Adéle de Lange, the Cement & Concrete Institute’s human settlements technical marketing consultant, has advised that contractors need to understand the role different constituents play to produce quality concrete for housing.

Contractors often misunderstand the role of constituents in producing quality concrete for housing.

According to Adéle de Lange, the Cement & Concrete Institute’s human settlements  technical marketing consultant, using the correct proportions in the mixing process and ensuring good site practice will affect the strength, durability and economy of the finished concrete.

Producers and importers of cement are legally required to have a letter of authority from the National Regulator for Compulsory Standards (NRCS) for each different cement type sold in South Africa. The letter is issued if the cement standard complies with SANS 50197-1 or SANS 50413-1 regulations.

De Lange identifies common errors that often occur when producing concrete for housing. The ratio between the water and the cement in a mix that determines the strength of the concrete is an important factor in the production. Errors occur when contractors use a builder’s wheelbarrow as a unit of measurement. This results in inconsistent concrete mix proportions. De Lange notes: “The contractor should ensure that the wheelbarrow is always levelled off at the top when measuring materials for mixing, to ensure that the correct, consistent mix proportion is achieved throughout. Two bags of 50kg cement is the equivalent to one builder’s wheelbarrow.”  

Another common mistake that contractors make is to add extra water to improve the workability of the concrete after an extended period of time. This significantly reduces the strength of the concrete.

De Lange notes that concrete is often not cured using the proper technique or is not cured long enough. She advises: “Newly cast concrete must be cured to ensure that hydration continues until the full potential strength of the hardened concrete is achieved and to minimise the tendency to crack. The concrete should be kept damp and not allowed to freeze during this time. The concrete should be cured for at least five days after placing it and longer in cold weather.”  

The most common problem on sites, however, remains the occurrence of cracks in plaster and floors. This can be avoided or reduced through the correct use of expansion joints to allow for movement of the structure at appropriate intervals. “Care should also be taken to allow for movement joints between different material types, such as clay bricks and concrete blocks,” states De Lange.

Cement and Concrete Institute
Tel: 011 315 0300
Website: www.cnci.org.za