Natural stone is perhaps the oldest, most abundant and most durable "readymade" building material, found predominantly in hilly areas. Various types and forms of natural stone can also be processed to produce other building materials.
The main stones used in building are divided into three geological categories:
1. Igneous rocks, generally crystalline, formed by the cooling of molten magma forced up through cracks in the earth's crust. It, therefore, cannot contain fossils or shells. Most common examples: granites and volcanic stones.
2. Sedimentary rocks, commonly found in layers, formed by the disintegration and decomposition of igneous rocks due to weathering (water, wind, ice), or by accumulations of organic origin. Most common examples: Sandstones and limestones.
3. Metamorphic rocks, which are structurally changed igneous or sedimentary rocks, caused by immense heat and pressure. Most common examples: Slates (derived from clay), quartzites (from sandstone) and marble (from limestone).
Extraction of rocks is possible with simple tools such as drills, wedges and hammers, but skill and experience is essential to ensure accurate cuts. Harder rocks, such as granite, require more sophisticated mechanized equipment. Natural stone can be used as quarried, ie irregularly shaped, or can be shaped with simple tools or machines, depending on the ultimate construction. The material can be used completely, without wastage.
· Rubble (undressed stone) for foundations, floors, walls, or even corbelled roof structures, in all cases with or without mortar.
· Ashlar (squared or shaped stone) for regular course masonry, window sills, lintels, steps and paving.
· Impermeable stone (eg granite) as damp proof courses; also as external cladding of walls, though less suited for low-cost constructions.
· Slate for roofing.
· Gravel and stone chippings as aggregate for concrete and terrazzo.
· Granules for surfacing bituminous felts.
· Powders for extending paint.
· Limestone for lime and cement production.
BUILDING STONE MATERIALS AND APPLICATIONS
(from United Natons: Stone in Nepal, 1977)
Walling end Cladding
Walling, Cladding plinths, surrounds and steps
Largely calcium carbonate
Quartz in all mica and felspar grains in some. Bonded largely with silica or calcium carbonate
Mainly felspar, quartz and mica
Method of production
Quarried, cut to size (masoning and sawing), finish as required, eg patterned, rock faced, fair picked, fine axed, rubbed, eggshell or polished
Specific weight kg/m3
1900 - 2700
1950 - 2550
2400 - 2900
Compressive strength MN/m²
9 - 59
Water absorption %
2.5 - 11
2 - 8.5
0.1 - 0.5
Effect of fire
Moisture expansion %
Effect of chemicals
Attacked by acids
Resistant to most acids except calcareous types which are attacked
Resistant to most chemicals
Resistance to effect of soluble salts
Poor to very good
Poor to good
Poor to good
Thermal expansion co-efficient (per °C approximations)
4 x 10-6
12 x 10-6
11 x 10-6
Thermal conductivity (W/m.°C approximations)
Resistance to frost
Poor to very good
Poor to excellent
Good to excellent
Dependent on thermal performance, resistance to chemicals and application in construction
Ease of working
Easy to hard
Liability to become dirty
Become soiled in urban atmosphere
Resistant to soiling
Ease of cleaning
Fairly easy to clean
Difficult to clean
Difficult to clean
Window surround, floors and stairs
Cladding sills, coping steps and paving
Cladding plinths, floors, paving and stairs
Mainly calcium carbonate
Mainly silica, alumina and iron oxides
Same as limestone, sandstone, granites
Finish natural, riven
2725 - 2900
2400 - 2900
75 - 200
0.1 - 0.5
0.1 - 0.5
Attacked by acids
Mainly resistant to acids
Resistant to most acids
11 x 10-6
11 x 10-6
Good to excellent
Good to excellent
Good to excellent
Dependent on thermal performance, resistance to chemicals and application in construction
Fairly resistant to soiling
Resistant to soiling
Difficult to clean
· Usually abundantly and easily accessible in hilly regions; extraction generally requiring low investment cost and energy input.
· Immense strength and durability of most varieties of stone; negligible maintenance requirements.
· Impermeability of most stone varieties, providing good rain protection.
· Climatically appropriate in highland and arid zones, due to high thermal capacity of stone.
· Deterioration may result from atmospheric pollution, eg when sulphur compounds dissolved in rainwater produce sulphuric acid, which reacts with carbonates in limestones, causing skin formation and blisters.
· Efflorescence and spelling caused by certain salts and sea spray.
· Damage due to thermal movement of some stones, especially when fixed rigidly to materials with differing thermal movement, eg concrete.
· Surface damage due to water, which slowly dissolves limestones; or by prolonged wetting and drying of certain sandstones; or by freezing of water trapped in cracks.
· Low resistance to earthquake forces, thus likelihood of destruction and endangering lives.
· Avoidance of using limestones and calcareous sandstones close to sources of atmospheric pollution eg where sulphur dioxide is emitted (from burning coal and oil).
· Avoidance of surface treatments that seal in salts; occasional sponging of affected stones helps to remove salts, especially in coastal areas.
· Construction of movement joints to accomodate differences between the thermal movements of adjoining materials.
· Construction details that will allow water to be removed by evaporation or drainage, to avoid frost damage or washing out of limestones.
· Careful building design, especially with corner reinforcements, ring beam, etc., in earthquake prone areas; especially avoidance of stone vaults or corbelled roofs.
When referring to earth or soil in building construction, both terms mean the same material. Mud is a wet, plastic soil mixture, with or without additives, which is used to make mud bricks (adobe) or monolithic mud walls.
Soil is the loose material that results from the transformation of the underlying parent rock by the more or less simultaneous interaction of climatic factors (sun, wind, rain, frost) and chemical changes, brought about by biological agents (flora and fauna) and migration of chemical substances through rain, evaporation, surface and underground water.
Of the various soil types that occur in the tropics and sub-tropics, laterites are of special interest in conjunction with building construction. These are highly weathered soils, which contain large, though extremely variable, proportions of iron and aluminium oxides, as well as quartz and other minerals. They are found abundantly in the tropics and sub-tropics, where they generally occur just below the surface of wide grasslands or forest clearings in regions with high rainfall. The colours can vary from ochre through red, brown, violet to black, depending largely on the concentration of iron oxides.
The special characteristics of laterites, by which they differ from other soils, are:
· Soft occurances tend to harden
on exposure to air, which is why blocks have traditionally (eg in India) been
cut in situ, allowed to harden and then used for masonry wall construction
(hence the name was derived from "later", the latin word for "brick").
· The darker the laterite, the harder, heavier and more resistant to moisture it is.
· Some laterites are found to have a pozzolanic reaction when mixed with lime (which can be explained by the high clay content), producing hard and durable building materials (eg stabilized blocks).
However, irrespective of the type of soil, it is always composed of particles of different size and nature, as summarized in the following chart.
60 to mm
Coarse pieces of rocks like granite, lime, marble, etc., of any shape (round, flat, angular). Gravel forms the skeleton of the soil and limits its capillarity and shrinkage.
2 to 0.06 mm (ie the smallest grain size that can be discerned by naked eye).
Particles mainly comprising silica or quartz; beach sands contain calcium carbonate (shell fragments). Sand grains lack cohesion in the presence of water, and limit swelling and shrinkage.
0.06 to 0.002 mm
Physically and chemically the same as sand, only much finer. Silt gives soil stability by incresing its internal friction, and holds together when wet and compressed.
Smaller than 0.002 mm (2 m)
Clay results from chemical wathering of rocks, mainly silicates. The hydrated aluminosilicate particles are thin plates of extremely great specific surface area, causing strong cohesion in the presence of water, alsoexcessive swelling and shrinkage.
Smaller than 0.002 mm (2 m)
Fine particles resulting from decomposition of minerals and organic matter (clay is the chief mineral colloid), forming a gluey substance.
Several mm to several cm
Micrograins and fibres resulting from decomposition of plants and soil fauna. It has a spongy or stringy structure and smell like wet decaying wood.
In addition to the solid particles, soil also comprises:
· Air, which is a weakening factor and undesirable in building construction, as it also entraps micro-organisms and water vapour, both of which can cause deterioration of the building component.
· Water, without which the soil cannot be used for building, but which can carry dissolved substances (salts) that may create problems.
Most soils are suitable for use as building materials, though in various cases, the addition or removal of certain constituents is required to improve their quality. Several tests need to be carried out in order to identify the characteristics of the soil and its appropriateness for building construction. The procedures are described under Soil Testing.
It must be stressed that, contrary to common belief, building with earth is not a simple technology. The mere fact that natives of many countries have been building their houses with earth since thousands of years does not mean that the technology is sufficiently developed or known to everyone. It is indeed the lack of expertise that brings about poor constructions, which in turn gives the material its ill reputation. However, with some guidance, virtually anyone can learn to build satisfactorily with earth, and thus renew confidence in one of the oldest and most versatile building materials.
Soil constructions are found in all parts of the world, though to a lesser extent in areas of extreme rainfall.
Buildings can consist entirely or partially of soil, depending on the location, climate, available skills, cost and use of the buildings. The construction can be monolithic or made of various components (bricks, renders, infills).
In areas where there is a large diurnal temperature variation (arid zones or highlands) the walls and roofs are preferably thicker than in more uniform climates (humid zones), where the need for materials of high thermal capacity is less.
The various earth construction methods (Bibl. 02.19)
Soil can be used for all major parts of the building:
· Hard varieties of laterite, with good particle size distribution (sand to gravel), lightly compacted, for small buildings in dry regions.
· Similar laterite as aggregate in concrete.
· Stabilized air-dried soil blocks, with 10 % cement content, laid in laterite-cement mortar, only in dry regions.
· Base course same as for foundations.
· Direct moulding, without shuttering, just by pressing moist earth by hand.
· Rammed earth construction by tamping lightly moistened soil in shuttering (similar to concrete) for monolithic walls. Stabilization with straw, cement, lime, bitumen, cow dung, etc. as required.
· Straw clay construction, similar to rammed earth, but with straw (any kind) as the major ingredient and clay as the binder. (Good thermal insulation, eg for highland regions).
· Daubed earth applied on a supporting substructure, eg wooden or bamboo frame with wickerwork or plaited straw (wattle and daub).
· Masonry constructions, using air-dried mud blocks (adobe) laid in a mud mortar (with addition of some sand). Rain protective rendering required.
· Masonry constructions, using compressed, air-dried stabilized soil blocks laid in soil-cement or soil-lime mortar. In areas of moderate rainfall, no rendering required.
· Renders, using soil with or without additives, such as binders (cement, lime, gypsum), waterproofing agents (bitumen, plant extracts, chemicals), fibrous material (plant or animal fibres, cow dung), or using plain cow dung.
· Paints based on soil mixes.
· In reasonably dry areas, with good drainage and low water tables: subbase of well compacted, clay-rich soil, covered by large sized gravel (to break capillary action), topped by small sized gravel and a layer of sand, the surface layer made of a silty soil, mixed with 5 % linseed oil and compacted with tamper or vibrator.
· Same as before, but surface layer of stabilized soil bricks or tiles, laid on the sand bed and jointed with soil cement mortar.
· Traditional rural house floors (Asia, Africa) made of compacted stone or earth and smoothened with a mixture of soil and cow dung, or only cow dung (for resistance to abraison, cracks and insects).
· Other surface hardeners: animal (horse) urine mixed with lime, ox blood mixed with cinders and crushed clinker, animal glues, vegetable oils, powdered termite hills, crushed shells, certain silicates and other synthetic products.
· Traditional flat roof with timber sub-structure covered with soil (same as for rammed earth walls) and compacted well, only suitable for dry regions.
· Fibre-soil reels laid moist between timber purlins, on flat or sloped roofs, evened out with a fibre-soil layer and covered with roofing felt or bitumen coat; not recommended in termite prone areas.
· Grass roofs, requiring a water and rootproof membrane, gravel to drain water and ventilate roots and a soil layer on which grass grows, providing favourable indoor climate and sound-proofing, as well as air-purification; suitable for all climates.
· Soil brick vaults and domes, constructed with or without formwork, such that each brick rests on the layer below, passing on the compressive forces in a curved line within the thickness of the structure; a traditional construction found in most arid and semi-arid regions.
Soil brick vault construction (Bibl. 00.56)
· Availability in large quantities in most regions,
· hence low cost (mainly for excavation and transportation) or no cost, if found on the building site.
· Easy workability, usually without special equipment.
· Suitability as construction material for most parts of the building.
· Fire resistance.
· Favourable climatic performance in most regions, due to high thermal capacity, low thermal conductivity and porosity, thus subdueing extreme outdoor temperatures and maintaining a satisfactory moisture balance.
· Low energy input in processing and handling unstabilized soil, requiring only 1 % of the energy needed to manufacture and process the same quantity of cement concrete.
· Unlimited reuseability of unstabilized soil (ie recycling of demolished buildings).
· Environmental appropriateness (use of an unlimited resource in its natural state, no pollution, negligible energy consumption, no wastage).
· Excessive water absorption of unstabilized soil, causing cracks and deterioration by frequent wetting and drying (swelling and shrinkage) as well as weakening and disintegration by rain and floods.
· Low resistance to abraison and impact, if not sufficiently stabilized or reinforced, thus rapid deterioration through constant use and possibility of penetration by rodents and insects.
· Low tensile strength, making earth structures especially susceptible to destruction during earthquakes.
· Low acceptability amongst most social groups, due to numerous examples of poorly constructed and maintained earth structures, usually houses of the underprivileged population, thus qualifying earth as being the "poor man's material".
· On account of these disadvantages, lack of institutional acceptability in most countries, which is why building and performance standards often do not exist.
· Avoidance of excessive water absorption can be achieved by selection of the most appropriate type of soil and/or correcting the particle size distribution; also by adding a suitable stabilizer and/or waterproofing agent; good compaction; and more important, by good design and protective measures.
· Resistance to abraison and impact is generally improved by the same measures as above; waterproofing agents, however, do not necessarily impart higher strength and hardness; hence special additives may be needed and special surface treatment.
· Soil constructions in earthquake zones require careful designing to minimize the effect of destructive forces, but also the use of additional materials, which possess high tensile strength (especially for reinforcements).
· Building important public buildings and high standard housing with earth can be convincing demonstrations of the advantages of the technology and thus improve its acceptability.
· By eliminating the major disadvantages, the lack of institutional acceptability can be overcome. Because of the importance of the material, methods of testing and improving soils for building construction are dealt with in more detail.
Extracting soil samples with an auger (Bibl 02.10).
Whether the aim is to build a single house or to start a production unit for stabilized soil blocks, it is essential to test the soil used, not only in the beginning, but at regular intervals or each time the place of excavation is changed, as the soil type can vary considerably even over a small area.
Basically there are two types of tests:
· indicator or field tests, which are relatively simple and quickly done,
· laboratory tests, which are more sophisticated and time consuming.
In certain cases, soil identification on the basis of experience can be sufficient for small operations, but normally some indicator tests are indispensable. They provide valuable information about the need for laboratory tests, especially if the field tests give contradicting results. Not all the tests need to be carried out, as this can be tiresome, but just those that give a clear enough picture of the samples, to exclude those that show deficiencies. This is not only necessary to achieve optimum material quality, but also to economize on costs, material, stabilizers, manpower and energy input.
It should further be remembered that soil identification alone does not provide assurance of its correct use in construction. Tests are also necessary to evaluate the mechanical performance of the construction material.
· The soil is best excavated directly at the building site and several holes are dug in an area that is big enough to supply all the required soil.
· First, the topsoil containing vegetable matter and living organisms is removed (unsuitable for construction).
· The soil samples are then taken from a depth of up to about 1.5 m for manual excavation, or up to 3 m if a machine will be doing the work.
· A special device, an auger, is used to extract samples from various depths. Each different type of soil is put on a different pile.
· The thickness of each layer of soil, its colour and the type of soil, as well as an accurate description of the location of the hole should be recorded on labels attached to each bag of soil taken for testing.
Indicator or Field Tests
The implementation of these simple tests should preferably follow the order presented here.
Duration: few minutes
Immediately after removal, the soil should be smelt, in order to detect organic matter(musty smell, which becomes stronger on moistening or heating). Soils containing organic matter should not be used or tested further.
Duration: few minutes
After removing the largest particles (gravel), a sample of soil is rubbed between the fingers and palm of the hand. A sandy soil feels rough and has no cohesion when moist. A silty soil still feels slightly rough, but has moderate cohesion when moist. Hard lumps that resist crushing when dry, but become plastic and sticky when moistened indicate a high percentage of clay.
Similar tests can be done by crushing a pinch of soil lightly between the teeth (soils are usually quite clean!).
Duration: few minutes
A slightly moist ball of soil, freshly cut with the knife will reveal either a dull surface (indicating the predominance of silt) or a shiny surface (showing a higher proportion of clay).
Duration: few minutes
When the knife easily penetrates a similar ball of soil, the proportion of clay is usually low. Clayey soils tend to resist penetration and to stick to the knife when pulled out.
Equipment bowl of water or water tap
Duration: few minutes
When washing hands after these tests, the way the soil washes off gives further indication of its composition: sand and silt are easy to remove, while clay needs to be rubbed off.
Equipment: two screens with wire mesh of 1 mm and 2 mm
Duration: half an hour
With the help of the screen the dry gravel and sand particles should be separated on a clean surface to form two heaps. Crushing of clay lumps may be necessary beforehand. By comparing the sizes of the heaps a rough classification of the soil is possible.
A. The soil is either silty or clayey if the "silt + clay" pile is larger; a more precise classification requires further tests.
B. Similarly the soil is sandy or gravelly, if the "sand + gravel" pile is larger.
C. and D. Further sieving with a 2 mm mesh screen will reveal whether the soil is gravelly or sandy.
In the case of sandy or gravelly soil, a handful of the original material (before sieving) should be moistened, made into a ball and left to dry in the sun. If it falls apart as it dries, it is called "clean", and thus unsuitable for earth constructions, unless it is mixed with other materials.
If the soil is not "clean", the silt and clay pile should be used for the next tests.
Water retention test
Duration: 2 minutes
A sample of the fine material is formed into an egg-sized ball, by adding just enough water to hold it together but not stick to the hands. The ball is gently pressed into the curved palm, which is vigorously tapped by the other hand, shaking the ball horizontally.
· When it takes 5 - 10 taps to bring the water to the surface (smooth, "livery" appearance), it is called rapid reaction. When pressed, the water disappears and the ball crumbles, indicating a very fine sand or course silt.
· When the same result is achieved with 20 - 30 taps (slow reaction), and the ball does not crumble, but flattens on pressing, the sample is a slightly plastic silt or silty clay.
· Very slow or no reaction, and no change of appearance on pressing indicate a high clay content.
Dry strength test
Equipment: oven, if no sun available
Duration: four hours for drying
2 to 3 moist samples from the previous test are slightly flattened to 1 cm thickness and 5 cm F and allowed to dry completely in the sun or in an oven. By attempting to pulverize a dry piece between thumb and index finger, the relative hardness helps to classify the soil:
· If it is broken with great difficulty and does not pulverize, it is almost pure clay.
· If it can be crushed to a powder with a little effort, it is a silty or sandy clay.
· If it pulverizes without any effort, it is a silt or fine sand with low clay content.
Equipment flat board, approx. 30 x 30 cm
Duration: 10 minutes
Another moist ball of olive size is rolled on the flat clean surface, forming a thread. If it breaks before the diameter of the thread is 3 mm, it is too dry and the process is repeated after re-moulding it into a ball with more water. This should be repeated until the thread breaks just when it is 3 mm thick, indicating the correct moisture content. The thread is re-moulded into a ball and squeezed between thumb and forefinger.
· If the ball is hard to crush, does not crack nor crumble, it has a high clay content.
· Cracking and crumbling shows low clay content.
· If it breaks before forming a ball, it has a high silt or sand content.
· A soft spongy feel means organic soil.
Duration: 10 minutes
With the same moisture content as the thread test, a soil sample is formed into a cigar shape of 12 to 15 mm thickness. This is then progressively flattened between the thumb and forefinger to form a ribbon of 3 to 6 mm thickness, taking care to allow it to grow as long as possible.
· A long ribbon of 25 to 30 cm has a high clay content.
· A short ribbon of 5 to 10 cm shows low clay content.
· No ribbon means a negligible clay content.
Equipment: cylindrical glass jar of at least 1 litre capacity, with a flat bottom and an opening that can be just covered with the palm; centimetre scale
Duration: 3 hours
The glass jar is filled quarter full with soil and almost to the top with clean water. The soil is allowed to soak well for an hour, then with the opening firmly covered, the jar is shaken vigorously and then placed on a horizontal surface. This is repeated again an hour later and the jar then left standing undisturbed for at least 45 minutes.
After this time, the solid particles will have settled at the bottom and the relative proportions of sand (lowest layer), silt and clay can be measured fairly accurately. However, the values will be slightly distorted, since the silt and clay will have expanded in the presence of water.
Linear shrinkage test
Equipment long metal or wooden box with internal dimensions 60 x 4 x 4 cm (l x b x h), open on top; oil or grease; spatula
Duration: 3 to 7 days
The inside surfaces of the box are greased to prevent the soil from sticking to them. A sample of soil with optimum moisture content is prepared (ie when squeezing a lump in the hand, it retains the shape without soiling the palm, and when dropped from about 1 metre height, breaks into several smaller lumps). This soil mix is pressed into all corners of the box and neatly smoothened off with the spatula, so that the soil exactly fills the mould. The filled box is exposed to the sun for 3 days or left in the shade for 7 days.
After this period, the soil will have dried and shrunk, either as a single piece or forming several pieces, in which case they are pushed to one end to close the gaps. The length of the dried soil bar is measured and the linear shrinkage is calculated as follows:
((Length of wet bar) - (Length of dried bar))/(Length of wet bar) x 100
To obtain good results in construction, the soil should shrink or swell as little as possible. The more the soil shrinks, the larger is the clay content, which can be remedied by adding sand and/or a stabilizer, preferably lime.
Wet sieving test
Equipment: a set of standardized sieves with different meshes (eg 6.3 mm, 2.0 mm, 0.425 mm and 0.063 mm); flat water container below the sieves; 2 small buckets, one filled with water; stove or oven for drying samples; 2 to 5 kg balance with an accuracy of at least 0.1 g
Duration: 1 to 2 hours
A 2 kg soil sample is weighed dry, placed in the empty bucket and mixed with clean water. The water-soil mix, well stirred, is poured into the sieves, which are placed in descending order one on top of the other, with the finest mesh at the bottom, below which is the flat container. The bucket is rinsed clean with the remaining water, which is also poured into the sieves.
Each sieve will have collected a certain amount of material, which is dried by heating on the stove or in the oven, then weighed accurately and recorded. The fine particles in the bottommost container is a mixture of silt and clay, which cannot be separated by sieving. This is carried out by the next test.
