XXIII Sympozjum Rogów, Polska Akademia Nauk, Komitet Technologii Drewna PAN 5.-7 Sept. 2007

XXIII Sympozjum Rogów, Polska Akademia Nauk, Komitet Technologii Drewna PAN 5.-7 Sept. 2007

Protection of Bamboo Structures

Walter Liese

Prof.em. Dr. Dr. h c. mult., Department of Wood Science, Division Wood Biology, University Hamburg, Leuschnerstr. 91, 21031 Hamburg, Germany

Introduction

The bamboo culm is a unique building material for allkinds of structures. Most important are bamboo houses of different types, which provide worldwide homes for many millions of people, not only in rural areas, but also in urban environments. Its use as a substitute or alternative for wood is accelerated by the increasing shortage of timber in most bamboo producing countries. The application of bamboo is promoted by international activities, as presently by the Workshop on Bamboo Constructions, August, 2007,Cali, Colombia, the International Conference on Modern Bamboo Structures, October 2007 in Changsha, China, the 2007 International Workshop on Bamboo, November, Hangzhou, China and an International Bamboo Building Design Competition with 250 designs in 12 building categories , submitted from 25 countries. The wider acceptance of bamboo for structural uses, however, is often hindered by its liability to biological degradation. These hazards have their impact on restrictive building regulations for bamboo in several countries. The protection of bamboo structures against biological hazards is an important requirement for utilizing this valuable woody resource (Liese, Kumar 2003, Liese 2004). Compared with the protection of timber in tropical countries there are certain similarities, but also considerable differences (Willeitner, Liese 1992).

Bamboo in Service

To utilize bamboo for construction, it is important to consider the technological properties of the culm but also its low durability against organisms. In a suitable environment, the culms are easily attacked by fungi and beetles. In ground contact, bamboo structures are destroyed in less than two years, under cover after 4-7 years, but may last for a long time in a favourable situation.

Bamboo culms do not produce any toxic substances during their lifetime, unlike the heartwood of many trees, so that the whole culm can be destroyed. In addition, the culmcontains about 50% parenchyma cells, filled with starch as stored energy,which is needed for the development of new culms. Since their growth starts generally after the rainy season, the starch content will then be lowest, resulting in a reduced liability to attack. An influence of culm cutting around the full moon on the durability has not been proven.

Non Chemical Protection

For any utilization of bamboo, the possibilities of non-chemical methods should be considered first to avoid the danger of environmental pollution. The main principle should be reducing the high moisture content and avoiding any later uptake by rain or ground contact (Moran, 2002).

As a traditional method clump curing has to be mentioned. Culms are cut at the bas and left leaning against neighbouring ones. The ongoing transpiration by the leaves reduces the moisture content and also the starch, thus enhancing some resistance against borers and staining fungi. For the common water-storage fresh culms are immersed in water for at least a month, whereby the starch content is reduced mainly by bacterial action. The susceptibility to deteriorating fungi and termites is however hardly reduced.Another common method for protection without biocides is the plastering of bamboo mats with mud, clay or cement. Plastering is not only applied by villagers, but also for urban houses. The tight seal by mortar protects against moisture and prevents the entry of beetles

A long standing tradition exists to protect bamboo structures by constructional methods, which prevent the accessof moisture. The main advantage is that no chemicals with their possible environmental impact have to be applied. Well-known are base supports by stones, pre-formed concrete footings or even durable wood blocks as well as an overhanging roof. If the environment is unsuitable for beetles and fungi, impressive structures can be built, like in the Cordillera region of Colombia, where in an altitude above 1.5000 m termites do not exist. (Jayanetti,1998,Janssen, 2002, Vantomme et al. 2003)).

The bamboo smoking system, originally applied inJapanby storing fresh culms above a fireplace has been developed in Colombia for commercial operation. A standing furnace is filled with semi-dry culms and heated by burning organic residues at around 55 C° for about two weeks until moisture is reduced to about 12%. For the impressive ZERI Pavilion at the EXPO 2000 in Hannover about 3.500 culms of 9 m length had been smoke-treated in Colombia. However, the technical parameters for this process have still be refined and long time results are wanted.

