Brick
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- This page is about clay bricks used for construction. For other uses of the word "brick" please see Brick (disambiguation).
Brick is an artificial stone made by forming clay into rectangular blocks which are hardened, either by burning in a kiln or sometimes, in warm countries, by sun-drying.
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[edit] History
In the Near East and India, bricks have been in use for more than five thousand years. The Tigris-Euphrates plain lacks rocks and trees. Sumerian structures were thus built of plano-convex mudbricks, not fixed with mortar or with cement. As plano-convex bricks (being rounded) are somewhat unstable in behaviour, Sumerian bricklayers would lay a row of bricks perpendicular to the rest every few rows. They would fill the gaps with bitumen, straw, marsh reeds, and weeds.
The Ancient Egyptians and the Indus Valley Civilization also used mudbrick extensively, as can be seen in the ruins of Buhen, Mohenjo-daro and Harappa, for example. In the Indus Valley Civilization particularly, all bricks corresponded to sizes in a perfect ratio of 4:2:1, and made use of the decimal system. The ratio for brick dimensions 4:2:1 is even today considered optimal for effective bonding.
The Romans made use of fired bricks, and the Roman legions, which operated mobile kilns, introduced bricks to many parts of the empire. Roman bricks are often stamped with the mark of the legion that supervised its production. The use of bricks in Southern and Western Germany, for example, can be traced back to traditions already described by the Roman architect Vitruvius.
In the 12th century, bricks from Northern Italy were re-introduced to Northern Germany, where an independent tradition evolved. It culminated in the so-called brick Gothic, a reduced style of Gothic architecture that flourished in Northern Europe, especially in the regions around the Baltic Sea which are without natural rock resources. Brick Gothic buildings, which are built almost exclusively of bricks, are to be found in Denmark, Germany, Poland and Russia. However, bricks were long considered an inferior substitute for natural rock.
During the Renaissance and the Baroque, visible brick walls were unpopular and the brickwork was often covered with plaster. It was only during the mid-18th century that visible brick walls regained some degree of popularity, as illustrated by the Dutch Quarter of Potsdam, for example.
The transport in bulk of building materials such as bricks over long distances was rare before the age of canals, railways, roads and heavy goods vehicles. Before this time bricks were generally made as close as possible to their point of intended use (it has been estimated that in England in the eighteenth century carrying bricks by horse and cart for ten miles over the poor roads then existing could more than double their price).
The use of brick in construction was not limited solely to regions that lacked stone or other materials suitable for building, bricks were often used, even in areas where stone was available, for reasons of speed and economy. the buildings of the Industrial Revolutin in Britain were largely constructed of brick and timber due to the unprecedented demand created. Again, during the building boom of the nineteenth century in the eastern seaboard cities of Boston and New York, for example, locally made bricks were often used in construction in preference to the brownstones of New Jersey and Connecticut for these reasons.
The trend of bulding upwards for offices that emrged towards the end of the 19th century displaced brick in favor of cast and wrought iron and later steel and concrete. Some early 'skyscrapers' were made in masonry, and demonstrated the limitations of the material - for example, the Monadnock Building in Chicago (opened in 1896) is masonry and just sixteen stories high, the ground walls are almost 1.8 meters thick, clearly building any higher would lead to excessive loss of internal floor space on the lower floors. Brick was revived for high structures in the 1950s following work by the Swiss Federal Institute of Technology and the Building Research Station in England. This produced eighteen story structures with bearing walls no thicker than a single brick (150-225 mm). This potential has not been fully developed because of the ease and speed in building with other materials, in the late-20th century brick was confined to low- or medium-rise structures or as a thin decorative cladding over concrete-and-steel buildings or for internal non-loadbearing walls.
[edit] Methods of manufacture
Bricks may be made from clay, shale, soft slate, calcium silicate, concrete, or shaped from quarried stone.
Clay is the most common material, with modern clay bricks formed in one of three processes - soft mud, dry press, or wire cut.
The soft mud method is the most common, as it is the most economical. It starts with the raw clay preferably in a mix with 25-30% sand to reduce shrinkage. The clay is first ground and mixed with water to the desired consistency for forming in a mould. The clay is pressed into steel moulds with a hydraulic press. The shaped clay is then fired ("burned") at 900-1000°C to achieve strength. In modern brickworks, this is usually done in a continuously fired tunnel kiln, in which the bricks move slowly through the kiln on conveyors, rails, or kiln cars to achieve consistent physical characteristics for all bricks. The bricks often have added lime, ash and organic matter to speed the burning.
The dry press method is similar but starts with a much thicker clay mix, so forming more accurate, sharper-edged bricks. The greater force in pressing and the longer burn make this method more expensive.
In wire cut the clay mix is 20-25% water, this is forced through a die to create a long cable of material of the demanded width and depth. This cable is then cut into bricks of the desired length by a wall of wires. The majority of structural bricks are made by this method as hard dense bricks are the result and any needed holes or other perforations can be introduced by the die. The introduction of holes reduces the needed volume of clay through the whole process, with the consequent reduction in costs per brick. The bricks are also lighter and so easier to handle and have different thermal properties compared to solid bricks. The cut bricks are hardened by drying for between 20 and 40 hours at 50-150°C before being burned. The heat for the drying is often the waste heat of the burning process.
The color of the finished bricks depends on the mineral content of the bricks and the temperature at which they are burned. Pink bricks are the result of a high iron content, white or yellow bricks have a higher lime content. Most bricks burn to various red hues, if the temperature is increased the color moves through dark red, purple and then to brown or grey at around 1300°C. A wider range of shades is produced from calcium silicate bricks.
