Week 12: THE SCIENTIFIC VERNACULAR

RESEARCH QUESTION                                                    

How can a composite of natural and man-made material increase the durability of bamboo construction in the modern industrialize construction?

The scientific Vernacular explores the turning points when inputs of scientific knowledge boost the creative urges of architects for new forms of architectural expression which were traditionally concentrated in Vernacular Architecture; taking the leap from a stylist approach to knowledge based and innovative approach with the use of technological science.

Bamboo has its disadvantages such as decaying, susceptible to insect and fungal attack. The service life of bamboo is generally considered as being too short for any worthwhile investment. A problem that compounds the low natural durability is the hollowness of the bamboo culm, particularly when compared with the end-to-end massive cross section of wood. If fungi or insects attack and destroy the outer layer of the bamboo, it may mean the loss of one quarter of its thickness compared to the total thickness of wood. The hollowness also offers a relatively safe hiding place for the pests of destruction. In most tropical countries, the high relative humidity of the air adds to durability problems. High moisture content in the bamboo also creates problem in drying process. Besides, bamboo does not contain cross fibers and is consequently, not designed to bear weight width-wise, with the exception of the points at the nodes. Bamboo is prone to splitting, especially when standard construction fasteners, such as bolts, screws and nails are inserted. Special fastening techniques are required when joining pieces of bamboo.

The diagram below shows the life cycle of an untreated bamboo:

Since Bamboo itself brings threats towards the mission and vision of our future development, a combination of kaccha and pukka styles can be used to increase its characteristic thus improve durability and feasibility of the various periods stated above. How? What is kaccha and pukka?

Kaccha is a building made of natural materials such as mud, grass, bamboo, thatch or sticks and is therefore a short-lived structure; Whereas pukka is a structure made from materials resistant to wear as forms of stone or brick, clay tiles, metal or other durable materials, such structures are expensive to construct as the materials are costly and more labour is required.

To simplify the research question, the research will be researching on the type of natural and artificial material used to increase the durability of bamboo.

Community in villages know by experience the durability of the bamboos in their homeseeds. The lower part of the culm is said to be more durable; and so is the outer part of the culm wall. The starch in the bamboo attracts fungi and insects, therefore the selection of harvest time during the dry season will have a better durability. Also, bamboo with flowers is more resistant to these agents due to its depletion of starch.

The correlation between natural durability is difficult to establish; most of the facts are based on culture and tradition from the villages through experience than on physical reality (Kirkpatrick and Simmonds 1958).

There are various durability can be found in bamboo in terms of its life span and service as a structural member.

Starch makes bamboo vulnerable to be attacked by fungi and insects. Therefore, reducing start content of bamboo is the best way to make it less vulnerable. You can reduce starch content by:

- keeping the culm in a vertical position under a shade for a week after harvesting

- harvesting only the mature bamboos

- harvesting in winter season

- by soaking the bamboo for 3 to 6 weeks in running water or permanent water sources

The durability of bamboo is directly related to how well it is treated at all stages of its use, including how it is grown, harvested, dried, stored, transported and installed. 

Bamboo fails most commonly through rot from excessive water contact, and attack from pests seeking out the sugars in the starch of the bamboo, hence most pest infestation occurs at the joints and through cracks and holes. Therefore, prevent direct moisture contact and reducing cracking through careful handling will greatly increase life expectancy as will surface treatments such as paint and oils.

Harvesting

When harvesting, cut the bamboo neatly and evenly immediately above a node that is one or two nodes above the ground, to reduce the potential for fungus to infest and degrade the plant. Using the basic principles stated above to increase the durability of bamboo.

Leaching

In many cultures freshly cut bamboo is soaked in rivers or streams to leach out sugars and saps to reduce pest infestation. Leaching is more effective on smaller sections of bamboo; therefore it should be done after cutting to length or split while vascular cells are still open. Knocking a hold through the centre of each node along the length of bamboo will allows saps to 

wash away more rapidly.

