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Issues of ...
Issues of visual design and environmental design are introduced in the text, and case-studies are included throughout.
Throughout, the book is clearly illustrated with many photographs and diagrams showing materials and building components both individually and in use. Where relevant the environmental aspects of the building materials are considered. Each chapter lists the up-to-date British and European Standards together with related Building Research Establishment publications and suggested further reading. A selection of colour images illustrates the appropriate use of different construction materials within the context of quality architectural design. Essential reading for students of building, architecture and construction
Originally, bricks were hand-moulded from moist clay and then sun-baked, as is still the practice in certain arid climates. The firing of clay bricks dates back well over 5000 years, and is now a sophisticated and highly controlled manufacturing process; yet the principle of burning clay, to convert it from its natural plastic state into a dimensionally stable, durable, low-maintenance ceramic material, remains unchanged.
The quarrying of clay and brick manufacture are high-energy processes, which involve the emission of considerable quantities of carbon dioxide and other pollutants including sulphur dioxide. The extraction of clay also has long-term environmental effects, although in some areas former clay pits have now been converted to bird sanctuaries or put to recreational use. However, well-constructed brickwork has a long life with low maintenance, and although the use of Portland cement mortar prevents the recycling of individual bricks, the crushed material is frequently recycled as aggregate in further construction.
The elegant cathedral at Evry near Paris (Fig. 1.1), designed by Mario Botta, illustrates the modern use of brickwork. The cathedral of Saint Corbinian, built with 670,000 bricks, was dedicated in 1997. The building exhibits fine detailing both internally and externally. Externally the cylindrical form rises to a circle of trees. Internally the altar is surmounted by a corbelled structure leading one's view upwards to the central rooflight. Three-dimensional internal brickwork is finely detailed to generate the desired acoustic response.
The wide range of clays suitable for brick making in the UK gives diversity to the products available. This variety is further increased by the effects of blending clays, the various forming processes, the application of surface finishes and the adjustment of firing conditions. Earlier this century most areas had their own brickworks with characteristic products; however, ease of road transportation and continuing amalgamations within the industry have left a reduced number of major producers and only a few small independent works. Most UK bricks are defined as high density (HD) fired-clay masonry units with a gross dry density greater than 1000 kg/m3. The European standard (BS EN 771-1: 2003) also refers to low density (LD) fired-clay masonry units, and these blocks are described in Chapter 2.
The main constituents of brick-making clays are silica (sand) and alumina, but with varying quantities of chalk, lime, iron oxide and other minor constituents such as fireclay, according to their source. The largest UK manufacturer uses the Lower Oxford clays of Bedfordshire, Buckinghamshire and Cambridgeshire to produce the Fletton brick. This clay contains some carbonaceous content that reduces the amount of fuel required to burn the bricks, lowering cost and producing a rather porous structure. Other particularly characteristic bricks are the strongly coloured Staffordshire Blues and Accrington Reds from clays containing high iron content and the yellow London stocks from the Essex and Kent chalky clays with lower iron content.
Within Europe, the dimensions of clay masonry units (BS EN 771-1: 2003) have not been standardised, but in the UK, the standard metric brick referred to in the National Annex (informative) to BS EN 771-1: 2003 is 215 x 102.5 x 65 mm, although this size is not a specified requirement. Dimensions are given in the order, length, width and height, respectively. These UK dimensions match those in BS 4729: 2005, which relates to special shapes and sizes of bricks. The standard brick weighs between 2 and 4 kg, and is easily held in one hand. The length (215 mm) is equal to twice its width (102.5 mm) plus one standard 10 mm joint and three times its height (65 mm) plus two standard joints (Fig. 1.2).
The building industry modular co-ordination system (BS 6750: 1986) is based on the module (M) of 100 mm and multimodules of 3M, 6M, 12M, 15M, 30M and 60M. For metric brickwork, the base unit is 3M or 300 mm. Thus four courses of 65 mm brickwork with joints give a vertical height of 300 mm, and four stretchers with joints co-ordinate to 900 mm.
Table 1.1 illustrates the two types of dimensional tolerance limits set for clay masonry units including the metric brick, which relate to the square root of the work size dimension. Measurements are based on a random sample of ten bricks. The calculation based on the use of the square root of work size ensures that the dimensional tolerance limits are appropriate for the wide range in size of clay masonry units used within the European Union (BS EN 771-1: 2003).