Equipment: a 1-litre graduated glass measuring cylinder, with an inside diameter of about 65 mm; a circular metal disk on a stem, which can be lowered down inside the cylinder; a rubber tube and heat resistant drying dishes for siphoning; a watch; a pinch of salt; stove or oven and balance, as in previous test
Duration: 1 to 2 hours
A dry sample of 100 g of the fine material from the previous test is carefully weighed and put into the cylinder. A pinch of salt is added, to improve dispersion of the clay particles, and water is filled up to the 200 mm mark. With the cylinder kept firmly closed with the palm of the hand, the contents are shaken vigorously until a uniform suspension of the grains is achieved. The cylinder is placed on a firm level surface and the time taken.
After 20 minutes, the metal disk is carefully lowered down to cover the material that has settled at the bottom of the cylinder, without disturbing it. The clay, which is still in suspension, is removed by siphoning off the liquid, which is subsequently dried out and the residue weighed. The weight in grams is also the percentage of clay in the sample.
Grain size distribution analysis
With the results of the wet sieving and siphoning tests of one sample showing the relative proportions of the various constituents, as defined by their particle sizes, several points can be plotted on a chart. A curve is then drawn so that it passes through each point successively, giving the grain size distribution of that particular soil sample. This can tee repealed for other samples on the same chart, showing the range of soil types analyzed.
The chart below shows an example of a gravelly soil (G) and a clay soil type (C). The horizontally shaded area indicates the types of soils that are suitable for rammed earth construction, while the vertically shaded area shows appropriate soils for compressed block production. The overlapping area is thus good for most soil constructions, so that a curve (I) running through the middle symbolizes a soil of ideal granulation.
The purpose of this exercise is to determine whether the available soil is suitable for building. If the soil is too gravelly, the gaps between the particles are not properly filled, the soil lacks cohesion and is consequently very sensitive to erosion. If the soil is too clayey, it lacks the large grains that give it stability, and is thus sensitive to swelling and shrinkage. An optimum grain size distribution is one in which the proportion of large and small grains is well balanced, leaving practically no gaps, and sufficient clay particles are present to facilitate proper cohesion.
If the tests reveal a poor grain size distribution, it can be corrected to some extent by:
· sieving the gravelly fraction, if the soil contains too much coarse material;
· partly washing out the clayey fraction, if finer particles are in excess;
· mixing soil types of different granular structure.
FIGURE (Bibl. 02.34)
Atterberg limit tests
These tests, developed by the Swedish scientist Atterberg, are needed to find the respective moisture contents at which the soil changes from a liquid (viscous) to a plastic (mouldable) state, from a plastic consistency to a soft solid (which breaks apart before changing shape, but unites if pressed), and from this state to a hard solid. While the previous tests determined the quantity of each soil constituent, the Atterberg tests show which type of clay mineral is present. This has an influence on the kind of stabilizer required.
For all practical purposes, the determination of the "liquid limit" and "plastic limit" is sufficient, the other Atterberg limits are not so important. However, the determination of the Atterberg limits is usually carried out with the "fine mortar" fraction of the soil, which passes through a 0.4 mm sieve. This is because water has little effect on the consistency of larger particles.
Liquid limit test
Equipment a curved dish, about 10 cm in diameter and 3 cm deep, with a smooth or glazed inner surface; a grooving tool (as illustrated); a metal container with tightly fitting cover (eg large pill box), a drying oven which maintains a temperature of 110° C; a balance, accurate to at least 0.1 g, preferably to 0.01 g.
Duration: about 10 hours
A sample of fine soil (about 80 g) is mixed with drinkable water to a consistency of a thick paste and evenly filled into the dish such that the centre is about 8 mm deep, gradually diminishing towards the edge of the dish.
This is divided into two equal parts by drawing the grooving tool straight through the middle, making a V-shaped groove (of 60° angle) and a 2 mm wide gap at the bottom. Alternatively, a knife can be used.
The dish is held firmly in one hand and tapped against the heel of the other hand, which is held 30 to 40 mm away. The motion must be a right angles to the groove. If it takes exactly 10 taps to make the soil flow together, closing the gap over a distance of 13 mm, the soil is at its liquid limit.
If it takes less than 10 taps, the soil is too moist; more than 10 taps means that it is too dry. The moisture content must then be corrected, whereby moist soils can be dried by prolonged mixing or adding dry soil. The process is repeated until the liquid is found.
With an accurate balance, it is sufficient to take just a small sample of soil, scraped off from a point close to where the groove closed. The sample is put into the container, which is tightly covered and weighed before the moisture can evaporate. The soil container is then put into the 110°Coven until the veil is completely dry. This may take 8 -10 hours and can be checked by weighing several times, until the weight remains constant.
Knowing the wet (W1) and dry weight (W2) of the soil and container, and the weight of the clean dry container (WC), the liquid limit, expressed as the percentage of water in the soil, is calculated as follows:
Liquid Limit=Weight of Water/Weight of oven dried soil x 100
L=(W1-W2)/(W2-WC) x 100 %
Some examples of liquid limits are:
Sand: L = 0 to 30
Silt: L = 20 to 50
Clay: L= over 40
Plastic limit test
Equipment: a smooth flat surface, eg glass plate 20 x 20 cm; a metal container, drying oven and balance, as for the liquid limit test.
Duration: about 10 hours
About 5 g of fine soil is mixed with water to make a malleable but not sticky ball. This is rolled between the palms of the hands until it begins to dry and crack. Half of this sample is rolled further to a length of 5 cm and thickness of 6 mm.
Placed on the smooth surface, the sample is rolled into a thread of 3 mm diameter (see illustration for Thread test). If the sample breaks before the diameter reaches 3 mm, it is too dry. If the thread does not break at 3 mm or less, it is too moist. The plastic limit is reached, if the thread breaks into two pieces of 10 - 15 mm length. When this happens, the broken pieces are quickly placed in the metal container and weighed (W1).
The next steps of drying and weighing the soil and container are the same as for the liquid limit test, determining the values W2 and WC. The whole procedure is repeated for the second half of the original sample. If the results differ by more than 5 %, the tests must be repeated one again.
The plastic limit is calculated in the same way as the liquid limit:
Plastic Limit = Weight of Water/Weight of oven dried soil x 100
P=(W1-W2)/(W2-WC) x 100 %
The plasticity index (PI) is the difference between the liquid limit
(L) and plastic limit (P):
The simple mathematical relationship makes it possible to plot the values on a chart. The advantage is that the areas can be defined in which certain stabilizers are most effective.
It should, however, be noted that laterite soils do not necessarily conform to this chart. There is in fact no substitute for practical experimentation, using the recommended stabilizers to begin with, and starting with small dosages. The choice of soil stabilizers is dealt with in detail in the next chapter.
Soils that do not possess the desired characteristics for a particular construction can be improved by adding one or more stabilizers.
Each stabilizer can fulfil one (or at the most two) of the following functions:
· Increase the compressive strength and impact resistance of the soil construction, and also reduce its tendency to swell and shrink, by binding the particles of soil together.
· Reduce or completely exclude water absorption (causing swelling, shrinking and abrasion) by sealing all voids and pores, and covering the clay particles with a waterproofing film.
· Reduce cracking by imparting flexibility which allows the soil to expand and contract to some extent.
· Reduce excessive expansion and contraction by reinforcing the soil with fibrous material.
The effect of stabilization is usually increased when the soil is compacted. Sometimes compaction alone is sufficient to stabilize the soil, however, without an appropriate stabilizer, the effect may not be permanent, particularly in the case of increased exposure to water.
But, before considering the use of a stabilizer the following points must be investigated:
· Does the available soil satisfy the main requirements even without stabilization? This is largely dependent on the local climate, natural hazards and type of construction.
· Does the building design take into account the characteristics and limitations of the material? Building on a high level and incorporating damp-proof courses (to minimize damage by rising water) and providing wide roof overhangs (for protection against rain and solar radiation) are examples of appropriate design.
· Is the stabilization of the entire construction really necessary, or can a good surface protection (egg stabilized render) be sufficient?
By reducing the need for stabilization, considerable costs, time and effort can be saved.
Kinds of Stabilizers
A great number of substances may be used for soil stabilization, and much research is going on to find the most suitable stabilizer for each soil type. But, despite these research efforts, there is no "miracle" stabilizer that can be used in all cases. Stabilization is not an exact science, so that it is up to the builder to make trial blocks with various kinds and amounts of stabilizers which can be tested.
The most common naturally available stabilizers used in traditional constructions are:
· sand and clay
· straw, plant fibres
· plant juices (sap, latexes, oils)
· wood ashes (cinders)
· animal excrete (mainly cow dung, horse urine)
· other animal products (blood, hair, glues, termite hills).
The most common manufactured stabilizers, (ie products or by-products of local village industries or large industrial processes) are:
· lime and pozzolanas
· portland cement
· commercial soil stabilizers
· sodium silicate ("water glass")
· whey (casein)
The listed stabilizers are briefly described below. The choice of the most suitable stabilizer will mainly depend on local availability and costs, but also to some extent on social acceptance.
Sand and clay
· These are used to correct the quality of soil mix, that is, addition of sand to clayey soils or addition of clay to sandy soils.
· Mixing should be done in the dry state, otherwise it cannot be uniform.
· Dry clay is usually found in the form of hard lumps, which have to be well crushed before mixing.
Straw, plant fibres
· These act as reinforcements, especially to check cracking in soils with a high clay content.
· They also make the soil lighter, increase its insulating properties (good in arid and highland regions) and accelerate the drying process (by providing drainage channels).
· Straw is universally the most common soil reinforcement; almost any type is acceptable (wheat, rye, barley, etc.), also the chaff of most cereal crops.
· Other fibrous plant materials are sisal, hemp, elephant grass, coir (coconut fibre), bagasse (sugar cane waste), etc.
· To achieve satisfactory results, the minimum proportion of plant reinforcements is 4 % by volume; 20 to 30 kg per m3 of soil are common.
· Since plant reinforcements tend to weaken the end product and increase water absorption, excessive use should be avoided.
· The straw and fibres should be chopped to lengths of not more than 6 cm, and mixed thoroughly with the soil to avoid nests.
· The juice of banana leaves precipitated with lime improves erosion resistance and slows water absorption.
· Reduced permeability is also achieved by adding the latex of certain trees (eg euphorbia, hevea) or concentrated sisal juice in the form of organic glue.
· Vegetable oils and fats must dry quickly to be effective and provide water resistance. Coconut, cotton and linseed oils are examples; castor oil is very effective, but expensive.
· Kapok oil can also be effective. It is made by roasting kapok seeds, grinding them to a fine powder and mixing it with water (10 kg powder: 20 to 251 water).
· Ash from hardwood is usually rich in calcium carbonate and has stabilizing properties, but is not always suitable for clayey soils. Some ashes can even be harmful to the soil.
· The addition of 5 to 10 % (by volume) of fine, white ashes from fully burnt hardwood appears to be most effective, that is, improvement of the dry compressive strength.
· Ashes do not improve water resistance.
· These are mainly used to stabilize renderings.
· Cow dung is the most common stabilizer, which is valued mainly for its reinforcing effect (on account of the fibrous particles) and ability to repell insects. Water resistance is not significantly improved, while compressive strengths are reduced.
· Horse or camel dung are less common alternatives.
· Horse urine as a substitute for mixing water effectively eliminates cracking and improves resistance to erosion. Even better results are obtained by adding lime.
· Despite their advantages, these materials face low social acceptance in most regions, while in others (mainly rural areas in Asia and Africa) they are well accepted traditional materials.
Other animal products
· Fresh bull's blood combined with lime can gready reduce cracking, however, here again low social acceptance.
· Animal hair or fur is often used to reinforce renders.
· Animal glues, made from horn, bone, hooves and hides, improve moisture resistance.
· Termite hills, which are known to resist rain, can be pulverized and used as a stabilizer for sandy soils.
Lime and pozzolanas
(see also chapters on Lime and Pozzolanas)
· Clayey soil (with liquid limits in the region of 40 % or more) can be stabilized only with lime, as it reacts with the clay particles in the soil to form a binder.
· For soils with a lower clay content, a suitable pozzolana (eg fly ash, rice hush ash) can be added to the lime, to produce a cementitious binder.
· Quicklime (CaO), produced by burning limestone, can be used for stabilizing, but has several drawbacks: it has to be well crushed before use; it becomes very hot (up to 150° C) and can burn the skin; the heat of hydration tends to dry the soil quickly, with the risk of delayed hydration after several months.
· Hydrated or slaked lime (Ca[OH]2), made by adding water to quicklime, has less drawbacks. It can be used as a dry powder (available in bags), as milk of lime (slaked lime with excess water) or as lime putty (a viscous mass).
· The correct proportion of lime (with or without a pozzolana) cannot be generalized and needs to be found by a series of tests. The required amount can range between 3 and 14 % by dry weight, depending largely on the clay content (more clay requires more lime).
· Dry soil must be crushed (as clayey soils usually contain hard lumps) and thoroughly mixed with the lime. Most soils can be dried and broken with quicklime.
· The wet soil-lime mix is best kept in that state under cover for a day or two, after which the lime will have broken the remaining clay lumps. The soil is mixed again (if necessary, with addition of a pozzolana) producing a homogenious mass, which can immediately be used in construction. (Proportion of lime: pozzolana can range between 1: 1 and 1: 3).
· The curing of lime-stabilized soil takes about six times that of cement-stabilized soil. High temperatures and humidity help to improve the ultimate compressive strength. This can be achieved by curing under a plastic sheet, or in an enclosed space built with corrugated iron sheets, for at least two weeks. Final strength is gained after two to six months.
· Curing can be accelerated by adding cement just before use in construction.
· Limestone with a high clay content produces a special type of lime, called hydraulic lime, which sets and hardens like cement. Soil stabilization with hydraulic limes reduces the period of curing, but may not achieve sufficient strengths.
(see also chapter on Cement)
· Soils with low clay contents are best stabilized with portland cement, which binds the sand particles and gravel in the same way as in concrete, that is, it reacts with the water in the soil mixture to produce a substance which fills the voids, forming a continuous film around each particle, binding them all together.
· The reaction of cement and water (known as hydration) liberates calcium hydroxide (slaked lime) which reacts with the clay particles to form a kind of pozzolanic binder. If the clay content is too low the lime remains free. This can tee remedied by replacing a proportion (15 to 40 % by weighs) of the cement with a pozzolana, which is usually cheaper than cement.
· Just as in cement-sand mortars, soil-cement mixes become more workable by adding lime. If the clay content is high, the additional lime reacts with it to further stabilize the soil.
· The appropriate cement content will vary according to the aspects mentioned above. A minimum of 5 % is recommended, while cement contents exceeding 10 % are considered unsuitable, because of the high cost of cement.
· Soil and cement must be mixed dry, and the water added and thoroughly mixed just before use, as the cement begins to react with water immediately.
· Once the cement has begun to harden, it becomes useless. Soil cement cannot be recycled.
· The more thoroughly the soil is mixed, the higher the ultimate strength, which is obtained by compaction (eg with a ramming device or block press).
· Portland cement is the stabilizer that provides the greatest strength as well as resistance to water penetration, swelling and shrinkage.
· Soil stabilization with gypsum is not common practice and information on its performance is very limited.
· Gypsum is abundantly available in many countries, either as natural gypsum or as an industrial by-product, and is cheaper than lime or cement (produced with less energy and equipment).
· Since gypsum mixed with water hardens rapidly, adobe blocks stabilized with gypsum require no lengthy curing period, but can be used for wall constructions soon after production. Gypsum contents around 10 % are best.
· The advantages of stabilization with gypsum are low shrinkage, smooth appearance and high mechanical strength. In addition, gypsum binds well with fibres (particularly sisal), is highly fire resistant and is not attacked by insects and rodents.
· The main disadvantage of gypsum is its solubility in water, which requires careful protective measures: protection from rain on outer walls by plastering, cladding or wide overhanging roofs; protection from indoor moisture development by avoiding steam (in kitchens) and condensation; protection against rising water by means of waterproof membranes.
(see also chapter on Binders)
· For soil stabilization, bitumen can either be used as a cutback (ie mixed with a solvent such as gasoline, kerosene or naphtha), or as an emulsion (ie dispersed in water).
· After mixing a soil with bitumen cutback, it should be spread out to allow the solvent to evaporate before the material is used for blockmaking. It is best to mix the cutback with a small quantity of soil, which is then mixed with the remaining soil.
· Bitumen emulsions are usually very fluid and mix easily with moist soil. Excessive mixing must be avoided to prevent a premature break-down of the emulsion, leading to increased water absorption after drying. Emulsions should be diluted in the mixing water.
· Soil mixes required for compaction should not be too moist, hence a less quantity of stabilizer should be added.
· The bitumen content should be between 2 and 4 %. Higher proportions result in dangerously low compressive strengths.
· Bitumen stabilized soils should be cured in dry air at temperatures around 40° C.
· While bitumen stabilization does not improve the strength of the soil, it significantly reduces water absorption. In other words, while the dry strength of the soil is not very high, the strength is not reduced when wet.
· Bitumen stabilization is most effective with sandy or silty soils with a liquid limit between 25 and 35 % and plasticity index between 2.5 and 13 %.
· The presence of acid organic matter, sulphates and mineral salts can be very harmful. The addition of 1 % cement is a possible remedy.
Commercial soil stabilizers
· These are mainly industrially produced chemical products, which were developed primarily to stabilize the soil used in road construction.
· These chemical stabilizers work mainly as a waterproofer. In general, they do not improve the compressive strength of the soil.
· The required quantities of these stabilizers range between 0.01 and 1 % by weight, hence very thorough mixing is required to achieve a uniform distribution.
· A long list of commercial stabilizers is given in Bibl. 02.19.
· Sodium silicate, known as "water-glass", is cheaply available in many parts of the world.
· It works best with sandy soils, like clayey sands and silty sands, but is not suitable for clay soils.
· Sodium silicate works as a waterproofer, and also prevents fungal growth.
· If it is mixed with the soil, the usual quantity is 5 %.
· However, it is best to use it as a surface coating, made of 1: 3 parts of commercial sodium silicate: clean water.
· Soil blocks are dipped into the solution for about a minute, after which the solution is applied with a stiff brush. The procedure is repeated a second time and the blocks are left to dry in a protected place for at least 7 days.
· Deeper penetration of the solution is achieved by adding a very small amount of a surfactant (surface active agent).
· Resins are either processed plant extracts, such as sap from trees, or by-products of various industrial processes.
· Much research work is being undertaken on these materials and extraordinary results have been obtained with resin stabilization.
· The main advantages are water resistance (though not in all cases), rapid setting and solidification of very moist soils.
· The main drawbacks, however, are high cost, sophisticated production technology and the need for larger quantities than conventional stabilizers. Resins are often toxic and degradable by biological agents.
· Whey (casein) is the protein-rich liquid formed by making curd. Its use for building will be very limited in most developing countries, on account of its nourishing value. However, in regions where a surplus of whey is produced, its use as a surface stabilizer for soil constructions is well worth considering.
· By adding whey to a soil-lime plaster or to a limewash, a weather-proof surface protection is achieved, without forfeiting the capability of the soil to breathe.
· In order to achieve good adhesion and avoid cracks, the limewash should be applied in two or three thin coats. The use of whey as a primer can also give good results.
· Molasses are a by-product of the sugar industry.
· Adding molasses to the soil improves its compressive strength and reduces the capillarity of the soil.
· They work well with silty and sandy soils. In the case of clayey soils, small quantities of lime should be added to the molasses.
· The quantity of molasses normally added to the soil is about 5 % by weight of soil.
How to Use Stabilizers
Although the use of each stabilizer is mentioned above, some general rules are summarized here:
· The full benefit of using a stabilizer is achieved only if it makes contact with each particle of soil, hence, thorough mixing is necessary.
· Much preparation and testing is required to find the best combination and proportions of stabilizers for a given soil. It is certainly worth the time and effort, even if it takes one or two months of preparation.
· The only way to determine the correct proportion of stabilizer is to make 5 to 7 trial blocks from each mix and subject them to a series of tests, such as compression strength tests after different periods of drying, prolonged wetting and drying tests, and immersion in water.
· Portland cement and lime stabilized blocks need to be moist cured for at least 7 days to gain strength.
· Testing programs should take into account the local climatic conditions, the possible occurance of frost, and the like. The choice of stabilizer will also differ between arid and humid regions.
· It should be remembered that trial blocks need only a small amount of soil, which is easy to mix. During the actual construction or mass block production, the mixing of large quantities of the soil is more difficult, so that a slightly higher proportion of stabilizer should be added (except in the case of cement).
· The aim of the tests should always be to find the lowest amount of stabilizer to satisfy the requirements. Very often the specified requirements are unjustifiably high, leading to unnecessarily high costs.
The technique of firing clay to produce bricks and tiles for building construction is more than 4000 years old. It is based on the principle that clayey soils (containing 20 to 50 % clay) undergo irreversible reactions, when fired et 850 -1000° C, in which the particles are bonded together by a glassy ceramic material.
A large variety of soils are suitable for this process, the essential property being plasticity to facilitate moulding. While this depends on the clay content, excessive proportions of clay can cause high shrinkage and cracking, which is unsuitable for brickmaking. The qualities of fired clay products vary not only according to the type and quantity of other ingredients of the soil, but also to the type of clay mineral. For the production of good quality bricks and tiles, careful testing of soils is necessary.
Burnt brick production has reached a high level of mechanization and automation in many countries, but traditional small-scale production methods are still very widespread in most developing countries. Thus there is a great variety of non-mechanized and mechanized methods for clay winning, preparation, moulding, drying and burning, which can only be dealt with briefly in this manual.
· Clay deposits are found at the foot of hills or on agricultural land close to rivers (which naturally generates conflicting interests between the use of land for brickmaking and for agriculture).
· The criteria for choosing a suitable location are the quality of clay, availability of level ground and closeness of a motorable road for transports.
· Hand-digging in small and medium-sized production plants is usually done to a depth of less than 2 m. (After excavation of large areas, they can be returned to agricultural use.)
· Mechanical methods, using drag-line and multi-bucket excavators, are required for large-scale brickmaking plants. These methods require proportionately less excavating area, but make deep cuts in the landscape.
· This includes sorting, crushing, sieving and proportioning, before the material is mixed, wetted and tempered.
· Sorting is done by picking out roots, stones, limestone nodules, etc., or in some cases by washing the soil.
· Crushing is required because dry clay usually forms hard lumps. Manual pounding is common, but laborious. However, simple labour-intensive crushing machines have been developed (see ANNEX).
· Sieving is needed to remove all particles larger than 5 mm for bricks, or 0.6 mm for roof tiles.
· Proportioning is required if the clay content or grain size distribution is unsatisfactory. In some cases, rice husks, which serve as a fuel, are added to the clay, in order to obtain lighter and more uniformly burnt bricks.
