Out of a total of 27 bamboo species, 13 species are considered to be commercially important in Bangladesh. The physical and mechanical properties of these species have been evaluated. The properties are found to be affected by the age of the culms. All the species reach the highest specific gravity and strength values at the age of about three years.
The younger culms, however, show the higher percentage of moisture content, while the variation of shrinkage does not follow any definite trend with the age. The top of the bamboos is the most dense and strong in compressive strength and modulus of elasticity, but the bottom possesses the highest value in modulus of rupture.
Melocanna baccifera, Dendrocalamus longispathus, D. hamiltonii and Oxytenanthera nigrociliata are thin-walled forest bamboo. These are used for mat, craft, housing material and as industrial raw material for paper and rayon. The rest species are thick-walled village bamboos which are used for making post, beam, roof, other construction materials, agricultural implements and components for transport like boat, bullock-cart and rickshaw.
There are 22-27 bamboo species under 9 genera in Bangladesh. It occurs both in the natural and cultivated states. The forest bamboo is a natural one which grows as a pure stand or understory in association with tree species.
The hill forests in the eastern part of the country occupy the entire stock. Bamboo is cultivated in the plain land and homesteads in almost all the villages throughout the country. It occupies the backyard and periphery of homesteads. Morphologically, there is a difference between the forest and village bamboos.
The forest species are thin-walled and comparatively smaller in size. The village bamboos are tall and branchy with thick wall. It is estimated that the village forests supply about 528 million bamboos, while the national forests supply about 128 million culms in 1993. The total demand is estimated at 709 million culms resulting in a net deficit of 53 million bamboos in 1993.
Bamboo is an essential item of day-to-day use of the rural people in Bangladesh. Its use, except raw material for pulp, is mostly based on traditional experience and conventional practice. There is hardly any scientific basis for its utilization.
Information on physical, mechanical and other properties is essential for assessing the suitability for various end products. Lack of these properties may lead to over or under designing of the products which cause wastage. To fill up the gap in this aspect, successive studies have been undertaken in the Bangladesh Forest Research Instituted (BFRI) for the last 7 years. Ten important species of bamboo have been investigated.
There are two important species, Dendrocalamus strictus and Bambusa nutans which have not yet been investigated. The present article incorporates the physical and mechanical properties along with the uses of all the important bamboo species.
Materials and Methods:
Three culms, each from five age groups of three species, Melocanna baccifera, Bambusa balcooa and Dendrocalamus longispathus were collected from the Bambusetum of the BFRI. Three culms, each from three years old of nine species, B. vulgaris, B. tulda, B. nutans, B. polymorpha, B. iongispatlius, B. arundinaceae, D. hamiltonii, D. strictus and Oxytenantliera nigrociliata were collected from the same Bambusetum. Each culm was divided into three portions- bottom, middle and top, which represent three height positions.
The specimens for moisture content and specific gravity were taken in the form of a pair of 2.5 cm wide rings from each of the height positions. The moisture content was determined by the oven drying method. The specific gravity was determined on the basis of oven dry weight, and green and oven dry volumes which were ascertained by the water displacement method.
The specimens for wall thickness were also prepared in the form of 2.5 cm wide ring, while the specimens for diameter thickness were from the internodes of the culms bounded by nodes at the extremity. Mechanical properties, viz., compressive and static bending strengths were evaluated following the Indian Standard Specifications.
The static bending test was performed on a full diameter specimen of 75 cm length bounded by two nodes. The compressive strength parallel to grain was determined from the specimens on full diameter and the length was ten times of the average wall thickness of the culm. Paired specimens, one for the test in green condition and the other in air-dry condition, were taken from culms at each height position.
Discussion:
(a) Moisture Content:
The moisture content is found maximum in the younger bamboos; as the culms become older, it decreases. Bambusa balcooa and Oxytenanthera- nigrociliata possess the highest moisture content, while B. arundinacea contains the lowest value among the species. In all the species, the moisture content declines along the culm height.
Thus the highest and the lowest moisture content are observed at the butt and top portions of the culms respectively. A similar trend was noticed by other workers. The variation of moisture content along the culm height may be associated with a decrease in the proportion of parenchyma cell which acts as a site of water storage.
(b) Specific Gravity:
The specific gravity is observed to increase from the bottom to top in all species under both green and air-dry volumes. The reason for higher specific gravity at the top may be attributed to the presence of higher amount of sclerenchyma tissue ‘fibro-vascular bundle at this position’. The younger culm shows lower values of specific gravity.
The maximum specific gravity is found in the culms of 3 to 3.5 years old and the value decreases thereafter. Dendrocalamus longispathus is found to be the densest and O. nigrociliata least dense among the species.
The specific gravity is significantly affected by the age and height position. Its comparative values range from 85 to 115 percentage relative to teak wood. The higher values of specific gravity were noted in Dendrocalamus strictus at different age groups by Indian workers.
(c) Shrinkage:
Unlike wood, bamboo seems to shrink from the beginning of drying. The shrinkages in wall thickness and in diameter are quite appreciable in both air-dry and oven-dry conditions. The values decrease from the bottom to top significantly. The higher shrinkage at the bottom may be related to the higher initial moisture content at that position. The age affects these values significantly, but there is no any definite trend in the variation.
(d) Mechanical Properties:
The compressive strength of all the species is affected significantly by the age and height of bamboos. The highest value is observed at the 3 to 3.5 years old bamboo and like physical properties this strength decreases thereafter. Other investigators noted the highest value of compressive strength from bamboos of 2.5 to 4 years.
The compressive strength also increases as the height increases. The top of the culm is found most strong in respect of this property due to presence of the highest percentage of sclerenchyma tissue at this position.
In respect of modulus of rupture and modulus of elasticity, bamboo shows an increasing trend with the increase of age upto 3 to 3.5 years. Like other properties, strength properties are also found to decrease thereafter.
The modulus of elasticity increases with the increase of height in all the species. However, the height has an inverse effect on the modulus of rupture. A similar result is obtained by many other workers. No plausible reason is available for this. Limaye (1952) correlated it with the higher wall thickness. This does not seem to be reasonable because the strength is calculated taking wall thickness into consideration as well.
As regards relative to teak wood, the strength values vary from as low as 48 to as high as 175 percentages. The strongest species is observed to be Dendrocalamus longispathus while the weakest one is Bambusa polymorph.
Uses:
Bamboo has been preferred for a great variety of purposes due to its favourable properties, easy availability and low cost for ages.
The major uses are as follows:
M. baccifera, D. longispathus, D. hamiltonii and O. nigrociliata are thin-walled forest species. These are used for mat, craft and housing material. These are also employed for the industrial raw material for paper and rayon.
The rest nine species are village bamboos which are comparatively thick-walled bamboos. These are extensively used for making post, beam, roof and other constructional purposes. The species are also used for a wide variety of purposes including agricultural implements and components for transport like boat, bullock-cart and rickshaw.
Conclusions:
The following conclusions may be drawn from the study:
(i) The moisture content and shrinkage decrease as the height of the culm increases. The specific gravity has the direct relation with the height. The top of the culm shows the highest specific gravity.
(ii) The compressive strength and modulus of elasticity increase from the bottom to top, whereas, the bending strength has the reverse trend.
(iii) The culms show the highest specific gravity and strength at the age of 3 to 3.5 years. Thus, the maturity of bamboos takes place at this age in respect of physical and mechanical properties. Thus, bamboos of 3 to 3.5 years old may be harvested for getting the optimum result.
(iv) Each bamboo species has multiple end-uses. It includes from house construction to the fancy handicraft.