Glass Fibre Reinforced Concrete (sometimes called Glass
Fibre Reinforced Cement) is a mixture of Cement, fine aggregate,
water, chemical admixtures and Alkali Resistant Glass Fibres.
There are a number of different manufacturing processes, the most
common are Hand Spray and Premix.
Glass Fibre Reinforced Concrete (GRC) is a material which today
is making a significant contribution to the economics, to the technology
and to the aesthetics of the construction industry worldwide.
The Alkali Resistant Glassfibre is generally used at
the 3 to 5% level in Hand Spray GRC Process and 2 to 3% in Premix
GRC Process to manufacture of factory finished prefabricated products.
ROLE OF FIBRES:
When the loads imposed on concrete approach that for failure,
cracks will propagate sometimes rapidly, fibres in concrete provide
a means of assisting the growth. Reinforced Steel bars in concrete
have the same beneficial effect because they at as long continuous
fibres. Short discontinuous Glass Fibre has the advantage, however,
being uniformly mixed and dispersed throughout the concrete.
WHY "ALKALI RESISTANT" GLASS FIBRE MUST BE USED IN GRC?
When specifying the constituent materials of Glassfbire Reinforced
Concrete it is an essential part of the overall Quality and Quantity
Assurance in GRC products that a high zirconia alkali Resistant
Glass fibre is used to manufacture GRC products.
The reasons for this are clearly illustrated. Figure-1 shows the
loss in weight of Glass Fibres held in a saturated cements solutions
(PH=12.9) for 200 Hours at 80 degree Celsius ploated against the
Zirconia content. As is seen, the higher the Zirconia cotent the
lower the weight loss.
Figure-2 visually illustrates the difference between high Zirnonia
Alkali Resistant Glassfibres and standard E-glass, the type of
glassfibre generally used in polyester resins and gypsum, quickly
deteriorates and loses strength, where as the high Zirconia Resistant
Glassfibre remains virtually unaffected.
The cost of the Zirconia makes Alkali Resistant Glassfbires
considerably more expensive than E-glass but, as seen the price for
security is well work paying.
GRC promise of thinner and stronger elements, reduced weight (1/3rd
of Precast Concrete) and controlled cracking by simply adding a
small amount of fibres an attractive feature of Fibre Reinforced
Concrete.
HOW IS GRC MANUFACTURED?
Glassfibre Reinforced Concrete (GRC) is generally manufactured
by either the " Spray" process or the "Premix" vibration
casting process. The method chosen is normally dictated by factors
such as strength requirements, size of mould, architects specification
etc. As a general rule, larger items, such as building cladding
panels, are normally "Sprayed" where as small items are
manufactured from "Premix" GRC Process.
In "Spray" GRC Process only thinner section's products
are preferred where as in "Premix" GRC Process thicker
section's products are preferred.
SPRAYED GRC PROCESS:
The water and admixture
(and polymer if used) are placed in a "high
shear mixer" and the sand/cement are slowly
added until a smooth creamy slurry is achieved.
The consistency of the slurry is achieved in
1-2 minutes of Mixing Time.
When ready the mix is transferred to a "pump/spray
unit". The pump conveys the slurry at a
regular rate of flow to the Spray Gun. At the
Spray gun fibre, in the form of a roving, is
chopped to a length of approximately 32 mm and
added to the slurry. The two materials are projected
onto the mould surface using a compressed air
(pressure @ 5 to 8 kg/cm2) supply from Air Compressor.
The GRC material is sprayed and built up in the layers until the required thickness is achieved -normally 10-15mm. Simple hand rollers are used to compact the material between layers.
The product is left in the mould and covered with polythene to prevent moisture loss until the next day. The product is then demoulded.
After demoulding the units are covered with polythene and allowed to cure for approximately 2 to 4 days. Alternatively, if polymer curing compound is used in the mix the units can be exposed to the atmosphere immediately although it is advisable to keep them protected from direct sunlight or severe external conditions for a day or two. Reference should be made to the Polymer Supplier's instruction.
PRE MIX GRC
The sand and cement are mixed dry and then the water/admixture
and polymer (if used) are added. Generally a two-speed slurry/fibre
blender mixer is used. With this type of mixer, the fast speed
is designed to create smooth creamy slurry. This takes about
one-two minutes. The mixer is switched to slow speed and fibre
in the form of chopped strand (length approximately 13mm) is
added slowly. The fibre is blended into the mix for approximately
1 minute.
Once the mix is ready, it is poured into the moulds, which
are vibrating using a vibrating table.
The product is left into the mould to set and is covered with
polythene sheet to prevent moisture loss. The product is demoulded
the next day.
After demoulding the products are cured under polythene sheets
to maintain moist conditions for approximately 2 to 4 days. Alternatively
a polymer curing compound can be used as described for the sprayed
process.
PROPERTIES OF GRC
The final properties of a particular type of GRC materials
will depend on the mix design and the method of manufacture.
Typical Properties of GRC:
PROPERTY
HAND OR MACHINE SPARY GRC
VIBRATION CAST PREMIX GRC
Glassfibre (weight%)
3 to 5%
2 to 3%
Bending
Ultimate strength (MOR) Mpa
18 TO 30
10 TO 14
Elastic limit (LOP) Mpa
7 TO 11
4 TO 8
Dry Density t/m3
1.9 to 2.1
1.8 to 2.0
Sprayed GRC is generally stronger than Premix Vibration Cast GRC.
The reasons for this are firstly that with Sprayed GRC it is possible
to achieve a fibre content 3-5% easily where as Premix GRC is limited
to around 2-3%. Secondly, Sprayed GRC has a lower water content than
Premix GRC.
WHAT IS GRC USED FOR?
GRC provides the Architect with an end product range
that no other material can match in terms of shape,
form, texture, lightweight, mouldability and long term
technical properties. This diversity of properties
offers the Architect a material which has:
Lightweight, ease and speed of handling, with
reduced erection and transportation costs.
Low maintenance requirements.
Low permeability i.e. good waterproof property.
Excellent fire resistance property.
Environmental acceptability i.e. excellent anti
corrosive property.
Ability to reproduce fine surface details.
Reduce loading on building leading to substantial
reductions in structural and foundation cost,
especially useful for building renovation or restoration.
