Housing in Southern Africa October 2015

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debonding and lack of hardness. Crazing is usually the result of over trowelling a rich (high cement con- tent) mix, or the use of sand con- taining an excessive amount of clay material. Cracking and crazing can occur due to the excessive loss of water from plaster in the first hours after application, exposure to the sun, wind, or by suction into the walls. Cracking can also develop after the plaster has hardened as a result of the use of poor quality sand, or the application of thick layers. Debonding is generally a result of inadequate preparation of the sub- strate, such as not cleaning dusty, oily walls, or roughing up smooth surfaces, and not allowing the wall to reach the correct moisture con- tent. It can also be caused by poor technique and the incorrect use of bonding agents. Lack of hardness is associatedwith insufficient cement in themix, the use of sands contained excessive fines, a mixwith poor water retention proper- ties, the addition of extra water after the first mixing or rapid drying out which means incomplete hydration of the cement. GYPSUM-BASEDPLASTER Gypsum based plaster must not be mixed with a plaster made with com- mon cement, as gypsum is a sulphate compound, which attacks common cement paste. This will cause swell- ing, softening and disintegration of the plaster. CEMENT PLASTER FINISHES Smooth untreated cement plaster rarely retains a satisfactory appear- ance. It tends to become somewhat patchy in appearance through slight, unavoidable variations in surface texture. Steel trowelling increases the likelihood that the surface will craze and should be avoided in external applications. A wood float finish is preferablewhen painting the surface. PLASTERING Before plastering commences, all chases with the electrical, plumbing conduits, boxes should be fixed into position. The first coat of a three-coat application is sometimes referred to as a ‘scratch coat’ and is frequently

Inspector checks brickwork and levels

A subsidy house nearing completion

deeply scored to break the continu- ous surface to enable the second coat to be applied. Scoring comprises of parallel lines about 20 mm apart and 5 mm deep. A plaster comb can be used to score the plaster. Plaster coats should ideally have a

lower strength than thematerial onto which they are applied. Successive coats should be thinner than the coat to which it is being applied. Plaster should not be applied during extreme temperatures and moisture conditions as it may affect

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