Concrete, Cement and Mortars
Ancient Greek Temples were built as dry stone masonry and were held together by the force of gravity pulling on the weight of the stones: Roman architecture, which is much more ambitious, and constructionally far more inventive, is not. The Romans invented concrete and that is the key to their architectural success.
Lime - calcium oxide - is the essential element in making concrete. It is also used for dressing fields that are acidic. Since the middle of the eighteenth century when agricultural improvements were made on Irish estates, limekilns, where limestone was burnt to make lime, were built throughout the whole country. Cement is made by heating a mixture of lime and clay until it nearly fuses and by grinding the mixture to a fine powder. Cement powder mixed with sand and water creates a cement mortar. This is usually very hard and, over time, has a tendency to pull apart the structure of the stones or bricks which it was used to bond together. Mixing sand and lime without cement creates a softer bonding agent - lime mortar - which, until the mid-nineteenth century, was generally used in Irish historic buildings. Lime mortar is now preferred to cement for the restoration and repair of Ireland's older buildings.
The most expressive material of the modern age is concrete which exists in many different forms. It is used for the frames of modern multi-storey buildings and for the decorative textured panels set into them. It is used for the embankments of motorways, for bridges, kerbstones and lamp standards.
Concrete consists of gravel, sand, cement and water in which the chemical reaction of water on the sements produces a dense, heavy, cheap material. It is relatively strong in compression but has little tensile strength. The development in the later nineteenth century of reinforced concrete, which combined concrete's compressive strength with the relatively expensive tensile strength of steel by embedding rods in the concrete, revolutionised construction techniques. Variants of reinforced concrete include pre-cast concrete made in factories, an concrete pre-stressed so that when loads are applied the nett stresses are greatly decreased.
In many concrete buildings built between 1920 and 1960 the steel rods were placed too close to the surface for protection against moisture ingress and rusting. This led concrete near the rods to crack and even fall off, leaving the rusted steel exposed.
In contemporary building practice concrete is usually cast on site in wooden moulds or 'shuttering', made of timber, blockboard or steel, which surrounds the steel reinforcement. It is vibrated while it is wet to remove any air pockets and has to be held in place by 'shuttering' which is a box frame usually made of block-board or timber. In Modern Architecture, in the 1950s and 1960s, it became fashionable to form the timber framework with wood which had been sandblasted so that the grain of the timber was exaggerated and appeared on the face of the concrete when the shuttering was removed. This, which is done for aesthetic effect and has no structural significance, is known as 'rough-shuttered concrete'. Le Corbusier was a pioneer of this method.
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