This article deals with the service life of bamboo, the treatability of bamboo culms, the traditional methods for the protection of bamboo, such as non-chemical treatments (curing, smoking, whitewashing, soaking), construction methods, and chemical treatments (fumigation, brushing, and spraying, butt-end treatment, open-tank treatment, hot and cold treatment, sap replacement treatment, pressure treatment), fire protection. Finally environmental aspects of bamboo preservation are considered.
Service Life of Bamboo:
Bamboo is liable to the attack by fungi (brown rot, white rot, soft rot) and especially by insects (beetles, termites). Its durability depends mainly on the climatic conditions and the physical environment. Fungi can only attack bamboo culms with sufficient moisture content, generally above 20%.
Air-dry bamboo is protected against fungi. Beetles, especially the bamboo borer Dinoderus minulus depend on the starch content of the culm, whereas termites will attack any bamboo. Untreated bamboo culms may have an average life of less than 1-3 years when exposed to atmosphere and soil. Under cover, it may be expected to last 4-7 years or more, depending on the nature of its use and the conditions. Under favourable circumstances, bamboo has been in service as support or frames for more than 15-20 years.
In seawater it is destroyed by marine organisms in less than one year. Compared with wood, bamboo has generally a low natural resistance. Most of the observations on its durability are based on experiences made on full-sized structures. Only a few special service tests have been carried out so far, and results from graveyard tests are still rare.
It has been shown that splitted culms are more rapidly destroyed than un-splitted bamboo; the bottom part of the culm has on average a higher durability than the middle part or the top. The inner part of the culm is earlier attacked than the outer one, because of its anatomical structure. Laboratory experiments have revealed that bamboo is more rapidly deteriorated by white rot and in particular by soft-rot fungi than by brown rotters.
Although the natural resistance of bamboo is low, it varies somewhat according to species. For example, Dendrocalamus strictus appears less resistant to termites than Dendrocalamus longispathus. Deterioration of bamboo by powder-post beetles starts as soon as the culm is felled.
The beetles attack the bamboo for the starch, and the damage they cause – at least for the common Dinoderus minutus – has been found to be proportional to the starch content of the culm. Bamboos harvested during summer are more liable than those felled after the rainy season, because the starch is depleted when bamboo sprouts. Flowered bamboo has a higher resistance to beetles since there is no starch left. Its drawback is that the culms become also brittle.
It is often believed that the durability improves when the culms are cut before sunrise. This may refer to the starch/sugar content, which might be lowered during the night, leading to a slightly improved resistance against moulds and beetles, but the susceptibility against true wood destroying fungi and termites has certainly not changed.
Also the cutting at the descending moon phase is regarded as an increase of the durability. Such assumption is considered in other countries even for the durability of wood; nevertheless, intensive investigations have not shown any proof for such a belief.
For the use of bamboo in constructions a preservative treatment is widely regarded as necessary. However, it is seldom carried out. The reasons are mainly the lack of knowledge about possibilities of bamboo protection; the lack of treatment facilities and preservative chemicals; the uncertainty about the advantages of bamboo preservation, and the lack of a market for treated bamboo.
In any consideration of bamboo preservation, knowledge of the available methods, their advantages, and disadvantages is necessary, as is an understanding of the treatability of bamboo. Also the environmental effects have to be considered.
Treatability of Bamboo:
The treatability of bamboo is in general rather low. It differs according to species, moisture content of the culm, treatment method, and type of preservative. Most of these factors are influenced by the anatomical structure.
A bamboo culm can anatomically be differentiated from wood by the following characteristics, which are important for the penetration of any liquid preservative:
1. The vessels as the main avenues run strongly axial; they are isolated from each other by the parenchyma in the internodes and connected only in the nodal segments; they are very small at the periphery of the wall and become larger near the center.
2. A special layer of cells covers like a cuticula, the inside and outside of the wall and retards penetration from both sides.
3. There are no ray cells in the bamboo tissue to facilitate a movement of liquids in the radial direction.
The penetration of liquid into a culm takes preferentially place from both ends through the vessels in the axial direction. To a small extent, movement is also possible through the intercellular cavity and the sieve tubes of the vascular bundles. Some penetration occurs at-the nodes where leafs heats and branches originated.
The penetration through the cuticula is slow. However, diffusion is better from the inner wall than from the exterior. In the living culm, the cuticula provides protection against water loss, and it is obvious that a minimal rate of diffusion would persist also in the dead culm.
Even when bamboo is treated under pressure, the lateral penetration is small. For maximum penetration an incising of the inner part of the wall has been mentioned, but due to the structure of bamboo this method would improve the treatability of only short pieces.
