Sand‐cement plaster is used extensively in building work as a decorative or protective coating for concrete and masonry walls and concrete ceilings. When applied incorrectly, this will inevitably lead to problems, as shown by the consistently high level of queries directed at Cement & Concrete SA (CCSA) and its predecessors.
Proper surface preparation
Bryan Perrie, chief executive officer of CCSA, says an important aspect of plastering that is often neglected and causes problems later, is the preparation of the surfaces.
Where zones of the substrate surface deviate from the required plane (or curved) surface by more than about 10mm, Perrie says the first option is to remove high areas by hacking or cutting. If not practical, undercoats should be applied to low areas in such a way that the final coat is uniformly thick. In cases where above average thickness is required, it is advisable and safer to mechanically anchor the plaster to the substrate, for example with stainless steel studs. This is also recommended when plastering dense non‐absorbent substrates.
Background surfaces should ideally be as rough as coarse sandpaper or rough‐sawn timber. Perrie suggests suitable surface roughness can be achieved in the following ways:
- Using formwork with a rough surface, for example sawn timber, for substrate concrete.
- Stripping formwork early and wire brushing the concrete.
- Abrasive blasting (such as sandblasting).
- Raking out mortar joints in masonry substrates to provide a key (a depth of about 10mm is normally adequate).
- Applying a spatterdash layer.
Spatterdash is a mixture of one part of cement (preferably CEMI or CEM IIA) to 1,5 parts of coarse sand, with enough water for a sluggishly pourable consistency. A polymer emulsion may be substituted for part of the mixing water (usually a quarter to a third), but according to the manufacturer’s instructions. The mixture is then flicked onto the substrate as an initial coating to provide a key on dense or smooth substrates with poor suction.
The spatterdash should cover the substrate surface completely and form a rough texture, with nodules about 5mm high. The spatterdash must not be allowed to dry out for at least three days and if a polymer emulsion is included in the mix, then curing should be in accordance with the manufacturer’s instructions. It should be tested for adhesion and strength by probing with a screwdriver or knife before plaster is applied to it.
The surfaces to be plastered must be free of loose material, such as dust, and films that can interfere with bonding, such as curing compounds. Substrate surfaces may be cleaned by water jetting, blowing with oil‐free compressed air or vacuum cleaning. Brushing solvents should not be used to remove films formed by curing compounds, but rather be removed mechanically.
Assess absorptiveness by throwing a cupful of water against the surface. Either no water will be absorbed, some will be absorbed but most will run off, or most of the water will be absorbed.
If no water is absorbed, as will be the case of hard-burnt clay face bricks, glazed bricks and very dense high‐strength concrete, a spatterdash coat that includes a polymer emulsion should be applied. Such surfaces must not be pre-wetted. When most of the water runs off, the surfaces would not require any treatment to control suction.
Finally, if most of the water is absorbed, the surfaces should be wetted thoroughly for at least an hour and then allowed to become saturated surface dry before the plaster is applied. CCSA publishes two free advisory leaflets on plastering: Common Defects in Plastering and SuccessfulPlastering. Both provide technical information for successful plastering. The practical skills component of the CCSA School of Concrete Technology’s “Introduction to Concrete” (SCT10) training course also includes basic plastering skills.
For more information about the importance of proper plastering, contact the CCSA Information Centre:
Tel: +27 11 315 0300
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