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.
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