Sprayed concrete for repairing concrete structures - Concrete Repair - page 215

The entire spraying system should be able to deliver a constant stream of
concrete free from pulsations which can give rise to segregation of the mix
and over-dosing of the admixture(s).
The lines, of either flexible hoses or steel pipes, should be laid as straight as
possible. They should be of uniform diameter and well sealed. The actual size
used is dependent on the maximum aggregate size or the inclusion of a fibre
in the mix. Prior to concreting, the lines should be lubricated with a grout.
All equipment should be cleaned and maintained to prevent the build-up
of set concrete in the delivery system.
13.5 The dry process
Definition
Here only the aggregate and cement are batched, mixed together and fed
into a purpose-made machine in which the mixture is pressurised, metered
into a compressed air stream and conveyed through hoses or pipes to a
nozzle, where water is introduced as a very fine spray to wet the mixture,
which is then projected continuously into place, all as shown in Figure 13.2.
Additions and/or fibres may also be in the mixture.
General
Water added at the nozzle needs only to be sufficient to hydrate the cement
and give some fluidity to the mix. This produces a mix with a w/c ratio
potentially as low as about 0.35 and consequently zero slump, enabling it to
be placed without admixtures on vertical and overhead surfaces.
Should they be required, admixtures can be added, either dry in the mix
or wet at the nozzle. Fibres can, of course, be added to the dry ingredients.
Figure 13.2 Diagram of a typical dry process system.