The role that the various mix constituents play to produce quality concrete for housing is often not fully understood, Bryan Perrie, MD of The Concrete Institute, has cautioned.
Perrie says using the correct mix proportions and ensuring good site practice affects the strength, durability and economy of the finished concrete. “Firstly, the quality of the cement is crucial.
Contractors should note that all producers and importers of cement must have a Letter of Authority (LoA) from the National Regulator for Compulsory Standards (NRCS) for each cement type sold in South Africa.
The LoA is only issued if the cement quality complies with SANS 50197-1 or SANS 50413-1,” he explains.
Perrie says five errors often occur when producing concrete for housing:
•The ratio between water and cement in a mix determines the concrete’s strength. When site batching small quantities of concrete, contractors often use a builder’s wheelbarrow for measurement. Unfortunately this can produce inconsistent concrete mix proportions. “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. Note: two 50-kg bags of cement is the equivalent to one builder’s wheelbarrow,” Perrie states.
• Another common mistake is the addition of extra water to improve the workability of the concrete after an extended period. This significantly reduces the concrete’s strength.
•Incorrect curing and/or not cured long enough. “Newly cast concrete must be cured to ensure that hydration continues until the full potential strength 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 and longer in cold weather,” he advises.
•There is often confusion between client, specifier and contractor regarding finishing a concrete floor, specifically applying a cement screed to the finished concrete floor. Generally, a sand-cement screed should not be applied as the final wearing surface. The appropriate application of sand-cement screeds and concrete toppings is described in detail in The Concrete Institute publication: Sand-cement screeds and concrete toppings for floors, which is available free of charge from the Institute.
•Cracks in plaster and floors are a common problem on most sites. This can be avoided or minimised by the correct use of expansion joints at appropriate intervals to allow for movement of the structure. “It is important to allow for movement joints between different material types, such as clay bricks and concrete blocks,” Perrie adds.
Detailed information is available from The Concrete Institute’s publication Concrete basics for building. This publication, as well as several other specialised information leaflets on these issues can also be obtained directly from the Institute.
Courses on concrete suited to learners varying levels of experience are also presented by The Concrete Institute.
More information from Bryan Perrie, Tel: +27(0)11 315 0300 / www.theconcreteinstitute.org.za
