The Civil Engineering Contractor July 2019 | Page 27

FEATURE
Bryan Perrie, managing director of The
Concrete Institute.
Silos decant the washed river sand, aggregate, silica fume, fly ash, cement and water
into the mixer.
and operators — these could all
contribute to a floor with poor
durability, strength and aesthetics.
The flooring process relies on the
successful completion of successive
phases — understanding what is
being done in each phase, using the
right concrete and equipment, by
the right person at the right time,”
he adds.
Weather in the concrete industry
is regarded as cold when the ambient
temperature falls below 5°C. The
effect of concrete freezing at early
ages depends on whether the concrete
has set, and what strength the
concrete had attained when freezing
took place. If concrete which has
not yet set is allowed to freeze, an
increase in the overall volume of the
concrete occurs due to the expansion
of water, especially in the capillary
pores. When thawing takes place, the
concrete will set with an enlarged
volume of pores which will reduce
strength and durability.
If freezing takes place after the
concrete has set, but before it has
gained sufficient strength (about
3‒5MPa), expansion associated
with the formation of ice will cause
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disruption of the microstructure
and irreparable loss of strength and
durability.
When the concrete has achieved
a compressive strength of at least
3‒5MPa, it can resist a freezing cycle
without damage because it has a
higher resistance to the pressure of ice
and because a large part of the mixing
water will either have combined with
the cement or will be located in gel
pores and therefore unable to freeze.
Perrie says that because of their
slower setting and rate of strength
gain, the use of highly extended
cements or the partial replacement
of CEM I cement with significant
amounts of either ground-
granulated blast furnace slap or fly
ash is not recommended. It may be
advantageous to use CEM I 42, 5R or
52.5N cement in preference to 42.5N
or 32.5 cements.
“Water in aggregate may be
prevented from freezing by covering
stockpiles with tarpaulins. If aggregates
are likely to become frozen or contain
ice and snow, they may have to be
heated with steam injection or hot air
blowers. When using steam heating,
adequate draining must be provided.
CoreSlab operational manager, Martin
Potgieter.
Typically, the aggregate should be
heated to between 10 and 20°C. All
water pipes must be adequately lagged
to prevent supply pipes from freezing,
or even bursting.
“The most common and easiest
way to heat concrete is to heat
the mixing water but care must be
taken not to exceed 60‒70°C. At
higher temperatures, flash setting of
the cement and reduced workability
may occur.
“Finally, the main requirement in
cold weather concreting is to prevent
heat loss of the freshly placed concrete.
So, under no circumstances should
water-curing methods be used. Heat
may be retained by using insulated
forms, covering exposed surfaces with
insulating materials, or erecting covers
with internal heating. Combustion
type heating under covers should be
avoided,” says Perrie. nn
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