Tolerance limits are set for the difference between the stated work size (e.g. 215, 102.5 and 65 mm) and the measured mean from the samples, for each of the three brick dimensions (length, width and height).
These are categorised as T1, T2 and Tm where Tm is a tolerance quoted by the manufacturer.
T1 ±0.40 √(work size dimension) mm or 3 mm if greater
T2 ±0.25 √(work size dimension) mm or 2 mm if greater
Tm deviation in mm declared by the manufacturer
The maximum range of size for any dimension is designated by categories R1, R2 and Rm.
R1 0.6 √(work size dimension) mm
R2 0.3 √(work size dimension) mm
Rm range in mm declared by the manufacturer
There is no direct correlation between the limits on mean value (T) and those for the range (R); thus, a brick conforming to category T2 may be within the wider range R1. Category R2 bricks may only be required for very tight dimensional control, as in short runs of brickwork.
The metric standard evolved from the slightly larger Imperial sizes, which varied significantly, but typically were 9x4 3/8 x 2 7/8 in (229 x 112 x 73 mm) or 8 5/8 x 4 1/8 x 2 5/8 in (219 x 105 x 67 mm). Some manufacturers offer a range of bricks to full Imperial dimensions, alternatively to an appropriate height (e.g. 50, 68, 70, 73, 76 or 80 mm) for bonding in to Imperial brickwork for restoration and conservation work.
The 1970s also saw the introduction of metric modular bricks with co-ordination sizes of either 200 or 300 mm in length, 100 mm in width and either 75 or 100 mm in height. The popularity of these bricks has now declined but they did give the architect opportunities for increasing or reducing horizontal emphasis and scale within the context of traditional brickwork.
A recent development has been the production of longer bricks to offer enhanced horizontality to brick facades (Fig. 1.3). Lengths up to 440 mm are available in a range of colours and textures for normal, quarter or third bonding, alternatively stack bonding in non-load-bearing situations. Sizes include 240 x 115 x 50 mm and 290, 327 and 440 x 102 x 50 and 65 mm.
MANUFACTURE OF CLAY BRICKS
There are five main processes in the manufacture of clay bricks:
extraction of the raw material;
packaging and distribution.
Extraction of the raw material
The process begins with the extraction of the raw material from the quarry and its transportation to the works, by conveyor belt or road transport. Topsoil and unsuitable overburden are removed first and used for site reclamation after the usable clay is removed.
The raw material is screened to remove any rocks, then ground into fine powder by a series of crushers and rollers with further screening to remove any oversize particles. Small quantities of pigments or other clays may be blended in at this stage to produce various colour effects; for example, manganese dioxide will produce an almost black brick and fireclay gives a teak brown effect. Occasionally, coke breeze is added into the clay as a source of fuel for the firing process. Finally, depending on the subsequent brick forming process, up to 25% water may be added to give the required plasticity.
The handmade process involves the throwing of a suitably sized clot of wet clay into a wooden mould on a bench. The surplus clay is struck off with a framed wire and the green brick removed. The bricks produced are irregular in shape with soft arrises and interestingly folded surfaces. Two variations of the process are pallet moulding and slop moulding.
In pallet moulding, a stock board, the size of the bed face of the brick, is fixed to the bench. The mould fits loosely over the stock board, and is adjusted in height to give appropriate thickness to the green brick. The mould and board are sanded to ease removal of the green brick which, is produced with a frog or depression on one face. In the case of slop moulding, the stock mould is placed directly on the bench, and is usually wetted rather than sanded to allow removal of the green brick which, unlike the pallet moulded brick, is smooth on both bed faces (Fig. 1.4).
Soft mud process
The handmade process has now been largely automated, with the clay being mechanically thrown into pre-sanded moulds; the excess clay is then removed and the bricks released from the mould. These soft mud process bricks retain much of the individuality associated with true handmade bricks, but at a lower cost.
In the semi-dry process used for Fletton bricks the appropriate quantity of clay is subjected to a sequence of four pressings within steel moulds to produce the green brick. These bricks usually have a deep frog on one bed face. For facing bricks, texturing on both headers and one stretcher may be applied by a series of rollers. A water spray to moisten the surface, followed by a blast of a sand/pigment mixture, produces the sand-faced finish.
