Sunday, June 22, 2014

CLAY or MUD STRUCTURES - part - 3


FOUR: WALLS WITH FIRED CLAY SOLID BLOCKS

Chandigadh India Housing Bricks façade


Mud wall systems have one major drawback, their susceptibility to water. Other issues are their low load bearing capacity and heavy dead weight due to the substantial thickness. Mud wall systems are labour intensive and require frequent upkeep.
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Mud blocks when fired in kiln at various temperatures and duration turn into ceramic product -the bricks
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Bricks are the prime building blocks for constructing walls, roofs, lintels, floors and pavements. Bricks or similar blocks used in nearly every geographic region of the world. The materials used for making such blocks include clay, china clays , pozzolanic ash, cement and lime. Some of these materials are mined from nature or reprocessed. Brick are solids or hollowed masses. Bricks are cast in form-work or extruded. Bricks have many different sizes and shapes. Basically bricks have two faces, in wall construction the larger surface is placed parallel to the gravity. Exception to these rule include bricks used for arches, lintels or floors -where the depth of the block remains a critical consideration.
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Ziggurat Brick work

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Brick are used with or without the joint materials. Joint-less construction or composition exploits the flat placement -or stability offered by the pull of gravity. Intermediate fillers and joining materials are used for bonding and also for levelling the surface.
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Joint materials used for bonding are generically called cements. These materials provide bonding by surface attraction, phase change (liquid to solid) and chemical change. Some of these materials achieve bonding after a period of setting and hardening, or achieve an instant bond. Bricks in thin form -as tiles, are often mechanically joined by screwing, clamping, etc. Such joints are used mainly for cladding or surface applications.
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Bricks are cast mainly from soils with high content of alumina and free silica. Though calcareous, chalky and high silica (fire clay) soils are also used for many special types of bricks (kiln lining fire-bricks). 
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Sanskar Kendra Ahmedabad by Le Corbusier

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Bricks get their primary toughness due to removal of chemically bound water from the constituents of soil, oxidation of salts, and vitrification of silica matters. At temperatures above 300° C, some of the constituents start flowing producing a glassy mass. These and some of the oxides bind together all other materials, forming a hard mass. Later, on soaking the hydration of silicates and aluminates provide the extra strength. Over the years gradual carbonation of hydrated lime provides additional hardness. 
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Colour of the brick, an important criteria for surface finishing is influenced by contents of the soils, range and duration of burning temperature, ventilation system of kiln, casting techniques etc.
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Bricks are made from high alumina clays with small amount of colloidal or free silica. Many other additives are also used primarily to adjust the plasticity of the mass. Some of these additives are organic (grass, hay, rice husk in nature, which on burning impart porosity. Rice husk in addition provides siliceous substances which in combination with lime impart pozolana like cementitious bond. Powder of burnt broken bricks is added to clays in place of free silica sand to reduce shrinkage cracks during drying and burning.
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Bricks are variously permeable. Predominantly alumina bricks are more permeable than silica bricks. Under burnt and low temperature fired bricks are more absorbent compared to over burnt and high temperature (refractory) bricks. Hand pressed bricks (table mould) are less compact than machine made bricks, and as a result absorb more water. Hollow and perforated bricks are extrusion machine cast from a very wet and plastic mass. Roofing tiles and facing brick tiles are die mould cast with minimal mixing water and very high impact-pressure.

Indian Institute of Management Ahmedabad by L Kahn

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Bricks of exposed masonry surface, if permeable allow bacterial growth such as mould, fungi etc. on the surface. Soluble salts present in the clay, usually get decomposed during the burning but immediately after highest temperature of firing and while cooling, sulphate of sodium, calcium, potassium and magnesium are formed with the help of sulphur from the fumes of the fuels. These salts on contact with absorbed moisture leach out on the surface. Most of the sulphate get washed away from the masonry surface, but magnesium sulphate does not leach out readily. It expands and causes cracks in bricks. Calcium sulphate though difficultly leached out, settles on the surface to form whitish scum
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Porous and rough brick surfaces are better for mortar adhesion than impervious smooth surface of a very vitrified brick, Over burnt or highly vitrified bricks have very low suction capacity for mortar binding. Over burnt bricks are dimensionally deformed due to running of the mass and unsuitable for masonry work.
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Next to the colour, texture and quality of the brick the masonry surface is characterized by the colour, material and technique of its joints. Jointing and pointing material for brick surfacing have to surpass the overall performance of bricks. High adhesion, low permeability and suitable colour matching are some of the attributes of a good jointing-pointing material. Flushed, projected and grooved pointing are used for brick work, of which flushed with string mark type of pointing is best. Unlike the projected or grooved pointing, flushed pointing does not retain dirt or water in its holds. The string marks are adequate guide path for any hair crack that may develop.
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