Drying

Bamboo will achieve it is the greatest strength if it is being dried slowly, uniformly; prior to use and to prevent cracking. The curing process is being done in a cool, shaded, ventilated and organic material free area. The culms should be stored horizontally; rotating regularly to avoid splitting from uneven drying. In the process of drying, the bamboo will loses its leaves and start turning brown; going through the process of losing starch content and moisture. On the other hand, storing in a silo as another alternative for its drying place where shades and ventilation are present. Also, split, woven bamboo is another strategy for drying to take place quickly.

Kiln Drying

Using the present level of drying technology, it is not feasible to dry bamboo poles due to its incidence of cracking and collapse. However split bamboos are preferred instead.

Air Drying
Air drying takes up 6-12 weeks depending on the moisture content and the wall thickness.  Mature culms are preferable; thus it will not pose any problems of splitting under the sun.

Smoking (Baking over Open Fire)

The bamboo is stored above the fireplace for few days after applying oil on the surface; until the colour turns into slightly black. This causes rapid drying of outer shell and decomposition of starch. However, excessive heat might develop fissure or crack in fresh bamboo when you apply heat to the freshly cut bamboo; therefore using a gentle fire and rotating the surfaces constantly is recommended.

Transport and Storage

Bamboo culms should be loaded and unloaded by hands; not thrown from the truck. They should be stored carefully so that air can circulate around each culm and being protected from ground water and rain; keeping it away from direct contact with soil due to its route for pests and moisture retention in it. 

Surface Treatment

Surface treatments such as application of sumps oil, paint, varnish or other chemicals being applied in a diluted form to allow deep penetration although surface treatment are not as effective as penetrative treatments thus require regular top up treatments.. Focus should be given to joints, ends, cracks and nodes as the area has the most pest infestation. Starch coatings within the culm have to be removed by boiling or abrading with wet sand if a burning process is required. The Japanese have refined an artificial method of obtaining a similar colouring using sulphuric acid mixed with mud.

Treatment of Fresh Bamboo                        

Steeping

Freshly cut culms being placed upright in containers of concentrated solutions of water-borne preservatives (5-10%). The treatment takes between 7 and 14 days, losing the preservative solution in the container and made up to maintain the initial level of solution.

Sap Displacement

Bamboo culms are prepared to size and submerged in Borax/Boric Acid solution which is a water soluble preservative for several days. Preservative rises by wick action as the sap is sucked up. The preservative enters the culm through ends and holes drilled in internodes. The soaking process is long enough for the chemical to soak through the entire structure of the culm.

Modified Boucherie Process

The Boucherie method was pioneered by Dr Walter Liese of Hamburg University. Borax is water soluble; it will leach out with continued exposure to rain, losing its effectiveness. In this method, the preservative is passed under pressure through the culm till it comes out at the other end of the culm. This can be applied only to fresh bamboo within 24 hours after the harvest.

Dip Diffusion Process

Fresh culm with high moisture content (above 50%) are kept submerged in solution for a diffusion period of 10 to 20 days. This method can be applied only to split and swan bamboo strips since the moisture causes difficulties for the penetration to occur. Steaming and quenching, followed by diffusion under drying conditions is another variation of diffusion process.

Treatment of Dry Bamboo      

Soaking

Air dried bamboos have only to be submerged in the preservative solution (oil or solvent type) for a period. The penetration is predominantly by capillarity. If water-borne preservatives are used, the process is called ‘steeping’. But the soaking treatment with organic solvent (such as pentachlorophenol, copper, abietates) works better than steeping in water-soluble preservatives.

Hot and Cold Method

The bamboo to be treated is submerged in a tank of preservative that is directly heated by fire after being maintained at a constant temperature for a period, the tank is allowed to cool. During the cooling process, the preservative is drawn into the bamboo. The hot and cold method can be used for green or dry bamboo culms with either Boron or Creosote.

Pressure Treatment

Round and half split bamboos of thick walled species can be treated with creosote (fuel oil) under hydraulic pressure. The problem of under pressure can be resolved by drilling holes or notches between, but this leads to spillage. Therefore, a vacuum/pressure schedule helps to ensure more inform penetration.

Protection by Design

The design of bamboo is to ensure its good nodal placement and protecting the bamboo from weather and pests. 

