Last year we acquired the premises next door to us which
has been a great improvement for the smooth running of the
Although Jim Horton has retired, we have retained his
services in a consultative capacity. His expertise
particularly in the field of construction defects is being
passed onto Nathi Memela who joined the company early in
2011, and with the experience gained, now runs our field
WATER ADDITION TO CONCRETE
Water is a very necessary component of concrete as it allows
the chemical process of hydration of the cement to take
place. This causes the cement to react and harden thus
providing the concrete strength.
Typically, extra water is added to concrete to improve the
workability and therefore make it easier to move and
When designing a mix it is important to ensure the water :
cement ratio (W/C) is correct. The standard W/C ratio curve
of strength against W/C ratio for any cement shows how
strength increases with reducing W/C ratio, i.e. high
strength mixes have a low W/C ratio and lower strengths
higher W/C ratios.
From this it is obvious if extra water is added to the mix
the strength will be reduced and this will have consequences
if the design W/C ratio is exceeded.
An addition of 20 litres / m3 of concrete could reduce the
compressive strength by 5 to 10 MPa and a 10 litre addition
by up to 5MPa.
This is the main concern of uncontrolled addition of water
to concrete. There are however other dangers that the
should be aware of:
Excessive cracking due to plastic settlement
Weak wearing surfaces due to the presence of excessive
Increased drying shrinkage and creep which can lead to
long term cracking and deflection problems
Control of workability and therefore water is normally
carried out by means of the slump cone. (This is described
in SANS Standard method 862-1:1994)
Slump is not an exact test and also one cannot design
concrete to achieve the same slump for each batch
manufactured. The allowed variation is ± 25mm or
a third of the
target value, whichever is the greater.
DURABILITY INDEX TESTING
Testing of the potential durability of concrete using the 3
tests developed at the Universities of Cape Town and
Witwatersrand has become a common requirement for
particularly Civil Engineering structures such as bridges
The three tests are the oxygen permeability index (OPI), the
(WS) and chloride conductivity (CC). Details of the
tests and their development may be found in a series of
Monographs issued by the universities
as well as draft
standard SANS 5.16 - 1 to 4: 2006.
Simply, the OPI test measures the permeability of the
concrete which correlates well with the rate of carbonation
of concrete. High values of OPI lead to slow carbonation
front advancement whereas low values correlate to faster and
deeper carbonation occurring. The W/C ratio of the mix and
early age curing significantly affect the OPI value.
The water sorptivity measures the rate of uptake of water.
It also correlates fairly well with carbonation values and
is particularly sensitive to efficiency of early age curing.
Finally the chloride conductivity measures the ability of
the concrete to reduce chloride ingress. This is
particularly important for reinforced concrete exposed to
marine conditions or where chlorides are produced in
industrial environments. The chloride conductivity value is
most significantly affected by the cement type used,
although W/C ratio and curing will also impinge on this
If durability index testing is included in your
specification pay due attention to the requirements or run
the risk of reduced payments, the cost of remedial measures
or even demolition.
Contest can assist in ensuring that the right approach is
used in achieving your Durability Index targets.
Remember, two important factors;
1. Design of the mix
2. Early age curing