Bricks made from calcium silicate contain a mixture of lime with either sand, crushed flint or crushed siliceous rock and mineral colorants. The materials are mixed and left until the lime is completely hydrated, the mixture is then pressed into moulds and cured in an autoclave for two or three hours to speed the chemical hardening. The finished bricks are very accurate and uniform, although the sharp arrises need careful handling to avoid damage to brick (and brick-layer). The bricks can be made in a variety of colors, white is common but a wide range of "pastel" shades can be achieved.
Bricks formed fro concrete are usually termed blocks, and are typically pale grey in color. They are made from a dry, small aggregate concrete which is formed in steel moulds by vibration and compaction in either an "egglayer" or static machine. The finished blocks are cured rather than burned using low-pressure steam. Concrete blocks are manufactured in a much wider range of shapes and sizes than clay bricks and are also available with a wider range of face treatments - a number of which are to simulate the appearence of clay bricks.
An impervious and ornamental surface may be laid on brick either by salt glazing, in which salt is added during the burning process, or by the use of a "slip," which is a glaze material into which the bricks are dipped. Subsequent reheating in the kiln fuses the slip into a glazed surface integral with the brick base.
Natural stone bricks are of limited modern utility, due to their enormous comparative mass, the consequent foundation needs, and the time-consuming and skilled labour needed in their construction and laying. They are however very durable and considered more handsome than clay bricks. Only a few stones are suitable for bricks, common materials are granite, limestone and sandstone. Other stones may be used (e.g. marble, slate, quartzite, etc.) but this tend to be limited to a particular locality.
[edit] Dimensions, characteristics and strength
For efficient handling and laying bricks must be small enough and light enough to be picked up by the bricklayer using one hand (leaving the other hand free for the trowel). Bricks are usually laid flat and as a result the effective limit on the width of a brick is set by the distance which can conveniently be spanned between the thumb and fingers of one hand, normally about four inches (about 100 mm). In most cases, the length of a brick is about twice its width, about eight inches (about 200 mm). This allows bricks to be laid bonded in a structure to increase its stability and strength (for an example of this, see the illustration of bricks laid in English bond, at the head of this article. The wall is built using alternating courses of stretchers, bricks laid longways and headers, bricks laid crossways. The headers tie the wall together over its width.
The correct brick for a particular job can be picked from a choice of variables in color, surface texture, density, weight, absorption and pore structure, thermal characteristics, thermal and moisture movement, and fire resistance
In England, the length and the width of the common brick has remained fairly constant over the centuries, but the depth has varied from about two inches (about 50 mm) or smaller in earlier times to about two-and-one-half inches (about 65 mm) in more recent times. In the USA modern bricks are usually about 8 x 4 x 2.25 inches (203 x 102 x 57 millimeters) in size. In the UK the usual ("work") size of a modern brick is 215 x 102.5 x 65 mm (about 8.5 x 4 x 2.5 inches) which, with a nominal 10 mm mortar joint, forms a "coordinating" or fitted size of 225 x 112.5 x 75 mm (i.e. a ratio of 6:3:2).
Blocks have a much greater range of sizes. Standard coordinating sizes in length and height (in mm) include 400x200, 450x150, 450x200, 450x225, 450x300, 600x150, 600x200, and 600x225; depths (work size, mm) include 60, 75, 90, 100, 115, 140, 150, 1990, 200, 225, and 250. They are usable across this range as they are lighter than clay bricks; solid clay bricks are around 2,000 kg/m³, this is reduced by forgging, hollow bricks etc., but aerated autoclaved concrete, even as a solid brick, can have densities in the range of 450-850 kg/m³.
Bricks may also be classified as solid (less than 25% perforations by volume, although the brick may be "frogged," having indentations on one of the longer faces), perforated (containing a pattern of small holes through the brick removing no more than 25% of the volume), cellular (containing a pattern of holes removing more than 20% of the volume, but closed on one face), or hollow (containing a paattern of large holes removing more than 25% of the brick's volume). Blocks may be solid, cellular or hollow
The compressive strength of bricks produced in the USA ranges from about 1000 lbf/in² to 15,000 lbf/in² (7 to 105 megapascals), varying according to the use to which the brick are to be put. In England clay bricks can have strengths of up to 100 N/mm², although a common house brick is likely to show a range of 20-40 N/mm²
[edit] Use
Bricks are used for building and pavement. In the USA, brick pavement was found incapable of withstanding heavy traffic, but it is coming back into use as a method of traffic calming or as a decorative surface in pedestrian precincts.
Bricks are also used in the metallurgy and glass industries for lining furnaces. They have various uses, especially refractory bricks such as silica, magnesia, chamotte and neutral (chromomagnesite) refractory bricks. This type of brick must have good thermal shock resistance, refractoriness under load, high melting point, and satisfactory porosity. There is a large refractory brick industry, especially in the United Kingdom, Japan and the U.S.A..
In the United Kingdom, bricks have been used in construction for centuries. Until recently, many houses were built almost entirely from red bricks. This use is particularly prevalent in areas of northern England and some outskirts of London, where rows of terraced houses were rapidly and cheaply built to house local workers [citation needed]. These houses have survived to the present day, but some are in need of attention as their structure has deteriorated. Although many houses in the UK are now built using a mixture of breeze blocks and other materials, many houses are skinned with a layer of bricks on the outside for aesthetic appeal.
[edit] See also
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[edit] Gallery
 Brick sculpturing on Thornbury Castle, Thornbury, near Bristol, England. The chimneys were erected in 1514 |
 Brick sculpturing |
