The precise cladding industry has now
come of age and, as STEPHEN MADDALENA, Chairman of
the ARCHITECTURAL CLADDING ASSOCIATION, explains,
is able to produce virtually any finish and effect
called for by architects.
Thanks to the universality of the personal
computer, the acronym WYSIWYG has become part of
our language and is today used in fields other than
IT. When it comes to concrete finishes, however,
a better acronym would be WYWIWYG – what you
want is what you get. For the fact is that the precast-cladding
industry is now able to produce virtually any finish
and effect called for by architects. Achieving these
requires a wide range of techniques, starting with
mix design and embracing a whole host of pre- and
post-casting treatments.
In addition to the aesthetic argument,
casting concrete cladding off site improves quality
and safety, improving efficiency and productivity
on site and offering programme and price certainty.
Waste during production is negligible and as a building
ages – the life expectancy of such cladding
being at least 60 years.
Essentially, precast-cladding panels
to BS8297 have two generic types of surface finish
that, combined with ability to design robust three
dimensional shapes and profiles, offer the widest
choice of façade design and construction to
the architect. These generic types are ‘self
finishes’, where the concrete surface is on
view and ‘applied finishes’, where materials
other than concrete are applied to and supported
by the precast concrete.
Whichever technique is
used, false joints and other profiles may be formed
by timber ribs in the mould. False joints are particularly appropriate on
large panels to break up the surface into smaller
elements in appearance. This gives a natural effect
on acid etched panels and improves the weathering
properties.
SELF FINISH
Self finishes exploit the ability
of the precast manufacturer to design concrete mixes
by selecting the cement (white or grey) and the aggregate
with the option of adding a range of stable purpose-made
pigments. As part of the manufacturing and finishing
processes, the exposed surfaces are generally textured
to produce a range of effects Typical surface treatments
are:
acid etching, surface retarding
and wash-off
rubbing
grit blasting
bush hammering
mechanical
grinding and polishing.
Whichever technique is used, false
joints and other profiles may be formed by timber
ribs in the mould. False joints are particularly
appropriate on large panels to break up the surface
into smaller elements in appearance. This gives a
natural effect on acid-etched panels and improves
the weathering properties. Rubber mould liners produce
a huge range of textures and profiles.
ACID ETCHING
OR SURFACE RETARDING
Etching with dilute hydrochloric acid
removes surface laitance to expose the sand and tips
of the main aggregate in the mix to produce a stone-like
texture. The depth of exposure can be varied to produce
relatively fine texture to a deeper etch that exposes
the aggregate in a more pronounced way. Such etching
is a highly sensitive process that calls for experienced concrete finishers.
However, not all surfaces are suitable for such treatment. Vertical cast
surfaces, such as return ends, are likely to have
blow holes caused by air entrapped against the mould
side and etching will make these more visible. To
overcome this, the blow holes can be filled with
a paste of cement and fine aggregate in the same
proportions as the design mix of the main body of
the panel. A final acid etch completes the process
to produce a consistent surface. It is worth noting
also that the attitude of casting (ie: face down
or vertical surfaces) will produce subtle differences
in appearance of the finished surface.
Very shallow acid etching should be
avoided since consistency of finish is virtually
impossible to achieve and does not deal with the
removal of surface laitance,
Coarser textured surfaces can be obtained
by applying a proprietary retarder to the surface
of the mould to delay the hardening of the cement
in contact with it. When the unit is removed from
the mould, typically the day after casting, the retarded
surface is brushed and washed to expose the main
aggregate. Face-up surfaces do not require a retarder:
instead, shortly after initial set, the surface is
sprayed with a fine water jet to wash-out the top
portion of sand and cement to expose the main aggregate.
RUBBING
Surface laitance may be removed with
a hand stone or orbital sander The result is often
referred to as a honed finish.
GRIT BLASTING
Grit blasting uses proprietary pressure-jetting
equipment using air or water as the propellant. Grades
of fineness/coarseness of the grit employed will
determine the depth of exposure achieved. The effect
is similar to acid etching but as grit blasting is
more vigorous, the depth of exposure of the aggregate
is greater. In addition the aggregates are partially abraded, distinguishing
the result from acid etching.
BUSH HAMMERING
Even more vigorous than grit blasting,
bush hammering involves roughening the surface using
a mechanical pneumatic/electric hand-held machine
equipped with a variety of tools from needles to
various chisel star points. The finish produced is
visibly more aggressive owing to the exposure and
shattering of the surface of the main aggregate.
