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Bamboo
Bamboo is a perennial grass with over 550 species found in the tropical, subtropical and temperate zones. lt contains a large percentage of fibre which has high tensile, bending and straining capacity.
However, bamboos have some shortcomings which limit their application. The low durability of bamboo constitutes one of its most serious defects along with its flammability and its tendency to split easily. This usually prevents the use of nails. Cutting a notch or a mortise in a bamboo drastically reduces its ultimate strength. The remedy is the use of nodes as places of support and joints, and the use of lashing materials (strings) in place of nails. Dry bamboo is extremely susceptible to fire, but it can be covered or treated with a fire retarding material.
The strength properties of bamboo vary widely with species, growing conditions, position within the culm, seasoning, and moisture content. Generally bamboo is as strong as timber in compression and very much stronger in tension. However, bamboo is weak in shear, only about 8% of compressive strength where timber normally has 20 to 30%. It is mainly used in building construction for wall poles, frames, roof construction, roofing and water pipes, and after splitting, to form flattened boards or woven wall, floor and ceiling panels.
New stalks of bamboo are formed annually in clumps growing out of the spreading roots. The individual bamboo shoots complete their growth within a period of four to six months in the first growing season. A strengthening process takes place during the subsequent two to three years and the culm reaches maturity after the fifth or sixth year or even later depending on the species. It must be cut before blooming since it looses its resistance and dies after blooming. Some bamboos grow to 35 metres in height while others are no more than shrubs. Diameters may vary from 10 to 300mm.
Bamboo without proper seasoning and preserving treatment will rot and be attacked by insects particularly so if used in moist locations such as in earth foundations.
Bamboo Joints
As nailing causes splitting and notching, drastically reducing the strength of a bamboo culm, lashes are generally used as binding elements in framing. They may be split from the bamboo itself, or made from vines, reeds or bark of certain trees. Soft galvanized wire is also used for binding. When bending, bamboo can be kept from splitting by boiling or steaming and bending it while hot.
Figure 3.8 Lashing bamboo joints.
Figure 3.9a Make four cuts in the upper end of the culm with a splitting knife.
Figure 3.9b Split the calm the rest of the way by driving a hardwood cross along the cuts.
Splitting Bamboo Several methods can be used fos Flitting bamboo culms. The edges of the strips can be razor-sharp and should be handled carefully. See figure 3.9.
Bamboo Preservation
Immediately after cutting, the fresh-cut lower end of the culm should be dusted with insecticide. The bamboo is then air seasoned for 4 to 8 weeks depending on the ambient humidity. The bamboo should be stacked well off the ground so that air can circulate freely. When the culms have dried as much as conditions will permit, they should be trimmed and all cut surfaces immediately dusted with insecticide, The seasoning is finished in a well-ventilated shelter where the culms are protected from rain and dew. If the bamboo is to be stored for a long time, stacks and storage shelves should be treated with an insecticide every six months. Bamboo which has already been attacked by insects, fungus or rot should never be used for construction. Culms which have fissures, cracks or cuts in the surface should also be rejected.
Natural Fibres
Natural fibres have been used for building since ancient times. Fibrous materials can be used by themselves as roofing material or for walls and mats. Natural fibres can also be combined with hydraulic-setting binders to make various types of roofing boards, wall boards, blocks and shingles. Animal hair is often used as reinforcing in plaster.
Thatch
Thatch, whether grass, reeds, palm or banana leaves, is susceptible to decay due to attack by fungi and insects and to destruction by fire. Preservative treatment is desirable but expensive. A treatment combining copper sulphate, sodium chromate, and acetic acid reduces attack by rot and may considerably increase the life span of a thatched roof. See Chapter 5.
Grass
The use of thatched roofs is common in many countries and suitable grass can be found almost everywhere. When well laid and maintained it can last for 10 to 20 years or longer.
A good quality thatching grass must be fibrous and tough with a minimum length of one metre. It should also have thin stems without hollows, a low content of easily digestible nutrients and the ability to withstand repeated wetting without decaying.
An annual treatment with a mixture of the following chemicals will improve fire resistance of a thatched roof and also give some protection against decay; 14 kg Ammonium Sulphate, 7 kg Ammonium carbonate, 3.5 kg Borax, 3.5 kg Boric acid, 7 kg Alum, 200 kg water.
Reeds
Reeds must be dry before use as building material and can be impregnated or sprayed with copper-chrome preservatives to prevent rotting. Ammonium phosphate and ammonium sulphate are used to protect the reeds against fire. See Chapter 5.
Reeds can be woven into mats for use as wall or ceiling panels, shade roofs, etc. The mats can be easily plastered. In tropical areas thatch from untreated reeds may last only one year, but if well laid, treated and maintained, it can last 5 to 10 years.
Sisal Stems
Before dying the sisal plant will, at 7 to 12 years of age, shoot a pole to carry the flowers. The pole may reach a height of 6m or more and has a fibrous circumference, which makes it tough, but the inner parts are quite soft. Sisal poles have limited structural strength and durability, but are sometimes used for wall cladding in semi-open structures, such as maize cribs. The poles can be split and are joined in the same way as bamboo.
Sisal Fibre
Sisal fibre is one of the strongest natural fibres. It has traditionally been used as a reinforcement in gypsum plaster sheets. Sisal fibres have the ability to withstand degradation due to bacteriological attack better than other organic fibres, but are attacked by the alkalinity of cement. However, research has been carried out to make sisal fibre, like other natural fibre composites, a reliable cement reinforcement for long term use in exposed situations. See Section Fibre Reinforced Concrete.
Coir Waste
Coir is the by-product of coconuts. The husk is used for making coir mats, cushions and as fuel. It can be mixed with cement, glue or resins either to produce low density boards having good insulating and sound absorption properties, or be compressed to make building boards. It is also used as reinforcement in cement for making roofing sheets.
Elephant Grass
Elephant grass is a tall plant similar to bamboo, but with the difference that the stem is not hollow. The fibres of the grass can be used to partly or wholly replace the asbestos in net and corrugated roofing sheets. However the sheets are more brittle and have a slightly lower strength than asbestos-cement sheets.
Straw
Baled straw, if supported by a frame work of wooden poles, can be used to construct temporary walls. Straw has also been used as raw material for manufactured building boards. Straw and split bamboo can be cement-plastered to permanent structures such as vaults and domes at low cost.
Natural stones are strong in compression and are generally extremely durable, although deterioration may result from soluble salt action, wetting and drying, or thermal movement. According to the manner of their geological formation, all stones used in building fall into one of three classes: igneous, sedimentary or metamorphic.
Igneous rock are mostly very hard and cliff cult to cut to size and shape. However, the are very durable.
Sedimentary rocks such as sandstone and limestone are used extensively for building. They are not difficult to work and yet are quite durable. Coral stone is found in coastal areas where chips or small stones are used in mud walls. Coral stone is also cut into blocks, and although not very strong, can be used in foundations and walls in multistorey houses.
Metamorphic stones consist of older stones which have been subjected to intense heat and pressure causing structural change. Thus, clay becomes slate, limestone marble and sandstone quartzite. Slate develops cleavage planes during formation. Roofing slates are split along these planes. They make very durable roof surfaces, but require strong frames because of their heavy weight.
At the building site the stones can be dressed to obtain a smooth surface. Often only the side or sides that will be visible are dressed.
Stones may also be used in the forms and sizes in which they naturally occur and be imbedded in mortar for foundation and wall construction. Stones are also crushed and sorted for size and use. Small size crushed stone is used in making concrete. Large sizes are used as hardcore for filling purposes.
