Housing in Southern Africa October 2015

C oncrete plays an important role in housing foundations and the type of cement, how it is stored, used and mixed all play a role in the final strength of the foundation. THE CONCRETE FACTS Cement in bags must be stored and protected from elements, which cause deterioration. Where it is neces- sary to store in the open, bags should be stacked on a wooden platform set on supports of at least 100mmabove the ground. Cover with a tarpaulin or plastic sheeting to ensure that water runs off without wetting the bags. Bags should be stacked not ex- ceeding 12 bags and arranged so that consignments can be used in the order of delivery, as cement which is stored in bags lose about 30% of its potential strength after about four to six weeks. If ‘old cement’ is used, then the mix must have a richer cement content. Cement that has lumps in it that cannot be easily crumbled with a finger should not be used. Aggregates for concrete should be stored separately so as to prevent contamination. Cement and lime should be mea- sured by volume. A bag of common cement has a mass of 50 kg and a volume of approximately 33 litres when packed under air pressure at the factory. Cement, however, fluffs up when poured into a container, and 50 kg cannot readily be contained in a box of 33 litres. The volume of loose (bulked) cement, depending on its compaction, can be up to 20% more than in a bag. For this reason it is always preferable to use whole bags of cement when volume batching. Back to Basics

Inspectors Zondi Doro and Sinfingatwa Mantsha

The fully comprehensive NHBRC Home Building Manual is a valuable resource of information on every aspect of home building. The state regulatory body continues to raise the bar on housing and assists the sector. Here are some of the tips from the NHBRC Home Building Manual.

Concrete mixed on site using com- mon cements has a strength class of 32,5 MPa or more. Concrete left standing for a limited period must be covered with plastic sheets or wet stacks to prevent it from drying out. Wet concrete needs to be remixed before being placed, in the event that stone particles have settled at the bottom of wheelbar- rows during transportation. Concrete should be deposited ver- tically into its final position to avoid segregation of aggregates. Concrete must be compacted by mechanical vibrations or tamping, spading, rodding or forking so that the concrete is thoroughly worked against the formwork and around the reinforcement, and other embedded items without displacing them. This will ensure that the concrete is free from honeycombing and planes of weakness. Inadequate compaction can seriously affect the quality of the

concrete. The concrete may be as- sumed to be fully compacted when the air bubbles cease to rise to the surface of the concrete in being vibrated. Contact between the vibra- tor and formwork can damage the surface ‘off-shutter’ concrete. Where possible concrete should be placed in a continuous process. If this is not possible because of weather conditions, construction joints at the point of stoppage may be formed by sloping the edge of the poured con- crete at 45 degrees to the horizontal. The surface of the joint should, prior to continuing with concreting, be brushed with a steel wire brush, swept clean with a light brush, wet and covered with a 10mm thick layer of mortar, (composed of one part ce- ment to three parts of concrete sand) before placing new concrete. After the initial set, the concrete needs to be protected from con- tamination and loss of moisture by covering with waterproof sheeting.

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