Civil Engineering Reference
In-Depth Information
Aluminium sheeting, pre-coated
with organic coatings, is now
available; the visually acceptable life
of such material is essentially the life
of its organic coating, but it depends
on the type of coating, environment,
thickness and bond. In aggressive
environments it will be necessary to
consider eventual repainting to
restore appearance. Aluminium
extrusions as well as sheet can be
coated with organic paints, the more
successful of these being the
fluorocarbon and polyester based
materials. Powder coating is a very
common finish, applied by heating
the powder to fuse it to the surface of
the aluminium.
Resistance to impacts
This has been covered in Chapter 1.1
Resistance to ultraviolet and
sunlight
Many walling materials fade on
prolonged exposure to sunlight,
including some woods, and some
plastics yellow. Yet other materials
are unaffected. Whether such colour
changes are important depends on
circumstances. Material
specifications do change, and
inspection of old samples is not
necessarily a good guide to future
performance. It may be possible to
carry out laboratory assessments,
though manufacturers' advice should
also be sought.
Expected life of surfacing materials
There is a British Standard on
durability, BS 7543
(74)
, which applies
to walls. It gives general guidance on
required and predicted service life,
and how to present these
requirements when preparing a
design brief.
BS 8200
(28)
requires all panels and
secondary framing to have durability
equal to the life of the building. In
practice not all parts of the system
can be expected to achieve that
figure, but special care must be taken
to ensure adequate life for any
components which are difficult to
inspect and critical to safety.
Natural pollutants
Carbon dioxide, CO
2
, is present in
the atmosphere at a concentration of
about 350 parts per million. It
dissolves in pure water to give a
slightly acid solution 'carbonic acid'
with a pH of 5.6 (a neutral solution
has a pH of 7.0). Dissolved in
rainwater, CO
2
is able to react slowly
with the carbonate component of
calcareous stones and lime mortars
leading to long term deterioration.
Carbon dioxide can also cause
deterioration of concrete, in
particular reinforced concrete. Steel
reinforcement is protected against
corrosion by the alkaline
environment of fresh concrete.
However, CO
2
gradually penetrates
from the concrete surface inwards,
neutralising the alkalinity as it
progresses (carbonation). Further
discussion of this phenomenon will
be found in Chapter 2.4.
The chief source of carbon
dioxide from human activities is the
burning of fossil fuels. It has been
suggested that, at present, peak urban
concentrations reach
3000 ppm. The effect of CO
2
as a
greenhouse gas is an issue beyond
the scope of this publication.
However, the implicit climatic
changes would be likely to modify
the value of several other agents
affecting performance, most
obviously temperature.
Figure 1.66
Severe corrosion has taken place in the
reinforced concrete columns of this two
storey house
Industrial pollution
It is known that in sufficient
concentration, sulfur dioxide plays an
important part in the deterioration of a
number of building materials the most
sensitive being calcareous stones and
certain metals. There is evidence that
the considerable reduction in
emissions over the last few years has
resulted in a reduction of weathering
rates for steel.
There is little evidence to indicate
pollution attack on brickwork. The
effect of absorption of sulfur dioxide
from the atmosphere will be
significantly less than intrinsic sulfate
attack on mortar by transfer from the
brick itself. (Sulfates are only present
in some types of clay brick.)
There is further discussion on
pollutants in
Roofs and roofing
.