· Thorough mixing is needed and a correct amount of water. Since manual mixing (traditionally by treading with bare feet) is laborious and often unsatisfactory, motor-powered mixers are preferred. The effort of mixing can be greatly reduced by at/owing the water to percolate through the clay structure for some days or even months. This process, known as "tempering", allows chemical and physical changes to take place, inproving its moulding characteristics. The clay must be kept covered to prevent premature drying.
· Moulding is done by hand or by mechanized methods.
· Hand-moulding methods make use of simple wooden moulds: the clay is formed into a clot, thrown into the mould, and the excess cut off.
· There are two traditional techniques for releasing the brick from the mould: a. the slop-moulding method, by which the mould is kept wet and the clay is mixed with more water, and b. the sand-moulding method, by which the clot is rolled in sand to prevent the clay from sticking to the mould.
· Bricks made by slop-moulding are vulnerable to slumping and distortion, while sand-moulding produces firmer, well-shaped bricks. Where sand is not available, finely ground clay can also be used, according to a technique developed at the ITW (Intermediate Technology Workshop in the United Kingdom).
· With table moulds (as developed by ITW, United Kingdom, and Central Building Research Institute, India), less effort, more accurately shaped bricks and higher outputs are achieved. While the moulding is done in the same way as with simple wooden moulds, the bricks are ejected by means of a foot-operated lever.
· Roofing tiles are made with specially shaped moulds, but principally in the same way as bricks. The main difference is that other material characteristics, with regard to uniformity, particle size and clay content, are needed.
· Mechanized brickworks use machines which extrude the clay through a dye to form a clay column, which is wirecut into brick-sized pieces. This method produces denser and stronger bricks, which can also be perforated.
· An intermediate solution is brick and tile moulding with mechanical compression. Two machines produced in Belgium (CERAMAN and TERSTARAM) were specially designed for this purpose, but are also used to make air-dried, stabilized soil bricks. Mechanical compression allows for considerably lower moisture contents, thus shortening the drying period.
· Green bricks are likely to be crushed in the kiln, under the weight of those piled on top; they can shrink and crack during firing; the water driven off can condense on cold bricks away from the heat source; or steam is developed, building up excessive pressures within the bricks; and, finally, too much fuel is required to drive out the remaining water. Hence, thorough drying is vital.
· Drying should be relatively slow, that is, the rate at which moisture evaporates from the surface should not be faster than the rate at which it can diffuse through the fine pores of the green brick. Air should have access to all sides of the bricks, so that they must be stacked with sufficient gaps between them.
· Natural drying is done in the open under the sun, but a protective covering (eg leaves, grass or plastic sheeting) is advisable to avoid rapid drying out. If it is likely to rain, drying should be done under a roof. But traditionally, bricks are only made in the dry season.
· Artificial drying (as in large mechanized plants) is done in special drying chambers, which make use of heat recovered from the kilns or cooling zones.
· Drying shrinkage is inevitable, and causes no special problems if below 7 % linear shrinkage. 10 % linear shrinkage should not be exceeded, thus, if necessary, the clay proportion must be reduced by adding sand or grog (pulverized brick rejects).
Typical clamp in India: The crushed coal, being screened in the foreground, is the fuel used. On the right are green bricks stacked for drying (Photo: K. Mukerji)
· There are two types of kilns for burning bricks: intermittent and continuous kilns.
· Intermittent kilns include clamps and scove kilns (traditional field kilns), updraught and downdraught kilns. Their fuel efficiency is very low, but they are adaptable to changing market demands. They vary in size from 10000 to 100000 bricks.
· Continuous kilns include various versions of the Hoffmann kiln (particularly the Bull's trench kiln) and the high-draught kiln. These are very fuel efficient. Tunnel kilns, in which the bricks are passed through a stationary fire, are too sophisticated and capital-intensive to be considered here.
· Clamps are basically a pile of green bricks interspersed with combustible material (eg crushed coal, rice husks, cow dung). Some holes are left at the base of the clamp, where the fire is lit. The holes are closed and the fire allowed to burn out, which can take a few days or several weeks. The bricks near the centre of the clamp will be the hardest. Sorting out is necessary, as about 20 to 30 % are not saleable. These are refired or used in the clamp base, sides or top.
· Scove kilns, plastered on all sides with mud, are principally the same as clamps, except that tunnels are built across the base of the pile, in order to feed additional fuel. This is the best method for burning wood.
· Updraught kilns (also known as Scotch kilns) function in the same way as scoves, except that the tunnels and walls are permanent.
· Downdraught kilns have a permanent arched roof. The hot gases from the fuel burnt ant the sides of the kiln, rise to the arched roof and are drawn down between the bricks by the chimney suction, through the perforated floor and out through the chimney.
· The Hoffmann kiln, which was originally circular but now more commonly oval, is a multi-chamber kiln in which the combustion air is preheated by cooling bricks in some chambers, and passes through the firing zone, from which the exhaust gases preheat the green bricks. While the cooled bricks are removed from one side of the empty chamber, green bricks are stacked on the other side. The fuel is fed from the top, through holes in the permanent arched roof. The daily output is about 10 000 bricks.
· The Bull's trench kiln operates on the principle of the Hoffmann kiln, except that the expensive arched roof is omitted and the exhaust gases are drawn off through 16 m high moveable metal chimneys with a wide base, which fit over the openable vent holes set in the brick and ash top of the kiln. The fuel, generally crushed coal, is fed in through the holes on the top. Depending on the size of the kiln, daily outputs can be between 10 000 and 28 000 bricks, 70 % of which being of high quality.
· The high-draught kiln is a further development of the Bull's trench kiln, whereby temporary cross-walls of green bricks leave openings on alternate sides, thus making the hot air travel a longer distance in a zigzag fashion, achieving a larger transfer of heat from a given quantity of fuel (wood and coal). Fans are installed to provide the necessary draught. Daily outputs of 30 000 bricks are possible.
· Wood, coal and oil are the main types of fuel used. Coal is suitable for all purposes, while wood is less suited for clamps and oil is not used for clamps, downdraught, Bull's trench and high-draught kilns.
Working principle of the Bull's trench continuous kilns used in Pakistan and India (Bibl. 04.11)
High-draught kiln developed by the Central Building Research Institute, India (Bibl. 04.04)
Scales of production in brick manufacturing (Bibl. 04.04)
Scale of production
Number of bricks per day (average)
Example of process used
Appropriate for market area
Hand made, clamp-burnt
Mechanized press, Bull's trench kiln
Industrialized areas of high demand and well-developed infrastructure
Typical fuel requirements of kilns (Bibl. 04.04)
Type of kiln
Heat requirement (MJ / 1 000 bricks)
Quantity of fuel required
|| || ||
|| || ||
Note: Figures in brackets mean that the fuel is not suitable for that kiln.
· Solid or perforated bricks of all shapes and sizes for standard masonry constructions, including foundations, floors, and load-bearing walls, arches, vaults and domes.
· Roof tiles of various shapes and sizes for roof slopes ranging between 1: 3 (18°30') and 1: 1 (45°).
· Floor tiles and facing bricks for waterproof and durable surface finishes, and for improving appearance.
· Special products, such as engineering bricks which have high densities and compressive strengths: refractory bricks, with high heat resistance,used for lining kilns and furnaces; acid resisting bricks and tiles to withstand chemical attack; pipes and channel elements for various purposes.
· Specially shaped, hollow clay blocks for composite reinforced concrete beam slabs (for ceilings and roofs).
· Brick rejects can be used to construct kiln walls, as a filler in wall or floor cavities, as an aggregate in concrete, or, when finely ground, underfired rejects produce a pozzolana (surkhi) and others produce grogs for brickmaking.
· Fired clay products can have high compressive strengths, even when wet, and are thus resistant to impact and abrasion.
· The porosity of fired clay permits moisture movement, without significant dimensional changes. Brick and tile constructions can "breathe".
· Solid bricks have a high thermal capacity, required for most climates, except for the predominantly humid zones; perforated bricks can be used (with perforations running vertically) for cavity walls, which provide thermal insulation, or (with perforations perpendicular to the wall face) for ventilation or screen walls.
· Fired clay products provide excellent fire-resistance.
· Bricks and tiles are weather resistant and can remain without any surface protection, thus saving costs. However, exposed brickwork is often considered unfinished and hence not always accepted.
· Poor quality and broken bricks are useable for other purposes, hence no wastage.
· The production process can be extremely labour-intensive and thus create many jobs, even for unskilled workers.
· Relatively high fuel consumption of the firing process. In many countries, where firewood is used, large forest areas have disappeared causing serious ecological damage. Where firewood is still available, it is usually extremely expensive, but this is also true for other fuels. Therefore, good quality fired clay products tend to be expensive.
· Simple field kilns do not always produce good quality and uniform bricks, and generally operate with very low fuel efficiency. Capital investments for fuel efficient kilns that produce good bricks are often too high for small-scale producers. They are also not justified, if continuous or large supplies of bricks are not required.
· A common defect of bricks is "lime blowing" (or "lime bursting"), a weakening or breaking of bricks, which is caused by the hydration of quicklime particles, derived from limestone in brickmaking clays.
· Another defect is "efflorescence", which appears temporarily on the surface of the brick, and is caused by soluble salts inherent in the clay or process water.
· Fuel efficiency is primarily dependent on the design of the kiln: continuous kilns retain the heat longest and utilize the heat from the cooling bricks, while the green bricks are preheated by the exhaust gases. Intermittent kilns have to heat up the entire heap anew, each time a batch is fired.
· Firewood should not be used up faster than it can be regrown. Hence plantations of fast-growing trees are vital. Considering their lower calorific value, larger numbers of fast-growing trees are needed than slow-growing trees. However, such plantations can be difficult to maintain in dry regions or when the rains fail.
· Agricultural wastes and other biomass, such as rice husks, coffee husks, papyrus, are useful and cheap (partial) substitute fuels. Mixing them with the clay helps to burn the bricks uniformly, avoiding unburnt cores.
· The Bull's trench and highdraught kilns have a fuel efficiency comparable to sophisticated, mechanized kilns. They are also cheaper to build than the Hoffmann kiln. It is, therefore, worth considering using the first batch of bricks from a clamp to build a more fuel efficient kiln, whereby the size is tailored to suit the local market demands. A certain minimum size is nevertheless needed to provide the requisite draught.
· Lime blowing can be minimized by reducing the particle size of the raw mix and firing at 1000° C. The addition of 0.5 to 0.75 % of common salt (sodium chloride) before firing has also proved effective. After firing, the bricks can be soaked in water for 10 minutes, during which the lime is slaked. The process, called "docking", is not always successful.
· Improvements are possible and greatly needed in all phases of brick manufacture, so that a good deal of research is still required to find simple, inexpensive methods for proper clay preparation, fast and uniform moulding, and - most important of all - maximum fuel efficiency.
Binders are substances which are used to bind inorganic and organic particles and fibres to form strong, hard and/or flexible components. This is generally due to chemical reactions which take place when the binder is heated, mixed with water and/or other materials, or just exposed to air.
There are four main groups of binders:
· Mineral binders
· Bituminous binders
· Natural binders
· Synthetic binders.
These can be divided into three categories:
· Hydraulic binders, which require water to harden and develop strength.
· Non-hydraulic binders, which can only harden in the presence of air.
· Thermoplastic binders, which harden on cooling and become soft when heated again.
· The most common hydraulic binder is cement (see chapter on Cement).
· Hydraulic and semi-hydraulic limes (see chapter on Lime) are obtained from burning limestone, which contains a large or moderate amount of clay. This can be easily understood, since limestone and clay are the main raw materials for cement production.
· Pozzolanas (see chapter on Pozzolanas), when mixed with non-hydraulic lime, form a hydraulic cement.
· Hydraulic binders are usually available in the form of a fine powder: the finer they are ground (usually in a ball mill), the larger is the specific surface area (of the sum of the particles) per unit weight. And the larger the surface area, the more effective and complete is the chemical reaction with the water that it comes into contact with.
· On account of their affinity to water, hydraulic binders must be stored in absolutely dry conditions, to avoid premature setting and hardening. Even humid air can cause hydration.
· The most common non-hydraulic binder is clay, which is present in most soils, causing them to harden on drying and soften when wet. Its main uses are in earth constructions and in the manufacture of burnt clay products.
· Another common non-hydraulic binder is high calcium or magnesium lime (see chapter on Lime). Hardening depends on its combination with carbon dioxide from the air (carbonation), by which it again becomes calcium carbonate (limestone). But limes are rarely used as the only cementitious binder, and more usually react with clay or a pozzolana to form a hydraulic cement.
· Gypsum is a non-hydraulic binder which occurs naturally as a soft crystalline rock or sand. The chemical name is calcium sulphate all-hydrate (CaS04.2H20). By gentle heating up to about 160° C, calcium sulphate hemi-hydrate (CaSO4.1/2H2O) is produced, more commonly known as "Plaster of Paris", which when mixed with water sets in 8 to 10 minutes. Gypsum plaster has successfully been produced by means of solar energy. Further heating of gypsum, slightly beyond 200° C (not achieved by solar energy) produces anhydrite gypsum (CaSO4), which when mixed with water, sets very slowly.
· Gypsum is also abundantly available as an industrial by-product from the evaporation of seawater to produce common salt, or from the manufacture of fertilizer from phosphate rock. The latter is called phosphogypsum, which contains more water than natural gypsum, is more acidic and has more impurities, so that costly processing is required. It is also to some extent radioactive and therefore not recommended for use in building.
· Gypsum is used as a building material, mainly as a retarder to regulate the setting of various types of hydraulic cements, and in conjunction with a variety of other materials (eg lime, sand, sawdust, jute, sisal, linseed oil, paper) to produce renders, boards and masonry blocks.
· The main advantages of gypsum are the low energy input during burning to produce gypsum plaster; rapid drying and hardening, with negligible shrinkage (needing no form-work); good adhesion to fibrous and other materials; good fire resistance; good sound reflection (if made dense and hard); superior surface finish; resistance to insects and rodents.
· The principle drawback of gypsum plaster is its solubility in water (2 g gypsum per litre of water). Humid air can also soften gypsum plaster. Frost and sudden temperature changes can also cause damage.
· On account of this drawback, gypsum should not be used on external surfaces in humid climatic zones, unless it is well protected by wide overhanging roofs and a water resisting coating (eg hot linseed oil).
· Thermoplastic materials require heat in order to be processed, and harden on cooling. Their properties remain unchanged on reheating and cooling, so that they can be reclaimed and reprocessed numerous times.
· Probably the only thermoplastic mineral binder used for building is sulphur. For details, see chapter on Sulphur.
· Bitumens are mechanical mixtures of different hydrocarbons (compounds of carbon and hydrogen) and a few other substances, and is obtained as a residue in the distillation of crude oil, either in petroleum refineries or in nature (in pores of rocks or in the form of lakes, close to petroleum deposits). Bitumens are generally dark black, oily, fluorescent thermoplastic substances, which are highly viscous to almost solid at normal temperatures. Compounds consisting of at least 40 % of heavy hydrocarbons are called bitumens.
· Asphalts are defined as mixtures containing bitumen and a substantial proportion of inert mineral matter (sand, gravel, etc.). In the USA, bitumen is called asphalt, thus causing some confusion.
· Tar is the thick black substance produced by the destructive distillation (or carbonisation) of organic matter, such as wood or coal.
· Pitch is the residue after distilling tar from coal.
· Bitumen is not affected by either light, air or water individually, but in combination they can make it brittle, porous and susceptible to oxidation, forming blisters and cracks. It becomes soft at temperatures between 30° and 100° C (no sharp melting point),and therefore must be protected from exposure to heat. It is insoluble in water and fairly resistant to most acids. Although bitumen is combustible, composite products, such as mastic asphalt, are not readily ignited. Bitumen and coal tar products may be poisonous, hence contact with drinking water should be avoided.
· Bituminous products can be used as waterproofing materials (in soil stabilization, as paints, damp-proof membranes, roofing felt, joint fillers, etc.), as paving materials (roads and floors) and as adhesives (for wood block flooring, insulating linings and felts).
· When bitumen is used, it must be either heated; or mixed with solvents (eg gasoline, kerosene or naphta), which is called "bitumen cutback"; or dispersed in water, which is called "bitumen emulsion".
· A variety of binders are obtained from plants and animals, and can be used in their natural form or after processing.
· Examples of natural binders are plant juices (eg juice of banana leaves; latex of certain trees; sisal juice; coconut, cotton and linseed oils), animal excrete (eg cow dung; horse urine) and other animal products (eg bull's blood; animal glues from horn, bone, hooves and hide; casein or whey, made from milk).
· Natural birders have played an important role in traditional constructions since prehistoric times, but nowadays face low social acceptance. However, research today is giving such materials increasing importance, especially with a view to Cost effectiveness and environmental acceptability.
· These binders are generally produced by industrial processes and, therefore, often expensive. Some synthetic binders are toxic.
· They can either be used as admixtures, as adhesives or as surface coatings and are either applied hot, or as an emulsion, or with a solvent.
· Synthetic admixtures which bond loose particles together are mainly resins derived from plant materials or mineral oil. The variety of commercial products is very large and their use depends on the required performance (strength development, waterproofing, elasticity etc.).
· Adhesives are used to stick larger particles, components, membranes, sheets, boards, tiles, etc. on another surface. Some adhesives are designed specifically for one job, whilst others can be used for a number of applications. Adhesives can have one or two components. Some adhesives are thermoplastic and retain their properties when reheated and cooled.
· Surface coatings can be used as a protective film, as a decoration or even to achieve a surface bonding. Here again the variety of products is too large to be dealt with here.
The production of lime in kilns is a more than 2000 year old technology, believed to have been developed by the Romans around 300 B.C. The process of burning limestone at temperatures above 900° C to produce quicklime, which is subsequently slaked with water to produce hydrated lime, has since become traditional practice in most countries, as lime is one of the most versatile materials known, being used for numerous industrial and agricultural processes, environmental protection and building construction.
Lime is also obtained as a by-product in the form of lime sludge (which contains calcium carbonate and various impurities) from sugar manufacture, and from acetylene and paper industries.
The chemical reactions in lime burning are:
Reaction 1: (900° C, depending on type of limestone)
CaCO3 + heat CaO +CO2
Reaction 2: (at around 750° C):
CaMg(CO3)2 +heat CaCO3 +MgO+CO2
then Reaction 1 (at around 1100° C)
· The chemical process of lime burning shows that the main constituent in the raw material (limestone) is necessarily calcium carbonate (CaCO3). Limestone can have CaCO3 contents exceeding 98 % (as in chalk and various types of shells and coral) or as low as 54 % (in pure mineral dolomite).
· Each type of limestone yields a different quality of lime, depending on the type and quantity of impurities. The purest forms of lime are needed for chemical and industrial use, while impurities can be desirable in limes used for building and road construction. Limestones, called "kankar" in India, that contain 5 to 25 % of clay can produce a hydraulic lime, which hardens in the presence of water, like a cement.
· By-product lime sludge is moulded into bricks or briquettes before firing in kilns.
· The presence of impurities in the limestones influences its behaviour during burning, so that the kiln design and choice of fuel are largely dependent on the raw material and the kind of end product required. Expert advice is therefore essential at a very early stage, in order to achieve satisfactory results, both for the lime producer and user.
· Preparing the raw material is extremely important as only one size of stone (about the size of a man's fist) should be used, in order to facilitate an even gas flow and uniform burning of the lumps. Small-scale firing trials are important to study the behaviour of the raw material and the quality of quicklime it yields, and also to make sure that the lumps do not break apart until they leave the kiln.
Kiln for small scale firing trials (Bibl. 06.08)
· Wood and coal are the most common, traditional fuels. Wood firing produces some of the best quality limes, as it burns with long, even flames generating steam (from the moisture content of the wood), which helps to lower the temperature needed for dissociation (separation of CO: from the carbonates), thus reducing the danger of overburning.
· The wood must be seasoned (dried) and cut into relatively small pieces. The wood supply should be close to the kiln in order to avoid heavy transport costs. About 2 m3 bulk of wood is needed for each tonne of hydrated lime produced. This is a problem, in view of the rapid depletion of timber resources, but a possible solution is to establish fuelwood plantations for continuous replacement of the harvested wood.
· Charcoal gives a higher fuel efficiency, but the lime produced is not as good as that burnt with wood.
· Coal with a high carbon content produces a good lime and can show good fuel economy even in small kilns. Coke is preferable because of its low volatile content (hydrocarbons which can be driven off as vapour), but is hard to ignite, and is, therefore, often mixed with coal.
· Liquid and gaseous fuels, though more expensive, are easier to handle than solid fuels, and burn without producing ash which contaminates the lime.
· The main types are heavy fuel oils, often mixed with used motor oil. The fuel is vaporized, mixed with air and ignited in chambers located around the kiln, producing a fully developed flame before it comes into contact with the limestone.
· Liquified petroleum gases, mainly propane (C3H8) and butane (C4H10), are other useful liquid fuels. Natural gas, such as methane (CH4), and producer gas, which is made from wood, plant material or coal, are used in the same way.
· Whether oil or gases are used, the kilns will necessarily be more sophisticated than those needed for solid fuels.
· Possible alternative fuels are peats and oil shales, and biomass energy, derived from plant material including agricultural and forestry wastes. There are several ways in which they can be used.
· Solar and wind energy are unlikely to be used in the near future.
Kiln design and operation
· A lime kiln is a built structure, in which limestone is heated to a temperature at which CO2 is released, converting the stone into quicklime. The heat is provided by burning suitable fuels, which are either placed in layers between the limestone or mixed with it. Liquid or gaseous fuels are either injected from the sides of the kiln or burnt in adjacent chambers, from which hot gases are passed through the kiln.
· Careful control is needed to maintain the correct temperature long enough to burn the stone completely. Underburnt limestone will not hydrate, while overburnt material is too hard and dense for slaking, or hydrates very slowly.
· As the variety of kiln types is extremely wide, they can only be described here in general terms. The more sophisticated types (eg rotary and fluidized bed kilns) are not dealt with, although in certain situations their use may indeed be worth consideration.
· Batch or intermittent kilns are generally used in remote places, where continuous supplies are not needed (eg small building projects or road construction). They are loaded with limestone and fired until all the stone has been burnt. After cooling, the quicklime is extracted, the limestone reloaded and the kiln fired again. The fuel efficiency is naturally very low, as the kiln walls have to be reheated each time a new batch is fired. On the other hand, it requires little attention during firing. The fuel is burnt below the limestone (in updraught or flare kilns) or within the entire batch (in mixed feed batch kilns).
· Vertical shaft kilns are designed mainly for continuous production: the stone, fed in from the top, gradually drops into the burning zone, then into the cooling zone, and is finally extracted from below, making room for the next load, and so on. The top layer is preheated by the exhaust gases and the air intake below is preheated by the cooling quicklime, thus achieving maximum use of the available heat.
