete during concreting in a suspension form. In deflected components longitudinal operating reinforcement is mounted according to the curve of maximum bending moments; in columns, longitudinal reinforcement takes primarily compressive stresses and is situated along the perimeter of the cross section. In addition to longitudinal reinforcement, distribution, assembly, and transverse reinforcement (collars and bends) is installed in reinforced-concrete structural members and products. In certain cases so-called indirect reinforcement in the form of welded grids and coils is specified. All of these varieties of reinforcement are joined together and create a reinforcement frame, which is spatially unaltered during the concreting process. High-strength bar reinforcement and wire, as well as strands of rope and cables, are used for nonstretching reinforcement of prestressed reinforced-concrete structural members and products. The method of stretching the reinforcement against supports of stands or molds is used in producing precast structures; in producing precast-cast structural members, the reinforcement is stretched against the concrete of the structure itself. Methods of designing and constructing reinforced-concrete structural members and products have been developed in detail and published as normative documents in the USSR. Numerous aids in the form of instructions, directions, and supplementary tables have been established for project engineers.

The extensive formative and technical potentials of reinforced-concrete structural members have been extremely influential in 20th-century world architecture. New scales, architectonics, and spatial organization of buildings and installations have evolved based on reinforced-concrete structures. Rectilinear frame structures impart to buildings a strict geometry of form, a measured rhythm of segmentation, and a clarity of structure. Horizontal roofing slabs rest on thin supports; light walls, deprived of their supporting function, are frequently transformed into glass screens. The uniform distribution of static forces creates tectonic equivalence of the structure’s components. Curved structures have considerable plasticity and spatial expressiveness (particularly thin-walled casings of various, sometimes fantastic contours) with their complex tectonics of forms (sometimes close to sculpture) and continually changing rhythms of the components. Curved structures make it possible to cover extremely large halls without intermediate supports and to create spatial and volumetric compositions of unusual shape. Certain modern reinforced-concrete structures (for example, latticed structures) have ornamental and decorative qualities that create the appearance of facades and coverings. Modern reinforced-concrete structural members impart aesthetic expressiveness not only to apartments and public buildings but also to engineering and industrial installations, such as bridges, piers, dams, and cooling towers.

New, progressive methods of using reinforced-concrete structural members and products in housing construction and civil engineering (for example, construction from modular units or based on a catalog of standardized industrial construction products) create opportunities for rich diversification in planning buildings and their spatial and volumetric structure.