Civil Engineering Reference
In-Depth Information
Plain boards are available in fibre
reinforced cement or calcium silicate,
or with resin binders. Some boards
have smooth polymer finishes. Fibre
reinforced, predecorated cement
based boards consist of compressed
and autoclaved sheets of Portland
cement reinforced with natural and
synthetic fibres and fillers, with
surface coatings permanently fused
or bonded to the base sheet. (Chapter
3.5 deals with these materials).
Boards can usually be found with
the following properties.
Dimensions: 4 mm and 7.5 mm
thickness with aggregate finish
typically 6.5 mm; overall size 3070
×
1240 mm and 2520
centres of the panels, or they can be
used as corrugated sheets. Although
sheets are available in lengths of up to
20 m, care must be taken to provide
for thermal movements in the
lengthways dimension of the sheet;
movements are automatically
compensated for in the width
direction because of the inherent
flexibility given by the corrugations.
Sheet metals coated before
forming should not have sharp arrises
or sharp radius bends. The quality of
the cover on the back of panels or
sheets should be no less consistent
than that of the front, especially for
those components to be used in a rain-
screen design where the backs will be
wet for long periods. It is not
possible, however, to maintain the
same cover on cut edges, which are
likely to show deterioration earlier
than the rest of the sheet. Machine
sheared cut edges are less vulnerable
than sawn edges, so the latter should
be protected by a joint overlap
wherever possible.
Vitreous enamelled steel is always
coated after forming. Although it
offers an extremely durable surface,
the panels are prone to chipping
around fixings, which therefore need
to be carefully designed and installed.
1240 mm.
Weights: 4 mm, 7.4 kg/m
2
; 7.5 mm,
13.1 kg/m
2
; with aggregate finish,
20 kg/m
2
depending on type.
Flat boards should have a
distortion-free surface and are
obtainable in a wide range of colours.
Surface textured boards have a lightly
dimpled, sculptured surface. The
structure, size and colour of
aggregate finished boards is
determined by the nature of the
quarried and crushed stone.
×
Figure 3.54
Here boards are fixed over shaped metal
channels to catch and redirect rainwater
penetrating the ventilated rainscreen
Sheet metals
Sheet metals are usually of
galvanised steel or aluminium.
Galvanised steel is available with
polyester coatings, silicone polyester
coatings and vinyl coatings.
Aluminium is available with
polyester coatings, PVDF and
modified alkyd coated sheets.
Profiled steel and aluminium
sheets are not usable below 1.5 m
height above access level, since, in
their commonly available
thicknesses, they will not meet the
impact damage test. Steel greater than
0.8 mm is satisfactory above
1.5 m, but aluminium would need to
be thicker. The manufacturers should
be asked for test results.
Where long lengths of steel sheet
are to be used, the manufacturer
should specifically be asked
whether any curvature due to the 'roll
memory' has been taken into account.
Steel or aluminium sheets can be
pressed into panels with returned
edges, or shallow drawn
deformations can be pressed in the
Composites
Sheets or boards are available in
which the thermal insulation is
integrally bonded to the inside of the
outer protective layer. These are
sometimes known as sandwich
panels, although they may not always
be a true sandwich with the soft core
skinned on both sides. Such
composite panels were often used in
the spandrels of otherwise glazed
façades. They can of course be
subjected to high temperatures
because of the proximity of the
insulation, and behaviour very much
depends on the particular materials
and design (Figure 3.55).
Some aluminium or steel sheets are
supplied already bonded to thermal
insulation (eg expanded polystyrene,
polyurethane and polyisocyanurate)
and are designed to be fixed with a
cavity. The cladding is relatively easy
to install, but careful detailing is
required to avoid thermal bridges at
Figure 3.55
These composite spandrel infill panels have
remained relatively flat, though they are
discolouring
