The Civil Engineering Contractor July 2019 | Page 26

FEATURE
Speciality concretes
Concrete is consistently tested in a laboratory.
that will bind the materials together
until the mix hardens. The strength
properties of the concrete are
inversely proportional to the water/
cement ratio: the more water you use
to mix the concrete the weaker the
concrete mix.
CoreSlab operational manager
Martin Potgieter operates a
batch plant at CoreSlab’s precast
manufacturing facility in Polokwane,
where there are a series of silos above
a skip where all materials are blended
before being decanted into the mixer.
Potgieter explains the importance of
moisture to the mix: “The washed
river sand, aggregate, chemicals and
water go into the skip and then into
the mixer, where we have a probe
which measures the moisture level
at all times and informs whether it
matches the calibrated mix. There are
parameters within which the precast
factory works, and the calibration
report forms the basis of acceptance
or rejection of a batch, or a basis on
which to make an informed decision.”
The plant is fully automated
and monitored continually by a
computerised system, enabling
Potgieter to refine the mix, including
24 | CEC July 2019
Innovation is ongoing: the addition
of a new silo the next month means
fly ash can be added to the mix
which reduces the cement and makes
provides a durable concrete mix that
is also ‘greener’. CoreSlab already
adds silica fume to the mix, “the
Rolls Royce of additives”, he says.
“This makes the micro-structure
more dense and impenetrable. It is
a process of continual development:
our aim is to keep up with European
trends, which are far advanced
compared to South Africa.”
For quality control purposes, if
there is an error there is a paper
trail for every mix which can explain
where it went wrong. “The more
quality assurance we can give the
engineers, the fewer questions they
will ask.”
the volumes of water and admixtures,
to achieve the set parameters. It
is a bespoke system “designed
by us for us”, says Potgieter. The
programmable logistics controller
controls the speed of mixing and
level of agitation — giving ultimate
control over everything that happens
inside the mix.
If the mix goes out of sync, it sends
an alert of three different escalations
of severity, the final one shutting
down the plant. The mix is fed into
the system, and the correct quantity
of raw material is then calculated,
decanted into the skip and verified.
The system also determines how long
the mix must be in the mixer —
by presenting a graph which shows
the mix is complete when there are
no longer variations in the graph,
meaning there is no room for human
error.
The additives are diluted with
water before being administered,
using an agricultural doser, says
Potgieter — his own innovation
“as the equipment requires minimal
maintenance”. Despite all efforts, it is
a dusty environment that is regularly
cleaned.
The construction of industrial
floors, such as warehousing and
factory floors calls for concrete with
characteristics entirely different from
that required for vertical structures,
says Bryan Perrie, managing director
of The Concrete Institute, which
monthly receives appeals for advice
regarding faulty flooring.
Perrie says that the properties
required of the concrete for flooring
are governed largely by using correct
materials and in correct proportions.
Material specifications for all the
ingredients of the concrete exist
— and should be adhered to —
as well as South African Codes of
Practice for the actual placement
of concrete and the finishing of
floor slabs. Perrie stresses that
consistency is crucial during all the
stages of the construction process:
receiving the concrete, discharging,
placing, consolidating and finishing.
The importance of protecting the
concrete while it is being placed,
and curing thereafter, are also vital
factors.
“An incomplete brief from the
client, lack of attention to correct
proportioning, handling
and
finishing, inexperienced contractors
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