Chapter IV. The Properties of Bamboo

The fact that bamboo is so widely used in China and commands so much wealth in trade lends weight to the supposition that its characteristics are suited to manifold uses. A comprehensive glance at the wide range of its uses inspires us to inquire more closely into those qualities which render bamboo so valuable. 

There are first of all certain external features which help make bamboo useful. In the handling of it many of the difficulties encountered with other lumber are not met with. None of the complicated machinery necessary to the loading of logs in the timber regions of the United States is needed. Bamboo never becomes so large nor is it ever so heavy as timber logs. In China a simple skidway straight down the hillside to the bank of a stream in the bed of which a stone platform for making rafts has been constructed is all that is needed. No rough bark increases the friction in skidding and no teams of horses are required to pull heavy logs out of the woods to the landing. Though wet bamboo is extremely heavy, it is never unmanageable, and when dry it is very light. This is a virtue which brought the bamboo pole into such universal use in western track athletics to take the place of the wooden pole in the pole vault. In transportation lightness is a characteristic to be thankful for especially in a country like China where carrying is done so much by human labor. In regard to water transportation lightness is not the only, or the chief, virtue, Buoyancy rather is the quality of bamboo poles most helpful in such transportation. This is due to their internal construction. The poles are hollow except for the partition at every joint. Instead of being a simple tube a pole is thus divided off into a series of air-tight chambers ; for this reason bamboo poles when they float draw very little water. As a consequence, they are as good as life savers. It is for this reason alone that in valleys and on the plains at the base of the hills the rafts which are made up are capable of being floated out from very small streams to the canals and thence to the markets. 

The smooth polished exterior of bamboo poles is in itself a remarkable, almost unnatural, characteristic. No finish applied by human hands is so  smooth and hard. The immediate cause is the secretion by the epidermis of wax and silicon. The waxy coating is the basis of the polish, while the silicon compound is responsible for most of the hardness. Some idea of the quantity of silica contained in bamboos may be gathered from the fact recorded of one species, Bambusa Tabacaria, that it will emit sparks when struck with an axe. Though polish and hardness are sufficiently remarkable traits of the epidermis to warrant special mention, they are no more advantageous to the plant than its imperviousness to water. Even parasites find it very difficult to effect an entrance so long as the epidermis is left intact. In time, however, the combined action of moisture, weak mineral acids, mildews, and micro-organisms succeed in forcing an opening in the protective armor of the culm. For a good illustration watch the base of a bamboo fence post. In a year the fibres have rotted through. In the air, however, bamboo poles as they stand in the lumber yards seem able to resist decay for years. This applies more so if the poles are being used constantly as boat poles, clothes line supports, carrying poles, etc. In the bamboo forests pieces of canes have been found with the wood entirely rotted out, leaving only the epidermis as a shell. 

The saying that a chain is no stronger than its weakest link might be revised to apply to bamboo. A bamboo pole is no stronger than its weakest joint. It seems peculiar that the only solid parts of the cane should be the weakest, but a moment's examination will serve to convince us of the fact. In the first place, the fibres at the joints are not so compact as they are in the other parts. They are loose and spreading, interspersed with small fibres which as branches of the vertical strands weave in and out at right angles to them and finally pass inward as part of the supporting skeleton of the partition. The results of mechanical tests support our statement and verify our conclusions. Out of fourteen shearing tests carried out by the Whangppo Conservancy Board on dry specimens of bamboo, six were with straight fibre, while the other eight were with specimens which included a joint each. In the former case the average stress was found to be 1,183 lbs. per square inch; in the latter 1,155 lbs. per square inch. These results in general were corroborated by others. Another investigator found the average shearing stress for specimens without joints to be 2,740 lbs. per square inch and for specimens with joints 2,000 lbs. per square inch. Again, we notice that the specimens with joints average less than those without. The explanation is that the joints are the weakening element. They are less able to withstand shear than the other parts. In young specimens it is even more TRUE because of the fact that the hardening process reaches the fibres of the joint last of all. A young cane when bent over too far will snap off at the joint. Examine the branches and see where they usually break off. 

In some cases, however, weakness at the joint may be increased by rot. The scar or ring just below the raised ridge of the joint proper is a favorable place of attack from fungi and bacteria. As a tender shoot the joint is enclosed in a protective sheath-leaf. Later, as each section in order attains its full size, the sheath-leaves fall off, leaving behind a scar which like the leaf-scars on trees becomes thoroughly corked over as a protection against the loss of internal moisture and the invasion of pests. But this does not always take place without accompanying defect, nor can the chance always be avoided of attacks by boring insects in the wake of which spores and penetrating mycelia find their way into the tissues and eventually the joint. 

But. withal, the pole, by reason of the partitions, is stronger than if they were not there at all. They are not massive and their density is only half that of the main part of the cane, but because of their large number the cane is better able to resist certain strains since they are well distributed. The partitions help the canes to maintain their tubular form under bending strains. Because of this fact, together with the fact that it has a high tensile strength, bamboo is said to have a flexural strength similar to that of a fir pole. And if in addition we mention again its lightness and rapid growth, we are speedily brought to the realization of the great utility and value of bamboo. 

