Natural building materials

Materials that are used for structural purposes should meet several requirements. In most cases it is important that they should be hard, durable, fire-resistant and easily fastened together.

Timber is the most ancient structural material. In comparison with steel timber is lighter, cheaper, easier to work and its mechanical properties are good. On the other hand, timber has certain disadvantages. First, it burns and is therefore unsuitable for fireproof buildings. Second, it decays.

At present an enormous amount of timber is employed for a vast number of purposes. In building timber is used too.

Timber is a name applied to the cut material derived from trees. Timber used for building purposes is divided into two groups: softwoods and hardwoods. Hardwoods are chiefly used for decorative purposes, as for paneling, veneering in furniture, and some of them are selected for structural use because of their high strength and durability. In modern construction timber is often used for window and door frames, flooring, fences and gates, wall plates, for temporary buildings and unpainted internal woodwork.

Timber cannot be used for either carpenters’ or joiners’ work immediately it has been felled because of the large amount of sap which it contains. Most of this moisture must be removed, otherwise the timber will shrink excessively, causing defects in the work and a tendency to decay. Elimination of moisture increases the strength, durability and resilience of timber.

Stone has been used as a structural material since the earliest days. Almost all famous buildings of classic times, of the medieval and Renaissance periods and of the eighteenth and early nineteenth centuries were erected of stone. The art of making any structure in stone is called stone masonry. In some places stone was used because of the scarcity of timber, but in other places stone was preferred because of its durability. (1800)

Structural materials

Cement is made of limestone and clay. It is a binder which sets to hold together the materials of concrete, mortar, and hardens as a solid mass. A rendering used for facing exterior walls is also known as stucco. Various types of stucco were available in England from the 1700s. In 1794 Joseph Aspdin discovered Portland cement and patented it in 1824. Portland cement is also known as hydraulic cement as it hardens under water.

Concrete is a mortar mixed with small stones to produce a hard monolithic mass. It was used in Ancient Roman architecture and became a determining factor in its development. Unlike modern concrete, which can be mixed and poured, Ancient Roman concrete had to be laid in courses as it dried out quickly. But about the time of Augustus (early AD C I) a slow drying mortar was discovered and this revolutionized architecture. After the fall of the Roman Empire it was almost forgotten and only revived in the late 18 century. The revival started in France, where the word for concrete is beton. Among the pioneers in the late 18 century and early 19 century are F. Cointereau and F.M. Lebrun. The story became more consistent only after 1850. The leaders were Francois Coignet (1814 -1888), Joseph Monier (1823 -1906) and William Ward (1821 -1900). Scholarly calculations began in 1870s in the USA (W.E. Ward, T. Hyatt) and Germany (Wayss, Koenen). But the breakthrough was due to Francois Hennebique who started factories with a framework of reinforced concrete in 1895. The first exposed concrete (or rather reinforced cement and steel rods piercing bricks) appeared in the church of St. Jean de Montmartre in Paris by Baudot. The next step in the history of concrete was taken by Maillart in 1905. Maillart recognized that concrete could be handled for bridges, not trabeated but arctuated, arch and road being a structural unity.

Reinforced concrete. Since concrete is strong in compression and weak in tension, steel mesh or rods are inserted to take the tensile stresses which, in a simple beam, occur in the lower part, the concrete is thus reinforced. It is also called ferro-concrete, introduced by Joseph Monier in 1849. It resists fire better than naked metal, hence its importance in skyscrapers and shell constructions.

Prestressed concrete. The theory of prestressed concrete was originated in Germany about 1888, but because of the poor quality of concrete, the tests were unsuccessful. In the USA it was first used in 1920’s in the construction of tanks and pipes. It was in Europe, however, that the development of its structural application received greatest attention. Here the costs of materials were relatively high and labour costs were low. Eugene Freyssinet is responsible for the first practical prestressing. The reinforcing steel is replaced by wire cables in ducts, so positioned that compression can be induced in the tension area of the concrete before it is loaded. This is done by stretching or tensioning the cables before or after casting the concrete. It results in more efficient use of materials and greater economy. (3200)