bambootower@iitb - Treatments

BAMBOO TREATMENTS

Using bamboo as a construction material must undergo a preservation process. It is because bamboo is vulnerable to termites and fungal attacks. Several bamboo failures are due to natural actions by fungi, termites and borers.

How natural durability of bamboo can be increased?

Under ordinary conditions, bamboo has an average life of only 1-2 years when used exposed to the atmosphere and in contact with the ground; while a service life of 3-5 years can be expected when used undercover and out of contact with the ground and when used indoors it can sustain for 6-8 years.

For construction, species known to be less susceptible to insect attack should be used.
The best time for harvesting bamboo is during the rainy season, when the mother plant's starch content is minimum in the culms.
Culms are cut at the base, over the first node located above the ground, using a chainsaw machete. After cutting the culm, it is necessary to cut the stump levelling with the surface of the node.
Bamboo for constructions, should be felled when they become "mature", which means once they have completed 3 or 4 years in the grove.
Bamboos culms that will be used in permanent constructions must be seasoned before use. Seasoned or dried bamboos are stronger and more resistant to the attack of fungus and insects.
The use of appropriate construction technologies for protecting the bamboo structure is recommended.

Treating or curing bamboo before using it is necessary as bamboo is a natural material and gets easily infected by insects or termites. Treatments increase the longevity of the bamboo by protecting it from natural failures like fungal and termite attacks. There are two types of treatments: Physical and chemical.

Physical treatments or curing methods

Physical treatments also called curing, help remove, reduce or destroy the starch in the culm, consequently reducing the insect's attack. The rural community has traditionally practised curing as it is the most inexpensive way of increasing the life span of bamboo.

Curing in the clump

Culms are cut at the base of the culm above the first node, left vertically with all their branches, and left for four weeks leaning against other culms. The base of the culm is separated from the ground by a stone, thus, reducing the starch content.

Curing by immersion

The fresh culms are kept in stagnant or running water for several weeks. Stones have to be put on the bamboo to keep them underwater. During soaking, starch and sugar in the parenchyma cells are leached out or degraded by bacteria to improve the resistance to borers.

Curing by smoking

Smoking of bamboo can be done by generating gas from charcoal kilns from the beginning until it reaches the carbonization state. This treatment makes bamboo more resistant to rot, deterioration and cracking and extends its life of bamboo.

Curing with heat

A ditch of 30cm to 40cm depth is dug and filled with burning coal. The bamboo culms are rotated turn by turn from the edges. This method can be used for straightening curved bamboo culms.

Curing with mud

Freshly cut bamboo is soaked in a muddy pond for 1-8 weeks and slowly dried in the shade, reducing the starch content that passes from the bamboo into the muddy water, reducing the food for insects and fungi, thereby increasing the durability of bamboo.

Curing in beach sand

The bamboo used for construction has been traditionally cured by burying them in the beach sand for 2-3 months to protect them from insects.

Curing in seawater

Bamboos are immersed in seawater supernatants containing 2-7% salt or an electrolyte solution containing water and NaCl. The process helps pickled bamboo, remove sugars, preserve it, and make it less appealing to pests. The bamboo used for construction has been traditionally cured by burying them in the beach sand for 2-3 months to protect them from insects.

Protection by white washing

Bamboo culms used for construction are coated with slaked lime CA(OH), which increases the service life of the culm by forming a waterproofing layer on the culm, ultimately protecting it from fungal attacks.

Chemical treatments

There are two types of chemical preservatives used for treating Bamboo;

Oil types such as creosote and Water-borne salts or water-soluble salts are applied as water solutions such as Copper sulphate, zinc chloride, boric acid and borax, and sodium pentachloro phenate. The advantage of these preservatives over the oil type is that timber treated with them can be painted over and varnished. We will majorly talk about treatment with Boric borax solution because it has been considered the most reliable treatment

Boric-Borax Treatment

Bamboo soaked in Borate solution is resistant to insects, termites and fungal attacks. There are various methods of inserting borates inside the bamboo tissues, but the most reliable is soaking bamboo in the solution. The technique is called Full immersion or Soak immersion.

The solution used for immersion should contain 5-6% of borates, of which 3% should be borax, 2% should be boric acid, and 95% is the fresh water. This mixture can be used repeatedly by adding borates and water as needed, which helps in reducing waste and lowering costs.

The complete immersion can be done in two ways: Cold soak immersion and hot soak immersion

Cold soak immersion

The solution used in the process is cold.
The system requires an open tank with a shed to avoid diluting the solution from rainfall.
Bamboo is soaked for 7-14 days in the tropical climate. In cold weather, the process might take a longer duration.

Hot soak immersion

The system requires an open tank with a fireplace underneath.
The heat provided helps borates to enter the bamboo culm faster.
This whole process can be done within 8 hours at 100 degrees Celsius or can take 24 hours at 60 degrees Celsius.

Process for Treating Bamboo

Drilling holes in bamboo culm

STEP 1: Before immersion, the internodes are pierced by drilling small holes near the nodes or running a metal rod through the entire pole, breaking all the internal rods.

Cold or hot soak immersion in borate solution

STEP 2: The bamboo is soaked in a cold/hot solution for a required time. These holes allow the solution to penetrate inside the culm easily.

Cleaning the solution from the culm surface

STEP 3: After the immersion process, bamboo is taken out, and the culm surface is washed with water using a high-pressure cleaner because the reaction between salt crystals on the surface and the sun can create cracks on the culm wall. Cleaning also helps in taking off dust, slime and lichens from it.

Drying the treated culm

STEP 4: The bamboo can be vertically stacked and dried directly under the sun until the green colour turns yellow. The thick culm takes a week to fully dry, while a thinner one might take 2-3 days. Afterwards, bamboo is then kept under a shed for 6-12 weeks before they are suitable for construction.

Post treatment

After the construction, post-treatment is provided, which increases the service life of the bamboo by protecting it from weathering and enhancing the visual appearance of the culm. The following steps are required for the post-treatment of bamboo culm:

Step 1: Remove excess pins from each joinery.

Step 2:Adding cover to nuts or tying ropes around the connection to avoid corrosion.

Step 3:All holes or openings in the culm should be closed to prevent insects or other unwanted creatures from residing in it.

Step 4:Sanding the culm's surface makes it smooth to touch and remove dirt from it.

Step 5:Application of sodium helps in removing moulds from the surface.

Step 6:The surface is cleaned with water, and a coating of any wood finish is applied, which protects the culm from weathering and extends its longevity.

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