Uses of Plasticisers And Superplasticisers

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Plasticisers and superplasticisers can be used to improve workability, reduce the w/c ratio,or decrease the cementitious content without reducing strength.Plasticisers increase the workability of concrete, and are also known as water reducers because they reduce the water content needed for a given workability. Superplasticisers are more powerful, and will give greater increases in workability, but the term “plasticiser” may be taken to include all of them. Significant advances in the technology have meant that there are now large numbers of different plasticisers available, some of which will make the concrete free-flowing. They work by adsorbing onto the cement grains, and dispersing them, so they do not coalesce and inhibit the flow.

The control mix is in the centre of the table, and is a typical concrete with 75 mm slump, and a strength of 35 MPa.The mix at the top left shows the effect of adding water. The slump is increased, but the water/cement (w/c) ratio is also increased, so the strength is decreased.

Mix in the bottom left, where more cement is added to give the original w/c. However, the higher cement content will increase the cost, and may lead to problems with overheating or cracking.The mixes in the right-hand column have added plasticiser.

The flowing mix is at the top right. This mix has the same slump as the mix with added water, but it has no increase in water, so the strength is not reduced. This mix will cost less to place, due to the reduced requirement for labour to move it into position and compact it.

The high strength mix is in the middle row on the right. By adding plasticiser and reducing the water, it is possible to increase the strength, without loss of workability. Very high strengths can be achieved with these mixes.

The low cost mix is at the bottom right. By adding plasticiser and reducing both the cement and water contents, it is possible to have the same workability and strength with less cement.

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