The main design features and operational considerations with regard to vertical shaft mixed feed kilns are:
· Foundations and kiln base: built on a firm ground and dimensioned to carry the shaft and kiln contents; an engineer's advice is needed.
· Shaft dimensions and shape: the cross-sectioned area is related to the desired output (rule of thumb: 1 m2 produces about 2.5 tonnes per day): a circular plan provides better heat distribution; the ratio of height to diameter should be at least 6: 1 for optimum gas flow; the height must be related to the type of limestone, as soft stones tend to get crushed under the pressure, thus restricting the gas flow (kilns for soft chalk should not exceed 5 m height); shafts that taper towards the top (angle about 3°) minimize "hanging" (stone sticking to the sides and forming arches).
· Structural walls: must support the lateral pressure of the limestone (by provision of greater wall thickness at the base, or buttresses, or by means of steel tension bands at intervals of 80 cm, as developed by the Khadi and Village Industries Commission, Bombay); must resist cracking due to heat expansion (by using small bricks rather than big blocks, and lime-sand mortar in narrow joints); wall thicknesses of at least 50 cm for good thermal performance; weather resistant material (natural stone or well-burnt bricks) at least for the top wall courses.
· Linings: at least 22 cm thick, in the upper part of the kiln, resistant to abrasion (eg hard stone or blue engineering bricks); in the firing zone and below, resistant to heat and chemical action (hard, fine-textured refractory bricks laid with very fine joints of fireclay mortar).
· Insulation: usually 5 to 10 cm thick, between wall and lining to retain the heat in the kiln, especially around the calcining zone; different insulations are possible (eg air-gap, rice husk ash or other pozzolana, light-weight aggregate, rockwool).
· Openings: at the top for charging, preferably with lid, if a chimney extends beyond the opening, at the bottom for air to flow in and to remove the cooled quicklime, whereby with a single opening in the centre (inflow type) draught control is easier than with two or more openings (outflow type); around the kiln at different levels as pokeholes and inspection holes, usually the size of a brick (which is used for closing), to regularly loosen stuck limestone lumps and to monitor the temperature within the kiln.
· Chimney: between 2.5 and 6 m high, to improve the draught and thus provide sufficient oxygen for combustion, to cool the quicklime, and to draw the exhaust gases away from operators loading the kiln.
From Bibl. 06.07: Alternative discharge openings of vertical shaft kilns
· The type of lime that is used for building and numerous other processes is hydrated or slaked lime. This is obtained by adding hot water or steam to quicklime. Pure quicklimes react vigorously evolving considerable heat, while impure limes hydrate slowly, or only after the lumps are ground.
Three forms of hydrated lime are commonly produced:
a. dry hydrate, a dry, fine powder, formed by adding just enough water to slake the lime, which is dried by the heat evolved;
b. milk of lime, made by slaking quicklime with a large excess of water and agitating well, forming a milky suspension;
c. lime putty, a viscous mass, formed by the settling of the solids in the milk of lime.
· The most common form is dry hydrate, which is very suitable for storage in silos or airtight bags, and easy to transport. Lime putty, which is an excellent building material, can be stored indefinitely under moist conditions. Milk of lime is generally produced in conjunction with other process industries.
· In small limeworks, slaking is usually done by hand, either on platforms to produce a dry hydrate or in shallow tanks to make lime putty.
· Although the hydration of quicklime is a simple process, it must be carried out with special care, for instance, to see that all the quicklime is completely slaked. Pieces that hydrate too slowly and as a result are overlooked, can cause serious problems later on.
· If water is added too slowly, the temperature of the lime may rise too fast, forming an inactive white gritty compound ("water burnt" lime). If water is added too quickly, a skin of hydroxide may develop, preventing further hydration ("drowned" lime).
The Central Building Research Institute in India has developed a small hydration plant, which requires very little space and eliminates most of the problems of hydration, producing uniform qualities of dry hydrate in a relatively short time.
The location and layout of a lime-works are vital factors that influence the economy and quality of lime production. The illustration (from Bibl. 06.08) shows an appropriate site organization in which distances between successive operations are relatively short.
· Lime is used as a stabilizer in soil constructions with clayey soils, because the lime reacts with clay to form a binder.
· Lime is mixed with a pozzolana (rice husk ash, fly ash, blast furnace slag, etc.) to produce a hydraulic binder, which can partially or completely substitute cement, depending on the required performance.
· Hydraulic lime (made from clay-rich limestone) can be used without a pozzolana.
· Non-hydraulic lime (pure calcium hydroxide) is also used as a binder in renders. It hardens on reaction with the carbon dioxide in the air to change back to limestone (calcium carbonate). This process can take up to 3 years, depending on the climatic conditions.
· Lime is used in cement mortars and plasters to make it more workable.
· Limewash (diluted milk of lime) is used as an external and internal wall coating.
· Lime is produced with less energy input than cement, making it cheaper and environmentally more acceptable.
· In mortars and plasterwork, lime is far superior to portland cement, providing gentle surfaces which can deform rather than crack and help to control moisture movement and condensation.
· Since the strengths produced by portland cement are not always required (and sometimes can even be harmful), lime-pozzolanas provide cheaper and structurally more suitable substitutes, thus conserving the cement for more important uses.
· Limewashes are not only cheap paints, but also act as a mild germicide.
· Soil stabilization with lime requires more than twice the curing time needed for soils stabilized with cement.
· If quicklime is stored in moist conditions (even humid air), it will hydrate.
· Hydrated lime, stored for long periods, gradually reacts with the carbon dioxide in the air and becomes useless.
· Lime bursting (hydration of remaining quick lime nodules) can take place long after the component has dried, causing blisters, cracks and unsightly surfaces.
· Plain limewashes take a long time to harden, and are easily rubbed off.
· Traditional lime burning in intermittent kilns waste a great deal of fuel (usually firewood) and often produce non-uniform, low quality limes (overburnt or underburnt).
· The value of lime is greatly underestimated, especially since portland cement has become a kind of "miracle" binder almost everywhere.
· The curing time of lime stabilized soils can be shortened by using hydraulic limes or adding a pozzolana to non-hydraulic limes.
· Quicklime has to be hydrated before use in construction work, therefore this should be done soon after it is unloaded from the kiln, as hydrated lime is much easier to store and transport.
· To prevent rapid deterioration of dry hydrated lime, it should be stored in air-tight bags.
· It is advantageous to store the lime in the form of lime putty, This can be done indefinitely, as the quality of the lime putty improves the longer it is stored. By this method, even the slowest hydrating quicklime particles are slaked, thus avoiding lime bursting at a later stage.
· A great deal has to be done to disseminate information and assist local lime producers in constructing more efficient lime kilns (in terms of fuel consumption and lime output).
· Similar efforts are needed to rehabilitate lime as one of the most important building materials.
Of the large variety of cements available today, ordinary portland cement (OPC) is the most common, and usually the type referred to when speaking of cement. It is the fine, grey powder that can be mixed with sand, gravel and water to produce a strong and long lasting mortar or concrete.
Portland cement was developed in the 1 9th century and was so named because it resembled a popular building stone quarried in Portland, England. It has since been associated with high strength and durability, and has consequently become one of the most prestigious building materials.
Cement is usually produced in large centralized plants, which incur high capital costs and long transportation distances to most building sites. In most developing countries, production capacities are far below the demand and also on account of losses and deterioration in transports and storage, cement is generally associated with high costs and short supplies.
In order to improve the situation, efforts have been concentrated on the development of small-scale cement plants (also called "mini-cement" plants), particularly in China and India.
Large-scale cement production
· About 95 % of the world's cement is being produced in rotary kilos with daily outputs ranging between 300 and more than 5000 tonnes.
· Limestone (calcium carbonate) and clay (silica, alumina and iron oxide) are ground and mixed with water to form a slurry, which is fed into the upper end of the slightly inclined, refractory lined rotating furnace, which can be more than 100 m long. Hot air of temperatures between 1300° and 1400° C is blown in at the lower end, drying the slurry, which is then sintered and fused into hard balls known as clinker. These drop out of the kiln, are cooled and interground in a ball-mill with about 3 % gypsum to retard the setting of the cement. The finer it is ground, the higher is the rate of the setting and strength development reactions.
· The wet process, described here, has largely been superceded by the dry process, which needs less energy to dry the raw material feed.
· OPC is sold in 50 kg bags, preferably heavy quality multi-ply paper bags. However, in some countries (eg India) reusable jute bags are used, leading to great wastage and difficulties in maintaining quality control.
Small-scale cement production
· This production method utilizes small vertical shaft kilns, a technology that accounts for more than half of China's annual cement production.
· The kiln feed is made of crushed limestone, clay and coal, which are proportioned and finely interground in a ball mill and then made into nodules in a disc nodulizer.
· The nodules are fed into the top conical portion of the kiln, in which the rising preheated air causes the fuel in the nodules to ignite, forming clinker.
· The clinker nodules gradually drop into the cylindrical portion, where it is cooled by the air introduced from below.
· A rotary grate discharges the clinker, which is then interground with gypsum in a ball-mill. Since the nodules are porous, less energy is required for grinding.
· Daily outputs of a vertical shaft kiln can range between 2 and 30 tonnes of ordinary portland cement.
· Numerous varieties of cement are produced by altering the types and proportions of the raw materials to be calcined, or by blending or intergrinding portland cement with other materials. A few common types are:
· Rapid hardening portland cement (more finely ground than OPC; ultimate strength same as OPC).
· Sulphate resisting portland cement (made by adjusting the chemical composition of the raw mix).
· Portland-pozzolana cements (made by blending or intergrinding a pozzolana, eg rice husk ash or fly ash, in proportions of 15 to 40 % by weight, thus saving on cement and improving some of its properties).
· Portland blastfurnace cements (made by blending ground granulated blast furnace slag, thus achieving slower hardening and sulphate resistance).
· Magnesium oxychloride or sorer cement (obtained by calcining magnesium carbonate, achieving much higher strengths than OPC, but is attacked by water).
· High alumina cement (obtained by calcining limestone and bauxite, achieving high early strengths, optimum sulphate resistance, good acid resistance, and heat resistance up to 1300°C; but 3 times the cost of OPC and not suitable for structural concrete).
Hydration of cement
· Water reacts on the surface of the cement grains and diffuses inwards to reach unreacted cement. Therefore, the finer the grains the quicker the reaction.
· The water in the capillary space between the grains is filled with products of the hydration process. The more water used, the larger is the space that needs to be filled, and if there are insufficient hydration products, capillary pores remain, which weaken the cement. Hence, the correct water-cement ratio is important for strength development.
· During hydration, lime is set free. This hardens (by combining with CO2) very slowly and expands in doing so, causing cracking and failure of concrete. By adding a pozzolana, it forms a hydraulic binder, which sets and hardens like cement.
· Setting (which means stiffening) takes place within 45 minutes, but hardening (which means useful strength development) takes several weeks. Specifications are, therefore, based on strengths achieved after 28 days.
· Because they set quickly, cement mixes have to be used as soon as possible.
· In hot climates, cements dry out too quickly and must be kept wet for at least two weeks.
· Cement is used as a binder for several inorganic and organic materials, eg soil-cement, sand-cement blocks, cement-bonded fibre boards.
· It is primarily used together with sand and gravel (and reinforcements) to produce (reinforced) concrete.
· It is used with sand and chicken-wire mesh (or fibres) to produce ferrocement (or fibre concrete).
· Mortars and plaster are made from cement and sand, often mixed with lime for better workability. With a very fine sand it is used for screeding.
· A paint can be made from cement mixed with excess water.
· Cements can achieve extremely high strengths, generally remain unaffected by water, and do not significantly swell and shrink.
· Cements are resistant to fire and biological hazards, if kept clean.
· Cement constructions have a high prestige value.
· With regard to decentralized, small-scale cement production, the advantages are: low capital investment; use of cheaper quality coke or coal; lower transportation costs, due to shorter distances to consumer; lower technical sophistication, thus providing job opportunities even for unskilled labour; adaptability to market demands; capability of using different raw materials and producing a variety of cementitious products; increase of supporting industries around the plant.
· In most developing countries, cement is still too expensive for the majority of the population, and usually in short supply.
· Storage requires great care to avoid premature setting.
· Cracks occur in hot dry conditions due to rapid setting or due to temperature fluctuations.
· Sulphates and salts can cause rapid deterioration.
· Due to the high reputation of cement, it is often used to make over-strong mortars which cause brittleness, or porous mortars which lack durability.
· Increase of supplies and reduction of costs are possible by introducing decentralized, small-scale cement plants.
· Improved bagging and storage methods in dry conditions, but also quick turnover can avoid wastage through premature setting.
· Proper wet curing avoids cracking, and special cements are used to avoid damage by sulphates and salts.
· Unnecessary and wrong usage of cement can be reduced by increased dissemination of information and increased use of lime, eg to improve the quality of cement mixes.
Pozzolanas are natural or artificial materials which contain silica and/or alumina. They are not cementitious themselves, but when finely ground and mixed with lime, the mixture will set and harden at ordinary temperatures in the presence of water, like cement.
Pozzolanas can replace 15 to 40 % of portland cement without significantly reducing the long term strength of the concrete.
Most of the pozzolanic materials described here are by-products of agricultural or industrial processes, which are produced in large quantities, constituting a waste problem, if they remain unused. Even if there were no other benefits, this aspect alone would justify an increased use of these materials. But compared with the production and use of portland cement, these materials contribute to cost and energy savings, help to reduce environmental pollution and, in most cases, improve the quality of the end product.
Types of pozzolanas
· There are basically two types of pozzolanas, namely natural and artificial pozzolanas.
· Natural pozzolanas are essentially volcanic ashes from geologically recent volcanic activity.
· Artificial pozzolanas result from various industrial and agricultural processes, usually as by-products. The most important artificial pozzolanas are burnt clay, pulverized-fuel ash (pfa), ground granulated blast furnace slag (ggbfs) and rice husk ash (RHA).
· The first natural pozzolana to be used in building construction was the volcanic ash from Mt. Vesuvius (Italy), found closeby in the town Pozzuoli, which gave it the name.
· Although the chemical compositions are similar, the glassy material formed by the Violent projection of molten magma into the atmosphere is more reactive with lime, than the volcanic ash formed by less violent eruptions.
· The occurrence of suitable natural pozzolanas is therefore limited to only a few regions of the world.
· Good pozzolanas are often found as fine "rained ashes, but also in the form of large particles or tuffs (solidified volcanic ash), which have to be ground for use as a pozzolana. However, the qualities of such pozzolanas can vary greatly, even within a single deposit.
· Natural pozzolanas are used in the same way as artificial pozzolanas.
· When clay soils are burnt, the water molecules are driven off, forming a quasi-amorphous material which is reactive with lime. This is also true for shales and bauxitic and lateritic soils. This was discovered in ancient times and the first artificial pozzolanas were made from crushed pottery fragments, a traditional technology that is still being widely practiced on the Indian subcontinent, Indonesia and Egypt, using underfired or reject bricks. (In India it is called "surkhi", in Indonesia "semen merah", and in Egypt "homra").
· Alternatively, as reported from a project in India, soils which contain too little clay and too much sand for brickmaking, are cut and removed in blocks, forming circular pits. The blocks are then replaced in the pits, together with alternate layers of firewood. The residue obtained from firing is very friable and needs no pulverization. This is used as masonry mortar by just adding it to lime putty and mixing it, without sand or cement (Bibl. 05.10).
· A similar technique is reported from Java, Indonesia, where clay blocks are burnt in a clamp, disintegrated, sieved and used with lime and sand, sometimes also cement (Bibl. 05.11).
· The qualities of these traditional methods are very variable, but improved methods of calcination have been developed to produce pozzolanas of higher quality and uniformity.
· The illustration shows a vertical shaft kiln (after Thatte and Patel) developed in India. The feed consists of a mixture of clay lumps 50 to 100 mm in size and coal slack (comprising 48 % ash, 31 % fixed carbon and 20 % volatiles). Calcination takes place at 700° C for 3 hours, with the temperature monitored by thermocouples and controlled by an air blower and feed input. The capacity is 10 tonnes per day. A fluidized bed process has been developed by the National Buildings Organization, New Delhi, by which the clay feed is calcined within a few minutes, thus achieving high output rates in a continuous process (Bibl. 08.07).
Pulverized-Fuel Ash (Fly Ash)
· By comparing the production processes of pulverized-fuel ash (pfa), commonly known as fly ash, and ordinary portland cement (OPC), it becomes clear, why pfa can be used as partial replacement of the latter.
· Finely ground coal is injected at high speed with a stream of hot air (about 1500° C) into the furnace at electricity generating stations. The carbonaceous content is burnt instantaneously, and the remaining matter (comprising silica, alumina and iron oxide) melts in suspension, forming fine spherical particles on rapid cooling while being carried out by the flue gases.
· In the production of OPC, limestone and clay, finely ground and mixed, are fed into an inclined rotary kiln, in which a clinker is formed at 1400° C. The cooled clinker is finely ground and mixed with gypsum to produce OPC.
· Depending on the type of coal, pfa contains varying proportions of lime, low-lime pfa being pozzolanic and high-lime pfa having cementitious properties itself. As with other pozzolanas, the lime liberated by the hydration of OPC combines with the pfa to act as a cementitious material.
· The glassy, hollow, spherical particles of pfa have the same fineness as OPC, hence no further grinding is needed. The addition of pfa makes fresh concrete more workable (probably due to the ball-bearing effect of the spherical particles) and homogeneous (by dispersing the cement floes and evenly distributing the water).
Other advantages of using pfa are:
· With increasing age, higher strengths than concrete without pfa are developed.
· Pfa does not adversely influence the structural performance of concrete members.
· Compared to OPC concrete, pfa concrete is lighter, less permeable (due to denser compaction) and with a better surface finish.
· Pfa concrete is also more resistant to sulphate attack and alkali-silica reaction.
· Concretes in which 35 - 50 % by weight of OPC is replaced by pfa have shown satisfactory performances.
· Aggregates derived from fly ash show excellent bonding in pfa concretes, contributing favourably to their performance and durability.
Freshly mixed ordinary portland cement concrete
Dispersion of the cement grains by adding pfa
Ground Granulated Blast Furnace Slag
· Blast furnace slag is a molten material which settles above the pig iron at the bottom of the furnace. It is produced from the various input constituents in the furnace when it reaches 1400° to 1600° C.
· Slow cooling of the slag produces acrystalline material, which is used as aggregate. Rapid cooling with air or water under pressure forms glassy pellets (expanded slag > 4mm, suitable as lightweight aggregate) and granules smaller than 4 mm, which possess hydraulic properties when finely ground.
· The ground slag is blended with OPC to produce portland blast furnace cement (PBFC), whereby the slag content can reach 80 %. However, since PBFC is slower to react than OPC, the reactivity is reduced the higher the percentage of slag.
· Although the early strength of PBFC concretes is generally lower than OPC concretes, the final strength is likely to be higher. The slower reactivity of PBFC develops less heat and can be advantageous in situations where thermal cracking is a problem.
· Apart from improving the workability of fresh concrete, PBFC has high resistance to chemical attack, and its capability of protecting steel reinforcement makes it suitable for use in reinforced and prestressed concrete.
Rice Husk Ash
· The combustion of agricultural residues removes the organic matter and produces, in most cases, a silica-rich ash. Of all the common agricultural wastes, rice husks (also called paddy husks) yield the largest quantity of ash - around 20 % by weight - which also has the highest silica content - around 93 % by weight. It is this high silica content that gives the ash its pozzolanic properties.
· However, only amorphous (non-crystalline) silica possesses these properties, which is why the temperature and duration of combustion are of importance in producing rice husk ash (RHA). Amorphous silica is obtained by burning the ash at temperatures below 700° C. Uncontrolled combustion of rice husks, eg when used as a fuel or in heap burning, usually at temperatures above 800° C leads to crystallization of the silica, which is less reactive.
· The illustrated incinerator, first developed by the Pakistan Council of Scientific and Industrial Research (PCSIR) and later improved by the Cement Research Institute of India (CRI), is made of bricks with many openings to allow good air flow through the rice husk mass. The inner surface is covered with a 16 gauge fine-wire mesh. The husks are filled in from the top and the ash removed from the bottom discharge door. A pyrometer monitors the temperature, which can be controlled by shutting or opening the holes, maintaining a temperature around 650° C for 2 - 3 hours.
· The reactive ash is dark grey to white, depending on the residual carbon in it, which has no negative effects if below 10 %. To improve its reactivity, the ash is ground in a ball mill for about one hour, or longer if it contains crystalline silica. The ash can replace up to 30 % of cement in mortar or concrete. Alternatively, it can be mixed with 30 to 50 % of hydrated lime to be used like cement in mortars, renderings and unreinforced concrete.
· In another process, the ash obtained from heap burning or the production of parboiled rice, is mixed with about 20 to 50 % (by weight) of hydrated lime. This is ground for 6 or more hours in a ball mill to produce ASHMOH, a hydraulic binder suitable for masonry, foundations and general concreting work other than reinforced concrete. A variation of this is ASHMENT, in which the lime is substituted by portland cement (Bibl. 08.04).
· A method has also been developed, using waste lime sludge obtained from sugar refining. This is dried and mixed with an equal amount (by weight) of crushed rice husks and some water. Tennis ball sized cakes are made by hand and sun-dried. These are fired on a grating in an open clamp, to produce a soft powder, which is ground in a ball mill. The hydraulic binder is used in the same way as ASHMOH.
· A variation of this method utilizes soils with at least 20 % clay content instead of lime sludge. The resulting binder can be used as a 30 % mixture with portland cement to make portland pozzolana cement. Tests have shown that the pozzolana is best if the clay is bauxitic.
· At the National Building
Research Institute, Karachi, Pakistan:
The first low-cost house to be built predominantly with rice husk ash and lime, substituting cement completely in the production of hollow load-bearing block, mortar and plaster. 30 % of the portland cement in the precast concrete lintels and roof beams were substituted by RHA.
The essential ingredients of concrete are cement, aggregate (sand, gravel) and water. When mixed in carefully prescribed proportions, they produce a workable mass, which can take the shape of any formwork into which it is placed and allowed to harden in.
Concrete technology is one that requires a great deal of know-how and experience. Therefore, only very general aspects can be dealt with here. If detailed information is required, specialized literature should be consulted, or professional advice sought.
Preparation of concrete mix
· Depending on the use and desired performance of the concrete, careful selection of the type and proportion of cement, aggregates and water is necessary, which is best done by a series of tests (if the qualities of the materials are not standardized or well-known from experience).
· In most cases, a good grain size distribution of fine and coarse aggregate (sand and gravel) is necessary, in order to leave no voids, which weaken the concrete. The more voids, the more cement and water are needed.
· Aggregate particles with rough surfaces and angular shapes create more friction than smooth, rounded particles, which are easier to compact. Silt, clay and dust should be removed, as they interfere with the bond between cement and aggregate, and require more water.
