Admixtures is defined as a material, other than,
That is used as an ingredient of concrete and is added
to the batch immediately before or during mixing.
Admixtures are used to modify the properties of
ordinary concrete so as to make it more suitable for any
History of admixtures is as old as the history
of concrete. It embraces a very vast field,
but a few type of admixtures called Water
Reducers or High Range Water Reducers,
generally referred as plasticizers and
superplasticizers, are of recent interest.
Specially developed in Japan & Germany
Later on they were made popular in USA and
Europe even in middle East and Far
East.Unfortunately, the use of Plasticizers
and Superplasticizers have not become
popular in India till recently (1985).
To modify properties of fresh & hardened
To ensure the quality of concrete during
mixing, transporting, placing & curing
To overcome certain unexpected
emergencies during concrete operations (ie,
Increase slump and workability
Retard or accelerate initial setting
Reduce or prevent shrinkage
Modify the rate or capacity for bleeding
Retard or reduce heat evolution during early
Increase strength (compressive, tensile, or
Decrease permeability of concrete
Increase bond of concrete to steel
Increase bond between existing and new
Improve impact and abrasion resistance
Inhibit corrosion of embedded metal;
Produce colored concrete.
Most admixtures are supplied in a ready-to-
use form and added at plant or jobsite.
Pigments and pumping aids are batched by
hand in VERY small amounts
According to characteristics effects
produced by them chemical admixtures used
are as follows.
Retarders and retarding admixtures
Accelerators and Accelerating Plasticizers
Air – detraining admixtures
Gas – forming admixtures
Corrosion inhibiting admixtures
Shrinkage reducing admixtures
Water or damp proofing and permeability
These admixtures are used for following
To achieve a higher strength by decreasing the
water cement ratio at the same workability as
an admixture free mix.
To achieve the same workability by decreasing
the cement content so as to reduce the heat of
hydration in mass concrete.
To increase the workability so as to ease placing
in accessible locations
Water reduction more than 5% but less than 12%
The commonly used admixtures are Ligno-
sulphonates and hydrocarbolic acid salts.
Plasticizers are usually based on lignosulphonate,
which is a natural polymer, derived from wood
processing in the paper industry.
Portland cement being in fine state will have a
tendency to flocculate in wet concrete, these
flocculation entraps certain amount of water
used in the mix.
When its used, they get adsorbed on the cement
particles,creates particle to particle repulsive
forces which overcome the attractive forces.
This repulsive force is called Zeta Potential,
which depends on the base, solid content,
quantity of plasticizer used.
When cement particles are deflocculated, the
water trapped inside the flocs gets released &
now available to fluidify the mix.
The agents are organic by nature, thus they
lubricate the mix reducing the friction and increasing
Plasticizers get adsorbed on the surface of the
cement particles and form a thin sheath.
This sheath inhibits the surface hydration
reaction between water and cement as long as
sufficient plasticizers molecules are available.
Quantity of plasticizers will decrease as the
polymers become entrapped in hydration
The following mechanisms may take place
Reduction in the surface tension of water.
Induced electrostatic repulsion between particles of
Lubricating film between cement particles.
Dispersion of cement grains, releasing water trapped
within cement flocs.
Inhibition of the surface hydration reaction of the
cement particles, leaving more water to fluidify
Its developed in Japan & Germany during
1960 & 1970.
They are chemically different from
Reduce water requirement by 12-30%
The mechanism is more or less same as
With 0.3 per cent superplasticizer
They flowing concrete with very high slump
in the range of 7-9 inches (175-225 mm) to
be used in heavily reinforced structures
Significant water reduction
Reduced cement contents
Reduced effort required for placement
More effective use of cement
More rapid rate of early strength development;
Increased long-term strength; &
Additional admixture cost (the concrete in-
place cost may be reduced)
Slump loss greater than conventional concrete
Modification of air-entraining admixture dosage
Less responsive with some cement
Mild discoloration of light-colored concrete
Copolymer of carboxylic acrylic acid
with acrylic ester (CAE)
Cross linked acrylic polymer (CLAP)
Polycarboxylate ester (PC)
Multicarboxylate ethers (MCE)
Combination of above
Retarding admixtures delay hydration of cement.
