Housing in Southern Africa October 2015

Transporting bricks on site

REINFORCEMENT Reinforcementmust at the time of the placing of the concrete, be free from rust, scale, oil and other coatings that may reduce the bond between steel and surrounding concrete, or affect the durability, or initiate corrosion. Ensure that water is used as a release agent on the formwork, to prevent contamination of the reinforce- ment. It must be secured by tying at intersections with annealed wire of 1,6mmor 1,25mmor suitable clips or fixed in position bymeans of hangers or saddles and aligned by spacers or other supporting materials. CEMENT PLASTER Common problems with cement plaster include crazing, cracking, Continued ▶▶▶

Lack of curing affects the strength, durability and abrasion resistance of the concrete. FORMWORK Formwork should be erected with joints tight enough toprevent leakage of cement paste. Surfaces of forms that are in contact with fresh wet concrete ought to be treated with a coat of non-staining mineral oil, or a suitable releasing agent. In the case of timber forms, wetting the surface is acceptable to ensure easy release and prevent adhesion of the concrete to the formwork. The formwork can be removed after the concrete has attained suf- ficient strength to support its own weight and any loads that may be imposed on it.

The curing period takes at least three days and in cold weather this should be extended to five. In freezing con- ditions, the fresh concrete needs to be well insulated from the effects of cold weather. When cement andwater aremixed together to form a paste, a chemical reaction occurs and the paste sets and slowly hardens. In concrete, the paste binds the other constituent materials andwill continue to harden and gain in strength as long as water is present. This hydration will contin- ue as long as there is sufficient water available to support the process. Curing is important to ensure that concrete hydrates in a proper man- ner. The gain in strength is initially rapid but slows down progressively. The purpose of curing is to control the rate of loss of moisture and to prevent plastic shrinkage cracking.

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