admixture for concrete

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,ADMIXTURE FOR CONCRETE

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Admixture can be defined as materials other than cement, aggregates and water which are added

in small quantity during mixing to produce one or more modifications in the properties of the

concrete or mortar. These admixtures should not have any adverse effect on concrete. It may be

of different varieties and may produce different changes in the physical parameters of fresh and

hardened concrete. These admixtures may have combined effect also.

CLASSIFICATION OF ADMIXTURE

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The classification of concrete admixture is generally done with respect to their individual effect on

concrete. As per INDIAN CODE IS:9103,concrete admixture has been classified into four types,

whereas AMERICAN STANDARD INSTITUTE ASTM C 494,the total admixtures have been classified

into eight types. The BRITISH STANDARD (BS : 5075) has classified it into five types. As per IS:9103,

four types are accelerating, retarding, water-reducing ,and air-entraining types admixtures. As perASTMC-494, the classifications are : Plasticiser, retarder, accelerator, plasticiser and retarder,

plasticiser and accelerator, super plasticiser, super plasticiser and retarder, super plasticiser and

accelerator. ASTM C 260 gives the details about the air-entraining types of admixture. According

to BS:5075 (3 parts) the classifications are :accelerating, retarding, normal water reducing,

accelerating & water reducing ,and retarding & water-reducing types. Apart from the types of

admixtures mentioned above , ACI Committee 212 specifies a number of miscellaneous

admixtures such as :-

1. Gas forming and grouting compound,

2. Expansion producing compound,

3. Bonding agents

4. Pumping aids and pigments.

5. Flocculating compounds,

6. Corrosion inhabiting agents,

7. Fungicidal, germicidal &insecticidal agents,

8. Damp proofing and permeability reducing agents,

9. Chemical agents to reduce alkali-aggregate reaction.

Now a brief description of the important admixtures is being given below:-

1. Accelerating Admixture:- A substance, when added to concrete, mortar or grout, increases the

rate of hydration of hydraulic cement, shortens the time of set, or increases the rate of hardening

or strength development. The category of admixture is based on two major row materials,

calcium chloride and calcium format, with minor amount of other materials occasionally

being included in the formulation such as calcium nitrate ,calcium thiosulphate, and

triethanolmine. IS 7861(part II)-1981(4) specifies the use of calcium chloride in cold weather

concrete construction jobs, but chlorides coming into concrete from cement, aggregate, water

and admixture should be limited to 0.15 percent by mass of cement. They should not be used


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in pre-stressed concrete because of the potential danger of augmented stress corrosion. Where

sulphate resisting cement is required, calcium chloride should not be used, as it may induce

corrosion of reinforced steel. Other accelerating admixtures are calcium format, high

alumina cement, sodium sulphate, potassium sulphate, trivalent iron salts, sodium hydroxide

, and potassium hydroxide, sodium fluoride, aluminium chloride, potassium carbonate, sodium

aluminate. Each admixture compound has its specific qualities. So before its use, one hasto satisfy their requirement.

2. Retarding admixture:- This admixture retards the setting time of cement paste , mortar or

concrete. Retarders are used to overcome the accelerating effect of high temperature on the

setting properties of concrete in hot weather concreting. The use is recommended in IS : 7861

(PART I) 1975(5).The materials for this type of admixture are high sugar lignosulphate

,hydroxy carboxylic acid and hydroxylated polymer. Common sugar is one of the most

effective retarder. Generally this type of admixture increases the workability of

concrete, thereby reducing water-demand. When retarders are used at normal dosage level,

to obtain set retardation and higher workability for a given concrete mix, there is no

necessity in the original mix design.

3.Water-reducing admixture - This type of admixture either increases workability of freshly mixed

mortar or concrete without increasing water content ,or maintains workability with a reduced

amount of water. The chemicals used for this type of admixtures are lignosulphate

hydroxyl-carboxylic acid and hydroxylated polymer, water reducing agents act also as workability

aid & reducing cement content.

4. Air -entraining admixture - This admixture causes air to be incorporated in the form of minute

bubble in the concrete or mortar during mixing, used to increase workability, and resistance to

freezing ,thawing and the disruptive action of de-icing salts. The raw materials, for this type ofadmixture are abietic and pymeric acid -salts, fatty acid salts, alkyl-aryl sulphonate, alkyl sulphate,

phenol ethoxilates etc. Bleeding and segregation are also reduced by it ,but in case of high cement

content, the concrete becomes sticky and difficult to finish. This admixture usually reduces the

strength of concrete to some extent (usually 4 to 5 %)

5. Super plasticiser - These are high-range water-reducer .Chemically they are sulphonated

melamine formaldehyde condensate and sulphonated naphthalene formaldehyde condensate. At

a given w/c ratio ,the addition of a super plasticiser increases the workability of

concrete, typically by raising the slump from 75 mm to 200 mm; the mix remaining

adhesive. The resulting concrete can be placed with little or no compaction and isnot subject to excessive bleeding or segregation. Such concrete is termed as

'flowing concrete' and is useful for placing in very heavily reinforced section, and also where very

rapid placing is desired. It is also used for normal workability but with an extremely high strength

owing to a substantial reduction in the w/c ratio. Its addition in cement concrete gives a reduction

of upto 28 % in mixing water.

Gas forming admixtures (usually hydrogen peroxide), by chemical reaction with

cement, produce slight expansion of concrete and is useful for bonding to steel and grouting

purpose. It results light weight concrete.

Damp proofing /water proofing /permeability reducing agents are used when concrete are to bemade impervious to water under pressure. Chemically they are calcium soaps resin, vegetable oil,

fats, waxes, coal tar residue, bitumen, ammonium stearate or oleate, butyl stearate, soda or

potash soaps etc.


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Corrosion inhibitors are nitrates, benzoids, thioslphate, stannous salts and ferrous salts. Synthetic

polyelectrolytes are used as flocculating agents.

The addition of admixture in concrete provides many

benefits. However it is certain that admixture is not a cure for poor quality concrete .The benefits

of admixture can be obtained only with correct choice of concrete materials, good concrete mix

design, proper batching, placing and compaction followed by through curing. The performance ofadmixture must be assessed with the same materials which are proposed to be used in the

construction. This would help in selection of the most suitable type of admixture with the lowest

possible dosage for achieving the desired parameters at the end. The aggressive chemical content

in the admixture must also be checked so that the same does not have any adverse effect in the

long run.

References :

1. Neville A.M.Properties of concrete, third edition ELBS, England.

2. Ganguly, K.K. and Ray ArunachalConcrete admixture types, classification & their use, CE

& CR, May91

3.

Prof. S. KrishnamurthiClass notes of M. Tech ,I.I.T./DELHI

4.

Kaushal KishoreConcrete Mix Design containing chemical admixtureC.E. & CR MAY 91

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Er. Krishn Murti Jha

A.E., Dam & Gate Design Division

No. 03, Anisabad, Patna.


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admixture for concrete

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