Use and importance of Plasticizers and Super-plasticizers in concrete
In the
situation such as in concrete pumping, ready-made concrete industry, tremie
concreting, concreting in deep beams etc, high degree of workability in grading
or use of high percentage can be increased by the improvement in grading or use
of high percentage of fine aggregates or increasing cement content or others
but it is very difficult to apply in the field. Extra water can be used to
increase workability but it causes harm to the strength and durability of
concrete. Hence, nowadays, it is very popular to use admixtures named
plasticizers and super-plasticizers (available in various brand names) to
reduce water requirement and to make concrete strong and workable.
Plasticizers
are the organic or combination of organic and inorganic substances which
reduces water content for certain degree of workability, when added in mix. The
basic products consisting plasticizers are anionic surfactants (such as
lignosulphonates, salts of sulphonates hydrocarbon) nonionic surfactants (such
as polyglycol esters, hydroxylated carboxylic acid products) and others such as
carbohydrates, etc. among them calcium, sodium and ammonium lignosulphonates
are commonly used. Plasticizers are mixed from 0.1% to 0.4% by weight of cement
used and it reduces 5% to 15% of water with the increment of workability from 3
to 8cm slump. In the mix, the cement grains absorbs the plasticizers molecules
and results change in the surface charge of the same sign which causes
repulsive forces and makes the dispersion which increases plasticity and
workability. Some Plasticizer also entrails the air but a good plasticizer is
that which entrains air less than 2% only. The plasticizers are available in
market in various brands with specifications for composition, dosages etc.
However,
the super-plasticizers are the improvement of plasticizers. It increases
workability at same water cement ratio (w/c) and decreases w/c ratio at the
same workability level. The fluidizing property remains longer due to the
retarding property on cement hydration. Hence it is possible to obtain so
called “flowing concrete or self leveling concrete”, which is pumpable or
requires very little effort in the compaction. In the concrete with
super-plasticizer segregation and bleeding are nearly absent. In other words,
use of super-plasticizers reduces much more segregations and bleedings than any
normal plasticizers. Hence, super-plasticizers are most effective mix
ingredient for concrete. The super-plasticizers are normally grouped as
sulphonated melamine, Naphthalene sulphonate, modified sulphonates and others.
The super-plasticizers are also available in market in various brands with
specifications for composition, dosages etc.
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