· The water should be as clean as possible, as salts and other impurities can adversely affect the setting, hardening and durability of the concrete. Seawater should be avoided as far as possible, especially in reinforced concrete, in which the steel easily corrodes.
· In special cases, a variety of admixtures can be used, depending on whether the setting should be accelerated or retarded, waterproofing and chemical resistance should be improved, and so on. Correct dosage and quality control are vital to achieve satisfactory results and save costs.
· The aggregate and cement should be well mixed in the dry state. Just before the concrete is used, water is added gradually while the mixing continues. As the water: cement ratio determines the strength and durability of the concrete (excess water produces air voids! ), the addition of water requires special care.
· In ready-mixed concrete, supplied from a central batching/mixing plant, by truck mixers (which are still rare in developing countries) principally the same criteria apply. However, a study by the Cement Research Institute, India, recommends the transportation of "semidry" mixes in small non-agitating vehicles (cheaper! ) and completion of mixing prior to final placing.
· The uniformity of fresh concrete is usually measured by the slump test: filling a conical mould in four layers of equal volume and rodding each layer 25 times, smoothing the top, lifting off the mould and measuring the difference in heights of the mould and the fresh concrete specimen. Slumps between 25 and 100 mm are most suitable.
· Mixes are specified primarily by grade designations, eg C7, C10, C25, etc., which refer to their compressive (C) strengths in N/mm2 (MPa).
· Formwork, which can be reused many times, is usually made of timber boards or steel panels, with joints sufficiently tight to withstand the pressure of compacted concrete, and without having any gaps through which the cement paste can leak.
· The texture of the hardened concrete surface can be predetermined by the type of formwork. If smooth surfaces are needed, concrete remnants from previous castings should be scraped off the forms.
· In order to facilitate removal, the inner surfaces of the formwork should be oiled with a brush or spray.
· If reinforcement is required, it is placed in the formwork after oiling, and spacers (pieces of stone or broken concrete) are placed between the steel and the oiled surface, such that the formwork and steel do not come into contact with each other. This is needed to prevent the steel from remaining exposed on the concrete surface, where it can easily rust.
· The choice of formwork must take into account ease of assembly and removal. In some cases, the formwork can be designed to remain in place (permanent shuttering); for example, where an insulating layer or special facing is needed, these can constitute the formwork (or part of it).
Placing and curing
· The concrete is transported from the mixer to the formwork by cranes, dumpers, barrows, buckets, pipes, or other means, depending on the available facilities. In many developing countries, long chains of workers pass the concrete in small metal pans from one to another. If the concrete is not produced on the site, ready-mixed concrete is brought in a special truck.
· The concrete must be placed without interruption to fill complete sections each time, since joints between concrete placed at different times are weak points.
· After a certain amount of concrete is in the formwork it needs to be compacted to fill up all voids. This is most effectively done by means of a vibrator (either attached to the formwork, or immersed in the concrete) which releases the trapped air. However, for most low cost constructions, which do not need high strengths, hand compaction with a suitable rod can be quite sufficient.
· It is important to immediately wash all the equipment that has been in contact with the concrete, as it will be difficult to remove after hardening.
· The formwork is removed after a few days when the concrete it hard enough. But strength development (curing) takes place over several weeks and a vital prerequisite is that the concrete is kept wet for at least 14 days, eg by covering it with wet jute bags which are regularly watered.
· All the above points, from preparation of concrete mix to curing, apply likewise to in situ construction (at the building site) and to prefabrication.
· Plain mass concrete, with graded or predominantly small sized aggregate, for foundations, floors, paving, monolithic walls (in some cases), bricks, tiles, hollow blocks, pipes.
· No-fines concrete, a lightweight concrete with only single size coarse aggregate (dense or lightweight) leaving voids between them, suitable for loadbearing and non-loadbearing walls, in-fill walls in framed structures or base coarse for floor slabs. No-fines concrete provides an excellent key for rendering, good thermal insulation (due to air gaps), and low drying shrinkage. The large voids also prevent capillary action.
· Lightweight aggregate concrete, using expanded clay, foamed blast furnace slag, sintered fly ash, pumice, or other light aggregate, for thermal insulating walls and components, and for lightweight building blocks.
· Aerated concrete, made by introducing air or gas into a cement-sand mix (without coarse aggregate), for thermal insulating, non-structural uses and lightweight building blocks. Disadvantages are low resistance to abrasion, excessive shrinkage and permeability. However, it is easy to handle and can be cut with a saw and nailed like timber.
· Reinforced concrete, also known as RCC (reinforced cement concrete), which incorporates steel bars in sections of the concrete which are in tension (to supplement the low tensile strength of mass concrete and control thermal and shrinkage cracking), for floor slabs, beams, lintels, columns, stairways, frame structures, long-span elements, angular or curved shell structures, etc., all these cast in situ or precast. The high strength to weight ratio of steel, coupled with the fortunate coincidence of its coefficient of thermal expansion being about the same as concrete, make it the ideal material for reinforcement. Where deformed bars (which have ribs to inhibit longitudinal movement after casting) are available, they should be given preference, as they are far more effective than plain bars, so that up to 30 % of steel can be saved.
· Prestressed concrete, which is reinforced concrete with the steel reinforcement held under tension during production, to achieve stiffness, crack resistance and lighter constructions of components, such as beams, slabs, trusses, stairways and other large-span units. By prestressing, less steel is needed and the concrete is held under compression, enabling it to carry much higher loads before this compression is overcome. Prestressing is achieved either by pre-tensioning (in which the steel is stressed before the concrete is cast) or by post-tensioning (after the concrete has reached an adequate strength, allowing the steel to be passed through straight or curved ducts, which are filled with grout after the reinforcement has been tensioned and anchored). This is essentially a factory operation, requiring expensive, special equipment (jacks, anchorages, prestressing beds, etc.), not suitable for low-cost housing.
· However, the cold-drawn low-carbon steel wire prestressed concrete (CWPC) technology, developed in China, where about 3000 CWPC factories produce 20 million m3 of precast components annually, is a promising alternative. The tensile strengths of low-carbon steel wires (normal steel wires) of 0 6.5 to 8 mm are doubled by drawing them through a die at normal temperatures, producing 3, 4 or 5 mm 0 wires, and saving 30 to SO % of the steel. Concrete grades of C30 are used. The technology is easily understood and implemented, the equipment is simple (Bibl. 09.09).
· Concrete can take any shape and achieve compressive strengths exceeding 60 N/mm2.
· Reinforced concretes combine high compressive strengths with high tensile strengths, making them adaptable to any building design and all structural requirements. They are ideally suited for prefabrication of components and for constructions in dangerous conditions (earthquake zones, expansive soils, etc.).
· The energy requirement to produce 1 kg of plain concrete is the lowest of the manufactured building materials (1 MJ/kg, equalling timber; Bibl. 00.50), while reinforced concrete (with 1 % by volume of steel) requires about 8 MJ/kg.
· The high thermal capacity and high reflectivity (due to light colour) are especially favourable for building in hot dry or tropical highland climates.
· Properly executed concrete is extremely durable, maintenance-free, resistant to moisture penetration, chemical action, fire, insects, and fungal attack.
· Concrete has an extremely high prestige value.
· A variety of processed agricultural and industrial wastes can be profitably used to substitute cement and/or improve the quality of concrete.
· High cost of cement, steel and formwork.
· Difficult quality control on building sites, with the risk of cracking and gradual deterioration, if wrongly mixed, placed and insufficiently cured with water.
· In moist climates or coastal regions, corrosion of reinforcement (if insufficiently protected), leading to expansion cracks.
· Fire resistance only up to about 500° C, steel reinforcement begins to fail (if not well covered) and after fires, RCC structures usually have to be demolished.
· Demolishing concrete is difficult and debris cannot be recycled, other than in the form of aggregate for new concrete.
· Negative electromagnetic effects of reinforced concrete create unhealthy living conditions.
· Cement proportions can bereduced by careful mix design, grading of aggregates, testing, quality control and by substitution with cheaper pozzolanas; also, increased decentralized cement production with sufficient supplies and low wastage (by better bagging) can reduce costs.
· Saving in steel reinforcement can be achieved by good structural design and use of deformed bars or prestressing with cold-drawn low-carbon steel wire.
· Quality control is only possible with a well-trained team and continuous supervision.
· The improvement fire resistance of non-structural components is possible by using high-alumina cements with crushed Bred brick, which resist temperatures up to 1300° C (refractory concrete).
· Crushed fired brick (brick rejects) can be used to substitute gravel aggregate, where these are scarce (eg Bangladesh), resulting in a relatively lightweight concrete of slightly less strength but higher abrasion resistance. Since the brick aggregate absorbs water, more water is required in preparing the concrete mix.
· Expansion joints should be designed, if excessive thermal movement is expected.
Ferrocement is principally the same as reinforced concrete (RCC), but has the following differences:
· Its thickness rarely exceeds 25 mm, while RCC components are seldom less than 100 mm.
· A rich portland cement mortar is used, without any coarse aggregate as in RCC.
· Compared with RCC, ferrocement has a greater percentage of reinforcement, comprising closely spaced small diameter wires and wire mesh, distributed uniformly throughout the cross-section.
· Its tensile-strength-to-weight ratio is higher than RCC, and its cracking behaviour superior.
· Ferrocement can be constructed without formwork for almost any shape.
Ferrocement is a relatively new material, which was first used in France, in the middle of the 19th century, for the construction of a rowing boat. Its use in building construction began in the middle of the 20th century in Italy. Although its application in a large number of fields has rapidly increased all over the world, the state-of-the-art of Ferrocement is still in its infancy, as its long-term performance is still not known.
· The essential ingredients of the mortar which represents about 95 % of ferrocement are portland cement, sand, water, and in some cases an admixture.
· Most locally available, standard cement types are suitable, but should be fresh, of uniform consistency and without lumps or foreign matter. Special cement types are needed for special uses, eg sulphate-resistant cement in structures exposed to sulphates (as in seawater).
· Only clean, inert sand should be used, which is free from organic matter and deleterious substances, and relatively free from silt and clay. Particle sizes should not exceed 2 mm and uniform grading is desirable to obtain a high-density workable mix. Lightweight sands (eg volcanic ash, pumice, inert alkali-resistant plastics) can also be used, if high strengths are not required.
· Fresh drinking water is the most suitable. It should be free from organic matter, oil, chlorides, acids and other impurities. Seawater should not be used.
· Admixtures can be used for water reduction, thus increasing strength and reducing permeability (by adding so-called "superplasticizers"); for waterproofing; for increased durability (eg by adding up to 30 % fly ash); or for reduced reaction between mortar and galvanized reinforcements (by adding chromium trioxide in quantities of about 300 parts per million by weight of mortar).
· The recommended mix proportions are: sand/cement ratio of 1.5 to 2.5, and water/cement ratio of 0.35 to 0.5, all quantities determined by weight. For watertightness (as in water- or liquid-retaining structures) the water/cement ratio should not exceed 0.4. Great care should be exercised in choosing and proportioning the constituent materials, especially with a view to reducing the water requirement, as excessive water weakens the ferrocement.
· The reinforcing mesh (with mesh openings of 6 to 25 mm) may be of different kinds, the main requirement being flexibility. It should be clean and free from dust, grease, paint, loose rust and other substances.
· Galvanizing, like welding, reduces the tensile strength, and the zinc coating may react with the alkaline environment to produce hydrogen bubbles on the mesh. This can be prevented by adding chromium trioxide to the mortar.
· The volume of reinforcement is between 4 and 8 % in both directions, ie between 300 and 600 kg/m3; the corresponding specific surface of reinforcement ranges between 2 and 4 cm2/cm3 in both directions.
· Hexagonal wire mesh, commonly called chicken wire mesh, is the cheapest and easiest to use, and available almost everywhere. It is very flexible and can be used in very thin sections, but is not structurally as efficient as meshes with square openings, because the wires are not oriented in the principal (maximum) stress directions.
· Square welded wire mesh is much stiffer than chicken wire mesh and provides increased resistance to cracking. However, inadequate welding produces weak spots.
· Square woven wire mesh has similar characteristics as welded mesh, but is a little more flexible and easy to work with than welded mesh. Most designers recommend square woven mesh of 1 mm (19 gauge) or 1.6 mm (16 gauge) diameter wires spaced 13 mm (0.5 in) apart.
· Expanded metal lath, which is formed by slitting thin gauge sheets and expanding them in the direction perpendicular to the slits, has about the same strength as welded mesh, but is stiffer and hence provides better impact resistance and better crack control. It cannot be used to make components with sharp curves.
· Skeletal steel, which generally supports the wire mesh and determines the shape of the ferrocement structure, can be smooth or deformed wires of diameters as small as possible (generally not more than 5 mm) in order to maintain a homogenous reinforcement structure (without differential stresses). Alternatively, skeletal frameworks with timber or bamboo have been used, but with limited success.
· Fibres, in the form of short steel wires or other fibrous materials, can be added to the mortar mix to control cracking and increase the impact resistance.
Hexagonal wire mesh
Square woven wire mesh
Expanded metal lath
Square welded wire mesh
Woven mesh (undulated wires)
· The first step is to prepare the skeletal framework onto which the wire mesh is fixed with a thin tie wire (or in some cases, by welding). A minimum of two layers of wire mesh is required, and depending on the design, up to 12 layers have been used (with a maximum of 5 layers per cm of thickness).
· The sand, cement and additives are carefully proportioned by weighing, mixed dry and then with water. Hand mixing is usually satisfactory, but mechanical mixing produces more uniform mixes, reduces manual effort and saves time. The mix must be workable, but as dry as possible, for greater final strength and to ensure that it retains its form and position between application and hardening.
· After checking the stability of the framework and wire mesh reinforcement, the mortar is applied either by hand or with a trowel, and thoroughly worked into the mesh to close all voids. This can be done in a single application, that is, finishing both sides before initial set takes place. For this two people are needed to work simultaneously on both sides.
· Thicker structures can be done in two stages, that is, plastering to half thickness from one side, allowing it to cure for two weeks, after which the other surface is completed.
· Compaction is achieved by beating the mortar with a trowel or flat piece of wood.
· Care must be taken not to leave any reinforcement exposed on the surface, the minimum mortar cover is 1.5 mm.
· Each stage of plastering should be done without interruption, preferably in dry weather or under cover, and protected from the sun and wind. As in concrete construction, ferrocement should be moist cured for at least 14 days.
· Boat construction (one of the most successful uses. especially in China).
· Embankment protection, irrigation canals, drainage systems.
· Silos (above ground or underground) for storage of grain and other foodstuffs.
· Water storage tanks, with capacities up to 150 m3.
· Septic tanks and aqua privies, and even complete service modules with washing and toilet facilities.
· Pipes, gutters, toilet bowls, washbasins, and the like.
· Walls, roofs and other building components, or complete building, either in situ or in the form of precast elements.
· Furniture, such as cupboards, tables and beds, etc. and various items for children's playgrounds.
Some Applications of Ferrocement
Furniture, sanitary units, roofing elements at the Structural Engineering Research Centre, Madras, India.
Latrine squatting slab, washing basin, toilet flush cistern and water tank (made of 5 square elements, assembled on site) at the Housing & Building Research Institute, Dhaka, Bangladesh.
· The materials required to produce ferrocement are readily available in most countries.
· It can take almost any shape and is adaptable to almost any traditional design.
· Where timber is scarce and expensive, ferrocement is a useful substitute.
· As a roofing material, ferrocement is a climatically and environmentally more appropriate and cheaper alternative, to galvanized iron and asbestos cement sheeting.
· The manufacture of ferrocement components requires no special equipment, is labour intensive and easily learnt by unskilled workers.
· Compared with reinforced concrete, ferrocement is cheaper, requires no formwork, is lighter, and has a ten times greater specific surface of reinforcement, achieving much higher crack resistance.
· Ferrocement is not attacked by biological agents, such as insects, vermin and fungus.
· Ferrocement is still a relatively new material, therefore its long-term performance is not sufficiently known.
· Although the manual work in producing ferrocement components requires no special skills, the structural design, calculation of required reinforcements and determination of the type and correct proportions of constituent materials requires considerable know-how and experience.
· Galvanized meshes can cause gas formation on the wires and thus reduce bond strength.
· The excessive use of ferrocement for buildings can create unhealthy living conditions, as the high percentage of reinforcement has deleterious electromagnetic effects.
· Research on the condition of older ferrocement structures.
· Development of simple construction guidelines and rules of thumb which can be applied without special technical knowledge.
· Galvanized mesh can be immersed in water for 24 hours and then dried for 12 hours, in order to allow the salts used during galvanizing to come to the surface. The residue can then be brushed off.
· Problems with galvanized mesh can be reduced by adding chromium trioxide to the mixing water.
· Complete enclosure of dwelling units with ferrocement components (ie for floor, walls and roof) should be avoided.
Fibre concrete (FC) is basically made of sand, cement, fibres and water. In the case of micro concrete (MC) fine aggregate is used instead of fibre. It is one of the newest building materials used in low-cost building. However, through intensive research and wide practical experience in many parts of the world, it has become a mature technology.
The types and characteristics of fibre concrete are extremely
diverse, depending on the type and quantity of fibre used, the type and quantity
of cement, sand and water, the methods of mixing, placing and curing, and - not
least - on the skill of production, supervision and quality control.
The most well-known and, until recently, most successful fibre reinforced concrete was asbestos cement (ac), which was invented in 1899. The serious health risks (lung cancer) associated with mining and processing asbestos have led to the successive replacement of asbestos by a mixture of other fibres (fibre cocktail) in most places.
In the 1960s fibre reinforced concretes, using steel fibre, glass fibre, polypropylene and some other synthetic fibres, were developed and research on them is still underway. However, these can generally be considered inappropriate for applications in developing countries, due to the high costs and limited supplies of such fibres. This section, therefore, mainly deals with natural fibre concrete.
Depending on the available resources in different places, a wide range of natural fibres has been tested. These are essentially organic fibres, since the only practical example of a natural inorganic fibre is asbestos. The organic fibres are either of vegetable (cellulose base) or animal origin (protein base).
Vegetable fibres can be divided into four groups:
· Bast or stem fibres (eg jute, flax, hemp, kenaf)
· Leaf fibres (eg sisal, henequen, abaca)
· Fruit hair fibres (coir)
· Wood fibres (eg bamboo, reeds, bagasse).
Animal fibres include hair, wool, silk, etc., but are less recommended if not perfectly clean, as contaminants, such as grease, weaken the bond between fibre and matrix.
A variety of building elements can be made out of natural fibre concrete or micro concrete, but its most widespread application is in the production of Roman tiles and pantiles for roofing. After a few years of experimental work, large-scale applications in low-cost housing projects with FC sheets began in the late 1970s in several countries. However, the results of these field experiences with FC sheets were extremely diverse, ranging from "very satisfactory" to "complete failure" (leaking roofs, breakage of sheets, etc.), creating controversies and uncertainty about the viability of the new technology.
This situation led SKAT (Swiss Centre for Appropriate Technology) to undertake, together with a number of international experts, a systematic evaluation of production experiences in 19 developing countries, resulting in a state-of-the-art report on "FCR - Fibre Concrete Roofing" in 1986 (Bibl. 11.08). The main conclusions of the study were:
· Most failures in FCR production and application were due to the lack of know-how transfer, inadequate professional training, and consequently insufficient quality control.
· The present level of know-how is sufficiently advanced to ensure the provision of good quality and durable roofing, with a minimum life-span of 10 years or more.
· A square metre of FC sheets or tiles can be produced at a cost of 2 to 4 US$ (that is, 4 to 8 US$ for the FC roof including the supporting structure), which is cheaper than any comparable roofing material, but this cost benefit can be completely reversed, if certain minimum standards of production and installation are not observed.
· The fibre content of FCR is required primarily to hold together the wet mix during manufacture, to inhibit drying shrinkage cracking and to provide early strengths until the roof is installed. In normal portland cement matrices, the fibres decay within months or a few years on account of alkali attack. Hence, FCR must be installed and treated with the same care and precautions as for burnt clay materials or unreinforced concrete.
· The main advantage of the technology is that a cheaper, and thermally, acoustically and aesthetically more satisfactory substitute for galvanized corrugated iron (gci) sheeting can be manufactured locally on any desired scale (usually small or medium scale), with a relatively small capital investment and large job creating effect. Compared to asbestos cement (ac) one advantage is the absence of any health risk.
The FCR study also identified the need for a follow-up program to assist and advise potential and existing producers and users of FCR. So, in collaboration with ITDG, GATE and other AT organizations, a Roofing Advisory Service (RAS) was established in 1987, at SKAT, St. Gall. RAS issues manuals and periodicals and generally serves as a clearing house for information and technical assistance on all aspects of fibre and micro concrete roofing.
For a general understanding of the role played by the respective constituent materials, some of the main points are discussed here:
· The main purpose of reinforcing concrete with fibres is to improve its tensile strength and inhibit cracking. While steel and asbestos reinforcements fulfil this function over many years, natural fibres maintain their strength only for a relatively short period (quite often less than a year), on account of their tendency to decay in the alkaline matrix, especially in warm humid environments.
· For many applications (eg roofing), this loss of strength is not necessarily a drawback. The fibres hold together the wet mix, inhibit cracking while it is being shaped and during drying, and give the product sufficient strength to survive transports, handling and installation.
· When the fibres lose their strength, the product is equivalent to an unreinforced concrete. However, by then the concrete will have attained its full strength, and since cracking had been inhibited in the early stages, it might be stronger than a similar product made without reinforcement.
· The same end-strength of the product can be achieved without fibre (MC). However, during manufacture and transport greater care is required.
· The fibre content is generally about 1 to 2 % by weight, never by volume, as fibre densities can vary greatly.
· Fibre concrete products have been produced with long fibres as well as with short (chopped) fibres, both methods having advantages and disadvantages.
· With properly aligned long fibres higher impact resistance and bending strengths are achieved. The method of working several layers of fibre into the concrete, such that each fibre is fully encased in the matrix, is, however, relatively difficult, and thus rarely done.
· In the short fibre method, the chopped fibres are mixed with the mortar, which is easy to handle as a homogeneous mass. Since the fibres are randomly distributed, they impart crack resistance in all directions. The length and quantity of the fibres is of importance, since too long and too many fibres tend to form clumps and balls, and insufficient fibres lead to excessive cracking.
· Extremely smooth and uniform fibres (eg some varieties of polypropylene) that can easily be pulled out, are ineffective. On the other hand, too good a bond of mortar to fibre will result in a sudden, brittle mode of failure, when the fibres fail in tension.
· If methods can be found to overcome the weakening and decay of natural fibres, a wide range of semi-structural applications of natural fibre concrete will be possible, eg hollow beams, stair treads, etc. Therefore, intensive research is being conducted on fibre durability (see BIBLIOGRAPHY).
