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Methods of
Manufacturing
Spraying
Spraying of fibre and slurry simultaneously onto a mould, by
manual or mechanical means.
Typical products made using the spray process include
architectural cladding panels, channels, tanks,
facade elements, ducting and permanent formwork.
Sprayed GRC
In the manufacture of GRC by the spra y process, simultaneous
sprays of cement/sand mortar slurry and chopped Cem-FIL AR glass
fibre are deposited from a spray-head into or onto a suitable
mould. The spray-head may be hand held or mounted on a machine.
The mortar slurry is fed to the spray gun from a metering pump
unit and is broken into droplets by compressed air. Cem-FIL AR
fibre roving is fed to a glass fibre chopper/feeder, mounted on
the spray head which chops the fibre to predetermined lengths,
typically 25-40 mm and injects the chopped strands into the
mortar spray so that a uniform felt of fibre and mortar is
deposited on the mould. The slurry has typically a sand:cement
ratio of up to 1:1 and a water:cement ratio of 0.33.
The water:cement ratio should be kept
as low as possible consistent with satisfactory spray and
incorporation characteristics, as increasing the
water:cement ratio leads to a reduction in the strength of
the product. Admixtures may be used to obtain the required
workability. The proportion of fibre to slurry is adjusted
so that the resulting composite contains typically 5% by
weight of glass fibre.
Manual Spray Method
The operator holds the spray-head in
his hand and moves it to and-from across the mould,
directing the stream of material perpendicular to the
mould surface, until the required thickness of GRC has
been built up. Roller-compaction ensures compliance with
the mould face, impregnation of the fibre by the slurry,
removal of trapped air and development of adequate
density. The rolled surface may be finally trowelled
smooth. Thickness control is achieved by use of
pin-gauges. A typical output of a single hand-unit is
10-12 kg of GRC per minute. The process results in one
surface of the product having an ex-mould finish and the
other surface a rolled or trowelled finish Products are
covered with polythene sheet after spraying and normally
demoulded the following day and then cured. The process is
labour intensive but is capable of producing complex
shapes and is extremely versatile. The process is used for
manufacture of a wide range of components including
cladding panels, agricultural components, facade elements,
formwork and ducting.
during
Spraying
Good quality Cem-FIL GRC is produced when there
is a minimum of trapped air (low pressures, good
compaction) and when the fibres are well distributed
(correct percentage and good spray technique).
1. It is normal good practice to spray in
layers (roughly 3-4 mm thick) at a speed likened to
waltz-time.
2. Each layer should be compacted before the next
layer is sprayed.
3. Each layer consists of spraying in alternate
directions.
Premixing
Premixing the fibre and slurry and then
processing the mixture by vibration casting,
extrusion, injection moulding, etc., to produce the
end product form. Typical products made using the
premix process are sunscreens, planters, electrical
transformer housings, slates and tiles, junction
boxes and drainage components.
Premixed GRC
All premix processes involve the blending
together of the cement, sand, water, admixtures
and choppe d strands of Cem-FIL fibre in a mixer
prior to being formed. To produce a premix of the
correct quality it is necessary to mix in two
stages. The first stage is designed to produce a
high quality slurry to achieve the necessary
workability and allow for the uniform
incorporation of fibre. The second stage is the
blending of fibres into the slurry at a reduced
speed. It is more convenient to carry out both
stages in the same piece of equipment, but
separate mixers can be used for each stage. The
actual mix formulation used depends upon the type
of product being made, but a typical mix has a
sand:cement ratio of 2:3 and a water:cement ratio
of preferably less than 0.35. It is essential to
keep the water:cement ratio as low as possible
consistent with maintaining workability of the
mix, so admixtures are used.
Up to
4% by weight of chopped strands can be
incorporated into the mix, but typical fibre
content is 3%. The fibre length is
normally 12 mm because above this length the mix
becomes difficult to work. A fibre length of 25
mm is generally found to be the maximum useable.
Although the glass fibre strands are designed to
withstand the mixing action, it is normal, as
indicated above, to add the fibre at the end of
the mixing cycle to minimise fibre damage.
Curing of GRC Products
The hydration of cement is a relatively slow
process at ambient temperatures and for this
reason concrete products are usually allowed
to hydrate or ‘cure’ for several weeks after
casting to give full strength development.
GRC products are normally of comparatively
thin section, manufactured with a lower
water:cement ratio than most conventional
concretes, and are prone to rapid drying. If
this occurs before hydration is complete the
cement never achieves its full strength and
the properties of the GRC are adversely
affected, so more attention to curing
conditions is necessary.
Moist Curing of GRC Products
To ensure complete hydration it is
essential that products are kept moist
immediately after manufacture and during
the curing period. Several methods
of achieving this are currently in use,
mainly: storage in a humidity chamber or
fog room, sealing in polythene bags, or
total immersion in water. For all
products the curing period can be
divided into three parts:
(i) A pre-demoulding cure to give
sufficient strength to the product for
demoulding. This is important and is
carried out by covering the component
closely with polythene to minimise air
flow across the GRC surface thus
enabling the component to retain as much
water as possible.
(ii) The main cure as described above.
(iii) Post-curing during which the
product is normalised to the ambient conditions prior to
storage or use, particularly in extreme hot or cold conditions.
The rate of hydration will be
different in each of these periods,
but at the end of the curing cycle the
GRC should have been
brought up to the final strength
requirements. The particular
curing regime will depend upon the
product, manufacturing
process and mix design, and must be
such that the required
level of properties is achieved.
During the curing period the strength
of the GRC products will
be building up from an initially low
level and care is necessary in
demoulding, handling and particularly
in the main cure to
ensure that products are not
overstressed whilst in a relatively
weak state since they could be
permanently deformed or
subjected to damage which may not be
visible.
As with concrete, it is possible
to use accelerated curing
schedules, either by the use of
chemical accelerators or by a
higher temperature cure. This may
be commercially attractive, but
conditions must be carefully
controlled to achieve consistent
and acceptable strength levels. A
controlled post-curing regime is
important where the conditions in
storage or use will be
substantially different, in either
temperature or humidity, from the
main cure conditions. In
particular, the combination of
direct sun and low humidity could
cause problems with differential
drying shrinkage even though
the GRC strength is high at
this stage in its life.
GRC products will achieve a
substantial proportion of their
ultimate strength when the main
cure is carried out for 7 days, in
a humidity of greater than 95% RH,
and with a minimum temperature of
15°C. A suitable post-curing
regime will allow the remainder of
the strength to be realized.
Air Curing
An alternative method of
curing the GRC is to
incorporate polymeric
materials into the GRC mix.
The polymer formulation used
must be capable of forming a
film around the mix particles,
thus allowing the moisture in
the GRC to be retained and
hydration to continue. The
polymer materials are normally
added at dosage rates of
between 2% and 10% of polymer
solids to cement weight. After
demoulding the GRC product can
be allowed to cure in ambient
air conditions, but care must
be taken to ensure that the
air temperature is above the
minimum film formation
temperature of the polymer.
The properties of GRC cured in
this way are similar to those
of the same basic formulation
(i.e. sand:cement ratio,
water:cement ratio and glass
content) made without polymer
additions and moist cured as
described in section 3.4.1.
However the addition of
polymer materials to GRC may
affect the fire performance
properties.
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