Since in its mechanical equipment lie the principal virtues of bamboo, let us examine this problem a little more closely. As a preliminary statement it may be said that in mechanical construction the sky-scrapers of New York cannot compare with bamboo structures. 

To ascertain more completely the true nature of the mechanical properties of bamboo, physical experiments have been undertaken. The tests referred to above were conducted at the instigation of the Whangpoo Conservancy Board to discover the possibilities of bamboo as a material for reinforcing concrete. In all 220 tests were carried out for bending, shearing, elasticity tension, and compression respectively. The mean result of 150 bending tests was 13,300 lbs. The ultimate flexural stress varied between 11,000 and 14,003 lbs. per sq. in., depending on the way the loads were applied. The collapse of the bamboo was always sudden, the material first splitting into pieces parallel to the longitudinal axis. None of the fibres were torn. Collapse was evidently caused by the shearing force. The average stress in the shearing tests was 1,183 lbs. per sq. in. for dry specimens and 1,033 lbs. per sq. in. for green. The modulus of elasticity was found to be 1,660,000 lbs. per sq. in., about the same as that of pine. The ultimate tensile stress of bamboo was worked out in the same way as it is for steel reinforced concrete. Concrete beams reinforced with bamboo were tested for bending and the value of the tensile stress calculated by means of the appropriate formulae. The concrete cracked, due to the very large deflection, so that in the calculations the tensile stress of the concrete was entirely neglected. The ultimate tensile stress of bamboo was found to be 14,000 lbs. per sq. in. In this connection it may be stated that the cables used to tow the Junks up the Yangtse gorges are made of bamboo strips taken from the outer 1/8" and plaited together. It has been estimated that the tension on the ropes averages 10,000 lbs. per sq. in., but very often it is more than doubled. From the compression tests the ultimate stress was found to be 5,500 lbs. per sq. in. 

Compared with other woods we may quote the following values for bamboo from the report by Messrs. H, F. Meyer and B. Ekelund to the Engineering Society of China : --

"If the value of the compressive stress of the kind of wood in question is called 100, we will find that the value of tension, deflection, and shearing are as follows: -- 

Tension Deflection Shearing
Oak 400 200 29 
Beech 270 177 26 
Pine 210 160 22
Fir 270 163 23 
Bamboo 255 218

 "The ultimate stress for different kinds of wood in sq. in.: -- 

Beech   Oak Pine Fir Bamboo 
Ultimate compressive stress 4,500 4,900 3,500 3,900 5,500 
Ultimate tension stress 19,000 13,700 10,500 11,200 14,000 
Ultimate bending stress 9,400 8,400 5,900 6,600 13,000 
Ultimate shearing stress 1,200 1,050 560 700 450 

"As regards compression and bending, bamboo is slightly superior to all other kinds of wood here mentioned. The bending problem in the case of bamboo is different from that of the other kinds of wood because of the peculiar form of the section of a bamboo stem. 

"As to tension the strength given in the table above is exceeded by that of the beech; however, in order to do justice to the bamboo, it should be remarked that the outer layer of the stem such as used in the native ropes on the upper Yangtse has a strength of at least 25,000 lbs. per sq. in. 

"In compression as well as in tension bamboo excels most other kinds of wood. Its weakness toward shear limits its use in modem European structures but the same weakness renders the material suitable for the primitive needs of the Chinese farmer." 

Because of its superior qualities, bamboo has been considered promising material for the reinforcement of concrete. But its elasticity is a great disadvantage. A test beam, bamboo reinforced, when placed horizontally on its supports (one at each end) cracked before any load had been applied, but did not completely break down until it had carried a comparatively heavy load. This is a tribute to its high tensile strength. The Chinese Government Railways have used bamboo to reinforce concrete friction piles which are used as foundation for railway bridges. The chief object was to strengthen the pile during handling and driving operations. The Whangpoo Conservancy Board has used concrete plates reinforced with bamboo for vertical bunding below water. It was reported that in 1918, 1/4" square split bamboos were used as part of the reinforcement for a two-inch concrete wall designed as a protection for the ten-inch cork insulation of a cold stor- age for the International Export Co. at Nanking. It is recommended when using bamboo as reinforcement material to split it into small square or lath form, soaking it well before imbedding. 

Two factors, however, have yet to be investigated before a final judgment can be passed on the suitability of bamboo as reinforcement material. Tests have been planned but not yet carried out by the Whangpoo Conservancy Board on the question of the rate of decay of the imbedded bamboo strips. Also, the problem of shrinkage is serious, but so far no information on this subject is available. The latter comes to our attention particularly in consideration of the bond between concrete and bamboo. Ordinarily, satisfaction has been expressed on this question. The projections at the joints when the pole is split into strips offers enough corrugation to give mechanical bond. But up to date no pull-out tests have been made to find out just how firmly the concrete adheres to the bamboo strip. It is interesting to note that after certain reported bending tests the reinforcing strips of bamboo were taken out of the failing beams and tensile tests made. The result did not show the diminished strength expected. On the other hand, the recoil seemed to be complete and perfect. Another interesting fact was the dryness of the above strips ; moreover, there were no signs of decay or deterioration. The strips had been imbedded only four months, -- too short a time to determine the efficacy of concrete in preserving the bamboo. 