· Since natural fibre decay is caused by the alkaline pore water in the concrete, it is necessary to reduce the alkalinity. This is achieved by using high alumina cement or replacing up to 50 % of the portland cement with a highly active pozzolana (eg rice husk ash or granulated blast furnace slag). Best results were obtained by adding ultra-fine silica fume (a by-product of the ferro-silicon and silicon metals industries), but this pozzolana is unlikely to be available in most developing countries.
· In order to seal the pore system of the concrete matrix several methods were tested (eg use of higher proportion of fines, lower water-cement ratio, etc.), and interesting results were achieved by adding small beads of wax to the fresh mortar. When the set concrete is heated (eg by the sun), the wax melts and fills the pore system, thus reducing absorption of water which causes fibre decay.
· A vital requirement is that the fibres are free from all impurities, such as grease which interferes with the fibre-mortar bond, and sugar (as on bagasse fibres) which retards the setting of cement.
· The cementitious matrix of the earlier specimens of the composite contained a large proportion of cement (2 parts cement: 1 part sand), which was why it was named "fibre cement". The new generation of mechanically compacted fibre reinforced composites contains only 1 part cement: 1 to 2 parts sand (depending on the quality of cement, therefore the name "fibre concrete" became more appropriate.
· For MC a proportion of 1 part cement, 2parts sand and 1 part aggregate is usually suitable.
· The proportion of cement needs to be higher if the sand is not well graded and if compaction cannot be done by a vibrating machine. For manual compaction by tamping the cement: sand ratio should be 1: 1.
· Ordinary portland cement of the standard quality available in most places is usually suitable. For the production of roofing components, slow setting qualities should be avoided, as they delay demoulding and thus require far more moulds and working space.
· For applications in which the improvement of fibre durability is essential (and slow setting causes no problems), the cement should be partially replaced by a pozzolana (eg rice husk ash). Since the qualities of cement, pozzolana and fibres differ greatly, the proportion of cement replacement should be determined by laboratory tests.
Sand and aggregate
· In order to obtain as small a proportion of voids, angular sand particles of good grain size distribution should be used. The small particles fill the gaps between the large ones, requiring less cement and resulting in a less permeable matrix.
· For FC products only sand between 0.06 and 2.0 mm is used.
· For MC products between 25 and 50% aggregate is used. The maximum grain size should not exceede two thirds of the product's thickness.
· The sand and aggregate should be of silicious origin or have similar characteristics. They should not contain minerals which may react chemically with the cement.
· Fine particles of silt and clay should be reduced as far as possible, as clay interferes with the bond between sand and cement.
· The correct proportion of sand must be determined by sample tests. Too much sand will result in a brittle, porous product. Too little sand means a wastage of the far more expensive cement and a greater tendency to develop cracks on setting.
· In order to safeguard against corrosion of the steel reinforcements, clean drinkable water is used to prepare concrete mixes. In fibre concrete, impurities, such as salts, do not necessarily affect the fibres, and satisfactory results have already been achieved with brackish water. But it is always recommended to use the cleanest available water.
· A correct water to cement ratio is vital for the quality of the product. The tendency is to use too much water because it makes working with the mix easier. Excessive water gradually evaporates, leaving pores which weaken the product and increase its permeability. The correct water to cement ratio is 0.5-0.65 by weight.
· Admixtures may be useful to accelerate or retard setting, or to improve the workability of the fresh mix, but are likely to be expensive and difficult to get. Generally, no additives are needed for FC/MC products, except in cases where fibre durability requires improvement and waterproofing is vital.
· As discussed above (see Fibres), fibre decay can be retarded by reducing the alkalinity of the cement matrix. This is achieved by adding a suitable pozzolana, such as rice husk ash, fly ash or granulated blast furnace slag.
· Reducing the permeability of the product also retards fibre decay. An interesting method (also discussed above) is to add small beads of wax to the fresh mix. In the hardened concrete, the wax melts on heating, forming an impervious film in and around the voids (Bibl. 11.07).
· A variety of other waterproofing agents is also available, and their selection should be governed by availability, cost and effectiveness.
· The colour of FC/MC products can be changed as desired by adding a pigment (in powder form) to the fresh mix, approximately 10 % by volume of the cement for red pigments, but considerably more for other colours. However, pigments are usually more expensive than cement and constitute a significant cost increase in the end product (Bibl. 11.15).
Hydraulic press and drag mould, for the production of corrugated fibre-cement roofing sheets, reinforced with coir fibre or wood wool. In this method, developed at the Central Building Research Institute, Roorkee, India, the cast sheets are kept pressed in the form during the setting period (4 hours), after which they are demoulded and cured in vertical stacks (Photo: K. Mukerji).
· Corrugated roofing sheets and tiles.
· Flat tiles for floors and paving.
· Light wall panels and cladding elements.
· Render for masonry or concrete walls.
· Door and window jambs, window sills, sunshades, pipes.
· Most other non-structural uses.
· A large variety of cheap, locally available natural fibres (even agricultural by-products) can be used.
· If correctly manufactured and applied, FC/MC products can be the cheapest, locally produced durable material.
· The technology is adaptable to any scale of production, right down to one-man production units, as in the case of small-scale pantile production.
· The thermal and acoustical performance of FC/MC roofing is superior to that of gci sheets.
· The alkalinity of the concrete matrix prevents the fibres from being attacked by fungi and bacteria.
· In many developing countries, the limited availability and high price of cement can make FC/MC an inappropriate alternative to other locally produced materials.
· Good quality FC/MC products can only be made by well-trained workers, with great care in all stages of production and with regular and thorough quality control. Without these, failure is almost certain.
· The introduction of this relatively new material faces great reluctance and mistrust, on account of past negative experiences or lack of information.
· Incorrect handling, transportation and installation of FC/MC products can easily develop cracks or break, becoming weak or useless before beginning its service life.
· In areas of limited supplies, the local production and distribution of cement should receive special attention and support, as without the availability of sufficient, standard priced, good quality cement, the FC/MC technology is not viable.
· Know-how transfer in the form of training courses and technical assistance by experienced practitioners is an essential requirement at the outset of every FCR/MCR project (Information available through RAS at SKAT, St. Gall).
· Problems of damage during handling, transports and installation can be reduced by making smaller products. Roofing sheets should not be longer than 1 m, and they should be transported (eg in trucks) standing vertically and tied securely, rather than lying, to avoid breakage.
· FC/MC roofs must be treated like clay tile roofs, and moving on them should not be done without crawling boards.
· The more successful FC/MC applications there are in a country, the greater will be the acceptance of the new technology.
Considering that various living creatures build shelters out of leaves, grasses and natural fibres, these materials were perhaps the earliest building materials of mankind, where caves or other natural dwellings were not available.
These materials are available in continuous supply in all but the most arid regions. In some places, they constitute the only useful construction material available, in others they are used together with a variety of additional materials.
The common features of these vegetable (cellulose based) materials are their renewability and their low compressive strengths, impact resistance and durability. Single fibres, grasses or leaves are usually too weak to support their own weight, but in larger quantities, when twisted, interwoven, bundled or compressed, can be used for various structural and nonstructural applications in building construction.
Reed houses of the Uru-Indians, Lake Titicaca, Peru
Mudhif (guest house) of the Ma'dan (Marsh Arabs), Iraq: bundled giant reeds as frame structure and scaffold, reed mats as cladding
Sidamo dwelling, Ethiopia: basket-like structure
Examples of traditional dwellings made of grasses and leaves (Bibl. 23.17)
· Natural fibres (such as sisal, hemp, elephant grass, coir) as reinforcements in soil constructions or fibre concrete and other composite elements (eg fibre boards).
· Natural fibres, twisted to ropes, to tie building elements together or to produce tensile structural members, especially in roof construction.
· Straw for thatch roofs or for making particle boards. In an industrial process, compressed straw slabs (Stramit) are produced by heat and pressure, without any binders, but with paper on both sides.
· Reeds, bundled or tied together as boards, or split and woven as mats, for various uses as columns, beams, wall cladding, sun screens, or roofing material, ores substructure for wattle and daub constructions.
· Leaves, mainly palm leaves, for thatch roofs or for making mats and woven panels for floors, walls and roofs.
Production and installation of Raphia palm leaf tiles, Ghana
· Usually locally available abundant, cheap (or even no-cost), quickly renewable materials (which can also be grown in the backyard).
· Traditional techniques (in most cases), easily comprehended and implemented by local people.
· Thatch roofing, if properly implemented, can be perfectly waterproof and possesses good thermal and acoustical properties.
· Reed constructions have high tensile strengths, good strength-weigh ratio, hence usually good earthquake performance. In case of collapse, their light weight causes less damage and injuries than most other materials.
· Compressed straw slabs have high dimensional stability, and resistance to impact and splitting, are not easily ignitable, and (if kept dry) are not attacked by biological agents. The slabs are used like timber boards.
· In most cases, low life expectancy, about 2 to 5 years, though with good constructions and maintenance useful service lives of 50 or more years are achievable (in the case of reed thatching).
· Vulnerability to biological agents (attraction and nesting of insects, rodents, birds, and development of fungi and rot).
· Risk of fire, either originating within the building or spread through flaming or glowing fragments carried by wind.
· Tendency to absorb moisture, thus becoming heavy, accelerating deterioration and creating unsanitary conditions.
· Low resistance to destruction by hurricanes.
· Deformation and gradual destruction due to impact, structural stresses and fluctuations in temperature and humidity.
· Low acceptance due to general view that these materials are inferior, used only for "poor people's houses.
· Impregnation of materials against biological hazards and fire, either by pretreatment or surface application, similar to bamboo and timber preservation. (Caution: these are costly, and easily washed out by rain, contaminating surroundings and drinking water collected from roofs. Moreover, fire resistant treatments may promote mould growth, leading to rapid decay.)
· Wide roof overhangs and roof pitches of at least 45° help to protect exposed surfaces and drain off rainwater quickly.
· Reduction of fire risk on thatch roofs by application of a coat of stabilized soil on the exterior surface to prevent ignition by wind-borne fragments, and restrict air-flow through the thatch in the event of fire.
· Maintenance of dry conditions and good ventilation to avoid attack by biological agents. In many traditional dwellings, smoke is developed inside the houses to prevent rot and nesting of insects.
The use of bamboo as a building material probably dates back to the invention of the earliest tools for construction. Thus, being such an old and well established, traditional technology, it has produced a great wealth of forms and construction techniques, which resulted from all kinds of requirements and constraints governed by climate, environment, religion, security, social status and so on. But despite this immense variety of applications of a single material, it evidently possesses an almost unlimited potential for the development of new forms and methods of construction, making use of its characteristic properties.
· Bamboo is a perennial grass found in most tropical and subtropical regions, and also some temperate zones. Well over 1000 species of some 50 genera are known, the largest number occuring in Southern Asia and the islands between Japan and Java.
· Bamboos differ from grasses in the long life-span of the culms (hollow stalk), their branching and lignification (development of woody tissues). Like leaf-bearing trees, they shed their leaves annually and grow new branches, increasing their crown every year.
· Bamboo is the fastest growing plant, and has been reported to grow more than one metre in a single day. Bamboo culms can reach their full height (giant species grow 35 metres or more) within the first six months of growth, but it takes about 3 years to develop the strengths required for construction, and full maturity is generally achieved after 5 or 6 years of growth.
· Bamboos flower only once in their lifetime. Depending on the species this happens every 10 to 120 years, and every bamboo of the same species, even if planted in different countries, will flower simultaneously. The leaves that are shed before flowering are not replaced by new ones and the culms die. Regeneration takes place after 10 or more years. In places where a bamboo species constitutes a valuable natural resource, its death can have serious economic consequences for the people. But also animals, like the rare giant panda in Chinas's Sichuan Province, are threatened with extinction now that their food source, the arrow bamboo, is flowering and dying en masse.
· There are two main types of bamboo:
a) sympodial, or clump forming bamboo, found in the warmer regions, and
b) monopodial, or running bamboo, found in the cooler zones.
· The roots of bamboo are called rhizomes, which grow sideways below the ground. The rhizomes of sympodial bamboo multiply with short links symmetrically outward in a circle from which the bamboo shoots grow, forming clumps. Monopodial bamboo sends its rhizomes in all directions covering a large area with widely spaced culms.
· The hollow, cylindrical bamboo culms comprise a fibrous, woody outer wall, divided at intervals by nodes, which are thin, hard transverse walls that give the plant its strength. Branches and leaves develop from these nodes.
Harvesting and preservation
· Untreated bamboo deteriorates within 2 or 3 years, but with correct harvesting and preservative treatment, its life expectancy can increase about 4 times.
· Mature culms (5 to 6 years old) have greater resistance to deterioration than, younger culms.
· Since fungal and insect attack increases with the moisture content, bamboo should be harvested when the moisture content is lowest, that is in the dry season in the tropics, and autumn or winter in cooler zones.
· The culms should be cut 15 to 30 cm above the soil level immediately above a node, so that no water can accumulate in the remaining stub, as this could destroy the rhizomes.
· The freshly cut culms, complete with branches and leaves, should be left standing for a few days (avoiding contact between the cut surface and the soil), allowing the leaves to transpire and reduce the starch content of the culm. This method, called "clump curing", reduces attack by borer beetles, but has no effect on termites or fungi.
· When considering preservative treatments of bamboo, non-chemical methods should be given priority.
· Stacks of bamboo are smoked above fire places or in special chambers, destroying the starch and making the outer wall layer unpalatable to insects. However, cracks can occur, which eventually facilitate insect attack.
· Immersion of bamboo in (preferably flowing) water for 4 to 12 weeks removes starch and sugar which attract borer beetles. Large stones are needed to keep the poles submerged.
· Application of lime slurry or coat of cow dung, creosote (a product of coal tar distillation) and borax, though not indoors, because of strong odours.
· Effective resistance to termites, most types of fungus and fire is achieved mainly by chemical treatment. However, great care must be exercised in the choice of preservative, application method and security measures. In most industrialized countries, a number of highly poisonous preservatives are banned, but suppliers and government institutions in developing countries and even recent publications still recommend their use. No chemical preservative should be used without full knowledge of its composition, and those containing DDT (dichlor-diphenyl-trichlorethane), PCP (pentachlorphenol), Lindane (gamma-hexachloro-cyclohexane) and arsenic SHOULD BE AVOIDED.
· Research on non-poisonous preservatives is still underway and full clarity on the toxicity of the recommended and currently available chemicals has not yet been attained. However, it seems safe to use preservatives based on borax, soda, potash, wood tar, beeswax and linseed oil. Their resistance to biological agents is less than that of the poisonous chemicals mentioned above, but can be equally effective in conjunction with good building design (exclusion of moisture, good ventilation, accessibility for regular checks and maintenance, avoidance of contact with soil, etc.). Several methods of chemical treatment are possible:
· Brushing and spraying of culms, which has only a temporary effect, because of the low penetration of the preservatives.
· Immersing the lower portion of freshly cut culms (which still have leaves), in a preservative solution, which is drawn up the capillary vessels by the transpiration of the leaves. This method (called "steeping") only works with fairly short culms, as the liquid may not rise to the top of long culms.
· Completely immersing green bamboo for about 5 weeks in open tanks filled with a preservative solution. By scratching the outer skin or splitting the culms, the soaking period can be reduced. With alternate hot and cold baths, the process can be still quicker and more effective.
· Replacing the sap with a preservative solution, by allowing it to slowly flow from one end of the culm to the other, where the sap is forced out. When the sap is removed, the preservative solution can be collected and reused. The process (called the "Boucherie" method) takes 5 days, but can be reduced to a few hours by pressure treatment.
· Whole culms for pile foundations (but of low durability), building frame structures, beams, trusses, grid shell structures, stairs, ladders, scaffolding, bridge constructions, pipes, fencing, furniture, musical instruments.
· Half culms as purling, roof tiles, gutters, and for floors, walls, concrete reinforcement ("Bamboocrete"), grid shell structures.
· Split bamboo strips for matting and woven panels, ornamental screens, concrete reinforcement, grid shell structures, fencing, furniture.
· Bamboo boards (split and flattened whole culms) for floor, wall and ceiling panels, doors and windows.
· Bamboo fibres and chips for fibreboards, particle boards and fibre concrete.
· Bamboo is abundantly available, cheap and is quickly replaced after harvesting, without the serious consequences known from excessive use of timber (environmental acceptability! ). The annual yield by weight per unit area can reach 25 times the yield of forests in which building timber is grown. Bamboo can be grown in the backyard.
· Handling during felling, treatment, transportation, storage and construction work is possible with simple manual methods and traditional tools.
· No waste is produced: all parts of the culm can be used; the leaves can be used for thatching or as animal feed.
· The pleasant smooth, round surface requires no surface treatment.
· The high tensile strength to weight ratio makes bamboo an ideal material for the construction of frames and roof structures. With proper design and workmanship, entire buildings can be made of bamboo.
· Bamboo houses provide comfortable living conditions in hot climates.
· On account of their flexibility and light weight, bamboo structures can withstand even strong earthquakes, and in case of collapse, cause less damage than most other materials. Reconstruction is possible within a short time and at low cost.
· Bamboo has relatively low durability, especially in moist conditions, as it is easily attacked by biological agents, such as insects and fungus.
· Bamboo catches fire easily.
· The low compressive strength and impact resistance limit its application in construction. Wrong handling, bad workmanship and incorrect design of bamboo structures can lead to cracking and splitting which weaken the material and make it more vulnerable to attack by insects and fungus. Nails cause splitting.
· The irregular distances between nodes, the round shape and the slight tapering of the culms towards the top end makes tight-fitting constructions impossible, and therefore, cannot replace timber in many applications.
· Bamboo causes greater tool wear than timber.
· Bamboo preservative treatments are not sufficiently well-known, especially the high toxicity of some chemical preservatives recommended by suppliers and official bodies.
· Certain bamboo species have a natural resistance to biological attack, hence their cultivation and use should be encouraged.
· Only mature culms should be used, properly treated (see Harvesting and preservation), not stored for too long (if at all, then without contact with the ground), carefully handled (avoiding cracks or damage of the hard outer surface), and installed in carefully designed structures (ensuring dry conditions, good ventilation of all components, accessibility for inspection, maintenance and replacement of attacked members).
· Fire protection is achieved by treatment with boric acid (also effective fungicide and insecticide) and ammonium phosphate.
· Predrilling is essential to avoid splitting, if nails, screws or pegs are used. Fastening of joints by means of lashing materials is more appropriate for bamboo constructions.
· Bamboo should not be used where tight-fitting components are required. Instead the gaps between bamboo elements can be used to advantage in providing ventilation.
· Recommendations for preservative treatments with chemicals should not be followed blindly. Different opinions of experts should be sought. And irrespective of the type of preservative used, care should be taken to protect the skin and eyes from coming into contact with it. The need for thorough safety precautions cannot be overstressed.
Timber is not only one of the oldest building materials, along with stone, earth and various vegetable materials, but has remained until today the most versatile and, in terms of indoor comfort and health aspects, most acceptable material.
However, timber is an extremely complex material, available in a great variety of species and forms, suitable for all kinds of applications. This diversity of timber products and applications requires a good knowledge of the respective properties and limitations as well as skill and experience in order to derive maximum benefits from timber usage.
Although only a small proportion of the timber harvested is used for building, the universal concern about the rapid depletion of forests, especially the excessive felling of large old trees (which take hundreds of years to replace) and the great environmental, climatic and economic disasters that follow deforestation, has led to a great deal of research into alternative materials and rationalized utilization. Since timber cannot be completely replaced by other materials, it shall long remain one of the most important building materials, and hence great efforts are required to maintain and renew timber resources with continuous, large scale re-afforestation programs.
· The cross-section of a tree trunk or branch reveals a number of concentric rings, with the innermost ring being the oldest. The trunk thickness increases by the addition of new rings, usually one ring each year, but because of the exceptions to this rule, they are called growth rings (instead of annual rings).
· The rings comprise minute tubular or fibrous cells (tracheids) which transport moisture and nutrients to all parts of the tree. The early wood (springwood) formed during the growth period has large cells, while in the dry season the late wood (summerwood) grows more slowly, has thicker cell walls and smaller apertures, forming a narrower, denser and darker ring, which gives the tree structural strength.
· As each new ring forms a new band of "active" sapwood, starch is extracted from an inner sapwood ring (sometimes substituted by natural toxins) adding a further ring to the "inactive" heartwood core. Mechanically there is hardly any difference between sapwood and heartwood, but sapwood is usually lighter in colour and contains substances (eg starch, sugar, water) which attract fungi and some insects.
· The slower the tree grows, the narrower are the growth rings, and the denser and stronger is the timber. Its resistance to biological hazards is also usually greater.
STRUCTURE OF A TREE TRUNK (HARDWOOD AND SOFTWOOD)
USABLE PARTS OF A COCONUT PALM
· Timbers are generally classified as hardwoods or softwoods. Hardwoods are from broad-leaved trees, in the tropics usually evergreen, in temperature zones usually deciduous (shedding their leaves annually). Softwoods are generally from coniferous (cone-bearing) trees, found mainly in temperate zones. The differentiation is only in botanical terms, not in mechanical properties, as some hardwoods (eg balsa) are much softer than most softwoods.
· In recent years, coconut timber has been found to be a good substitute for the common timber varieties. While cocowood is related to hardwood, there are some basic differences in growth characteristics: cocowood has no heartwood and sapwood, no annual rings and hence no increase in diameter; the age is determined by circumferential demarcations along the length of the bark; it has no branches and knots; the density decreases from the outer part to the centre, and from the lower part to the upper portion of the trunk. Coconut timber is commercially useful only after 50 years of age, when the copra yield begins to decrease rapidly.
Types and properties of timbers
· Timber for building construction is divided into two categories: primary and secondary timber species.
· Primary timbers are generally slow-grown, aesthetically appealing hardwoods which have considerable natural resistance to biological attack, moisture movement and distortion. As a result, they are expensive and in short supply.
· Secondary timbers are mainly fast-grown species with low natural durability, however, with appropriate seasoning and preservative treatment, their physical properties and durability can be greatly improved. With the rising costs and diminishing supplies of primary timbers, the importance of using secondary species is rapidly increasing.
· Research activities in several Asia-Pacific countries have shown cocowood to be a viable secondary timber, which is abundantly available in most tropical costar areas. However, special knowledge and equipment is required in processing cocowood, as each portion of the coconut trunk has a different density and strength, and its high silica content and hard outer portion causes rapid dulling of sawteeth (requiring special tungsten-carbide blades).
· Without considering the many exceptions, the main properties of timber are: relative low density compared with other standard building materials; high strength: weight ratio with the highest tensile and compressive strengths displayed parallel to the grain; elasticity; low thermal conductivity; growth irregularities; tendency to absorb and release moisture (hygroscopicity); combustibility; renewability.
