Sand‐cement plaster is used extensively in building work as a decorative or protective coating to concrete and masonry walls and concrete ceilings. But incorrectly applied plastering inevitably leads to problems, as shown by the consistently high level of queries in this regard directed at The Concrete Institute (TCI).
TCI publishes two free advisory leaflets on plastering: Common Defects in Plastering and Successful Plastering. Both provide technical information for successful plastering for architects, building contractors, building inspectors, and anyone in need of guidance to achieve a satisfactory standard of work.
Here Bryan Perrie, MD of TCI, deals with one important aspect of plastering: the preparation of the surfaces.
Surface preparation:
Preparation of the surface to be plastered starts with accurate setting out and construction of walls and soffits. The aim should be to provide a surface that can be plastered to the required lines and levels by applying a coat (or coats) of uniform thickness. It should be remembered that excessively thick plaster - or plaster of uneven thickness – will never disguise inaccurate work.
Where zones of the substrate surface deviate from the required plane (or curved) surface by more than about 10 mm, the first option is to remove high areas by hacking or cutting. If this is not practical, undercoats should be applied to low areas in such a way that the final coat is of uniform thickness. In cases where above average thickness is required, it is advisable - and safer - to mechanically anchor the plaster to the substrate, e.g. with stainless steel studs. This is also recommended when plastering dense non‐absorbent substrates.
Roughness:
Background surfaces should ideally be at least as rough as coarse sandpaper or rough‐sawn timber. Suitable surface roughness can be achieved in the following ways:
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Using formwork with a rough surface, e.g. sawn timber, for substrate concrete
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Stripping formwork early and wire brushing the concrete;
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Hacking;
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Abrasive blasting (e.g. sand blasting);
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Raking out mortar joints in masonry substrates to provide a key. A depth of about 10 mm is
normally adequate; or
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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 consistence. A polymer emulsion should be substituted for part of the mixing water (usually a quarter to a third, but in accordance with the manufacturer’s instructions). The mixture is then flicked on to 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 5 mm high.
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.
Cleanliness:
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:
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Water jetting
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Blowing with oil‐free compressed air
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Vacuum cleaning
Bear in mind that brushing solvents should not be used to remove films formed by curing compounds: such films must be removed by mechanical means.
Absorption:
The simplest way to assess absorptiveness is by throwing about a cupful of water against the surface. The surface will respond in one of three ways:
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No water will be absorbed
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Some water will be absorbed but most will run off
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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.
Strength for new work:
Masonry units strong enough to survive handling and transport prior to being placed on site, must be strong enough to hold plaster. Similarly, in situ concrete should have ample strength.
