The role that the various mix constituents play to produce quality concrete for housing is often not fully understood , cautions Bryan Perrie , managing director of The Concrete Institute ( TCI ).
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Critical factors that affect concrete quality
The role that the various mix constituents play to produce quality concrete for housing is often not fully understood , cautions Bryan Perrie , managing director of The Concrete Institute ( TCI ).
Perrie says that using the correct mix proportions and ensuring good site practice affects the strength , durability and economy of finished concrete .
“ Firstly , the quality of the cement is crucial . Building 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 different cement type sold in South Africa . The NRCS issues a LoA only if the cement standard complies with SANS 50197-1 or SANS 50413-1 ,” he explains .
He says the most common errors that tend to occur when producing concrete for housing include :
• Incorrect ratio between the water and the cement – a ratio that determines the strength of the concrete . When site batching for small quantities of concrete , contractors tend to use a builder ’ s wheelbarrow as unit of measurement . But this practice often produces 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 bags of 50kg cement is the equivalent to one builder ’ s wheelbarrow ,” Perrie states .
• The addition of extra water to improve the workability of the concrete after an extended period . This practice significantly reduces the strength of the concrete .
• Often concrete is not cured using the proper technique and / or is not cured long enough . “ 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 be allowed to freeze during this time . The concrete should be cured for at least five days after placing it and longer in colder weather ,” he advises .
• There is often confusion between client , specifier and contractor when it comes to finishing a concrete floor , specifically relating to the application of a cement screed to the finished concrete floor . In general , a sandcement screed should not be applied as the final wearing surface . The appropriate application of sandcement screeds and concrete toppings is described in detail in The Concrete Institute ’ s publication
The Concrete Institute ( TCI ) says that there is still alarming ignorance about the roles that the various mix components play to produce top quality concrete for housing . Image : The Concrete Institute
Using the correct mix proportions and ensuring good site practice affects the strength , durability and economy of finished concrete .
Sand-cement screeds and concrete toppings for floors which is available free of charge from the TCI .
• Cracks in plaster and floors are a frequent problem on most sites – a problem that 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 ,” Perrie adds .
More detailed information on this subject is available from The Concrete Institute ’ s publication Concrete basics for building .
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