Materials through the ages: Structural material for housing

Materials World magazine
,
2 Apr 2012
timber blocks

Greek scholar Epictetus said, ‘Materials, of themselves, affect us little; it is the way we use them which influences our lives’. In this new series, Maria Felice illustrates how advances in materials science have contributed to the development of everyday objects.

There are countless examples. Single crystal turbine blades in aeroplane engines resist creep and make flying safer, non-stick materials such as Teflon in saucepans make cooking and washing up easier and Kevlar, with its high specific strength, makes body armour more comfortable. This month, the focus is on structural material for housing and the myriad materials used for constructing dwellings.

Building materials account for approximately half the cost of a house, in no small part thanks to the large amounts needed (about 200 tonnes of brick for a family home). The materials are used in three ways – structurally to hold the building up, as cladding to keep the weather out and internally to form accommodation. Structural materials must be stiff so that they do not move or bend significantly when loaded, and they must be relatively cheap, since they are used in such great quantities.

While early humans sought shelter in caves and other natural structures, their Stone Age successors used stone tools to make tents wrapped in leather and furs – an early example of the dependence of material selection on available tools and processes. And fabric remained the material of choice for nomadic groups.

Mud and clay have been used for thousands of years, and some buildings made from these materials have remained habitable for centuries. A variant on the mud hut is wattle and daub – a composite material that was in use for more than 6,000 years. A woven lattice of wooden strips (the wattle) is daubed with a sticky material that includes mud, clay and straw. Historic examples can be seen in many parts of the British Isles.

Wood is another material that has been used for thousands of years. It is naturally anisotropic, that is stronger along the grain than against it. Wood is probably the most aesthetically pleasing of all structural materials. Unfortunately, it poses a great fire risk, explaining why in places at great risk of forest fires, many historic buildings are coated with corrugated iron.

While concrete, brick and stone are stiffer and cheaper than wood, they can only be used in compression, whereas wood can be used in compression and tension, and also with greater freedom of section shape. Along with its nonflammable properties, this explains why it is common nowadays to see homes made primarily of stone, but with wood used for beams and roofs. Stone, like wood, has been in use for thousands of years. Dry-stone structures are made by piling stones on top of each other, whereas mortar structures use a liquid paste, such as cement, to hold stones together.

Stone has great strength, is readily available and long-lasting. However, it is very heavy and awkward to work with. A sedimentary rock such as sandstone, usually composed of quartz and feldspar, is easy to carve because it is soft, yet it is still resistant to weathering, so has been used extensively in construction since prehistoric times. Indeed, sandstone houses line the streets of the typical picture-postcard English village, topped with plaques demonstrating the rock’s durability: ‘Built in 1789’.

Bricks – essentially artificial stones made by forming clay into rectangular blocks – are usually laid using mortar. However, the first bricks were made of mud more than 9,000 years ago and the first sun-dried bricks date back to around 6,000 years ago. The first machine for extruding bricks came much later in 1875. And while many materials have come and gone, brick in its various forms has stood the test of time and is regarded as one of the strongest and longest-lasting building materials ever used.

Concrete is a favoured building material of our time due to its longevity, formability and ease of transport. But this belies the material’s origins, which date back to Roman times. Today it is made of an aggregate – usually gravel and sand – and a binder, usually Portland cement, which originated from Joseph Aspdin, a bricklayer from Leeds in the 1820s. But concrete is only strong in compression, so reinforcement bars (rebars) typically made of steel must be used to increase its tensile strength. Steel and other metals are also used for structural framework of apartment blocks, because of their strength, flexibility and longevity, if protected from corrosion.

The word ‘prefab’ might conjure images of temporary post-war housing, but the process has many advantages, including the reduction of site time and better quality of components, especially wooden ones. An interesting example of prefabrication includes The Crystal Palace built for The Great Exhibition of 1851 in London using wood, cast iron and plate-glass, and later moved from Hyde Park to Sydenham Hill. Meanwhile, in 1855, Isambard Kingdom Brunel developed a prefabricated hospital that was shipped to Crimea.

And while building your home might be conventional, it is possible to grow it, instead. Straw bales were used to build homes on the African plains hundreds of thousands of years ago, and are now a green building material. However, bales require a lot of space, which often makes them unsuitable for homes in densely populated areas.

Ice is another natural material that has been used for many years and is en vogue, not as a green material, but for creating ice bars and other tourist attractions. I will close with a fascinating fact about this seemingly primitive and precarious building material, which is in fact self-strengthening: when the interior of an ice structure melts slightly, it refreezes into a layer of ice that strengthens the structure. Sometimes nature really does know best.