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CONCRETE ADMIXTURES - PLASTICIZERS

There are essentially four ingredients in concrete, i.e. the coarse aggregates, fine aggregates, cement and water. However, in order to improve certain properties of concrete both in the plastic and hardened state, it is necessary to add the fifth ingredient in the concrete mix. This fifth ingredient can be a mineral or chemical admixture. 

If the concrete mix is properly designed, it does do not need any admixture. However, there may be a need to produce concrete with a particular result and using admixture could be the most convenient way. It is very important to note that any type of admixture should be used only when there is a valid reason for using it.

There are several types of admixtures available but the most popular and most often used is the plasticizer. The plasticizer is a chemical admixture and this publication covers in detail its classification, interaction, useful effects on concrete properties and precautions to be exercised in its usage. Plasticizers have been extensively used in concrete mixes for the past three decades the world over. However, in India this usage has considerably increased in the last five years. Plasticizers or other type of admixtures are not a substitute or badly designed concrete mixes or bad construction practices. They are aids to modify good mixes and good construction practices to achieve certain specific requirements which can be conveniently achieved. There are two types of plasticizers. The ordinary plasticizer is one type and the other is a superior plasticizer or superplasticizer.

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Concrete gets fluidised or attains high workability when plasticizers or superplasticizers are used. To achieve a homogeneous distribution of water and optimal utilisation of the cement particles, water get dispersed and flocculation is prevented due to the following effects :

Reduction of surface tension of water and lubrication of the film between the cement particles .
The induced electrostatic repulsion due to absorption of charged polymer on the cement particles causes dispersion of cement grains
and releases the trapped water from the cement floes.
Steric hindrance prevents particle-to-particle contact.
Change in morphology of hydration product.
Retardation due to the inhibition of the hydration reaction of cement particles leavs more water to plasticize the mix.

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Superplasticizers are clasified based on their chemical composition:

Sulphonated melamine - Formaldehyde condensates

Sulphonated napthalene - Formaldehyde condensates

Modified lignosulphonates

Carboxylated acrylic esters copolymers

Others like sulphuric-acid esters, amide polysaccharide mixtures, carbohydrate esters etc.

The Sulphonated melamine based plasticizers are generally preferable if ambient temperatures are low, while sulphonated napthalene based plasticizers are preferable in high ambient temperatures due to their good workability retention quality. Therefore they are more suitable for Indian conditions.

Lignosulphonates at times, also give air entrainment and set retardation in concrete. Polycarboxylate admixtures are high performance plasticizersJ_.., which also help in slump (workability) retention for a longer period.