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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.