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26
CONSTRUCTION WORLD
JUNE
2016
PROJECTS AND CONTRACTS
Just as important as understanding the concrete pumping
process is the role played by admixtures in controlling the way
in which concrete behaves frommixing to final curing.
Eddie Correia, executive vice president at Chryso Southern
Africa, says that without the use of admixtures pumping concrete would
not be an economically viable construction methodology.
“Without admixtures the concrete mix would have to be modified
to make it pumpable and this could include altering the sand/aggregate
ratio or even the water content. This is most undesirable as it could
adversely affect the concrete’s physical characteristics,” Correia says.
“The key to the overall success of concrete pumping is all about
getting the right mix in terms of paste, paste volume and admix-
ture selection, and this is where it is important to secure access to a
company with the necessary level of applications knowledge to ensure
that the most appropriate admixture is selected for the mix design and
the construction project,” Correia says.
One of the most important objectives that will ensure successful
pumping is to achieve a dense, cohesive concrete mix with sufficient
paste volume. This will enable the correct level of workability of the
concrete. Good aggregate coating is essential with the primary goal
to attain a high level of plasticity which will allow the material to be
moulded and shaped to hold the required form, thereby facilitating the
pumping operation.
Correia says that even where the available aggregates are not of a
high standard it is possible through the introduction of an appropriate
admixture to produce medium to high workability of the concrete. An
example would be use of Chryso’s Quad 20 which enables greater cohe-
sion and is not sensitive to mix designs. Where admixtures are sensitive
to mix designs the risk of material segregation is ever present.
“Segregation can be a serious risk and must be guarded against at all
costs. Where material segregates and the water separates out of the mix
– in other words where water washes out of the paste – there is a strong
possibility of blockages occurring in the pipeline,” Correia explains.
Superplasticisers from the Chryso® Fluid range will assist in creating
a favourable water/cement ratio by reducing the amount of water, and
this also prevents shrinking, cracking and porous concrete.
Significantly, Chryso® has developed Fill Free® technology which assists
contractors to produce concrete that is cohesive and yet still has a
moderate viscosity. This modified PCE-technology was developed to have
non-thixotropic properties and makes use of Chryso® superplasticisers.
Superplasticisers, otherwise known as water reducing agents,
enable good aggregate coating by dispersing the cement grains thereby
minimising agglomeration of the mix and increasing plasticity and
therefore workability of the concrete.
Pumping pressure and material grading
Other factors that could contribute to segregation are pumping pressure
and material grading. If the pumping pressure used is too high excessive
pressure is placed on the mix and this could force segregation of the
concrete during pumping. Likewise if the grading of the aggregates is
incorrect, they will be pushed out during pumping and this will result in
blockages in the pump and/or pipeline.
Correia explains that when being pumped concrete moves in the
form of a cylinder and is separated from the pipeline wall by a lubri-
cating layer made up of water, cement and fine aggregate or sand. It is
essential that this lubricating film is achieved on all sides of the pipe
and that the requisite workability is attained so the concrete can be
pushed or transported through this channel.
Chryso® Fill Free® technology facilitates a cohesive concrete paste
which is less sticky. This is important as stickiness can produce lower
cleaning efforts in the pipeline and also increase the resistance of the
flow of the concrete in the pipeline. A paste that is too sticky will also
make it difficult to maintain a constant rate of pumping.
“Again, this is where superplasticisers are invaluable is achieving
the plasticity and workability needed for the concrete paste to pass
easily through reducers and move through bends in the pump and pipe-
line without causing blockages,” he says.
Paste volume
Typically paste volume is determined by the aggregate type and shape as
well as the grading curves of the mix. In addition to this, both the water/
cement and water/binder ratios of the mix design and appropriate admix-
ture selection play critical roles. Correia cautions that simply changing
the ratio of components in a mix design is not the answer.
One of the ways to increase the volume of the paste is to increase
the air content in the mix through the use of an air-entraining admix-
ture. This will allow small stable bubbles to be developed in the paste
and enable sand particles to move by each other more freely, increasing
the plasticity of the mix without the need to add additional water.
Caution must be used, however, in the appropriate selection of
the air-entraining admixture. The introduction of too much air into
the mix can create a ‘concertina effect’ in the pipes that will affect
pumping distances.
Contruction sites
PUMPING
with
ADMIXTU
Pumping concrete is perhaps one of
the most challenging aspects within
the construction environment, and
with the increased move towards taller
structures, it is one which contractors
need to understand.
>
Without the use of admixtures pumping, concrete would
not be an economically viable construction methodology.
Chryso® Fluid Optima 100 was used on the Impala Platinum No 16 shaft to
give the mix extended workability without affecting the setting time.