**4.4. Bamboo natural durability**

Shear stress parallel to grain is 6–12 N/mm<sup>2</sup>

114 Bamboo - Current and Future Prospects

**Table 1.** Physical and mechanical properties of Indian bamboos (in round form) [14].

what-are-the-mechanical-properties-of-bamboo).

design are shown in **Table 3** (adapted from [14]).

which is approximately 10 times lower than com-

pressive strength and up to 20 times lower than the tensile strength of the same bamboo species. However, the shear strength of bamboo is often twice the value of popular timber species. (https://www.bambooimport.com/en/blog/what-are-the-mechanical-properties-ofbamboo). The bending strength of most bamboo species varies between 50 and 150 N/mm<sup>2</sup> (**Table 2**) and is, on average, twice the magnitude of most conventional structural timber materials. Interspecies variations can be caused by different test methods, sample quality and moisture content of the tested bamboo. (https://www.bambooimport.com/en/blog/

When bamboo is used in green condition, the mechanical values that should be used for

Laboratory testing of material properties [15, 16] showed that the compressive and shear strength parallel to the grain were most significantly affected by moisture content, followed by longitudinal tensile modulus and then bending modulus. Age had little effect on the sensitivity of the tensile modulus and bending modulus to moisture content change, while young

**Table 2.** Material properties of different categories of structural bamboo (adapted from BIS [14]).

Bamboo, like most lignocellulosic materials, has very low resistance to biological degrading agents. The culms are liable to attack by insects and termites, and above the fibre saturation point, they can be deteriorated by strain and rotting fungi [19].

Bamboo is more susceptible to decay than timber, due to a lack of natural toxins and its typically thin walls. This means that a small amount of decay can mean a significant percentage change in technical capacity [20].

The high sugar and starch content of the bamboo culm explains its low natural durability. There is not much data available on the natural durability of different bamboo species. The natural durability of raw bamboo is low and varies between 1 and 36 months depending on the species, age of culms and climatic conditions [21]. Bamboo is generally destroyed in about 1–2 years when used in the open and in contact with the ground. According to durability classification [22], bamboo falls in class III (non-durable category) with little variation among species. Data about natural durability of some bamboo species obtained from tests of untreated bamboo poles can be found in [21].

The most popular traditional treatment for enhancing bamboo natural durability is soaking the bamboo in running water for a period of time [19]. This treatment has a significant effect in enhancing durability against decay fungi because it washes off the starch content of the culms [19]. This treatment has little effect on termite attack because these organisms depend merely on cellulose rather than on starch as food source [23]. Traditional techniques for controlling starch content in felled bamboo include [21]:


When bamboo is used as an inert construction material, seasoning (drying) processes are important in order to carefully bring down its moisture content to levels closer to the in service equilibrium moisture content. Seasoning improves bamboo's resistance to biological attack and limits the amount of drying shrinkage in service [20].

The low natural durability of bamboo material can be perfectly accommodated in the bioengineering approach since, in this type of work, just an initial rigidity is pursued and, hence, only temporary structures are usually included in the work design. Once the introduced living plants become established, the vegetation gradually takes over the structural functions of the wooden supports [24, 25]. As time progresses, the inert bamboo culms will deteriorate, and the live bamboo pegs (or poles) will grow and assume the strengthening effect of the initial structure.

In the bioengineering design approach, the load transfer between the initial structural elements and the evolving structural vegetation elements can be calculated using an eco-engineering design scheme for durability [26]. The rapid growth pattern of bamboo species make them very suitable for this kind of work approaches and strategies.