· The shrinkage of wood is a common feature and varies according to the direction of shrinkage: radial shrinkage is about 8 % from the green to the dry state; the corresponding tangential shrinkage is about 14 to 16 %; in the longitudinal direction shrinkage is negligible (0.1 to 0.2%).
Seasoning and preservative treatment
· Seasoning is the process by which the moisture content of timber is reduced to its equilibrium moisture content (between 8 and 20 % by weight, depending on the timber species and climatic conditions). This process, which takes a few weeks to several months (depending on timber species and age, time of harvesting, climate, method of seasoning, etc.), makes the wood more resistant to biological decay, increases its strength, stiffness and dimensional stability, and reduces its weight (and consequently transportation costs).
· Air seasoning is done by stacking timber such that air can pass around every piece. Protection from rain and avoidance of contact with the ground are essential.
· Forced air drying is principally the same as air seasoning, but controls the rate of drying by stacking in an enclosed shed and using fans.
Solar timber seasoning kilns
designed by the Commonwealth Forestry Institute (CFI) and ITDG, UK: Solar heat is collected by a series of black-painted panels; hot air is circulated through the stacks by two large fans; the humidity is released through a series of vents.
designed by CBRI, Roorkee, India: two solar collectors transport heated fresh air into the seasoning chamber and the humid air escapes through the chimney; the kiln works without fans on the principle of thermal air circulation.
· Kiln drying achieves accelerated seasoning in closed chambers by heating and controlling air circulation and humidity, thus reducing the time by 50 to 75 %, but incurring higher costs. An economic alternative is to use solar heated kilns.
· Seasoning time is greatly reduced if the timber is harvested in the dry or winter season, when the moisture content of the tree is low.
· Seasoning alone is not always sufficient to protect timbers (particularly secondary species) from fungal decay and insect attack. Protection from these biological hazards and fire is effectively achieved by preservative treatments with certain chemicals.
· The chemicals and methods of application are generally the same for timbers, as are described in the previous section on Bamboo. Hence the comments about the avoidance of highly poisonous preservatives are equally valid in the case of timber.
· When considering preservative treatment of timber, it should be remembered that timber is the healthiest of all building materials and it is paradoxical to "poison" it, especially when other methods can be implemented to protect it, for instance, with non-toxic preservatives and good building design (exclusion of moisture, good ventilation, accessibility for periodical checks and maintenance, avoidance of contact with soil, etc.).
· Pole timbers, generally from young trees (5 - 7 years) with the barks peeled off, seasoned and treated as required. The cost and wastage incurred by sawing is eliminated and 100 % of the timber's strength is used. A timber pole is stronger than sawn timber of equal cross-sectional area, because the fibres flow smoothly around natural defects and are not terminated as sloping grain at cut surfaces. Poles also have large tension growth stresses around their perimeters and this assists in increasing the strength of the compression face of a pole in bending.
· Sawn timber, mainly from older trees with large diameter trunks, cut in rectangular sections as beams or boards. The part of the trunk from which they are cut and the slope of grain have a great effect on the quality of the product (as shown in the diagrams). The cutting of logs before seasoning is called conversion; re-sawing and shaping after seasoning is called manufacture.
· Plywood, made of several plies ("peeled" off a pre-boiled log by rotating it against a knife) glued together such that the direction of grain of each ply runs at right angles to the ply on either side, producing extremely large panels of greater strength and lower moisture movement than sawn timber boards. As the outer sides must have uniform strength and moisture movement properties, there must always be an odd number of plies. Thicknesses range from 3 to 25 mm. A major problem is the use of formaldehyde-based glues, which are highly toxic.
· Blockboard, comprising a solid core of (usually secondary timber) blocks up to 25 mm wide, faced each side with veneers (of primary timbers), with their grain at right angles to that of the blocks.
· Glue-laminated wood, composed of layers of wood with the orientation of the grain of each layer usually in the same direction, or varied according to the intended use of the product. By this method, straight or curved structural members of very large (even varying) cross-sections and great lengths can be produced with low grade timbers of small sizes, achieving high strengths, dimensional stability and very pleasing appearance.
· Particle board (also called chipboard), principally made of wood chips (but also from other fibrous or small-sized ligno-cellulose materials), which are dried, blended with a synthetic resin and hot-pressed (requiring about 8 % binder) or extrusion-pressed (requiring only 5 % binder) to almost any desirable shape. Hot-pressed boards are stronger than extruded boards; and moisture movement acts at right angles to the plane of hot-pressed boards, and parallel to the plane of extruded boards. To improve their strengths, extruded boards are invariably veneered.
· Fibre board (ranging from "softboards" having good thermal insulation, to "hardboards" having properties similar to plywood) principally made of wood (or other vegetable) fibres, which interlock mechanically, requiring no adhesive as the lignin in the fibres acts as the bonding agent. The sheets are either hot-pressed (cardboards) or simply dried without pressing (softboards), and may contain additives such as water repellents, insecticides and fungicides.
· Wood-wool slabs, comprising long wood shavings saturated with an inorganic binder (such as portland cement or magnesium oxychloride) and compressed (for 24 hours, before demoulding and curing for 2 to 4 weeks). Various wood species can be used, except those that contain appreciable amounts of sugar, which retards the setting of cement. Wood-wool slabs are relatively light in weight, elastic, resistant to fire, fungal and insect attack, can be easily sawn like timber boards and plastered.
· Saw dust, and other finely chipped forestry or sawmill by-products, as additive in clay brick production. The wood particles are burnt out, producing porous, lightweight fired clay bricks.
· Tannin based adhesives, extracted from the bark of certain trees, used in particle board production.
· Wood tar, obtained from the dry distillation of timber, and used as a timber preservative.
· Complete or partial building and roof frame structures, using pole timber, sawn timber beams, or glue laminated elements.
· Structural or non-structural floors, walls and ceilings or roofs, made of pole timber (block construction), sawn timber boards, or large panels from plywood, particle board, fibre board or wood-wool slabs; in most cases, suitable for prefabricated building systems.
· Insulating layers or panels made of wood-wool slabs or softboard.
· Facing of inferior qualify timber elements with timber ply or veneer, to obtain smooth and appealing surfaces, or facing of other materials (brickwork, concrete, etc.) with boards and shingles.
· Door and window frames, door leaves, shutters, blinds, sun-screens, window sills, stairs and similar building elements, mainly from sawn timber and all kinds of boards and slabs.
· Roof constructions, including trusses, rafters, purling, lathing and wood shingles, mainly from pole or sawn timber.
· Shuttering for concrete or rammed earth constructions and scaffolding for general construction work, from low grade pole and sawn timber.
· Furniture, using any or combinations of the timber products described above.
· Timber is suitable for construction in all climatic zones, and is unmatched by any other natural or manufactured building material in terms of versatility, thermal performance and provision of comfortable and healthy living conditions.
· Timber is renewable and at least secondary species are available in all but the most arid regions, provided that re-afforestation is well planned and implemented.
· Most species have very high strength: weight ratios, making them ideal for most constructional purposes, particularly with a view to earthquake and hurricane resistance.
· Timber is compatible with traditional skills and rarely requires sophisticated equipment.
· The production and processing of timber requires less energy than most other building materials.
· Timber provides good thermal insulation and sound absorption, and thicker members perform far better than steel in fire: the charred surface protects the unburnt timber, which retains its strength.
· The use of fast growing species helps to conserve the slow growing primary species, thus reducing the serious environmental problems caused by excessive timber harvesting.
· Using pole timber saves the cost and wastage of sawing and retains its full strength, which is greater than sawn timber of the same cross-sectional area.
· Since cocowood was previously considered a waste material with immense disposal problems, its utilization as a building material not only solves a waste problem but provides more people with a cheap, good quality material and conserves a great deal of other expensive and scarce timber resources.
· All the timber-based sheets, boards and slabs provide thin components of sizes that can never be achieved by sawn timber. Apart from requiring less material by volume (which generally consists of lower grade timber or even wastes), larger, lighter and sufficiently strong constructions are possible.
· Demolished timber structures can often be recycled as building material, or burnt as fuelwood, the ash being a useful fertilizer, or processed to produce potash (a timber preservative).
· High costs and diminishing supplies of naturally resistant timber species, due to uncontrolled cueing and exports, coupled with serious environmental problems.
· Extreme hardness of some dried timbers (eg cocowood) making sawing difficult and requiring special saws.
· Thermal and moisture movement (perpendicular to the grain) causing distortions, shrinkage and splitting.
· Susceptibility of cheaper, more abundantly available timber species to fungal decay (by moulds and rot) and insect attack (by beetles, termites, etc.).
· Fire risk of timber members and timber products with smaller dimensions.
· High toxicity of the most effective and widely recommended chemical preservatives, which represent serious health hazards over long periods.
· Failure of joints between timber members due to shrinkage or corrosion of metal connectors.
· Discoloration and embrittlement or erosion of surface due to exposure to sunlight, wind-borne abrasives or chemicals.
· Conservation of forest resources by comprehensive long-term re-afforestation programs, and use of fast growing timber varieties and forestry by-products, thus also reducing costs.
· Harvesting timber in the dry or winter season, when the moisture and starch content, which attracts wood-destroying insects, is lowest.
· Sawing of hard timber species (eg cocowood) when still green, since the moisture in the fresh logs lubricates the saw.
· Reduction of moisture content to less than 20 % by seasoning, in order to prevent fungal growth. Care should be taken to control and slow the rate of drying to avoid cracking, splitting or other defects.
· Temperatures below 0° C and above 40° C also prevent fungal growth, as well as complete submersion in water.
· Chemical treatment of timber against fungi, insects and fire should only tee done with full knowledge of the constituent substances, their toxicity (especially the long-term environmental and health hazards associated with their production and use), the correct method of application and the requisite precautionary measures. Opinions from different experts should be sought, in order to determine the least hazardous option. Proposals, such as facing of particle board with wood veneer or plastic laminate, are not always acceptable, as the emission of formaldehyde fumes is not reduced but takes place over a longer period.
· Indoor and outdoor uses of timber should be differentiated according to durability and degree of toxicity: under ideal (dry, well-ventilated, clean) conditions, even low-durability timbers can be used indoors; treated timbers that could represent a health hazard should only be used externally, but well protected from rain, if leaching out of toxic chemicals is expected.
· Good building design using well seasoned wood, good workmanship and regular maintenance can considerably reduce the need for chemically treated timbers.
· Good design of timber constructions includes: avoidance of ground contact: protection against dampness by means of moisture barriers, flashing and ventilation; avoidance of cavities, which can act as flues spreading fire rapidly; accessibility to all critical parts for regular maintenance; provision of joints designed to accomodate thermal and moisture movement; avoidance of metal connectors in places exposed to moisture, protection of exterior components from rain, sunlight, and wind by means of wide roofs and vegetation.
Metals are not generally considered appropriate materials for low-cost constructions in developing countries as they are usually expensive, in most cases imported, and very often require special tools and equipment. However, only a very small percentage of buildings are constructed without the use of metals, either as nails, hinges, roofing sheets or reinforcement in concrete components.
Metals used in construction are divided into two main groups:
· Ferrous metals: irons and steels
· Non-ferrous metals: aluminium (Al), cadmium (Cd), chromium (Cr), copper (Cu), lead (Pb), nickel (Ni), tin (Sn), zinc (Zn).
· All ferrous metals are made from pig iron, which is produced by heating iron ore, coke, limestone and some other materials, in a blast furnace.
· Cast irons are alloys of iron, carbon (in excess of 1.7 %), silicon, manganese and phosphorus. They have relatively low melting points, good fluidity and dimensional stability.
· Wrought iron is pure iron with only 0.02 to 0.03 % carbon content, is tough, ductile and more resistant to corrosion than steel, but is expensive and unsuitable for welding, so that it has almost completely been replaced by mild steel.
· Steels are all alloys of iron with carbon contents between 0.05 and 1.5 %, and with additions of manganese, silicon, chromium, nickel and other ingredients, depending on the required quality and use.
· Low carbon steels, with less than 0.15% carbon, are soft and used for wire and thin sheet for tin plate.
· Mild steels, with 0.15 to 0.25 % carbon, are the most widely used and versatile of all metals. They are strong, ductile and suitable for rolling and welding, but not for casting.
· Medium carbon steels, with up to 0.5 % carbon, are specialist steels used in engineering.
· High carbon steels, with up to 1.5 % carbon, have high wear resistance, are suitable for casting, but difficult to weld. They can be hardened for use as files and cutting tools.
· Aluminium, the the most common element, but difficult to recover as a metal (produced with very high energy input and high costs), is the lightest metal, has good strength, high corrosion resistance, high thermal and electrical conductivity, and good heat and light reflectivity. Aluminium and its alloys have numerous applications in building construction, but their high costs and limited availability in most developing countries makes them less appropriate building materials.
· Copper is an important non-ferrous metal, available in its pure form, or as alloys, such as brass, bronze, etc., and suitable for a large number of special uses, but with few applications in low-cost constructions.
· Lead, mainly used in its pure form, is the densest metal, but also the softest, and thus weakest metal. Its good corrosion resistance makes it useful for external applications, eg in roofing (flashings, gutters, etc.), but rarely in low-cost constructions. Its high toxicity makes it a less recommended material, especially where alternatives are available, as for pipes and paint pigments.
· Cadmium, chromium, nickel, tin, zinc and a few other metals are mainly used as constituents of alloys to suit a variety of requirements, or as coatings on less resistant metals to improve their durability, a common example being galvanization (zinc coating) of corrugated iron sheets (gci).
· Structural steel components (columns, beams, joists, hollow sections, etc.) for complete framed structures, or individual elements, such as lintels, trusses, space frames and the like.
· Sheets, usually corrugated for stability, for roofs (mainly galvanized corrugated iron, less commonly corrugated aluminium sheets), walls (infill panels or cladding), sun-shades, fencing, etc.
· Plates, strips or foil for flashings (eg steel, copper, lead), fastenings (as in timber trusses) and facing (for protection against physical damage or for heat reflection).
· Steel rods, mats, wire mesh for reinforcement in concrete and ferrocement. The use of deformed bars (twisted or ribbed) gives higher mechanical bond between steel and concrete, reducing construction costs by up to 10 %. Mild steel wires of 6.5 to 8 mm, drawn through a die at normal temperatures, producing 3,4 or 5 mm wires, have twice their original tensile strength and low plasticity, and are used (predominantly in China) in making prestressed concrete components, saving 30 to 50 % of the steel.
· Wire of various types and thicknesses, eg steel wire for tying steel reinforcements or other building components together, copper wire for electrical installations and thick galvanized
steel, aluminium or copper wire for lightning conductors.
· Galvanized steel wire mesh or expanded metal (made by slotting a metal sheet and widening the slots to a diamond shape) as a base for plaster or for protection of openings.
· Nails, screws, bolts, nuts, etc., usually galvanized steel, for connections of all kinds of construction components, formwork, scaffolding and building equipment.
· Rolled steel sections or extruded aluminium sections of various profiles for door and window frames, shading devices, fixed or collapsible grilles.
· Ironmongery of all kinds, eg hinges, handles, locks, hooks, various security devices, handrails, etc.
· Pipes, channels, troughs for sanitary, electrical, gas installation.
· Construction tools and equipment.
· Miscellaneous metal components for tanks, furniture, outdoor facilities.
· Most metals have high strength and flexibility, can take any shape, are impermeable and durable.
· Prefabricated framed construction systems of steel or aluminium are assembled extremely quickly. With strong connections, such systems can be very resistant to earthquake and hurricane destruction.
· Roofing sheets are easy to transport without damage, easy to install, require minimum supporting structure, permit large spans, are relatively light, are wind- and waterproof, and resistant to all biological hazards. In most developing countries they have a high prestige value.
· Many concrete constructions are only possible with steel reinforcements.
· Similarly, there are often no alternatives to certain uses of metals, eg electrical installations; screws, bolts, etc.; tools; security devices.
· High costs and limited availability of good quality metal products in most developing countries. As a result, inferior quality products are supplied, eg extremely thin roofing sheets, insufficiently galvanized components.
· With regard to roofing sheets: lack of thermal insulation (causing intolerable indoor temperatures, especially with extreme diurnal temperature fluctuations); condensation problems on the underside of roofs (causing discomfort, unhealthy conditions and moisture related problems, such as corrosion and fungal growth); extreme noise during rainfall; tendency of thin sheets to be torn off at nailed or bolted points (particularly those without or with only small washers) under strong wind forces; havoc caused by whirling sheets that have been ripped off in hurricanes.
· Poor fire resistance of most metals: although they are non-combustible and do not contribute fuel to a fire or assist in the spread of flames, they lose strength at high temperatures and may finally collapse.
· Corrosion of most metals: corrosion of ferrous metals in the presence of moisture and some sulphates and chlorides; corrosion of aluminium in alkaline environments; corrosion of copper by mineral acids and ammonia; corrosion of various metals by washings from copper; corrosion by electrolytic action due to contact of dissimilar metals.
· Toxicity of some metals: lead poisoning through lead water pipes or paints containing lead; toxicity caused by fumes emitted when welding metals coated with or based on copper, zinc, lead or cadmium.
· Cost reduction by limited use of metals and design modifications which permit the use of cheaper alternative materials.
· To counteract heat and condensation: avoidance of sheet metal roofs in areas of intense solar radiation and large temperature fluctuations; double layer roofs with ventilated air space and absorptive lower layer; reflective outer surface.
· To prevent corrosion: avoidance of use in moist conditions; periodic renewal of protective coating; in case of dissimilar metals, prevention of contact with non-metallic washers; avoidance of contacts between aluminium and cement products (mortar or concrete).
· For noise reduction: shorter spans and coating of bitumen on underside of roofing sheet; also careful detailing of suspension points, and application of insulating layers or suspended celling.
· For resistance to uplift: thicker gauged sheets and stronger connections.
· To reduce toxicity: avoidance of lead or lead compounds where they may come into contact with food or drinking water; good ventilation of rooms in which toxic fumes are produced.
Like metals, glass is a solidified liquid. It is produced by melting sand, soda ash, limestone, dolomite, alumina, feldspar, potash, borax, cullet (broken glass) and/or other ingredients, at about 1500° C, shaping it, and allowing it to cool slowly (annealing) to prevent cracking. Although the earliest forms of glass were produced a few thousand years ago, its large-scale production and use in buildings is less than two centuries old.
Glass is not an essential material for low-cost constructions in developing countries, but certain glass products or even waste glass can be quite useful in improving the quality of other materials, or indoor comfort in buildings.
· Flat glass, either as clear float glass (with undistorted vision and reflection), cast glass (usually translucent) or special variety (for solar control, thermal insulation, decoration, etc.) mainly for glazing of windows, sometimes doors, also for solar collectors, greenhouses, Trombe walls (thermal storage walls).
· Hollow glass blocks (made by fusing two trays of glass together) for non-loadbearing walls or screens to provide light and solar heat transmission.
· Glass fibre, in conjunction with other materials such as cement, polyester and epoxide resins, for lightweight roofing materials or infill wall panels, sun shades, cisterns and other items of any desirable shape.
· Glass wool, made of glass fibres sprayed with a binder and formed into boards or rolls, as thermal insulating material.
· Old bottles used as a substitute for hollow glass blocks.
· Waste glass, crushed to a fine powder and mixed with clay (7 parts powder: 3 parts clay), acts as a flux and reduces the temperature needed to fire the bricks by more than 50° C (saving nearly 50 % of the fuel). The bricks are tough and resistant to wind and rain. Very strong and resistant bricks are also made from 31 % crushed glass, 6 % clay, 7 % wafer end 56 % crushed old bricks.
· Durability, usually high in normal conditions, and good resistance to chemicals (with a few exceptions) and biological hazards.
· Sufficient strength and elasticity, so that an ordinary glazed pane will safely deflect up to 1/125th of its span.
· In regions with cold seasons, utilization of solar energy by trapping the heat within the building ("greenhouse effect"), providing indoor comfort and saving fuel for heating.
· Glass can be recycled.
· Glass is brittle and thus difficult to transport; incorrect installation, thermal stresses, sudden impact, etc. can lead to breakage.
· Broken glass can cause serious injuries.
· Most modern varieties of glass absorb most of the sun's ultra-violet rays, which is vitally important (especially for children) for the synthesis of vitamin D and to destroy harmful bacteria.
· Hydrofluoric and phosphoric acids, and strong alkalis (eg caustic soda, alkaline paint removers, cement products) attack glass; deterioration is also caused by prolonged action of water.
· Although glass is non-combustible, it breaks and later melts in fires.
· Small glass components are easier to transport end less likely to break. A good alternative to standard glazed windows are adjustable glass-louvred windows, especially in the humid tropics, where cross-ventilation is desirable.
· Cheaper, low quality glass, made primarily from quartz sand, does not permit undistorted vision, but allows the healthy ultra-violet rays to pass through.
· Water running off from fresh concrete or mortar must be properly removed from glass to prevent deterioration. In dry conditions, with regular cleaning, glass can be extremely durable.
Plastics are synthetic materials based on carbon compounds derived from petroleum and to a small extent from coal. All plastics materials are polymers (long chains of molecules loosely tangled together), the lengths and characters of which can easily be adjusted in manufacture, which explains the immense variety of plastics.
All plastics materials can be classified as either thermoplastic or thermosetting:
· Thermoplastics soften on heating without undergoing chemical change (if not overheated) and harden again on cooling.
· Thermosets undergo an irreversible chemical change during moulding, so that they do not soften on heating and thus cannot be recycled.
While some developing countries have their own plastics industries, many others have to import the raw materials or finished products, which consequently are expensive. This is not always a disadvantage in building construction, as plastics are not essential materials, but if available, they have numerous applications in building, either to substitute or protect other materials, or to improve comfort conditions.
· Rigid plastics for various uses in water supply and sanitation; transparent, translucent or opaque sheets for non-loadbearing wall and roofing elements, glazing, facing, etc.; extruded profiles for window frames, furniture elements, etc.; fibre reinforced plastics (eg with glass, jute or sisal fibres) for (double curved or folded) self-supporting wall and roof elements (complete building systems).
· Plastic films and membranes for damp-proof courses; covering for concrete curing; temporary rain and wind protection of openings; tent structures. Thicker varieties and tubes for electrical insulation.
· Synthetic fibres for high strength ropes and fabrics, and as alkali resistant reinforcements (eg in fibre concrete roofing elements).
· Foamed plastics mainly as thermal insulation materials, lightweight ceiling panels, or as aggregate in lightweight concrete components.
· Synthetic resins and adhesives for production of various composite materials, such as particle board, plywood, all kinds of laminated and sandwich panels.
· Emulsion paints, distempers, enamel paints, varnishes.
· Sealants for movement joints, weather and waterproof joints.
· Impermeability and resistance to most chemicals, hence no corrosion.
· Good strength: weight ratios of most plastics materials; lightness in weight makes handling and transportation easier and cheaper; no heavy supporting structure is required.
