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 appearance.
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 surface.
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 appearance.
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 Aquapel (siloxane).
The Resene architect's memo section provides technical information on a variety of topics relating to paints, finishes and coatings.