MECHANICAL GRINDING AND POLISHING
Although expensive and time-consuming,
polishing concrete can produce spectacular results.
Hand-held or specialist polishing machines initially
grind the surface of the concrete to section the
aggregate. Final polishing is achieved using fine
polishing wheels, the lubricant being water. The
degree of polish achieved varies according to the
hardness of the aggregate and ranges from a honed
effect to a high polish.
APPLIED FINISHES
Applied finishes include bricks, brick
slips, tiles, terracotta, slate, granite or limestone
pre-assembled in the mould before the concrete is
cast to produce large wall-cladding panels. The technique
can realistically replicate brickwork and stonework,
eliminating a large element of on-site trade requirements
and speeding construction. As skilled tradesmen become
ever more scarce, the use of such 'artificial' brickwork
is becoming more widespread. An advantage is that
any form of brick bonding can be achieved.
BRICKS, SLIPS AND TILES
Brick types are subject to a degree
of selection to satisfy tolerances and bonding issues.
A machine-made brick with three perforation holes
is typical. To produce a dovetail key effect, bricks
are cut longitudinally over centre and laid to a
grid in the mould. Special bricks are cut or formed
to produce shapes and keys. Grout-seal strips are
then inserted into the brick joints and the concrete
is cast. Panels are finished by removing the grout-seal
strips and gun-pointing the joints with a colour-matched
proprietary mortar Tiles and brick slips are treated
similarly to cut bricks and are cast face-down in
a mould with a grid of timber strip or a rubber mat
with a grid form. In every case, an effective bond
must be established. An individual precast cladding
panel may incorporate more than 1 000 brick slips.
Proprietary terracotta
cladding tiles with extrusion holes are split to give very effective bonding
grooves on the inside surface. The terracotta tiles are set in a gridded
mould and, when cast with open joints, the precast
panel acts as a rain screen.
STONE FACINGS
Typical stone facings are granite,
limestone and slate. Hard stone such as granite is
applied as a veneer at least 30mm thick, while other
stones such as Portland limestone are applied no
thinner than 50mm. This economy in material can often
enable a project to go ahead when traditional stonework
would be too expensive.
Stone-veneer panels, potentially incorporating
in excess of 100 individual facing stones that are
drilled in the back face, enable the precaster to
bond in 60mm stainless-steel dowels at a rate of
not less than 11 per square metre. The dowels are
set at 60° to the stone face and alternate
rows of dowels are reversed. A flexible grommet is fitted to the dowel at the
stone face and the rear face of the stone is treated with a de-bonding agent.
The combined effect of the grommet and the de-bonder enables the stone to move
relative to the backing concrete panel to accommodate differential thermal
movement. Stone-to-stone joints are a minimum of 5mm per BS8298: 1994 table
10 and are grout-sealed in the mould with waterproof tape to prevent penetration
by the backing concrete.
CHOOSING A FINISH
With self finishes, the precaster
has a library of samples from which a selection can
be made or which can be the starting point for the
preparation of a unique finish. After selection,
a panel at least 1 sq m should be cast to production
standards.
Samples must replicate actual manufacturing
conditions. For example, face-down finishes will
be subtly different from tall vertical-face finishes,
A similar
selection process must be used with applied finishes, involving the supplier
of the brick, natural stone, etc. The approved sample must be permanently
displayed in the factory to give production staff
a continuous reference.
On large contracts, the employer/architect
should instruct the precaster to construct a mock-up
to enable the finishes and details to be viewed at
full scale and perhaps modified as a result. The
viewing distance of finished units should generally
be not less than 3m. The exception is where the finished
units can be viewed on the building from a lesser
or greater distance. On large projects and particularly
for complex tight-tolerance units, a full-scale pre-manufacture
prototype should be considered. The architect should arrange regular inspection
visits during the manufacturing period.
WEATHERING
Varying conditions of lighting and
weather will cause different colour shading. Units
manufactured to what was deemed to be acceptable
at the time of production change tone during storage
and construction. When a unit is placed adjacent
to a component manufactured some weeks or months
earlier a distinct difference in colour shade should
initially be expected. Time must be allowed for the
units to mature to a consistent appearance.
STEPHEN MADDELENA is Joint Managing
Director of the MARBLE MOSAIC COMPANY LTD.
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