A large percentage of queries received by Cement and Concrete SA (CCSA) and its predecessors deals with defects in plastering. John Roxburgh, senior lecturer at the CCSA School of Concrete Technology, provides some insight on the types of cracks often encountered in plaster and their causes.
Crazing and map cracking are both cracks which usually occur as a result from over-trowelling a rich mix (one with a high cement content) or sand containing too much dust. Crazing can occur within hours of the plaster being applied, and cracks may hardly be visible until dust or moisture makes them noticeable.
• Crazing is a network of fine cracks, usually in a hexagonal pattern, which measure between 5mm and 75mm across each hexagon. The cracks are usually very shallow and do not extend through the whole depth of the plaster.
• Map cracking is usually deeper (sometimes going through the plaster) and the hexagons of the pattern may measure up to 200mm across. These cracks normally occur when a plaster mix with a high cement content is used or when the plaster is allowed to dry too quickly.
“When the cracks are noticed while the plaster is still plastic, they are often floated closed only to reappear sometime later. These cracks can be filled with a proprietary filler and painted over. Glass-fibre tissue can also be applied during painting,” says Roxburgh.
Cracking that results when an excessive amount of water is lost from the plaster in the first hours after application is known as plastic shrinkage cracking.
• Drying shrinkage cracks are the result of moisture loss after the plaster has hardened. Plaster with a very high cement content and made with poor-quality sand and a high volume of water, will tend to develop a few widely spaced cracks. Plaster applied in layers that are too thick will also crack this way. These cracks are normally stable and can be filled with a filler and painted over.
• Structural cracks are usually caused by cracking of the wall. A specialist should be called to establish the cause and for remedial measures.
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