What is concrete?

OPC, Blended
cement( PFA,
GGBS), Silica Cement
fume

Coarse agg. Aggregate

Fine agg.
CONCRETE
water
Plasticizers
Accelerators
Retarders
Air Entrainers
Admixture

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Admixtures

Used to modify/improve the properties of fresh and

hardened concrete

Admixtures may be chemical or

mineral products.

classified by their function

in concrete

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Concrete admixtures by classification (ref Portland Cement Association)

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Admixtures can be categorize into four
categories according to their mechanism of
action

Water reducing
Dispersion of the cement in the aqueous phase of concrete High range water-reducing or
super plasticizing

Alteration of the normal rate of hydration of cement, in Retarders

particular the tricalcium silicate phase. Accelerators

Supplementary cementitious
Reaction with the by-products of hydrating cement, such as
materials like Fly ash, slag,
alkalis and calcium hydroxide.
Silica fume

No reaction with either the cement or its by-products. Air entraining agent
Dispersion of cement particle

Cement

Flocculation of
cement particles
Water

Water reducing Admixture

- - Admixture
- - molecules

- - - -
Dispersion of Cement
particles due electrostatic
- - - - polynaphthalene and polymelamine
sulphonate molecules work
repulsion
- - - essentially under this mode of
repulsion
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Application of water
reducing admixtures

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 Advantages of Water –Reducing Admixtures

 Increase the workability

 Achieve higher compressive strength

 Cement saving

Important : not all three benefits can be obtained at the same time

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 Superplasticizers

 Superplasticizers belong to a class of water reducing chemicals different

from the normal water reducers and capable of reducing water contents by
about 30%.

 The admixtures belonging to this class are known as high range water
reducers.

 They were first introduced in Japan in the late 60's and in Germany in
early 70's. In North America they were used from 1974.

 The advantages derived by the use of superplasticizers include production

of concrete having high workability for easy placement, and production of
high strength concrete with normal workability but with a low water
content.

 A mix having a combination of better than normal workability and lower

than normal amount of water, or that with less cement but having the
normal strength and workability are other possible applications.
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Types of superplasticizers

polysulfonates,
purified lignosulfonates (LS),
polynaphthalene sulfonate (PNS), and
polymelamine sulfonate (PMS)

new family of superplasticizers, known as

polycarboxylates or polyacrylates,

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 Slump

Water content

Superplasticizers : 25 to 30 % water reduction for a given consistency

normal plasticizer : 5 to 10 % water reduction
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 No drastic slump loss a short time
Compatible with Cement after good initial workability has
been obtained.
Superplasticizers
Small dosage range does not produce either a very
Robustness rapid slump loss or an unacceptable bleeding and
segregation

How do we evaluate
cement/superplasticizer
compatibility
and robustness?
Cement/superplasticizer interaction can be studied using
two simple comparative tests with cement paste

Mini-slump method yield stress

Marsh cone method plastic viscosity Flow diameter

is measured

Time taken
to flow the
cement
paste is
measured

Ref:Binders for Durable and Sustainable Concrete, Aïtcin 2008.

 A higher superplasticizer dosage does not significantly
improve the rheology of the grout; it begins to retard
cement hydration instead. A much higher dosage can
result in the entrainment of an excessive amount
of large air bubbles

As some polycarboxylates have a

strong natural tendency to entrain
air, a detraining agent has to be
added.

Air-detraining admixtures(deformers) destabilize air in

fresh concrete, allowing it to escape during mixing, thus
reducing the air content.
Ref:Binders for Durable and Sustainable Concrete, Aïtcin 2008.
 0.8% polynaphthalene sulfonate (PNS),
Typical hehaviour of Mini slump

Significant
drop in slump
flow
 Retarders
 These are admixtures which delay the setting of concrete, as measured by
the penetration test. Such admixtures are prescribed in ASTM C 494
and in BS EN 934.

 Retarders are useful when concreting in hot weather, when the normal
setting time is shortened by the higher temperature, and in preventing
the formation of cold joints between successive lifts.

 Generally, with a retarder, a delay in hardening also occurs - a property

which is useful to obtain an architectural surface finish of exposed
aggregate.

 Retardation action is exhibited by the addition of sugar, carbohydrate

derivatives, soluble zinc salts, soluble borates and others.

 In practice, retarders which are also water-reducing are commonly used in

ready-mixed concrete. 22
 Accelerators

 An accelerating admixture is a material that is added to concrete for

reducing the time of setting and accelerating early strength
development.

 Accelerating admixtures are used in precast industry and also are

components of shotcreting mixes.

 The advantages include: efficient start and finishing operations,

reducing the period of curing and protection, earlier removal of
forms so that the construction is ready for early service, plugging
of leaks, and quick setting when used in shotcreting operations.

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The most common accelerator is calcium chloride (CaCl2),
which accelerates primarily the early strength development of concrete.

This admixture is sometimes used when urgent repair work is required

because it increases the rate of heat development during the first few hours
after mixing. Calcium chloride probably acts as a catalyst in the hydration
of C3S and C2S.

The use of calcium chloride reduces the resistance of cement to sulphate

attack, particularly in lean mixes, and the risk of alkali-aggregate reaction is
increased for a reactive aggregate. Other undesirable effects are that the
addition of calcium chloride increases shrinkage and creep, and promote of
corrosion of embedded steel.

However, there is a beneficial effect in increasing the resistance of concrete

to erosion and abrasion.
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 Air-Entraining Admixtures

Air entrainment is the process whereby many small air bubbles are incorporated into
concrete and become part of the matrix that binds the aggregate together into the hardened
concrete.

This effect is accomplished by the use of an air-entraining agent

Air bubbles formed between

concrete particles by chemical air
entraining agent act as “shock
absorbers for dynamic stresses of
expansion and contraction due to
changes in environmental
temperature.

Air entrainment is regarded as essential for the durability of concrete that will become
wetted and then exposed to freeze-thaw conditions.

Other benefits of the air entraining admixtures include improvement of workability, and
reduction in bleeding. However it reduces the strength.
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 Mineral Admixtures

Industrial by-products Natural Pozzolanic Materials

 fly ash  volcanic ash

 slag
 silica fume

Unlike chemical admixtures, these are used in relatively large

amounts as replacement of cement and/or of fine aggregate in
concrete.

These admixtures are available in abundance at a much lower

cost compared to chemical admixtures.

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