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Cladding and Curtain Wall Construction 451
fibre is chopped to lengths of about 35 mm before mixing. It is formed in moulds by spray
application of the wet mix, which is built up gradually to the required thickness and compacted
by roller. After the initial 3 mm thickness has been built up, it is compacted by roller
to ensure a compact surface finish. For effective hand spraying, the maximum width of
panel is about 2 m. For mass production runs, a mechanised system is used with dual spray
heads which spray fibre and cement, sand and water separately in the mould, which moves
under the fixed spray heads. The mechanised spray results in a greater consistency of the
mix and a more uniform thickness of panel than is usually possible with hand spraying.
The moulds for GRC are either timber or the more durable GRP lined, timber-framed
types. Spray moulded GRC panels have developed sufficient strength 24 hours after moulding
to be taken from moulds for curing. The size of GRC cladding panels is limited by the
method of production as to width and to the storey height length for strength, transport
and lifting purposes. It is also limited by the considerable moisture movement of the
cement-rich material, which may fail if moisture movement is restrained by fixings. The
usual thickness of GRC single skin panels is 10–15 mm.
As a consequence of moulding, the surface of a GRC panel is a cement-rich layer, which
is liable to crazing due to drying shrinkage and to patchiness of the colour of the material
due to curing. To remove the cement-rich layer on the surface and provide a more uniform
surface, texture and colour, the surface can be acid etched, grit blasted or smooth ground.
Alternatively, the panels can be formed in textured moulds so that the finished texture
masks surface crazing and patchiness. For a uniform colour finish that can be restored by
repainting on site, coloured permeable coatings are used which have microscopic pores in
their surface that allow a degree of penetration and evaporation of moisture that prevents
blistering or flaking of the coating. Textured permeable finishes such as those used for
external renderings, and microporous matt and glass finish paints are used. The thin single
skin of GRC does not have sufficient strength or rigidity by itself to be used as a wall facing
other than as a panel material of up to about 1 m 2 square, supported by a metal carrier
system or bonded to an insulation core for larger panels, as illustrated in Figure 7.31.
Rebate for gasket
and cover strip
Single skin GRC panel
Single skin GRC panel
with insulation core
Figure 7.31 Single skin GRC panel.