Bamboo has been traditionally in used from time immemorial, the important uses was being housing, scaffolding, ladder, fence post, tent pole, bridges, bullock cart and agricultural implements. However, bamboo is non-durable as it is liable to attack by fungi (brown-rot, white-rot, soft-rot and moulds) and also susceptible to attack by insects (termites and borers).
As its natural durability is low, the service life of structure made from it is often short. However, by treating the bamboo, its service life can be extended. An attempt has been made to review the different treatment methods (traditional and non-traditional) to enhance durability of bamboo in service, and is reported in this article.
Methods of Treatment to Enhance Durability of Bamboo:
Generally, the treatment of bamboo is divided into two categories, viz., (a) treatment of green bamboos and (b) treatment of dry bamboos. In addition to the established methods of treatment for wood, some traditional methods are also in use for the treatment of bamboos.
Such methods include leaching in water and white washing, and can be carried out without special equipment and technical know-how. Chemical preservation, on the Narayanamurthi et al., 1947 other hand, needs skill and a definite treatment schedule.
A. Traditional (Non-Chemical) Methods of Protection:
(a) Controlling Starch Content in Felled Bamboos:
In bamboos, soluble sugars are the principal nutrients for parasites. Thus, bamboos with depleted carbohydrates become reasonably resistant to the attack of borers and staining; fungi.
Methods adopted for lowering the sugar content in bamboos are:
(i) Felling of Bamboos during Low-Sugar Content Season:
Sugar content in almost all plants varies with seasons. In India, for example, it is higher in spring than in winter. Therefore, it is advisable to harvest, bamboos between August and December.
(ii) Felling of Bamboo at Maturity when Sugar Content is Low:
Sugar content in bamboos varies with age. It is lowest during the first year but felling of one-year-old bamboo is not desirable because of very low strength and yield. Normally, bamboo matures at 3-4 years.
(iii) Post-Harvesting Transpiration of Bamboo Culm:
Sugar content in bamboos can also be reduced by keeping culms upright or leaning them against trees for a few days. Parenchyma cells in plants continue to live for some time, even after felling. During this period, the stored food materials are utilized and, thus, the sugar/starch content in bamboos are lowered.
(iv) Water Soaking of Bamboo:
In Indonesia, Vietnam and Africa, an easy and widely followed practice for increasing the durability of bamboo is soaking bamboo in water. During soaking in water, most of the sap present in bamboo is leached out. Some workers have suggested that a soaking period of 4 to 12 weeks is sufficient. Experimental works on submerging in mud and other applications of water soaking have not yet resulted in additional recommendations.
(b) Baking Over Open Fire:
Baking over fire after applying oil on the surface is another traditional method for preservation of green round bamboos. This causes rapid drying of the outer shell and induces partial charring and decomposition of starch and other sugars.
Moist heating is reported to cause irreversible swelling in bamboo, which probably balances the shrinkage due to moisture loss, thus stabilizing bamboo. Baking is recommended over a gentle fire, taking care that the surfaces are rotated constantly. Excessive heating/ drying can cause severe collapse. This method is very useful for simultaneous straightening of bamboos in round form.
(c) Lime Washing and Other Coatings
A variety of coatings, such as tar, lime wash, tar and lime wash and tar sprinkled with sand, are used by house builders in Indonesia. These coatings are successful only when continuously applied at cut surfaces, exposed internodes, abrasions and splits.
B. Chemical Preservative Treatment Methods:
Chemical protection ensures a longer life for bamboos. Treatments can be given using a variety of chemicals, depending upon the culm condition (green or dry) and ultimate use to which the material is to be put. Both non-pressure and pressure treatment processes can be used effectively, the key being though penetration and distribution with recommended doses of preservatives. Penetration of chemicals can be checked by simple spot tests. Following usual laboratory analysis techniques recommended for different wood preservatives can do assay of preservative.
(a) Treatability of Bamboo:
The tissue of bamboos is built-up of parenchymatous cells and vascular bundles (vessels and thick-walled fibers). The vascular bundles are not uniformly distributed inside the culm. Numerous smaller ones are present towards the outer portion, while larger but fewer bundles are found towards the central part of the culm. Bamboo has no radial cell elements like the rays in wood.
The outer wall is covered by a thin and hard layer and is lees permeable than the inner layer. Nutrients are stored in the ground tissue of parenchyma cells, which constitute up-to 50% of the tissue. Bamboos behave entirely differently from wood during treatment with preservative.
The vascular bundles play an important role in preservative treatment. The axial flow is quite rapid in green bamboos, because of the end-to-end alignment of vessels. The degree of penetration decreases as the distance from the conducting vessel increases.
The larger vessels tend to get a larger amount of preservative than the smaller vessels. (Both larger and smaller vessels belong to metaxylem whereas the prototype consists of tracheid-like elements). Since vessels occupy a mere 10% of the culm column, the penetration of preservatives to other tissues surrounding the vessels assumes more importance because untreated pockets, especially in parenchyma tissues, can lead to early destruction by fungi.