For timber, a heat treatment has been developed in recent years withawider practicalapplication. Temperatures above 150 C° lead to a better dimensional stability and improved resistance against micro-organisms due to modification of organic matter. Trials with split bamboo at temperatures above 200 C° resulted in an enhanced durability against fungal attack, but the mechanical properties were severely reduced, as it has also been observed for timber (Leithoff, Peek, 2001).

Protection with Chemicals

In most cases a chemical treatment is needed. A key factor is the sufficient preservation of the culm. However, its treatability is rather low. Unlike timber, the bamboo tissue is quite resistant towards penetration of liquids due to its anatomical structure (Liese 1998). The culm is covered at its outsideby a water-tight layer, the cuticula, and inwards by a suberin layer, which hinders side-ways penetration by simple treatments. Radial pathways, like the rays in timber do not exist. The uptake of a preservative is therefore restricted mainly to the vessels at the culm ends. They run through the internodes like “water-pipes “ and become partly modified at the nodal region. The total vessel volume amounts to only 6-8 %, in comparison to about 70 % of softwoods. The main tissue of around 90 % parenchyma and fibers has to be penetrated and protected by diffusion. The passing of the vessels can be hindered by tyloses and slime blocking (Liese,1998).

The limited access of the bamboo tissue through the vessels only reduces the possibility for treatment with monomers, which has been considered for quality improvement of bamboo (Lawniczczak 1991, 1993; Liese 1994).

As preservativeswater-borne types are in use. They can penetrate by diffusion the tissue of moist culms for a complete protection. Boron compounds are most common for culms and products. Also CCA and CCB-types are used in Asia and other bamboo regions, preferably by factories which normally treat timber. Creosote had been applied earlier, mainly in a marine environment.

For prophylactic protection of culms in storage insecticides are common as sprays. Fire retardants to cover the surface are hardly used. Natural toxicants are tried from several plants, like Neem, for the protection of bamboo handicrafts.

The treatment methods for bamboo depend on the intended use, the type of material and the environmental aspects.

Painting with industrial lacquers is applied for furniture and handicraft. Beside the colouring it may have some hydrophobic effect, but does not provide protection.

As a non-pressure process the butt-end-treatment is common, whereby the base of fresh culms is immersed in a container. It is a simple process and used for the culm protection for supporting fruit trees or in vineyards.

The soaking method is often applied. Fresh culms, preferable with broken nodal walls and/or borings in the internodes at opposite sides are submerged in a boron-containing solution for 7-10 days. Recently, the Vertical Soak and Diffusion Process (VDS) has been developed by the Environmental Bamboo Foundation ( EBF), Bali. Freshly harvested culms are vertically stacked in a basin and all nodal walls fractured, except the lowest one. The internodes are filled upwith the preservative for about two weeks. Then the lower node is also ruptured and the solution released in the basin. By this method especially the inner, nutritious culm part is protected, whereby the culm skin is free from any preservative residues.

Special importance merits the classical sap-replacement method, which allows a complete penetration of a fresh culm by boric saltswhile providing a clean surface. Once developed in Francefor spruce stems by A. Boucherie, 1840, the method was modified for bamboo in India (Purushotham et al. 1954, Liese 1959), further improved and is now employed in many countries. The preservative is pushed from a closed container by a pressure pump into the base of the culm to withdraw the sap from the vessels, which flows off from the top end. Culms of 9 m length are treated within 30-50 minutes with a pressure of 1.9-1.3 bar. The sap-replacement method is a reliable and environmental friendly treatment for bamboo culms since the preservative is closed inside the culm (Liese et al. 2002).

The pressure treatment provides the best protection, but is hardly applied anymore, except for water-front structures. Creosote and CCA-salts are the main preservatives. Split bamboo and woven mats obtain a high absorption and which might be useful for special cases.

A remedial treatment of infested culms or construction parts is seldom to be recommended. It is quite uncertain to determine the extent of deterioration within the culm. Fumigation and bore-hole treatment are sometimes applied. If possible, the building component has to be replaced by a treated part.

Quality control is an essential tool for any bamboo construction (Janssen 2001). For bamboo preservation it is even more demanding, albeit most neglected. It concerns the quality of the bamboo material to be treated, the choice, concentration and uptake of the preservative and a sufficient distribution within the culm tissue.

