IELTS Academic Reading Practice Test 3: Passage 1

Across much of Asia, Latin America and Africa, bamboo has been used for building for thousands of years, in everything from houses and bridges to the scaffolding used to construct them. In recent decades it has also attracted growing interest from architects and engineers elsewhere, who see it as a greener alternative to steel and concrete. Despite its woody appearance, however, bamboo is not a tree at all but a giant grass, and many of its advantages and disadvantages as a building material stem from this fact.

Bamboo is among the fastest-growing plants on Earth. Some species can grow by almost a metre in a single day, and a new shoot reaches its full height within a few months. At this stage, however, it is soft and weak; the stem, known as a culm, needs a further three to five years to harden before it is ready to be cut. Even so, this compares very favourably with the decades needed to produce good-quality timber. Harvesting is also less destructive than felling a forest, since the plant's extensive underground root system survives the cutting of mature stems and continues to send up new shoots year after year.

The structure of the culm explains much of its strength. It is a hollow tube, divided at intervals by solid walls at the joints, or nodes, which help to prevent it from buckling. Its fibres run along its length and are concentrated towards the outer surface, where they are most effective at resisting bending. As a result, bamboo is remarkably strong for its weight, and in tension, that is, when it is being pulled, its strength in relation to its weight compares favourably with that of steel. Its lightness and flexibility also mean that well-built bamboo houses tend to perform well in earthquakes, swaying rather than collapsing.

Bamboo also has serious weaknesses. Its stems contain large amounts of starch, which makes them attractive to beetles and fungi, and untreated bamboo that is exposed to rain or in contact with the ground may last only two or three years. It burns easily. And because each culm is round, hollow and slightly tapered, with a diameter that varies from one stem to the next, it is difficult to join pieces together securely: nails, the standard fixing for timber, tend to split it along its length. Traditional builders overcame this by tying joints with rope or strips of bamboo, but such methods are slow and hard to standardise.

There are well-established ways of making bamboo last. The simplest is to cut the stems in the dry season, when their starch content is lower. In many regions the culms are then left to soak in water for several weeks, which allows much of the remaining starch to dissolve, or are heated over a fire, which also hardens their outer surface. The most effective modern treatment involves soaking the stems in a solution of boron salts, which are poisonous to insects but relatively harmless to people; bamboo treated in this way can last for decades, provided that it is kept dry. Design is therefore as important as chemistry. Builders who work with bamboo often say that a building needs 'good boots and a good hat': columns are set on concrete bases to keep them clear of damp ground, and roofs are given wide overhangs that protect the walls from rain.

Bamboo's lightness makes it ideal for temporary structures. In Hong Kong, scaffolding made of bamboo poles tied together with plastic strips has long been used even on high-rise towers, because it can be put up quickly, cut to length on site and carried by hand, although the authorities have recently begun to replace it with metal scaffolding on safety grounds. More ambitious uses have been pioneered in Colombia, where the architect Simón Vélez has designed large buildings using a local species called guadua. To create strong joints, he fills the hollow ends of the culms with cement mortar, so that they can be fastened with steel bolts without splitting.

Another way of overcoming bamboo's irregular shape is to turn it into engineered products. The culms are split into strips, which are flattened, dried and glued together under pressure to form boards and beams of uniform size and strength. These can be cut and fixed like ordinary timber. Engineered bamboo loses some of the material's environmental appeal, however, since the processing consumes energy and many of the adhesives used are made from fossil fuels.

Perhaps the greatest obstacles are not technical but institutional and cultural. Because bamboo varies so much between species and even between stems, engineers have found it hard to give it the reliable strength ratings that building regulations require, and in many countries there are still no codes covering its use. An international standard for designing bamboo structures was first published in 2004, but progress since then has been slow. Meanwhile, in many places where bamboo is plentiful, it is still seen as 'poor man's timber', a material for those who cannot afford anything better. Changing that perception may prove harder than solving any engineering problem.