Boring axially through the normally impermeable diaphragm of the nodes has also been tried, so that the culm serves as a vessel for the preservative and the liquid penetrates the inside wall. In this case it must, be secured that the preservative is thoroughly removed from the inside of the culm after treatment to avoid pollution from the remaining liquid.
For a satisfactory treatment of the bamboo tissue it is necessary that the preservative diffuses from the vessels into the surrounding fibres and the parenchyma cells. The vessels occupy only a small portion of about 5-10% from the cross section.
Even if the vessels are entirely filled, the bamboo culm can be destroyed by fungi or insects when the preservative does not diffuse enough into the tissue, so that the main part of the culm is being left untreated. Consequently, preservatives for bamboo must have good diffusion properties.
Methods for the Protection of Bamboo:
Much damage can be avoided by using simple construction methods. The placing of bamboo posts on stones or on cement walls instead of putting them directly on the ground, keeps the bamboo dry, so that at least deterioration by fungi is prevented.
It is strongly recommended that the basement of bamboo house constructions is made of concrete work or by using stones. To keep bamboo constructions dry during service life is a necessity to avoid fungal deterioration. The cover of bamboo mats by cement or plaster can restrict the access of beetles, thus providing a protection.
There are two ways of treatments which are used to prolong the life span of bamboo:
1. Non-chemical treatments (without using preservatives) and
2. Chemical treatments.
Whereas, non-chemical treatments have been used by tradition for a long time in the villages of many countries, a regular chemical treatment is applied only in few cases.
Non-chemical treatments are generally, the so-called traditional treatments. They are quite often used, although, not much is known about their real effectiveness. They cost almost nothing and can be carried out by the villagers themselves without using special equipment.
(a) Curing:
Culms are cut at the bottom but are left for some time with branches and leaves at the clump (clump curing). Because the respiration of the leaves still continues, the starch content in the culm is reduced and thus the durability against infestation by borers may increase. There is no influence on the resistance to attack by termites or fungi Also the moisture content is reduced by transpiration.
(b) Smoking:
The culms are stored above a fireplace inside the house for some time so that the smoke turns the culm surface black. It is possible that this process produces toxic agents that lead to some resistance. Also due to the heating, the starch within the parenchyma cells may be destroyed.
In Japan, bamboo materials are kept in a heating chamber at 120-150°C for 20 minutes. This treatment is considered to be in some way effective against insect attack, although it may cause cracks which would lead to increased susceptibility.
(c) Whitewashing:
This traditional treatment is used more for its ornamental effect than for its preservative value. Bamboo culms and bamboo mats for housing constructions are painted with slaked lime (CaOH2), which may delay the water absorption and thus lead to a higher resistance against fungi. In Indonesia, bamboo mats are tarred and later sprinkled with fine sand. As soon as the sand clings to the mat and the tar is dry, whitewash is applied (up to four times). Plastering is also commonly done by the people, using cow dung mixed either with lime or mortar.
(d) Soaking:
The freshly cut culms are put into stagnant or running water or mud for several weeks. Because the specific gravity of the culms is less than 1, stones have to be put on the bamboos to keep them under the water surface. Subsequently, the bamboo is dried in a shady place. During the soaking, the starch content of the parenchyma cells is reduced by bacterial action.
This treatment is said to improve the resistance against borers. It has to be emphasized that soaking does not increase the resistance against termites and fungi and that the submergence in water may lead to staining and bad odor of the bamboo culms.
Chemical Treatments of Bamboo:
Treatment with chemicals for the preservation of bamboo is more effective than no chemical treatments, but it is not always economic.
(a) Fumigation:
Fumigation with methyl bromide or some other chemical is applied in Japan for insect control.
(b) Brushing and Spraying:
Brushing and spraying are rarely done except for prophylactic treatment because, these methods have only a temporary effect, reflecting the very low penetration of the preservatives. A prophylactic treatment may be advisable in bamboo depots and paper mills. These methods cause considerable environmental pollution.
(c) Butt-End Treatment:
The bottom part of a freshly cut bamboo culm with branches and leaves is placed into a barrel containing a preservative. The transpiration of the leaves, which is still in progress, makes it possible that the preservative solution penetrates into the vessels of the culm.
Harvesting the culm and subsequent handling, however, may allow air to penetrate into the vessels at the bottom of the culm and thus reduces the suction effect of transpiration. Moreover, this treatment is only possible with rather, short culms with a high moisture content. It takes a long time, and in most cases the vessels do not take-up enough liquid to preserve the surrounding fibres and the parenchyma.
(d) The open-tank treatment or soaking has a good protective effect, is economic, and simple in its application. Culms prepared to size are soaked in a solution of a water-soluble preservative for several days. The solution penetrates into the culm by diffusion through the ends and partly through the sides. Practical investigations carried out with the open-tank treatment show that in nature bamboo allows much better penetration through both the outer and inner skin than mature bamboo.
