Civil Engineering Reference
In-Depth Information
with Types 302 and 304 stainless
steels in marine conditions corrosion
is a risk. And whatever the location,
local industrial pollution may also be
a problem. Because the lee side of a
building is the most susceptible, there
should be as few ledges as possible to
harbour run-off ponds.
In system built dwellings are, no
problems with loss of integrity of the
structural frame have been reported
on Atholls and Telfords, but some
BISFs have had serious corrosion of
stanchion bases (Figure 3.67). Also in
BISFs, corrosion of the sheeting rails
from condensation within the cavities
has led to protrusion of hook bolts.
Extensive corrosion of sheeting on
Atholls built post-1945 has occurred
to a much greater extent than on
interwar examples. Some Atholls
also had timber framing in the
external wall which is at risk of decay
if rainwater penetrates. The structural
frames of Open System dwellings
were galvanised - a comparative
rarity in dwelling systems, although
less of a rarity in other building types.
In Livett-Cartwrights jointing
material has deteriorated, allowing
rain penetration.
All systems in this category had
some breakdown of protective
coatings to the structural steel
components, though in no case had
this progressed sufficiently to allow
more than superficial corrosion of the
steel. However, the situation was
rather different with steel sheet
claddings, where some severe
corrosion was found (Figure 3.68).
Rot in ply sheathing following rain
ingress has been reported in Arcals
and 5Ms, and in the latter case, also
the plywood box beams at eaves
level.
Hawthorne Leslies normally had
galvanised steel framed external
cladding panels. On some examples
the galvanising was missing, leading
to corrosion. In this system too there
has been deterioration of the surface
finish on the cladding boards. Repair
has not proved practicable, and
removal and replacement has been
the preferred alternative. This system
originally used felt pitched roof
coverings which have not proved
durable. In any replacement
programme, care must be exercised
not to increase the loads on the roof
structure. On some 5M dwellings, an
excessive number of unprotected
steel shims was used to pack column
bases to level. These were rusting and
required attention.
Figure 3.69
Deteriorating asbestos cement sheet
cladding to an industrial building
Durability of boards
Asbestos cement sheets used to be
judged to have lives of between 30
and 45 years, depending on the
exposure and pollution to which they
were subjected, before becoming
unacceptably brittle and subject to
impact and other damage - for
example from movements under wind
loads or corrosion or loosening of
fixings (Figure 3.69).
Undecorated replacement sheets of
other fibre cements will have similar
lives, though applied finishes will
have shorter lives; perhaps as little as
10 years.
Fibre reinforced corrugated and
flat sheets are relatively fragile; some
of their properties are described in
BRE Information Paper IP 1/91
(152)
.
The characteristics of these sheets are
different from the asbestos cement
sheets they replace. Cracking may
occur, even in sheets as little as two or
three years old, though its cause may
be as much due to fixing deficiencies
or inappropriate specification as to
material deficiencies. Adequate
allowance for movements is
necessary to avoid cracking.
Durability of fixings
Durability of fixings into concrete
must be assessed carefully. The risk
of corrosion is greatest with chloride
bearing concretes: in these concretes,
in the presence of moisture, even 18-8
stainless steels are subject to crevice
corrosion. Type 316 stainless steels
should be specified.
Particular care should be given to
the specification of self-tapping
Figure 3.68
Steel sheet cladding to housing in Glasgow. Significant corrosion has occurred at low
levels, and rather less under the protection of the verge