With clays that require a slightly higher water content for moulding, the stiff plastic process is used in which brick-size clots of clay are forced into the moulds. A single press is then required to form the brick. Engineering bricks made by this process often have shallow frogs on both bed faces. In all cases the size of the mould is calculated to allow for the anticipated drying and firing shrinkage.
Extruded wire-cut bricks
In this process clay with a water content of up to 25% is fed into a screw extruder which consolidates the clay and extracts the air. The clay is forced through a die and forms a continuous column with dimensions equal to the length and width of a green brick (Fig. 1.5). The surface may then be textured or sanded, before the clay column is cut into brick units by a series of wires. The bed faces of wire-cut bricks often show the drag marks where the wires have cut through the extruded clay. Perforated wire-cut bricks are produced by the incorporation of rods or tines between the screw extruder and the die. The perforations save clay and allow for a more uniform drying and firing of the bricks without significant loss of strength. Thermal performance is not significantly improved by the incorporation of voids.
To prevent cracking and distortion during the firing process, green bricks produced from wet clays must be allowed to dry out and shrink. Shrinkage is typically 10% on each dimension depending upon the moisture content. The green bricks, laid in an open chequer-work pattern to ensure a uniform loss of moisture, are stacked in, or passed through, drying chambers which are warmed with the waste heat from the firing process. Drying temperatures and humidity levels are carefully controlled to ensure shrinkage without distortion.
Both intermittent and continuous kilns are used for firing bricks. The former is a batch process in which the single kiln is loaded, fired, cooled and unloaded. In continuous kilns, the firing process is always active; either the green bricks are moved through a fixed firing zone, or the fire is gradually moved around a series of interconnecting chambers to the unfired bricks. Both continuous systems are more energy efficient than the intermittent processes. Generally, for large-scale production, the continuous tunnel kiln (Fig. 1.6) and the Hoffman kiln (Fig. 1.7) are used. Clamps and intermittent gas-fired kilns are used for the more specialised products. Depending on the composition of the clay and the nature of the desired product, firing temperatures are set to sinter or vitrify the clay. Colour variations called kiss-marks occur where bricks were in contact with each other within the kiln and are particularly noticeable on Flettons.
In the tunnel kiln process the bricks are loaded 10 to 14 high on kiln cars which are moved progressively through the preheating, firing and cooling zones. A carefully controlled temperature profile within the kiln and an appropriate kiln car speed ensure that the green bricks are correctly fired with the minimum use of fuel, usually natural gas. The maximum firing temperature within the range 940°C to 1200°C depends on the clay, but is normally around 1050°C, with an average kiln time of three days. The oxygen content within the atmosphere of the kiln will affect the colour of the brick products. Typically a high temperature and low oxygen content are used in the manufacture of blue bricks. A higher oxygen content will turn any iron oxide within the clay red.
Introduced in 1858, the Hoffman kiln is a continuous kiln in which the fire is transferred around a series of chambers which can be interconnected by the opening of dampers. There may be 12, 16 or 24 chambers, although 16 is usual. The chambers are filled with typically 100,000 green bricks. The chambers in front of the fire, as it moves around, are preheated, and then firing takes place (960°-1000°C), followed by cooling, unloading and resetting of the next load. The sequence moves on one chamber per day, with three days of burning. The usual fuel is natural gas, although low-grade coal and landfill methane are used by some manufacturers.
Intermittent gas-fired kilns
Intermittent gas-fired kilns are frequently used for firing smaller loads, particularly specials. In one system, green bricks are stacked onto a concrete base and a mobile kiln is lowered over the bricks for the firing process. The firing conditions can be accurately controlled to match those within continuous kilns.
The basis of clamp firing is the inclusion of coke breeze into the clay, which then acts as the major source of energy during the firing process. In the traditional process, alternate layers of unfired bricks and additional coke breeze are stacked up and then sealed over with waste bricks and clay. The clamp is then ignited with kindling material and allowed to burn for two to five weeks. After firing, the bricks are hand selected because of their variability from under- to over-fired. Currently some handmade bricks are manufactured in gas-fired clamps which give a fully controlled firing process but still produce bricks with the characteristic dark patches on their surfaces due to the burnt breeze content.
Excerpted from MATERIALS FOR ARCHITECTS AND BUILDERS by ARTHUR LYONS Copyright © 2010 by Arthur Lyons. Excerpted by permission of Butterworth-Heinemann. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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