This effect may be correlated with the amount of lignification. Both, the outer and inner skin are permeable to some extent to preservatives during long soaking; with the inner skin a little more permeable. Consequently, split culms can more easily be treated than round culms, and the use of split bamboo could, reduce the soaking period by as much as one-half. In air-dried culms, diffusion in the axial direction has been found to be about 20 times more than in the transverse direction, and radial .diffusion is slightly more than the tangential.
Because, water absorption in bamboo is generally slower than in treatable wood species, the soaking period has to be considerably longer than for wood of the same dimension. The duration of treatment depends mainly on the kind of preservative chosen, the bamboo species, and the condition of the culm.
Using hot dipping or the hot and cold treatment, the time of treatment can considerably be reduced; scratching of the outer skin may speed up penetration, especially for slow-diffusing preservatives.
(e) Sap-Replacement Treatment:
The classical sap-replacement treatment, also called Boucherie treatment, has been adapted to bamboo by Dr. A. Purushotham about 50 years ago. It has been modified (developed) further and is now applied on a larger scale for bamboo building programs in Costa Rica and on Bali, Indonesia.
With this treatment the water-transporting part of the culm can be penetrated completely, and the treatment itself needs only a few installations. With the classical form of the treatment the preservative is forced through pipes by gravity into the stem-base from a container placed higher than the stem itself, it is also possible to hang the culm vertically and to scratch the inner wall of the top internode so that it acts as a reservoir for the treatment.
The internode has to be filled several times. At the beginning of the treatment, the solution dripping from the top-end consists mainly of sap. The solution can later be collected and may be used again after filtering and new salts have been added for maintaining the concentration. The treatment is finished when the solution at the dripping end shows a sufficiently high concentration of chemicals.
The treatment is much improved if pressure by a pump is used. At the bottom of a container a small metal tube is fixed from which several other tubes lead to the bottom end of the bamboo culms. The culms are tightly fastened by rubber cuffs.
The preservative is forced axially through the culm by an air pressure in the container. Compared with the classical Boucherie treatment, the time of treatment has been reduced from several days to 1-2 hours for culms of 4-5 m length. The duration of the treatment and its effectiveness depends on the pressure applied, the bamboo species, the moisture content of the culm, and the preservative used.
The moisture content of the culms must be as high as possible. The treatment should be done on the day of cutting. Otherwise the culms have to be stored in water. Since at the cross end a blockage of the vessels by tylosis may rapidly develop, a fresh cut has to be made immediately prior treatment.
The best results are obtained during the rainy season or after it, failures may occur during the dry season. Younger culms with a higher water content are more suitable for this than over-mature culms with less moisture content.
The kind of preservative chosen, in particular its diffusion capacity is also of great importance. Preservatives with quick fixation generally stop flowing through the culm in a relatively short time, so that the vessels and pits get blocked. Good results have been obtained with boron solutions.
The modified Boucherie treatment has been further developed by the National Bamboo Project of Costa Rica and is applied at a larger scale. Also the Environmental Bamboo Foundation, Bali, uses such installation.
(f) Pressure Treatment:
Pressure treatment of bamboo has been carried out in some countries with water-home preservatives and earlier also with creosote. It is, however, expensive and in many cases not economic for material such as bamboo. Moreover, there are not enough preservation plants in operation.
Pressure treatment penetrates air-dried culms well but may cause cracks or collapse for round bamboo. Cracks or collapse do not only reduce the strength of the culm, but also lead to inaccurate results when the amount of the absorbed preservative is measured. Cracks and collapse arise especially in thin- walled bamboos, so that only the thick-walled species should be treated in their round form.
In round culms the preservative solution mostly penetrates from the cut surfaces and at the nodal region into the culm. Split bamboos show a much higher absorption and penetration than round bamboos. Here the preservative can penetrate also from the sides which make the treatment more effective. In both cases, however, it is necessary for the preservative to have the potential to diffuse into the tissue later on.
(g) Fire Protection:
Fire causes disastrous losses in bamboo constructions because of their easy inflammability. It spreads fast over larger settlements. Theoretically it would be possible to apply a certain protection by brushing the surface with a fire retarding coating. However, the costs for such a treatment would be much higher than for protection against insects and fungi. At present, fire protection is hardly applied for bamboo structures.
Environmental Aspects of Bamboo Preservation:
Any bamboo preservation with chemicals has to consider the possible side effects on the environment. Since, preservatives have to be poisonous towards fungi and insects; they are also harmful for animals and human beings.
Aspects to be considered right from the beginning are selection and storage of the preservative, pollution potential of the different treatment, methods, training and protection of the workers, procedure of the treating process, storage and handling of the treated bamboo, disposal of the Leftovers (treating solution, off-cuts from bamboo), disposal of treated bamboo after use.