Civil Engineering Reference
In-Depth Information
screws. Self-tapping screws are
generally manufactured from carbon
steel in order to achieve the desired
mechanical properties. Corrosion
protection has to be applied without
dulling the cutting edge. This is
normally provided by a thin zinc or
cadmium coating. Unfortunately the
life of such a coating can normally be
measured in months rather than
years when exposed externally. In
dry conditions these fixings may
perform adequately, but in damp
conditions their durability is suspect.
The outer (or head) part can be
protected, given an adequate
standard of workmanship, with an O-
ring and plastics cup, but the back is
vulnerable, and so long term
durability could be threatened.
Repeated loading under wind action
may cause partial unscrewing.
Self-tapping screws should not be
considered as a permanent fixing,
therefore, where regular inspection
would be difficult and the
consequences of failure might be
serious.
Maintenance
Few surfaces are truly self-cleaning
under the action of rainwater since
streams of run-off will preferentially
follow certain routes rather than
others. Early in the specification
process, therefore, it will be
necessary to consider whether uneven
dirt adherence can be tolerated,
whether periodic cleaning will be
needed, and whether or not it is likely
to be carried out - some systems
require washing down several times a
year.
Trapezoidal profiled or sinusoidal
corrugated sheet will self-clean much
better if the corrugations run
vertically than if they run
horizontally.
Anodised or mill finish aluminium
should be washed periodically in any
case to preserve its integrity. In
practical terms this will often exclude
it from the range of cladding options,
though it has performed well in
particular circumstances.
The potential side effects of
overcladding an existing structure
should be examined. Vapour barriers
in the wrong place, leading possibly
to condensation, have been
mentioned already, but there will be
other side effects. Damage may also
be done to the panels while
determining fixing points, and the
overcladding may spall or lose
exposed aggregate or surface finish
after completion of the fixing.
It is sometimes thought that
overcladding can provide some
protection to pedestrians from falling
concrete or aggregates, and some
systems have been installed
especially to do this, with a mesh
secured over the original cladding
prior to overcladding. While this
practice may help in some
circumstances, it must be
remembered that any case of
dislodgement becomes a potential
source of failure in the overcladding,
leading to detachment of insulation
and surface material, and then to
leakage of the envelope. The original
structure must therefore be repaired
to a satisfactory standard before any
overcladding is undertaken,
irrespective of the design to be used.
In spite of taking all reasonable
precautions to ensure longevity in the
base structure, it may still be prudent
to choose a replacement sheet
cladding or overcladding system
which allows access for inspection
and monitoring. However, certain
sheet cladding techniques do not
facilitate inspection at all, so that this
requirement will limit the options
available. The frequency of
inspections will need to be based
on engineering judgement. (BRE
suggests intervals not exceeding
3 years.)
HSE Guidance Note EH 36
(149)
,
dealing with the cleaning of old
asbestos cement roofs, also applies in
some degree to walls. One of the most
important considerations is that
asbestos cement must not be brushed
when it is in a dry state. It needs to be
thoroughly wetted to minimise fibres
being released from the material, and
dust masks of the appropriate rating
should be worn.
Redecoration of board finishes
could be required. Rain alone will not
wash surfaces clean and
periodic washing with a mild
detergent may be necessary. Stains
can be treated with high pressure
hot water.
Sealants
No sealant can be expected to match
the full 30 year life of the cladding
without replacements. Choice of
sealant together with its renewal
dates will need therefore to be made
against a knowledge of the
remaining design life of the building.
Provided protection can be
afforded from ultraviolet or direct
sunlight (eg by cover mould or shape
of the profile), lives of up to 20 years
can be expected from one or two-
part polysulfides or polyurethanes
and one part silicones. The elastic
sealants such as the polyurethane
and silicones will be found to
perform best with the relatively
lightweight sheet materials of high
thermal conductivity, though
compatibility should be checked
before specifying.
The range of materials and profiles
for gaskets is very wide, and it is not
practicable to give guidance on their
durability in this topic. Information
should be sought from
manufacturers.
Work on site
Ease of repair is likely to be critical
near to the ground or at access levels
at any height in the building, and it
should be possible to take out and
replace individual panels, for
example, without removing and
replacing a whole run. Special one-
off replacement panels with unique
surface profiles can be very costly
to supply.
Some means of access by
operatives to the cladding will be
needed for the replacement of
damaged panels; indeed the action
of access in itself could increase the
risk of damage. There may
therefore be a case for installing a
permanently available access
system as an integral part of any
new cladding, preferably including
some means of anchoring
suspended cradles against
sideways movements.
