Civil Engineering Reference
In-Depth Information
concrete structures
Graham Taylor
13.1 Introduction
Sprayed concrete was first used over a century ago, mainly for structures
where formwork and normal placing methods were not possible. During the
Second World War its potential for repair and rehabilitation was realised.
Since that time respect for it has grown as more research has been
undertaken, new materials and equipment have been developed and
contractors, banding together in trade associations, have worked towards
reassuring their potential clients of the integrity of sprayed concrete.
Spraying concrete involves a fine-aggregate concrete, made with small-
sized aggregate, projected at high velocity onto a hard surface. Impelled
by compressed air or pump pressure, the material is rapidly placed and
compacted and, with a high cement content and a low w/c ratio, its potential
strength is high.
European harmonisation has required the system to be called sprayed
concrete but the original process was patented as 'gunite' and this has been
used as a generic term in the UK, whilst in America it is called 'shotcrete'.
13.2 History
The orginal Cement Gun was used by Dr Carlton Akeley, Curator of the
Chicago Field Museum of Natural Science, in the late 1890s, to apply
mortar over skeletal matrices to form replicas of prehistoric animals because
the results of trowel-applied mortar were not acceptable.
The original cement gun, with a single chamber pressure vessel, into
which a mixture of sand and cement was placed and then compressed air
was applied, was the forerunner of today's twin-chamber dry mix method;
the mixture travels down a hose under pressure, passes through a spray of
water and then exits through a nozzle and then into place.
Gunite, as it was then known, came to Britain in the 1920s and healthy
competition between rival firms gave rise to technical developments. During
the Second World War, gunite, like everything else, was used as an expedient.
One notable use was that basements of bombed buildings were cleared of
