Civil Engineering Reference
In-Depth Information
difficult as they are dependent on the types and sources of both the cement
and the accelerator. Therefore, trials should be conducted each time an
accelerator is to be added, either at a different dosage level or with a new
mix design. There are a variety of types, the more recent being low-caustic
to minimise adverse health effects.
Retarders and stabilisers
Traditional retarders are not common in sprayed concrete work. However,
stabilisers are often incorporated (in conjunction with an activator added
at the nozzle) to control hydration where long periods of standing-time are
expected in the spraying process, such as in underground work where the
concrete is batched above ground and then conveyed large distances to the
spraying pump.
Fibres
Both steel and synthetic fibres are used to satisfy a performance requirement
of the finished concrete. Steel fibres, which may be included for abrasion
resistance, have the ability to cause blockages in pipes and hoses, hence the
general requirement to keep their length down to not more than 0.7 of the
pipe/hose diameter until it has been proved that longer fibres can be sprayed
without causing blockages. Synthetic fibres, used, for example, to control
cracking, are flexible and, provided they are well-distributed, flow easily
through the hose.
Cementitious additions
Silica fume, pulverised fuel ash (pfa) and ground granulated blastfurnace slag
(ggbs) can all be used in sprayed concrete; generally up to the percentages
given in
Table 13.2.
Condensed silica fume, microsilica or silica fume, as it can be known, is
now commonplace in sprayed concrete. It is formed from the production
of silicon or ferrosilicon alloys and the average particle size is typically 0.15
microns. It has a high SiO
2
content and is a very reactive pozzolan. This
extremely fine material is available in powder form for the dry process, or in
wet, slurry form for wet process mixes.
Material
Max addition
(% of Portland cement)
Silica fume
15
pfa
30
ggbs
30