If bamboo proves to be a practicable reinforcement material generally it will cut down the cost of construction considerably. The cost of bamboo reinforcement in Shanghai is about 20 per cent of that of a corresponding iron reinforcement. This shows quite a substantial reduction. But according to present information it can be recommended only where its strength during construction matters. Independent investigators have brought out other physical characteristics of bamboo. Among its other properties might be mentioned its specific gravity. Of split bamboo the specific gravity was found to be 0.862. But it varies in different regions of the cross section. The specific gravity of the outer silicious layer, which is one-sixteenth of an inch thick, of the bamboo is in old dry canes 50 per cent greater than that of the inner or main part. The average for the coefficient of friction was found to be .279. Tests were carried out to ascertain if possible the coefficient of expansion. These were apparently not successful. It was thought, however, that expansion due to heat within the temperature limits to which concrete is ordinarily subjected is negligible. 

A histological examination of a thin cross section of bamboo will show that it is extremely porous at regular intervals toward the inner layers. This is due to the extreme width of the water vessels of the vascular strands. The tissues surrounding the vascular tubes are very different. They are composed of fibres with thick walls and narrow cavities packed together very tightly. These predominate toward the periphery. In young shoots which have been allowed to dry out, the inter-vascular tissues have not yet hardened so that they dry out, break down and disappear, thus isolating the fibro- vascular bundles. This alters the space relations within the cane and accounts for the shrinkage and distortion of green culms which have been cut while very young. A study of the distribution arid arrangement of the vascular bundles shoves that they are smaller toward the outer edge of the cross section and much more numerous. At the same time the size of the water vessels is very much smaller. These facts explain the greater density and hardness of the outer "1/16" of a bamboo cane. All this compact fibrous tissue, protected on the outside by the hard silicious epidermis together with the peripheral layer, not only forms a protective barrier around the inner and less dense region occupied by the larger non-mechanical vascular strands, but is also responsible for its stiffness and resistance to compression. 

Lastly, not only from practical experience but also from a view of the longitudinal section of a cane, we see that the grain of bamboo is very straight. From the tangential section the fibres may be seen to run up and down throughout the whole length of the pole in a straight line. From the radial section, however, there is a slight crook at every joint which varies with the position of the particular fibre toward the inner or the outer edge. At any rate, from the practical standpoint, the strands run straight from toe to bottom. This is of real benefit to the farmer who can go out to his light grove with a dull cleaver and with a few well-directed whacks at the right angle fell a pole in no time at all and split it throughout its entire length before an ordinary person could saw through a two-inch limb. As we learned from a study of the pole in relation to shear, the partitions offer on difficulties. These partitions, which are about the consistency of compressed pith, though adequately supported by a network of cross fibres, are easily knocked out. A single blow with a dull tool is usually sufficient. A good idea of this aspect of bamboo may be had by watching a Chinese laborer at work building one of the ubiquitous bamboo fences. 

There are external factors that combine to make bamboo a plant of great economic importance. Up to the present we have emphasized the internal characteristics. Now let us see what influence its general distribution and habits have on its usefulness. In the first place, as we see from the map, bamboo occurs everywhere south of the Yellow River with the exception of Shantung, most of Honan, and the mountains of the west and southwest at altitudes over 13,000 feet. Its common occurrence enhances its usefulness. Then, also, it grows thickly. Unlike the woods with which most of us are familiar, bamboo forests are very dense, the culms coming up so close together at times that a man can scarcely push his way between them. There are two distinct types of bamboos which are characterized by their habit of growth. There are the clump-forming kinds and the more loosely separated types. In the latter case the culms occur singly and separate from each other, while in the former the culms come up close together, sometimes touching. There is a third type which is inter, mediate. One plant consists of many small clumps connected by an underground stem. It is more like the second of the two major types except that instead of single separated culms, there are four or five culms close together in the place of each, In any case, therefore, a comparatively small plot of ground can be very productive, proportionately much more than the land occupied by Western woods. For instance, a plot of ground 45 feet by 15 feet had 111 culms on it according to our investigation, or one culm to every six square feet of land. According to an estimate made by the Whangpoo Conservancy Board, from a dense plantation eight years old about 400 culms per mow can be cut yearly ( 1 mow=7,26o sq. ft.). At this rate a comparatively small area can produce a great quantity of poles. We must be reminded of the fact, however, that only one out of every four poles is fit to cut, since the practice is to allow four years for hardening. In estimating, therefore, the stand can be taken as about four times the yearly production. If the yearly cutting per mow is to be 400 culms, the stand per mow must be close to 1,600. 

The third and last characteristic is its rapid rate of growth. This has been discussed in a previous chapter but we mention it here in this connection to correlate and bring out especially those external factors that help make bamboo economically such a valuable plant. What more could be desired in a useful plant than that only a small bit of land need be used for it to grow on, that a dense growth can be successfully maintained on it without much trouble, and that the full growth of the most used parts may sometimes be attained in less than seven weeks.