Architects memo no.
76: June 2004
Whilst concrete can have a certain rugged beauty when first poured,
generally, unlike paint chemists, it rarely ages well. In our temperate
humid climate a combination of surface erosion, laitance, dirt and mould
result eventually in less than pristine surfaces. Less natural pollution,
coming out of an aerosol can and representing the artistic energy of
a rising generation, can also leave its indelible mark.
There is often a desire to preserve the appearance of new concrete
especially when the aesthetic of a building has been designed around
it. Clear coatings are often assessed as a potential means of achieving
this, often with great success.
One class of materials, solventborne acrylics, have had a long history
in the area. Although currently being phased out by waterborne analogues,
solution acrylics' most stunning successes have been as the factory-applied
clear coats over concrete roofing tiles. In closely controlled environments
these products are able to be applied over the wet, uncured tile to
provide a glossy durable surface. In fact the high water content of
the 'green' tiles is a benefit in preventing the absorption of these
solventborne acrylics and leaving the beneficial film right on top of
the tile where it will do the most good.
We highlight this because, over dry concrete, the normal tendency for
these products is to absorb into the somewhat porous surface. This again
is useful in that they can consolidate weak, porous surfaces and enhance
the colour of a surface, providing a 'wet look'. This is especially
useful for re-vitalising pressed concrete paving.
A drawback to penetrative clear coatings over concrete is that this
ability can also emphasise any variations in porosity of a concrete
surface. Invariably this is aesthetically unpleasant. In aesthetically-challenged
environments, such as industrial floors, these penetrating polymers
can provide very useful anti-dusting coatings. Small amounts of colourant
can be added which, over uniformly porous surfaces, can provide a pleasant
Clear two-component systems, usually based on polyester or solventborne
acrylic/isocyanate blends are an extremely useful defence against graffiti.
These tightly cross-linked urethane systems resist the penetration of
graffiti media and allow for easy cleaning. They are only of real value
over smooth, pore-free concrete as graffiti lodged in pores will always
be difficult to remove. The challenge with these systems is to achieve
a finish that is flat enough to be in harmony with the original concrete
concept whilst giving a surface smooth enough to make for easy cleaning.
Waterborne products, particularly acrylics, are known for their excellent
performance over concrete and for their durability. The vast majority
of this genre do not penetrate porous surfaces at all so they tend to
produce more uniform surfaces over variably porous substrates. While
this leads to better appearance, it does demand a sound substrate, as
lack of penetration means there is absolutely no consolidation of weak
friable substrates. These materials can be flatted so that a very 'sympathetic'
coating can be designed to replicate, but protect, the new concrete
Application of waterborne acrylics over porous surfaces can generate
microfoam if not applied very carefully. Microfoam appears as a 'milkiness'
within the clear coating, which permanently detracts from the overall
There has, over the past year or two, been a request from the market
for high-build, elastomeric, flat and clear waterproof coatings for
use over concrete, especially masonry. Such technology is available
but, in common with all the above coatings, they can be completely ruined
by efflorescence. Readers of Architects Memo No.
58, if they have still retained their sanity, will know that if
there is water around concrete, efflorescence will not be far behind.
If efflorescence appears on uncoated concrete, it will eventually weather
off. If it occurs on clear-coated concrete it will deposit under the
film and be a salutary reminder for the life of the clear coat - which
could be a very long time.
The best defence against efflorescence is, as stated before, a deeply
penetrating oligomeric siloxane and this should be used before any suggestion
of clear coating is contemplated. In fact, in the opinion of your humble
scribe, the masonry water-repellent treatment, in itself, is the ideal
clear treatment for architectural masonry surfaces.
Useful products in the Resene range for clear coating concrete are: Resene
Concrete Stain (coloured solventborne acrylic), Resene
Multishield+ (waterborne acrylic), Resene
Uracryl (anti-graffiti coating) and Resene