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Concrete

Reinforced concrete is also a major structural material in these buildings. Indeed, outside of North America and western Europe, it is the dominant industrialized building material. Its component parts are readily available throughout the world at fairly low cost. Portland cement is easily manufactured by burning shale and limestone; aggregates such as sand and crushed limestone can be easily obtained. Steel minimills, which use scrap iron to feed their electric furnaces, can mass-produce reinforcing bars for regional use. In industrialized countries the mixing and delivery of liquid concrete to building sites has been mechanized with the use of central plants and mixing trucks, and this has substantially reduced its cost. In barely 100 years, reinforced concrete has risen from an experimental material to the most widespread form of building construction.

There are two methods of fabricating reinforced concrete. The first is to pour the liquid material into forms at the building site; this is so-called in situ concrete. The other method is called precast concrete, in which building components are manufactured in a central plant and later brought to the building site for assembly. The components of concrete are portland cement, coarse aggregates such as crushed stone, fine aggregates such as sand, and water. In the mix, water combines chemically with the cement to form a gel structure that bonds the stone aggregates together. In proportioning the mix, the aggregates are graded in size so the cement matrix that joins them together is minimized. The upper limit of concrete strength is set by that of the stone used in the aggregate. The bonding gel structure forms slowly, and the design strength is usually taken as that occurring 28 days after the initial setting of the mix. Thus there is a one-month lag between the time in situ concrete is poured and the time it can carry loads, which can significantly affect construction schedules.

In situ concrete is used for foundations and for structural skeleton frames. In low-rise buildings, where vertical gravity loads are the main concern, a number of framing systems are used to channel the flow of load through the floors to the columns for spans of six to 12 metres (20 to 40 feet). The oldest is the beam and girder system, whose form was derived from wood and steel construction: slabs rest on beams, beams rest on girders, and girders rest on columns in a regular pattern. This system needs much handmade timber formwork, and in economies where labour is expensive other systems are employed. One is the pan joist system, a standardized beam and girder system of constant depth formed with prefabricated sheet-metal forms. A two-way version of pan joists, called the waffle slab, uses prefabricated hollow sheet-metal domes to create a grid pattern of voids in a solid floor slab, saving material without reducing the slab’s strength. The simplest and most economical floor system is the flat plate, where a plain floor slab about 20 centimetres (eight inches) thick rests on columns spaced up to 6.7 metres (22 feet) apart. If the span is larger, the increasing load requires a local thickening of the slab around the columns. When these systems are applied to spans larger than nine to 12 metres (30 to 40 feet), a technique called posttensioning is often used. The steel reinforcing takes the form of wire cables, which are contained in flexible tubes cast into the concrete. After the concrete has set and gained its full strength, the wires are permanently stretched taut using small hydraulic jacks and fastening devices, bending the entire floor into a slight upward arch. This reduces deflection, or sagging, and cracking of the concrete when the service load is applied and permits the use of somewhat shallower floor members. Concrete columns are usually of rectangular or circular profile and are cast in plywood or metal forms. The reinforcing steel never exceeds 8 percent of the cross-sectional area to guard against catastrophic brittle failure in case of accidental overloading.

Precast concrete structural members are fabricated under controlled conditions in a factory. Members that span floors and roofs are usually pretensioned, another prestressing technique, which is similar in principle to posttensioning. The reinforcement is again steel wire, but the wires are put into tension (stretched) on a fixed frame, formwork is erected around the taut wires, and concrete is poured into it. After the concrete has set and gained its full strength, the wires are cut loose from the frame. As in posttensioning, this gives the precast floor members a slight upward arch, which reduces deflection and permits the use of shallower members. Precast prestressed floor elements are made in a number of configurations. These include beams of rectangular cross section, hollow floor slabs 15 to 30 centimetres (six to 12 inches) deep and spanning up to 18 metres (60 feet), and single- and double-stem T shapes up to 1.8 metres (six feet) deep and spanning up to 45 metres (150 feet). Precast concrete columns are usually not prestressed and have projecting shelves to receive floor members. At the building site, precast members are joined together by a number of methods, including welding together metal connectors cast into them or pouring a layer of in situ concrete on top of floor members, bonding them together. Precast prestressed construction is widely used, and it is the dominant form of construction in the Soviet Union and eastern Europe.

Masonry finds only a limited structural use in these buildings. Concrete block walls with brick facing and punched openings (discrete windows entirely surrounded by the facing material) spanned by concealed steel lintels can be used for exterior bearing walls where the interior is a skeleton frame of steel or timber. The use of interior bearing walls so greatly reduces the flexibility needed in these buildings that they are only rarely found.

Enclosure systems

Enclosure systems in these buildings range from rather simple forms in industrial uses to quite sophisticated assemblies in the commercial and institutional sectors. Most have in common the use of flat roofs with highly water-resistant coverings, the traditional one being a built-up membrane of at least four layers of coal-tar pitch and felt, often weighted down with a gravel ballast. Such roofs are pitched at slopes of 1 : 100 to 1 : 50 toward interior drains. In recent years the single-ply roof, made of plastic membranes of various chemistries, has found wide application. The seams between the pieces of membrane are heat- or solvent-welded together, and they are either ballasted with gravel or mechanically fastened to the underlying substrate, which is usually rigid foam insulation. Sometimes standing-seam sheet-metal roofs are also used; the best quality is continuously welded stainless steel.

