This month we continue the important subject of surface preparation, discussing concrete surfaces. Concrete may be considered a permanent material but the performance of concrete as a weathering surface leaves much to be desired.
Concrete is the complex reaction product of aggregates, silica sand, and calcium silicates present in cement. Lime is produced during the setting reaction which, slowly, further reacts with the silica sand to form more cement. This lime production accounts for the high initial alkalinity of concrete and is influenced by the amount of water added, type of aggregate, and additive used. Excess lime can migrate to the surface as laitance or efflorescence. The cement matrix is slowly eroded by acids normally present in rain, so that old concrete has a weak, unbound layer of sand on the surface.
The above holds true for all concrete products although the different forms do have some individual differences.
More likely to have efflorescence and laitance due to higher slump mixes. Strong likelihood of form oils and/or curing agents being present.
The very low slump mixtures used in this area reduce alkalinity but increase porosity. Mortars used for jointing and reinforcing are generally highly alkaline.
Cement readers (including stucco, rough cast and EIFS)
Can have weak surface layers due to water loss from relatively thin layers leading to poor curing. Many renders have added lime reading to very high alkalinity.
Fibre reinforced cement
Low incidence of alkalinity and efflorescence in flat sheet; greater danger in moulded sheet. Some possibility of mould release oil contamination.
Concrete roofing tiles
The factory applied cementitious coating breaks down on extended weathering to a fine powder which, if untreated, provides a weak substrate for paint.
Old fibre reinforced cement
(Prior to 1982) may contain asbestos. Local Health Authorities must be contacted for advice on preparation work.
Glass reinforced cement (GRC)
These very dense and glossy surfaces can also be contaminated with form release oils and defoamers.
New concrete, concrete masonry and fibre reinforced concrete
Remove any laitance or efflorescence by dry wire brushing or grinding; remove any remaining form oil or curing agents by scrubbing and rinsing off with water; remove all dust and dirt.
Old concrete, concrete masonry, fibre reinforced cement, new and old cement renders (including stucco) weathered roof tiles
In order to assess the condition of these surfaces prior to painting, scratch the surface with a pen-knife blade - if the surface can be scratched off as a powder the whole area must be thoroughly cleaned. Treat efflorescence as in a); remove any powdery layer by vigorous wire brushing or preferably water-blasting; sand off any protruding fibres from asbestos cement. When dry, the surface must be conditioned by using a penetrating sealer such as Resene Sureseal . Mouldy surfaces should be treated with Resene Moss & Mould Killer.
Smooth/glossy concrete and old cement floors
New cement floor slabs must be allowed to cure for a minimum of twenty eight days prior to painting. Acid etching is the preferred surface preparation. Some new floors may have had a curing membrane applied, or may contain additives that harden the surface. These treatments result in a surface that is usually resistant to dilute acids and require alternative preparation by mechanical abrasion or captive shot blasting.
All new concrete must be checked for excess moisture before coating. In the case of new concrete the test is carried out after the minimum recommended curing period.
Allow surface to dry out thoroughly, then seal with a full coat of Resene Smooth Surface Sealer applied at an approximate spreading rate of 7.5-12.5 square metres per litre depending on surface texture.
Refer D83 in the Resene One-Line Specification manual for detailed instructions.
The Resene architect's memo section provides technical information on a variety of topics relating to paints, finishes and coatings.