Civil Engineering Reference
In-Depth Information
rubble and then lined with gunite, in very short time spans, to produce water
tanks for fire fighting.
After the war there was much to be repaired and gunite was used
extensively as it gave a fast and effective answer to the problems of damaged
and deteriorated jetties, warehouses, factories, grain silos, water towers,
bridges, etc. It was also realised that gunite could be used as a strengthening
medium, not merely as a means of repair.
Modern dry process machines do not have pressurised chambers but have
a hopper, open to the air, into which the mixed material is fed. It then falls
into one of a circle of chambers where it is pressurised with compressed air,
and hence down the hose. These rotating barrel guns are almost constant
feed-and-supply machines. In the 1960s the wet process was introduced,
where cement, sand and water are mixed together before being put into
the machine. Materials for repair are now often supplied pre-bagged and
include various carefully selected sands and admixtures which give additional
properties, such as non-shrinkage, and make the process more reliable.
Over the past forty years we have also seen the development of several
codes of practice, specifications and methods of measurement, now largely
superceded by the EFNARC specification and European standards.
13.3 Definition of sprayed concrete
Sprayed concrete is a mortar or concrete pneumatically projected at high
velocity from a nozzle to produce a dense homogenous mass. It often
incorporates a combination of admixtures and additions or fibres.
The material, which has zero or very low slump, is compacted by the force
of the jet impacting on the surface, and can support itself without sagging
whether on a vertical, sloping or, within certain limits, an overhead surface.
Rebound, material which bounces off the surface, is inevitable when
concrete is projected at a relatively hard surface at high speed. It consists
mainly of the larger aggregate particles, and should be discarded because it
is of unknown grading.
13.4 The wet process
Definition
Cement, aggregate and water are batched and mixed together and then fed
into a purpose-made machine. The mixture is then conveyed through a hose
to a nozzle where the mixture is pneumatically and continuously projected
into place. This can be seen diagrammatically in
Figure 13.1.
The mixture
normally incorporates a set accelerating admixture and possibly other
admixtures, and may also contain additions and/or fibres.