The choice of transparent surfaces in these enclosures is based on three major considerations: conductive heat transfer, radiant energy transfer, and safety. All the transparent materials used in the low-rise residential sector are found, plus a number of others. In buildings with fully controlled atmospheres, double glazing is common to reduce heat transfer and both interior and exterior condensation on the glass. Commercial and institutional buildings tend to have large internal sources of heat gain, such as people and lighting, so it is desirable to exclude at least some solar gain through the transparent surfaces to reduce energy consumption in cooling. This can be done by reducing the light transmission or shading coefficient of the glass by integrally tinting it in various colours; grey, bronze, and green are common tints. This can also be accomplished by vacuum-plating partial reflective coatings of varying densities to an inner surface of double glazing; this can reflect up to 90 percent of the incident energy. Two kinds of reflecting metal are used: aluminum, which is silver in tone, and rubidium, which is gold-toned. These coatings are perceived as strong tints when the outside world is viewed through them by day: grey for aluminum and green for rubidium.

Skylights or horizontal transparent surfaces have found wide application in these types of buildings. These installations range from purely functional daylighting in industrial uses to elaborate aesthetic forms in commercial structures. In horizontal applications, and in vertical walls where people might blunder into glazed panels, safety glazing is required. Safety glazing is of four types: certain plastics that are flexible and difficult to break; wire-embedded glass, which holds together when broken; tempered glass, which is very strong and breaks into tiny and relatively harmless fragments; and laminated glass, which consists of two layers of glass heat-welded together by an intermediate plastic film. Laminated glass can also be made with tinted lamination film, producing many colours not available in integrally coloured glass.

Because many of these buildings have skeleton structures, their vertical surfaces are enclosed in nonstructural curtain walls that resist wind forces and provide weatherproofing. Curtain walls are of several types; the most common is one supported by a metal (typically aluminum) gridwork attached to the building structure. The vertical members, called mullions, are attached to the building at every floor and are spaced 1.5 to three metres (five to 10 feet) apart; the horizontal members, called muntins, are attached between the mullions. The rectangles between the grid of mullions and muntins are filled with transparent or opaque panels. The transparent surfaces can be any of those just described, and the opaque panels include opaque coloured glass, painted or anodized aluminum sheets, porcelain enameled steel sheets, fibreglass-reinforced cement, and stone wafers of granite, marble, or limestone cut with diamond-edged tools. All of these materials are usually backed up by rigid insulation to slow heat transfer. Metal sandwich panels are also used for economy of material; two thin layers of metal are separated by a core of different material, often with a high U-value for insulating effect. The separation of the thin layers of strong metal greatly increases the overall stiffness of the panel. The joints between panels and the supporting grid are weatherproofed with elastomeric sealants (cold-setting synthetic rubbers) or by prefabricated rubber gaskets. In glazed areas of curtain walls, mullions of structural glass are an alternative to metal mullions; they are more expensive, but they give an effect of greater transparency where this is desired.

Another type of curtain wall is the panel type. It has no gridwork of mullions and muntins but is made of large prefabricated rigid panels connected to the floors and spanning between them, with transparent openings made as holes cut out of the panel. The panels can be made of precast concrete, aluminum, or steel, often in sandwich form; elastomeric sealants are used to close the joints.

The finishes of metals in curtain walls include anodizing of aluminum, an electrolytic process that builds up the natural colourless oxide of aluminum into a thick adherent layer; it often includes the introduction of colour into the oxide layer itself. Durable paint coatings (with lifetimes of up to 40 years) can be applied to the metal in the factory; more conventional paints that must be renewed at shorter intervals are also used.

Interior finishes

Partitions

Space-division systems in these buildings make use of gypsum board partitions, usually applied to a framework of formed sheet-metal members attached to the building structure. They are readily demolished and rebuilt at relatively low cost, meeting the need for flexibility in such buildings. They are often used for fire-resistive protective enclosures, for which a number of layers are laminated to achieve the specified fire resistance. Transparent and translucent partitions are also used, with different types of glass set in metal frames. Office buildings may contain prefabricated movable metal partitions, which typically use metal sandwich panel construction to create panels with both transparent and opaque surfaces as well as doors. These partitions are expensive compared with gypsum board and must be moved often to justify the greater initial cost. Concrete block is used in unfinished spaces and for fire-resistive partitions. Glazed ceramic block or ceramic tile applied over concrete block or gypsum board is used in wet areas and where cleanliness is a problem, such as in kitchens and toilet rooms. Occasionally walls with wood paneling or stone veneer are used for aesthetic effect. Doors are usually set in formed sheet-metal frames, although some wood frames are used. The doors themselves are usually made of solid timbers glue-laminated together and covered with thin decorative wood veneers; painted hollow sheet-metal doors are used for exterior doors and in areas of hard use.

Ceiling finishes

Ceiling finishes in these buildings create a sandwich space below the roof or floor slab above, which conceals projecting structural elements, recessed light fixtures, electrical wiring conduits, and air-handling ductwork. The ceiling must be accessible to change or maintain the service elements located above it, and the most common ceiling system is composed of wet felted mineral fibre panels, painted and perforated on one side for sound absorption. The removable panels are supported on a grid of formed sheet-metal tee bars or zee tracks, which are suspended by wires from the structure above. Where accessibility is not important and a smooth finish is desired, suspended gypsum board ceilings can be used.

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