· Capability to take on a wide variety of forms, colours and other physical properties; imitation and substitution of scarce and expensive materials.
· Generally good resistance to biological hazards.
· Excellent electrical insulation.
· High costs and limited availability in many developing countries.
· Flammability of most plastics, with development of noxious fumes and dense smoke.
· High thermal expansion, up to ten times that of steel, and rapid decline of mechanical properties at elevated temperatures.
· Deterioration of most plastics due to prolonged exposure to the sun's ultra-violet rays.
· Use of plastics only for special purposes, eg for waterproofing, thermal and electrical insulation, easier and cheaper transports or for use in earthquake prone areas.
· Avoidance of combustible materials installed close to plastics, and provision of sufficient ventilation openings to remove smoke and fumes in case of fire.
· Provision of sufficient movement joints for plastics components.
· Avoidance of uses of plastics exposed to sunlight.
Although there are several very useful applications of sulphur as a building material, the technology is not yet widely known. This is probably because research and development has taken place almost exclusively in Canada and the United States and only few prototype buildings have been constructed in developing countries. However, the increasing supplies of sulphur, mainly from the desulphurization of petroleum and natural gas, are causing disposal problems in some countries, problems that can be solved if sulphur is used extensively as a building material.
Sulphur also occurs naturally in volcanic regions and has since long served as a basic material for the chemical industry, particularly for producing sulphuric acid, a primary material for large-scale industrialization. Sulphur is also used in the production of fertilizers and insecticides.
At normal temperatures, pure sulphur is a yellow crystalline material, which melts at about 119° C and hardens rapidly on cooling. In the molten state it adheres firmly to a wide range of materials rendering them waterproof and resistant to salts and acids. Sulphur can be stored indefinitely and recycled any number of times by heating and recasting.
The use of sulphur also has several limitations which must be recognized. Further research is needed, preferably in sulphur producing developing countries, especially with a view to the use of low-cost additives, development of practical, inexpensive equipment and simple construction methods.
· Sulphur concrete, comprising elemental sulphur (about 30 % by weight) and coarse and fine inorganic aggregate (about70 %), forming a concrete-like material that can tee moulded and which is impervious to water. It contains neither water nor cement. The powder sulphur and aggregates can be mixed in a conventional mixer equipped with a heater, which raises the temperature of the mix to 140° C in a matter of minutes. Preheating the aggregates to about 180° C and addition of silica flour produces a more homogeneous flowable mixture and more uniform products. The colour can be varied with different aggregates. Sulphur concrete can be cut with a saw and drilled.
· Sulphur coating on weak, flexible and porous materials makes them strong, rigid and waterproof. By dipping, spraying or painting, almost any material can be impregnated with sulphur.
· Sulphur bonding, by using molten sulphur as an adhesive, or applying it externally over non-adhering joints, can produce extremely strong bonds between two components.
· Sulphur foams, produced by introducing small amounts of foaming agents, are light (weighing about 170 kg/m3), rigid, and have excellent thermal resistance, low shrinkage and water absorption.
· Sulphurized asphalts, in which either the aggregate or the asphalt (as used in road and pavement construction) is partially replaced by sulphur, thus raising the viscosity at high temperatures or lowering it at lower temperatures.
· Sulphur-infiltrated concrete, produced by introducing molten sulphur into moist-cured lean concrete, in order to increase its strength and water resistance.
· Blocks, bricks and tiles of any desired shape mace from sulphur concrete for load-bearing floor and wall constructions. Blocks are most appropriately made hollow and interlocking, facilitating accurate and quick constructions, and the cavities to be filled with reinforced concrete (eg in earthquake regions) or with insulating material (eg in colder climates).
· Impregnation of weak and porous materials (such as thatch roofs; panels of reeds, woven mats, cloth or paper stretched on wooden frames; timber components; and even low-strength concrete) to provide strength and water resistance. For example, a large piece of cloth, stretched on a frame and impregnated with sulphur, forms a bowl shape, which hardens and - when turned upside down - becomes a strong, waterproof dome-shaped panel.
· Rigid walls made by laying bricks or concrete blocks dry and then applying a sulphur coating onto the internal and external surfaces. Strong lintels have also been made by laying hollow concrete blocks in a row and bonding them by applying molten sulphur across the joints on the two vertical outer surfaces.
· Thermal insulation of buildings with sulphur foams, or production of lightweight, non-loadbearing wall and ceiling panels.
· Paving of courtyards and other outdoor surfaces, walkways, etc. with sulfurized asphalts.
· Pipes, cisterns and a variety of precast elements made of sulphur-infiltrated concrete for better chemical resistance, higher mechanical strength and impermeability, despite lower proportion of cement.
· Pure elemental sulphur is abundantly available in many regions; can be stored indefinitely and reused any number of times; requires relatively little energy and only simple equipment to melt; adheres to a wide range of materials; has no taste or smell (except when heated or cut with an electric saw) and does not act on the skin; and is a poor heat and electricity conductor.
· Sulphur concrete gains 90 % of its ultimate strength in 6 to 8 hours (normal portland cement requires 30 to 60 days to gain the same strength); it is not attacked by salts (hence unwashed aggregates and even sea sand can be used); it does not require water (of special significance in desert regions, which incidentally also produce large amounts of by-product sulphur from oil refining); it can be cast to produce building components with precise dimensions and sharp edges (especially suitable for the manufacture of interlocking blocks, which can be assembled without the use of mortar or special skills); it has a chemically resistant, non-absorbing, smooth, hard and appealing surface (which is easy to keep clean by merely washing), eliminating the need for plastering or painting; and it retains most of the characteristics of pure elemental sulphur.
· Sulphur coating can considerably increase the strength and prolong the service life of many materials.
· Sulphur surface bonding reduces construction time, saves cement and produces strong, waterproof bonds.
· Sulphur foams have similar thermal insulation characteristics, but higher compressive strengths than conventional rigid foams, such as expanded polyurethane.
· Sulfurized asphalts can be stronger and cheaper than standard paving materials.
· Sulphur-infiltrated concrete requires less cement than concretes of the same strength and impermeability.
· Sulphur has a low melting point (about 119° C) and ignites at about 245° C. Sulphur combustion is self-sustaining and thus, once ignited, will continue to burn until extinguished. Burning sulphur produces sulphur dioxide, a toxic gas.
· Pure sulphur becomes brittle and powdery (orthorhombic crystalline form) on cooling, making it unsuitable for a variety of applications.
· Sulphur has a much higher coefficient of thermal expansion than portland cement concrete, and sulphur concrete tends to contract on cooling.
· Under humid or wet conditions, reinforcing steel tends to corrode in the presence of sulphur, making sulphur concrete unfit for structural uses.
· Sulphur should not be used as a building material where temperatures are likely to exceed 80°C.
· A sulphur fire in an enclosed structure can be smothered by closing all entrances and denying it air; it can also be extinguished with water or sand.
· Apart from avoiding all potential sources of fire (eg cookers, heaters) close to sulphur-based components, a precautionary measure is to add a fire resistant material to the molten sulphur. A suitable material is dicyclopentadiene.
· The tendency of sulphur to become brittle and powdery is overcome by adding a plasticizer which retards the crystallization of sulphur. Dicyclopentadiene was also found to be effective for this purpose, as well as to increase the thermal stability of sulphur concrete.
· Shrinkage of sulphur concrete in precast components (eg hollow blocks) is best overcome by overfilling the mould, and after cooling, sawing off the extra concrete.
· Thermal expansion of sulphur concrete should be taken into account by providing sufficiently wide joints.
· The brittleness and thermal movement of sulphur-based materials can be reduced by fibre reinforcement, but further research is needed on this aspect.
Although the term "Wastes" is in common use, it may be misleading. Not all wastes are useless rubbish and freely available. It is also mainly a matter of definition: from one point of view a material can be of no use, while it is a valuable resource from another.
In this context, wastes can be defined as by-products (of agricultural, forestry, industrial or even household processes), which do not essentially have anything to do with building, but which, with special processing and treatment, or in conjunction with other materials, can economically substitute (or even improve the quality of) conventional building materials. Exceptions to these wastes are recycled materials from demolished buildings, which continue to serve as building materials, though perhaps in a different way.
Discarded consumer goods (such as bottles, tins, car tyres), which have been experimented with in several industrialized countries, are of less significance in developing countries, as such materials already have numerous other uses (eg household articles, musical instruments, shoes).
The materials referred to in this section are extremely diverse, but are basically of two types: organic and inorganic wastes. As a further sub-division, organic wastes are generally agricultural or forestry by-products and also household and urban wastes, while inorganic wastes are mainly obtained from industrial processes and demolition of old buildings, but there are several exceptions.
· The outer skin of rice grains can be used in the dry state, chemically treated, or in the form of ash.
· Full or crushed husks mixed with clay in brick production, help to burn the brick uniformly, creating voids, and thus producing lightweight bricks.
· Water glass (sodium silicate), a useful binder, can be manufactured from rice husks. This can be used in the bonding of full or crushed husks to produce particle boards. Other binders can also be used.
· Rice husk ash (RHA) is a useful pozzolana, which can be mixed with lime to produce a cementitious binder. (Details are given in the section on Pozzolanas).
· RHA mixed with soil, nodulized and sintered in a kiln, makes lightweight aggregates for concrete.
· These include fresh husks, coconut shells and waste from the coir industry.
· The husks consist of 15 - 35 cm long fibres (about 60 % of husk), with high tensile strength, which is affected by moisture. The fibres, and more so the pith (soft cork-like material), are chemically reactive, as long as they are kept dry. During the resting process (softening by soaking in water) they become inert. The difference in reactivity between rested and fresh husks necessitates different methods of conversion into building materials.
· Unretted husks, hot-pressed (at 150° C, 1 MPa pressure for 15 to 25 minutes) without any additives, produce strong particle boards.
· Unretted pith, obtained by defibrating mature husks, hot-pressed without additives, produce strong, moisture resistant boards. Lighter, resilient boards are made in the same way, but with addition of rested pith (low density, highly elastic granular material).
· Retted pith mixed with cashew nut shell liquid resin (rubbery substance) produces an expansion joint filler, which is resistant to temperature and moisture fluctuations and to insect and fungal attack.
· Retted pith granules as an aggregate in concrete are useful for thermal insulation.
· Unretted fibres, mixed with paraffin wax and hot-pressed, make strong and flexible hardboards (fibre boards).
· Coir shearing waste, containing fibre, pith and dust, bonded with an adhesive, produces particle boards with an attractive mottled appearance.
· Coir waste, mixed with portland cement and moulded under compression, produces large corrugated roofing sheets (see section on Fibre concrete).
· Coconut shell chips and conventional adhesives make good quality particle boards.
· Coconut shell tar, obtained during the destructive distillation of the shells, is a slightly viscous liquid with anti-microbial properties.
· Sawdust, woodchips, wood shavings and other wood residues from sawmills can be used in the conventional ways to produce particle, fibre and woodwool boards.
· With sawdust as aggregate in concrete, preferably with magnesium oxychloride cement, precast lightweight concrete components (eg door and window frames) can be made.
· Wood waste, mixed with inorganic materials (cement, trass, lime, pozzolana) in a mixer/ pulper machine, produce pulp cement boards for various non-loadbearing uses.
· Tannin is extracted from the bark of various timber species (obtained in timber processing) to produce tannin-based adhesives for the manufacture of particle board.
Reeds and straw
· Straw from wheat, barley, rice and other plants are hot-pressed, without any binders, to produce rigid boards, faced with paper on both sides (Stramit process).
· Flexible boards are also made by placing reeds (or stiff varieties of straw) side by side and then stitching them across with ordinary galvanized wire.
· Straw and other dried fibrous material, chopped to lengths of 10 to 20 cm, softened in water, and mixed with wet clayey soil, can be compacted in formwork to make stiff, thermal insulating walls (straw clay construction).
· This is the fibrous residue from sugar cane processing. It is not suitable for reinforcement of cement based products, as the residual sugar retards the setting of cement.
· With a suitable organic adhesive, particle boards and fibre boards can be made from bagasse.
Banana stalks and leaves
· Banana fibres have been successfully used in fibre concrete.
· Stalks and leaves, chopped up and boiled in water, form a thick liquid, which is applied on soil walls and roofs for waterproofing and higher resistance to abrasion and cracking.
Cashew nut shell liquid
· A by-product from cashew nut processing is a viscous liquid extracted from the mesocarp. The CNSL severely blisters the skin of any person coming into contact with it, but is a useful anti-microbial and waterproofing agent. It is therefore used to protect materials which are susceptible to biological decay (eg thatch roofing), and is applied with a brush. It can also be sprayed if mixed with kerosene to reduce viscosity.
· This beautiful plant, originally found only in Brazil, has become a serious problem, clogging tropical waterways worldwide and invading paddy fields in Southeast Asia. It is now widely used to produce biogas, mulch for soil improvement and silage as animal feed.
· Research in India and Bangladesh has shown that tough, flexible hardboard can be made from a fibrous pulp of chopped water hyacinth stems.
Miscellaneous vegetable wastes
· A large variety of other agricultural wastes (eg jute and corn stalks, peanut shells) can be used in similar ways to those mentioned above. The most common uses are in the manufacture of particle board or fibre board.
· If used with cement as a binder, this is only possible if the waste material contains no cement "poison" (which retards setting), if the material has no cavities (which entrap and thus waste cement), and if the particles or fibres are long enough to provide strength by interlocking.
· Some non-edible grains are suitable for carbonization (conversion into carbon by slow burning) to produce particles of a fine cellular structure containing entrapped air. They are similar to, and used in the same way as, conventional lightweight aggregate (eg polystyrene beads), are biologically inert, fire resistant (up to 2000° C) and highly resistant to water and chemicals.
Waste paper and textiles
· While these are collected for other uses (such as recycled paper, packaging material, shoddy, bags, rag dusters, mats, etc.), shredded waste paper and cloth strips can serve as thermal insulations, for instance, in wall cavities and sandwich panels. Fire resistance can be achieved by soaking in a solution of borax, and drying.
· Asphalted corrugated sheets are produced by making a pulp out of washed and beaten paper and textile wastes, forming the pulp into sheets, drying in the sun or drying chamber, trimming, passing through an oven with corrugating rolls and finally dipping in a bath of hot asphalt.
· Sludge from wastewater treatment plants is normally dewatered and used for land-filling. This causing a serious disposal problem in the small island-state of Singapore led to research on utilization of the sludge as building materials (at Nanyang Technological Institute).
· Burnt bricks made of clay mixed with 40 % dried sludge or 50 % sludge ash showed better results with the ash, though higher percentages are not advisable.
· By adding pulverized sludge ash, to replace up to 20% of the cement in concrete, its workability improves, the setting time remains unaffected, but the compressive strength decreases with increasing proportions of sludge ash.
· The sludge ash can be partially crushed and used as graded aggregate in lightweight concrete, or as coarse aggregate in no-fines concrete, with satisfactory results.
· Coal is an organic material, but the wastes referred to here are largely inorganic, and can thus be ascribed to either group.
· Gangue is a by-product of coal production and is chiefly composed of silicon and aluminium with 75 % oxide. In China large amounts are used as building material: mainly as masonry blocks, aggregate in lightweight concrete, end es a cement replacement material.
· The burning of coal in thermal power plants produces basically two types of residues: cinder (or clinker), formed by burning lump coal, or pulverized coal which fuses to lumps and falls to the bottom of the furnace (also called "bottom ash"); fly ash (or pulverized-fuel ash) formed by burning pulverzed coal, producing a fine dust, which is carried upwards by the combustion gases. Coal ashes can contain unburnt carbon in varying proportions.
· Cinder and sintered fly ash are used as lightweight aggregate in concrete construction and blockmaking.
· Fly ash and/or crushed cinder can be used in making burnt clay brick, masonry mortars and aerated concrete. (For further details about fly ash see section on Pozzolanas.)
Blast furnace slag
· This is the molten material which settles above the pig iron at the bottom of the furnace. (Details are given in the section on Pozzolanas.)
· The washings of bauxite ore in the production of alumina are collected in ponds, which dry out leaving a residue called red mud.
· The red mud can be mixed with clay to make fired bricks and tiles, or pelletized and fired to produce lightweight aggregate for concrete. The fired pellets can also be finely ground to produce a high quality pozzolana.
· The sludge, in the form of finely precipitated calcium carbonate (with varying amounts of free lime), is obtained from fertilizer plants, sugar and paper factories, tanneries, soda-ash and calcium carbide industries.
· Lime sludges are used for the manufacture of portland cement and to produce sand-lime bricks.
· The lime sludge can also be moulded into bricks and fired in kilns to produce quicklime (calcium oxide).
· Dried lime sludge mixed with rice husks and fired in an open clamp produce a hydraulic binder (see section on Pozzolanas).
· Phosphogypsum (calcium sulphate, contaminated with phosphates) is produced as a slurry in the manufacture of fertilizers and phosphoric acid. It contains several impurities, which have to be removed by expensive washing, thermal or chemical treatments. It is also to some extent radioactive and thus not recommended for building.
· If the amount of impurities and radioactivity is sufficiently low, the purified gypsum can be used as a set-retarder in portland cement, or to produce gypsum plaster, fibrous gypsum plaster boards or gypsum blocks.
· Cements from phosphogypsum have delayed setting and slow rate of strength development at early ages, but strengths at later ages (28 days) are comparable with those of ordinary cements.
· Demolished buildings can provide a vast number of materials that can be recycled in new constructions. Careful dismantling and separation of various individual components (metal parts, timber boards and beams, windows, doors, tiles, pipes, etc.) help to conserve limited resources and save the immense costs and energy required to produce new components.
· Brick waste can be finely ground and used as a pozzolanic binder (see "Burnt clay" in section on Pozzolanas). It can also be crushed to a maximum size of 20 mm and used as coarse aggregate in concrete construction (especially important in countries, like Bangladesh, in which natural aggregates are scarce). Brick aggregate absorbs water, so that more water is required in preparing the concrete mix.
· Broken concrete serves well as aggregate in new concrete.
· The collection and reuse of metal scrap is one of the world's largest industries with regard to the number of companies, people employed, weight of material handled and value of equipment used. Metal scrap can be collected at construction sites (eg off-cuts of reinforcing steel and mesh, wire and nails), demolition sites, engineering workshops (off-cuts from lathes, drills, etc.), garages and factories (scrap cars, oil drums, disused machinery, etc.), households (tin cans, domestic appliances, broken tools, furniture, etc.) and refuse dumps.
· The collected and sorted metal scrap can be melted in small decentralized foundries to produce new metal components; reshaped on a forge; cut into suitable pieces; welded together to form new products; or reused without special processing.
· Discarded beverage cans, of which large quantities accumulate in industrialized countries, are less common in the Third World. In places where they are abundantly available, they have been successfully used as bricks to construct light, thermally insulating masonry walls.
· Swarf (metal off-cuts from lathes, drills, etc.), if it is not contaminated with oil, can be used as aggregate in concrete, especially where increased resistance to cracking, impact and abrasion is needed (eg road and pavement construction).
· Flattened cans, drums, car body material, serve as cheap jointing plates in timber constructions (eg for roof trusses).
· In most developing countries, clean, used bottles have a high resale value and will hardly be considered as material to build with. In more affluent countries, where the bottles have no value, they have been used for wall construction as bricks, permitting light to pass through and presenting an attractive appearance.
· Broken glass (cullet) can be recycled in glass manufacture, but also has some uses as building material.
· Waste glass, crushed to a fine powder and mixed with clay (7 parts powder: 3 parts clay), acts as a flux and reduces the temperature needed to fire the bricks by more than 50° C (saving nearly 50 % of the fuel). The bricks are tough and resistant to wind and rain. Very strong and resistant bricks are also made from 31% crushed glass, 6% clay, 7 % water and 56 % crushed old bricks.
· Crushed glass, with a continuous grading of about 3 mm to 2 micrometres can be used as aggregate in concrete, but certain types of glass (eg soda and pyrex glass) have been found to expand in the alkali environment of portland cement, causing cracks and ultimate disintegration of the concrete.
· Large amounts of sulphur are produced in the desulphurization of petroleum and natural gas. On account of its many applications as a building material, it has been dealt with in a separate section on Sulphur.
· Components, mainly boards, made with organic or inorganic binders, from rice husks, coconut wastes, wood residues, bagasse, banana fibres and other vegetable waste.
· Boards made by hot-pressing without binders from straw, coconut husks, wood fibres, water hyacinth.
· Thermal insulation material and lightweight aggregate in concrete from rice husk ash nodules, coconut pith, sawdust, straw, carbonized grains, paper and cloth strips, sewage sludge ash, cinder and sintered fly ash, blast furnace slag, sintered red mud pellets, foamed sulphur.
· Replacement of aggregate in concrete by brick waste and broken concrete (demolition waste), crushed glass.
· Materials for cement production and replacement (pozzolanas) from rice husks, fly ash, blast furnace slag, bauxite, lime sludge, phophogypsum, pulverized burnt clay.
· Additives in clay brick production from rice husks, wood residues, sewage sludge, cinder, bauxite waste, crushed glass.
· Corrugated roofing sheets using coir waste, woodwool, vegetable fibres, paper and textile waste.
· Adhesives and surface protection coating made from tannin, banana stalks and leaves, cashew nut shell liquid, lime sludge, sulphur.
· Conservation of scarce and expensive resources, and utilization of locally available materials, reducing costs and transportation.
· Reduction of pollution by the use of materials that are difficult to dispose of, and avoidance of excessive production of new materials in polluting industrial processes.
· Considerable saving of the energy required to produce new materials.
· Improvement of the quality of some materials (eg by using certain artificial pozzolanas in concrete).
· Handling of wastes can be dangerous, eg inhaling of fine particles; blisters, burns and illness from toxic substances; severe cuts from broken glass and metal scrap.
· Although the total amount of available waste is large, it may be produced in numerous decentralized units, making collection extremely difficult.
· Once a by-product becomes a useful building material, higher prices are charged, so that the benefit of using cheap materials is quickly lost.
· Not all building materials based on wastes provide the same strength and durability as the materials they were designed to substitute (but if the price is low, this drawback can be accepted).
· The concept of using wastes and the fear of future problems that may arise due to inferior qualities of materials makes builders reluctant to use them.
· Careful supervision and strict observance of safety precautions (eg use of gloves, goggles, protective clothing) in handling waste is of vital importance to reduce injuries and health problems.
· Producers of useful by-products need to be well instructed on appropriate methods of handling and storage of the material in order to facilitate collection.
· Especially in the case of lesser known but promising waste utilization, considerable efforts are needed to demonstrate the technology and its benefits. Prototype structures (preferably important public buildings) that are constantly used can convince most doubters.
· The use of wastes for building offers a wide field of research and should be given priority - even in the more affluent countries - as there is a great need to save resources, energy and costs, and at the same time provide more shelter for the homeless.