![]() ![]() Glass fibre tissue can also be applied during the painting process. These cracks can be filled with a proprietary filler and be painted over. It is often floated closed, only to reappear sometime later. Roxburgh adds, “When the cracks are noticed while the plaster is still plastic. Not using building lime or a masonry cement when the sand lacks fine material.Use of sand that is badly graded and lacks fine material (less than 5% by mass passing the 0,075 mm sieve).Suction into the walls, if the masonry units are absorbent and have not been dampened. ![]() Evaporation – if the wall is not protected from sun and wind.These cracks normally occur when a plaster mix with a high cement content is used or the plaster is allowed to dry too quickly.” Causes of such excessive early moisture loss may include: “ Map cracking is similar to crazing except that it is usually deeper – sometimes going right through the plaster – and the hexagons of the pattern may measure up to 200mm across. This can be done during the painting operation,” Roxburgh explains. If necessary, glass fibre tissue can be applied. It can be covered using reasonable quality paint. The good news is that craze cracks are of relatively little importance. Cracks may hardly be visible until dust or moisture covers it. Crazing often occurs within a few hours of the plaster being applied to the wall. More than 15% by mass passing a 0,075 mm sieve. Or using sand containing an excessive amount of dust. Cracks are usually the result of over-trowelling a rich mix (one with a high cement content). They do not extend through the whole depth of the plaster. Craze cracks are usually very fine and shallow. Usually in a hexagonal pattern, which typically measures between 5 and 75mm across each hexagon. “ Non-structural cracking is a network of fine cracks. Perhaps the most obvious defect in plastering: CRACKS – those that are non-structural and the more challenging and nasty type: structural cracks This means that it may be difficult to force the contractor to make repairs if the plaster is clearly not acceptable,” Roxburgh states. Most contract documents also are not clear on the subject. “Neither SABS specifications nor the National Building Regulations have measurable requirements covering hardened plaster quality. How can a property owner be sure that the plaster provided by the smaller or relatively inexperienced building contractor is acceptable? And if not, what can be done about it? It’s not a simple question to answer, says John Roxburgh, lecturer at The Concrete Institute’s School of Concrete Technology. A ‘Throwback Thursday’ to a cement and concrete top solution. Posted under Cement & concreteĬauses of plaster cracks, originally published on November 2016. ![]()
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