Large roof overhangs prevent direct wetting of bamboo walls in heavy rains; thus using drainage channels or gutters to discharge water from the building at a certain safe distance. Bamboo flooring can be protected through building on a slightly sloping site and using raised masonry or concrete footings. This can be applied on bamboo columns and wall panels to reduce the risk of termite infestation through the direct contact towards the ground surface.

Bamboo has got a round profile therefore creating connections will be leading to difficult geometric structures at the knot. Various connections can be used; for example, friction tight rope connections, nuts and bolts connection, positive connections, interlocking connections. Besides that, bamboo is hollow in the middle of the cane; therefore it can be reinforced by pouring concrete inside. 

Traditional Methods

Joint Bracing

While bamboo has been used for centuries, the traditional methods of lashing bamboo together are not appropriate for the design of long span trusses. These lashed connections also do not fully utilize its strength; it solely rely on friction, transferring load between members is limited thus require more members to do the same job. Therefore joints in bamboo always have been difficult because it is hollow, has nodes and resists only a little shear stress. Many traditional joints suffer from weakness or deformation. Many joints cannot take advantage of the strength of the culm itself and a special problem is to design a joint for tensile forces.

Modern Methods         

Bolting Joints

Modern connections have been proposed by Huybers, Shoei Yoh and Renzo Piano. These connections solve the issues of complex geometries by joining the members in various ways. These connections require puncture of the bamboo walls sine all fibres in a bamboo culm run parallel once a bolt is placed through it and the connection loaded in tension, the bolt acts like a wedge and splits the bamboo. Also the puncture allows moisture to enter the culm and accelerate decay.

Nodal Placement

With an understanding of the bolt alone concentrate much on the wall of the bamboo, the void between the solid internal nodes is filled with solidifying mortar where every bolt penetrates each of the bamboo culm. Members of a truss come together at angles and tension forces are anticipated, a steel strap is placed to bridge the pieces. It is important to design with redundant system that is capable of both compression and tension. This frame truss system is certainly preventing bamboo from bending and buckling in the middle of the culm. Larger holes were drilled into the bamboo to insert the mortar into the joint segments. The mortar was mixed with the optimal quantity of water; the bamboo pieces where then duck taped together to maintain their positions as they were filled with mortar and left to cure for a week. Once mortar is cured, the tape can be removed.

Bamboo + Mortar + Rebar

Though holes were drilled into bamboo segment to insert re-bars. This re-bars will absorb most of the stress moments and allow the bamboo structure to sustain high stress impacts as occur during earthquakes. Bamboo tends to shrink with time making the string joinery slack and insecure. Therefore the joinery system relies on bolts and mortar-fillings maintaining their shape over time.  Bamboo filled with mortar are reinforced by an additional the rebar is embedded in mortar, the load is transferred evenly across the member's cross section and can transfer high axial loads to the bamboo. 

Bamboo + Mortar + Rebar + Clamp

The bamboo is cut at the end axially crosswise to generate a cone by tightening a steel clamp. The cone is filled up with mortar so that the cone is able to transfer its force to the outer high tensile fibres. A common steel clamp is also placed at the end of the member to provide confinement of the bamboo and prevent splitting of the bamboo. Because the rebar is embedded in mortar, the load is transferred evenly across the member's cross section and can transfer high axial loads to the bamboo. Finally, the incorporation of the steel gusset plate makes the bamboo easy to connect in any configuration desired.

New Joints

In order to allow bamboo for spatial and light structures, it is important to have a durable joint which does not bring much weight to the structure and which works under high forces to take advantage of the high resistance of bamboo fibers. The alternative joints for bamboo and by some techniques which are already used in steel constructions. The joint is light, strong and appropriate for easy assemble and disassemble.

Based on the various methods to increase the durability of the bamboo from its own solid wall towards its connectivity on the building structure, bamboo can almost become a universal element. This research has provided a proof of concept for the durability of bamboo itself can be developed by using the natural methods and artificial methods; thus increasing it durability on structure by roughening the inner surface of the bamboo member, filling a portion with mortar, embedding re-bar and welding several of these members to gusset plate. Having both of these characteristics can then be used to construct and span moderate distances, thus providing a safe and predictable behavior. This research will help facilitate the future use of bamboo in a cost effective solution to many parts of the world.