Civil Engineering Reference
In-Depth Information
specially formulated undercoat,
bridge the cracks.
Where walls are relatively porous,
attempts have been made to increase
their resistance to rainwater by using
a water repellent treatment which
does not completely seal the pores of
the surface. Nevertheless, some
closing of the surface is inherent with
most treatments. The principle is that
a greater quantity of water is turned
away from the surface than is
prevented from evaporating.
Principles of modern building
(6)
quotes examples where a reduction of
one hundredfold in rainwater entering
the wall resulted in a reduction of
fivefold in the rate of evaporation.
Against this of course is the risk that
cracks in the surface might allow
water to enter which cannot then
evaporate due to the surface
treatment. Clearly a balance has to be
struck.
The performance of water repellent
treatments is at present determined in
the UK by test methods described in
BS 6477. These do not, however, take
into account either the crucial
influence of mortar joints or the effect
of water applied with forces similar to
wind-driven rain. In BRE
experiments, treatments on clay and
calcium silicate brickwork were
investigated using BS 6477 methods.
The silicone based treatments were
the most effective, with no leakage
through the treated masonry and
moisture content of the walling
remaining low. Polyoxoaluminium
stearate treatments were moderately
effective but a treatment based on
acrylic polymers gave poor test
results on brickwork.
Main performance
requirements and defects
Choice of materials for structure
It is important to try to establish
exactly what paint has been used on a
surface before specifying additional
coats. Laboratory examination may
be worthwhile.
Case study
Investigation of paint failure on
galvanised steel windows
The BRE Advisory Service was requested to
investigate problems of premature paint
failure on galvanised steel windows and sills
at a large block of flats, and to report on the
paint system used and the reasons for
failure. Peeling and loss of adhesion of alkyd
gloss paint had become a problem within
eighteen months of completion of
redecoration.
Samples showed a white undercoat,
87.5
Dimensional stability
Some paints have very limited ability
to accommodate movements in the
substrate. Likely performance under
severe conditions should be checked
with manufacturers.
For coefficients of linear thermal
and moisture expansion of
background materials, see
Chapter 1.2.
m white topcoat and a total film
thickness of 137.5
µ
m for the new system
over bare metal. There was no trace of a
zinc tetroxychromate layer beneath the
undercoat. However, it was noted that one
flake did have a small patch of zinc
tetroxychromate fragments loosely
adhering to the underside.
From examination of the paint samples
removed from the windows it was concluded
that, where sampled, the new paint system
applied to bare galvanised surfaces had a
thickness consistent with use of the
specified undercoats and topcoats, but the
specified zinc tetroxychromate etch primer
was not present. Only traces of unbonded
fragments of zinc tetroxychromate from
previous decoration were found on the
underside of the samples of the new paint
system. This was basically why the paint
system had failed.
µ
Dampness and condensation
Paints of course have been used on
stucco for centuries, generally with
good results. The main problem
seems to be that cracking of paints on
masonry allows water to penetrate the
cracks, but subsequent drying is
inhibited by the paint film leading to
an accumulation of water and
potential frost or sulfate damage. A
paint must be selected which will,
perhaps with reinforcement or a
On timber, durability of painted
finishes depends very heavily on the
adequacy of preparation, but even the
best of specifications in exposed
situations will not last more than a
few years (Figure 9.32).
The zinc on galvanising reacts with
most oil based paints, forming
soluble salts (zinc soaps) beneath the
paint film, which cause it to become
embrittled and lose adhesion.
Pretreatment primers
(259)
are
necessary to prevent this process.
Colourless treatments to porous
wall surfaces to resist rain
penetration may penetrate only
shallowly, and may cause salts buried
deep in the wall to crystallise at the
depth of penetration of the
waterproofing liquid leading to
crypto-efflorescence (see Chapter
2.2). Silicone based treatments tend
to be more durable than stearates. For
further information, see BRE Digest
177
(115)
.
Durability
Failures of paint films on walling
materials were commonplace in the
1930s. Many of these failures were
due to entrapment of construction
water as a result of premature
painting of surfaces, but another
cause of failure had already been
recognised - the destruction of oil
based paint applied to alkaline
backgrounds from saponification of
the paint.
Figure 9.32
Painted shiplap which has exceeded its
maintenance interval
