TECHNICAL TALK
Achieving quality concrete for housing
The role that the various mix constituents play to produce quality concrete for housing is often not fully understood , Bryan Perrie , CEO of Cement & Concrete SA ( CCSA ), has cautioned .
Perrie says using the correct mix proportions and ensuring good site practice affect the strength , durability , and economy of the 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 an LoA only if the cement complies with SANS 50197-1 or SANS 50413-1 ,” he explains .
Perrie says five errors tend to occur when producing concrete for housing :
• The ratio between the water and the cement in a mix 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 .” Unfortunately , 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 also that two bags of 50kg cement is the equivalent of one builder ’ s wheelbarrow ,” Perrie states .
• Another common mistake on site is 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 .
• All too frequently , 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 allowed to freeze during this time . The concrete should be cured for at least five days after placing it and longer in cold 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
CCSA surface . The appropriate application of sand-cement screeds and concrete toppings is described in detail in the CCSA publication : ‘ Sand-cement screeds and concrete toppings for floors ’ which is available free of charge from the association .
• Cracks in plaster and floors are very common – 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 .
“ The NRCS issues an LoA only if the cement complies with SANS 50197-1 or SANS 50413-1 .”
More detailed information on concrete for housing is available from the CCSA 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 association . CCSA ’ s School of Concrete Technology also presents a variety of educational courses on concrete for all levels of experience .
For further information , visit www . cemcon-sa . org . za or phone 011 315 0300 or email info @ cemcocn-sa . org . za .
Concrete in mass housing
www . saaffordablehousing . co . za NOVEMBER - DECEMBER 2021 21