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Concrete

- aggregates, usually a mixture of sizes, bound together by a cement. In situ concrete is concrete "cast in place", a fairly straightforward operation. Re-inforced concrete contains metal strengthening. It is the placing and dimensions of the metal reinforcing bars that are the most important element of concrete construction. Best known method is probably the French Hennebique system (Francois Hennebique 1843-1924) developed from 1870 onwards. Rusting and subsequent expansion are now posing problems, best known currently is probably the Lion Chambers, Glasgow, by James Salmon and John Gillespie, 1904-7. Prestressed concrete invented by another Frenchman, Eugene Freysinnet, involved replacing the metal reinforcing bars in reinforced concrete with tensioned rods or wire, making it better able to carry loads ie prestressed concrete lintel. "No fines" concrete, contains no small aggregate. The initial setting time of concrete is the time taken for it to harden sufficiently to bear a man's weight.