To delay or extend the setting time of cement
paste in concrete.
These are helpful for concrete that has to be
transported to long distance, and helpful in placing
the concrete at high temperatures.
Used in casting and consolidating large number
of pours without the formation of cold joints
Used in grouting oil wells.
Commonly known retarder is Calcium sulphate.
Appropriate amount of gypsum to be used must
be determined carefully.
Use of gypsum is recommended when adequate
inspection and control is available.
Otherwise addition of excess amount may cause
undesirable expansion and indefinite delay in
setting of concrete.
Many a time this extent of retardation of
setting time offered by natures will not be
sufficient. Instead of adding retarders
separately, retarders are mixed with
plasticizers or super plasticizers at the time
of commercial production.
Such commercial brand is known as
retarding plasticizers or retarding
Retarding plasticizers or superplastizers are
important category of admixtures.
Used in Ready mixed concrete Industry for
the purposes of retaining the slump loss,
during high temperature, long transportation,
to avoid construction or cold joints, slip form
construction regulation of heat of
Accelerating admixtures are added to
concrete to shorten the setting time &
increase the rate of hardening & strength
development of concrete.
Its increase in the rate of hydration of the
These are used in cold weather conditions
(below 5˚C or 41˚F)
Some widely used and effective chemicals
that accelerate the rate of hardening of concrete
nitrites, nitrates,& formates.
Improved protection against early exposure
to freezing and thawing,
Earlier use of structure,
Reduction of protection time to achieve a
Early removal of form, and
In some case, early load application.
Certain Ingredients are added to accelerate the
strength development of concrete to plasticizers or
super plasticizers. Such accelerating super
plasticizers, when added to concrete result in faster
development of strength
The accelerating materials added to plasticizer or
superplasticizers are triethenolamine chlorides,
calcium nutrite, nitrates and flousillcates etc.,
The accelerating plasticizers or accelerating super
plasticizers manufactured by well known companies
are chloride free
Microscopic air bubbles intentionally incorporated in
mortar or concrete during mixing, usually by use of a
surface-active agent; typically between 10 &1000 μm in
diameter and spherical or nearly so.
♫ It is the process whereby many small air bubbles are
incorporated into concrete and become part of the
matrix that binds the aggregate together in the
♫ These air bubbles are dispersed throughout the hardened
The air voids present in concrete can be
brought under two groups:
a) Entrained air
b) Entrapped air
Natural wood resins
Animal and vegetable fats and oils such as
tallow, olive oil and their fatty acids such as
stearic and oleic acids
Various wetting agents such as alkali salts or
sulphonated organic compounds
Water soluble soaps of resins acid
Miscellaneous materials such as sodium salts of
petroleum sulphonic acids, hydrogen peroxide
and aluminium powder, etc
Type and quantity of air entraining agents used
Water cement ratio of mix
Type and grading of aggregates
Type of cement
Influence of compaction
Admixtures other than air entraining agents used
Air entrainment will effect directly the
following three properties of concrete
Increased resistance to freezing and
Improvement in workability
Reduction in strength
Incidentally Air entrainment will affect
the properties in following ways:
Reduces the tendencies of segregation
Reduces the bleeding and laitance
Decreases the permeability
Increases the resistance to chemical attack
Permits reduction in sand content, water
content, cost, & heat of hydration
Reduces unit weight, alkali aggregate reaction,
the modulus of elasticity
Plasticizers, superplasticizers, & Air-
entraining admixtures is mostly used.
Superplasticizers is widely used all over the
India is catching up with the use of
superplasticizer in the construction of high
rise buildings, long span brides
Because It reduced cement content,
increased workability, & increased strength.