Moisture has a great influence on tractability of bamboo, especially in the green condition, where the movement of the preservative occurs via diffusion. For a Boucherie treatment, high moisture content is conditional. Treatability is thus regulated by age (6-9 years old bamboos contain less moisture than young bamboos of 3-4 years), season of felling (maximum moisture is present during the rainy season), and position (the upper portion of the culm has always a lower moisture content than the bottom). Such differences are of great consequences in uniform and adequate treatment of bamboos by no-pressure methods.
During drying, a number of anatomical changes occur, which reduce the treatability of bamboo. In contrast to wood, bamboo starts shrinking from the moment it starts losing water. Sap in the vessels is precipitated, clogging the openings to the adjacent tissues.
The pit canals in the parenchyma cells become covered with protoplasmic substances, rendering them somewhat less permeable to fluids. The entrapped air in various tissues increases their interfacial tension between the penetrating fluids, restricting the flow. The epidermal layers containing waxy and siliceous material repel incoming preservative solutions.
Although, the anatomical structure of some bamboos has been well studied, there are not many studies on the flow channels and distribution of the preservative chemicals in different structural parts in dry bamboo. However, it appears that diffusion across the wall decreases with increasing wall thickness. Diffusion rates are highest in the longitudinal and lowest in the radial direction.
Recently, a study conducted on Dendrocalamus strictus using organic and inorganic chemicals to determine the flow paths, showed that, creosote was better distributed than water soluble inorganic chemicals. Similar studies on some other species indicated that parenchyma cells are not easily penetrated in bamboos, owing to deposits of protoplasmatic substances, as mentioned above. Studies have indicated that there is variation in tractability of bamboo along the wall thickness as well as height.
C. Treatment of Fresh Bamboo:
Traditional methods increase the resistance of bamboos to borer attack but are ineffective against termites and fungi. Moreover, such methods are best suited to a small-scale user, who is, as well a producer. Since, bamboos have a large variety of uses and are required throughout the year, traditional methods are of limited value. Alternatives to these methods are treatments with chemical preservatives.
(i) Steeping:
Freshly cut culms are immediately placed upright in containers of concentrated solution of water-borne preservatives (5-10’’ %). The butt end, up-to 25 cm, is kept immersed in the preservative solution. Generally, drops of preservative solution are observed at the nodes.
The treatment takes between 7 and 14 days, depending on the length of the culm. Losses in preservative solution in the container are made upto maintain the initial level of solution. Bamboos can be satisfactorily treated by this method without any equipment and technical skill.
(ii) Sap Displacement:
Round, half, quarter and 1/8 split fresh bamboos are immersed vertically up-to 25 cm, in 10% aqueous solution of water-borne wood preservatives, i.e. boric acid-borax, acid- copper-chrome, copper-chrome-arsenic, etc., in suitable containers. The preservative solution rises by wick action as the sap is sucked-up.
Solution level is maintained by adding fresh quantity at intervals. Adequate loading exceeding 10 Kg/m3 are obtainable in two meter long bamboos in just six days. Longer pieces can be treated over a slightly longer period. In many cases, such treatments are not favoured because of environmental pollution.
(iii) Diffusion Process:
In the diffusion process, freshly felled culms or bamboos with high moisture content (above 50%) are kept submerged in solution of water-borne preservative for a period of 10 to 20 days is satisfactory. The absorption and penetration of the chemicals is more in split than in round bamboo.
The outer layer of bamboo is more or less impervious and the inner cuticle is permeable to diffusing ions. Therefore, boring holes near the nodes or increasing the diffusion time results in better penetration and higher loading. Preservatives that fix slowly, or have higher diffusion coefficients like boron-based preservatives, penetrate better. Dip diffusion, steaming-quenching followed by diffusion, and double diffusion, are all variations of the diffusion process.
Experiments have shown the effects of dip-diffusion. For instance, diffusion with copper sulphate solution (20%) for 96 hours followed by dipping in sodium-dichromate solution (20%) for 96 hours, a do then storing the bamboos under non-drying conditions of for one month, result in over 40% penetration with loading around 13 Kg/m3 in Dendrocalamus strictus.
Steaming and quenching, followed by diffusion under drying conditions, is another variation of the diffusion process but this is still only experimental and not yet applied widely. Steaming bamboos at about 100°C for 2.5 hours and quenching in 20% solution of water- borne wood preservatives (CCA), and subsequent storage for about one month, lead to almost complete penetration with very high split bamboos is 60 per cent higher than in round bamboos.
It is, therefore, recommended that in round bamboos, if this method is to be applied, that, either the septa should be punctured, or small notches should be made near the nodes, to allow free access of the solution to the inner epidermis layer of the bamboo, enabling subsequent drainage of the preservative solution from the culm. Treatment with 8-10% solution of CCA, ACC or CCB following the above schedule would give retention of about 10-12 kg/m3 in most bamboo species.
