Construction World April 2017

CEMENT AND CONCRETE TECHNOLOGY

The key to MAKING QUALITY mortar and plaster With so many reports of poor workmanship in the construction of houses and other buildings, it is worth going back to basic truths about how to achieve a high standard of mortar and plaster.

Brenton Brouard, technical manager: concrete at CHRYSO Southern Africa, explains that an acceptable standard mix design demands the use of ingredients that are of an acceptable quality. “The choice of materials is vital,” he says. “Use good cement; one with the South African Bureau of Standards (SABS) mark that complies with the SANS 50197 standard. It is probably most practical to use cement in the 32 N and the 42,5 N (preferred choice when making plaster) strength class.” One should also use clean, well-graded sand. This, Brouard explains, is because washed plaster sand will ensure that clay levels have been reduced. “Sand with too much clay is not good for plastering as the clay greatly increases the water demand of the mix and this increases the potential for shrinkage and cracking. He says that even water quality is important and says clean, potable water should be used. This should give a good quality mortar and plaster that is soft and plastic; cohesive without being sticky and can be placed easily. Using sub-standard materials in plasters may result in cracking, dusting, poor cohesion and crazing – those narrow, shallow, interconnected cracks that appear on plastered surfaces within a few days of casting.

Brouard says one of the biggest problems on site is the mixing of quantities of mortar and plaster which are too large to be accommodated by the rate of application. This results in the rapid loss of workability due to evaporation (caused by sunlight, heat and wind). On many sites, the solution is to increase the workability to the original level by re- tempering with additional water. “In almost all cases, this both reduces the strength and increases the potential for shrinkage cracking of the mix,” he says. Brouard says mixed batches should be protected by immediately placing the mortar or plaster into watertight containers with lids and only quantities that will be applied relatively quickly should be removed from these containers. This will reduce the risk of evaporation from the mix and as a result, increase the pot life of the mix. Chemical solutions company CHRYSO Southern Africa recommends the use of CHRYSO ® Stab rather than water in order to retain workability of plaster or mortar. CHRYSO ® Stab acts as a retarder and an air entrainer, imparting micro-bubbles into the plaster or mortar, significantly reducing the amount of water in the mix. This reduces the risk of shrinkage and the potential cracking resulting from it.

Brenton Brouard, technical manager: concrete at CHRYSO Southern Africa,

CHRYSO ® Stab also makes the mix light and fluffy and for an optimum workability, with sufficient entrained air, a targeted slump of 120 to 150 mm is recommended. In conjunction with the appropriate protection, typical dosages of CHRYSO ® Stab will produce brick laying mortars with pot lives of eight to 36 hours and plasters with pot lives of four to six hours. Brouard cautions that it is important to remember that extended pot lives are not recommended for plasters because additional bleeding from large exposed surface areas will increase the risk of shrinkage cracking. In addition to CHRYSO ® Stab, using CHRYSO ® Fibrin Fibres will inhibit surface cracking. Once the plaster has been applied, it is also important to cure the surface using fog sprayers as well as a curing agent from explains that an acceptable standard mix design demands the use of ingredients that are an acceptable quality.

CHRYSO ® Stab will produce plasters with pot lives of four to six hours.

The Vodacom Data Centre where a waterproof plaster was used to prevent tiles from falling off the façade.

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CONSTRUCTION WORLD APRIL 2017