Health and environmentalrisks are important aspects for bamboo preservation, but often neglected. Although regulations may exist at varying degrees, they are often not known or considered. Strict regulations have to be applied wherever bamboo is chemically treated. This should be permitted only at sites where the required skill, experience and control are available to avoid danger for the workers and the environment.

Marketing and economics are key factors for a demand of treated material and essential for the economical success of the treatment installation. Most factories treat on demand only so that stock-keeping of treated material does not exist.

Economics is the major component considered by the consumer for the choice of chemically treated bamboo. The wide-spread use of bamboo products is often based on minimal wages for the poorest and an easy replacement. The progress towards better living will raise simultaneously the cost for harvesting and processing.

In addition, excessive harvesting and the increasing demand of bamboo processing factories, like for pulp and parquet have led regionally to a shortage of bamboo. Also a displacement of bamboo by competitive materials, like plastic is to be observed. Nice looking walls and commodities from plastic are less expensive, durable, with no toxic effects and more colourful then the local ones from bamboo.

Bamboo protection against deterioration can contribute considerable to utilize this gift of nature efficiently for the welfare of the ruralpeople and as a significant contribution to the biomass resource.

Bibliography

Janssen, J.A. Design and Building with Bamboo. INBAR Techn. Rep. No 20, 2002, 207 pp.

Jayanetti, D.K.L; Folett,P.R. Bamboo in construction. TRADA Techn. Ltd and INBAR Techn. Rep. No 15, 1998, 120 pp.

Lawniczak, M. Bamboo-polymer composite new construction material. Intern. Bamboo Workshop, 1991, Chiangmai, Thailand.

Lawniczak, M..Method of production of the composite bamboo-polysterene elaborated in Poland. National Bamboo Conv.26.11.1993,Bandung, Indonesia.

Leithoff, H.,R.; Peek, R..Heat treatment of bamboo. IRGWP 01-40216, Stockholm, 2001, 11pp.

Liese, :W. Bamboo preservation and soft rot. FAO Report No 1106 to the Government of India, Rom, 1959, 45 pp.

Liese, W. Biological aspects of bamboo and rattan for quality improvement by polymer impregnation. Folia Forestalia Polonica, Ser. B., zesyt 25, 43-56, 1994.

Liese, W. The anatomy of bamboo culms. INBAR Techn. Rep. No. 18, 1998, 204 pp.

Liese, W. Preservation of bamboo structures. Ghana J. Forestry. 15&16,2004, 40-48.

Liese, W., J. Gutierrez, G. Gonzales. Preservation of bamboo forthe construction of houses for low income people. In: Bamboo for Sustainable Development. VSP and INBAR, 2002, 481-494.

Liese, W. Kumar, S. Bamboo Preservation Compendium. INBAR Techn. Rep.No 22, 2003, 231 pp.

Moran, J.A. Traditional Bamboo PreservationMethods in Latin America. INBAR Techn. Rep.No 25, 2002, 70 pp.

Purushotham,A., Sudan, S, Vidya Sagar. Preservative treatment of green bamboos under low pneumatic pressure.Ind. For. Bull. No.178,1954, 21 pp.

Vantomme, P., Braunlin, P., Chioetto, V., Liese, W. Public constructions made with bamboo. Lessons learnt from the “Vergiate Bamboo Pavilion” in North Italy. Journ. Bamboo and Rattan, 2, 2003, 369-380.

Willeitner, H.W. Liese, Wood Protection in tropical countries. A manual on the know-how. GTZ Schriftenreihe No.27,1992,228 pp.

Abstract: Bamboo culms are an excellent material for countless applications. Their wider use for construction is encouraged by the growing scarity of timber. Since bamboo has a low natural resistance, protection against biological degradation is of vital importance for longer service life. Protection measures without chemicals are preferable, but often limited in their effectiveness. When using preservatives, the restricted permeability of culm tissue,choice of preservative and treatment method as well as environmental effects and economical aspects have to be considered.

Keywords: Bamboo, deterioration, permeability, protection, preservatives, treatment technics, environment, economics.

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