Diffusion rates almost double with every 10-20°C rise in temperature. However, CCA, ACC, or CCB solutions start precipitating on heating. Diffusion rates also depend on the nature of the diffusing ion and its interaction with the diffusion media. Preservatives based on ammonical solution not only diffuse faster, but can also be heated to get better penetration and loading in shorter periods. Ammonical-copper-arsenite can be used for treating green bamboos by diffusion, taking advantage of better penetration of the ammonium ions.
The diffusion processes are, perhaps the best and simple for treatment of green, split and round bamboos. Infact, these could be universally specified as preservative treatments of bamboos for all purposes.
(iv) Boucherie Process:
This is a widely recognized process which does not require detailed description here. It is suitable for freshly felled green bamboos with branches and leaves intact. Just chopping off about 15 cm from the butt end can treat even one-day-old bamboo. In the process, the preservatives pushed through the bamboo by gravity from a container placed at a height.
This method has been modified to use a simple hand pump by means of which air pressure of 1.0 to 1.4 kg/cm2 could be applied to the preservatives in a suitable container kept at the ground level. This reduces the period of treatment significantly. The penetration and absorption of the preservative depend upon several factors like – concentration of solution, treatment time, nature of chemical used, dimensions of bamboo, its age and moisture content.
It usually takes 30-60 min. to treat short bamboo lengths using pressure up-to 2 kg /cm, the equipment at the Forest Research Institute, Dehra Dun and used for the past 35 years. Earlier studies on a number of bamboo species indicated that treatability of green bamboos with non-pressure methods was highly variable. The modified Boucherie process is a process, which can be used with confidence.
In order to get more uniform distribution of preservative from bottom-to-top, it is possible to use initially a concentrated solution (-6%) until the solution appears at the dripping end. This should be followed by pumping in a solution of a lower concentration (-2%) for the same period.
D. Treatment of Dry Bamboo:
(i) Soaking:
Air-dried bamboos have only to be submerged in the preservative solution (oil or solvent type) for a period depending upon the species, age, thickness and absorption required. Such treatments are used in India, but not elsewhere. The penetration is predominantly by capillarity. The soaking method requires little equipment and technical knowledge, provided the schedule of treatment, such of dripping, and is worked out. Nonetheless, there are dangers of pollution due to spillage.
If water-borne preservatives are used, the process is called “Steeping”. Results of investigations using specimens of D. strictus and B. ploymorpha soaked in water-soluble preservatives (5% CCA composition) indicated that the penetration of the solution was fast in the initial stages and gradually slowed with time. Adequate amount of preservatives, however, were absorbed in all the specimens in about two weeks. The absorption of preservative was more in half-round specimens in comparison to round ones.
Soaking treatments with organic solvent type preservatives, such as pentacholorophenol, copper/zincnapthenates/abietates, work better than steeping in water-soluble preservatives. Such treatments may, however, be more expensive in some countries because of the cost of the solvents. Cold soaking in diesel oil for 7days, reported to prevent borer attack, works out to be cheaper than treatment with copper-sulphate solution in Indonesia.
Results above are experimental and if the process becomes more widespread, practical application such as, moisture contents and length of samples need to be worked out.
(ii) Hot-Cold, Process:
At FRI, Dehra Dun, hot-cold tank process has been developed. However, it is not widely in use elsewhere due to the economic aspects. Air-dried material is loaded in the tank fitted with steam coils or more some other heating arrangement. The tank is then filled with a hot creosote-fuel oil mixture and heating is continued to raise the temperature to about 90°C, which is maintained for a period of about 3-6 hours.
The preservatives are allowed to cool, after which oil is drained out. The round bamboos with holes are then kept inverted in the tank, to allow the preservatives to flow out from the hollow portions of the bamboo. Split bamboos do not require such a practice.
(iii) Pressure Treatment:
It is theoretically possible to use pressure treatment but general applicability in a number of countries has not yet been developed. Species of bamboos having thin walls are susceptible to cracking, even when treated under low pressure (5-7 Kg/cm2). Round and half-split bamboos of thick walled species like Dendrocalamus strictus can be treated with creosote – fuel oil (1:1) under hydraulic pressure of 14 kg/ cm. Retentions ranging between 88 and 92 kg/m3 are obtainable in round or half-split bamboos.
Treatment with water- borne preservatives like CCA can also be done effectively. A pressure of 3.5 Kg/cm2 for one hour sufficient to achieve retention of more than 8 Kg/m3 (dry salt) in split bamboos. The preservative loading varies significantly from bottom-to-top (Increase) and from inner layers to outer layers (decrease). Treatment with 6% CCA under 7 Kg/cm2 yielded absorptions between 14-18 kg/m3.
The problem of collapse of round bamboos under pressure treatment can be solved by drilling holes or notches between septa but this leads to considerable spillage. Drilling holes not only equilibrates the pressure on both sides of the bamboo wall, but also ensures better treatment from both surfaces. A vacuum/pressure schedule helps to ensure more uniform penetration.