40 Building materials Concrete "Concrete is an artificial conglomerate or pudding stone, in which the pebbles which make up the greater part of the bulk are cemented together by lime mortar" E. Dobson, A Rudimentary Treatise on Foundations and Concrete Works Concrete is a manufactured stone and is made with water, aggregate, and cement in different proportions and often with chemical admixtures. Concrete is usually a mixture of 10­15 per cent cement; 15­20 per cent water; about 60­75 per cent aggregates, such as gravel, sand and limestone filler; about 5 per cent entrained air (trapped air bubbles); and less than 0.1 per cent chemical admixtures. Aggregates are often quarried stone or gravel--but may be natural (like stone and gravel), recycled (like crushed brick or concrete), or manufactured material (like furnace clinker). Cement binds together the aggregate. It is often said concrete has a relatively short history as a building material; however, forms of cement and concrete were used by ancient Egyptian and early Chinese builders, and certainly the Romans used concrete construction in the dome of the Pantheon. 3 Concrete is described by Vitruvius in his De Architectura and in standard architectural manuals of the Renaissance by Alberti and Palladio. Later, it fell out of favor in Europe largely owing to the problem of obtaining good cement. Curiously, it was experience with pisé (rammed earth) construction in France (see "Compressed earth, cob and pisé," page 95) which led to its being used again at the end of the eighteenth century. 4 Above An early form of concrete: Roman lime mortar and rubble fill from the late fourth century AD, Pevensey Castle, England. Right Early concrete gate pier of 1873, London, built by Charles Drake's London Patent Concrete Company. Far right Concrete is a freeform sculptural material, as used in this 1930s acoustic mirror, Dungeness, England.