The role that the
various mix constituents play to produce quality concrete for housing is often
not fully understood resulting in a series of costly errors, Bryan Perrie,
managing director 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. This is particularly important at present when tons of
imported cement of questionable quality are arriving at our ports. 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 the required South African standards,” he explains. The
validity of a LoA can be checked with NRCS.
Perrie says five errors
in particular tend to occur when producing concrete for housing:
- Not realising that 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 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. It should also be noted that two bags of 50 kg cement is the
equivalent to 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 of time. Simply adding more and more water significantly reduces the
strength of the concrete; - Concrete is often 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 cooler 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 sand-cement screed to the
finished concrete floor. In general, 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 and
downloadable from its website www.theconcreteinstitute.org.za; - Cracks in plaster and floors is a very common
problem on most sites – a problem that can be avoided or reduced through the
use of the correct type of 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’. This publication, as well as
several other specialised information leaflets on these issues can also be
obtained directly from the Institute. The Concrete Institute’s School of
Concrete Technology also presents a variety of educational courses on concrete
for all levels of experience.
More information from
Tel: +27(0)11 315 0300 / www.theconcreteinstitute.org.za
