A superplasticizer is a chemical admixture that helps to improve the flowability of concrete mixes without reducing cement content or adding water. They are primarily used in ready-mix concrete, precast concrete, and self-compacting concrete (SCC).
Most concrete superplasticizers reduce the water to cement ratio by increasing the amount of aggregate or using less water in the mix, which increases strength and durability. These admixtures can also improve the workability of concrete by improving its consistency and helping to reduce air holes or voids.
They may also retard setting time and help to increase the early compressive strength of concrete (50 to 75%). This is especially useful in the production of concrete for structural applications or under water concreting, where the set time is important for the optimum performance of the concrete.
However, superplasticizers also can cause bleeding or segregation if they are overdosed or the water content is too low for the desired consistency. In this case, viscosity modifying admixtures (VMAs) may be needed to correct the effect of excess superplasticizer or water.
These admixtures are usually high molecular weight polymers based on modified polysaccharides like hydroxyethyl cellulose or welan gums, or synthetic copolymers. They are generally soluble in water and adsorb onto cement particles.
This helps to increase the fluidity of the concrete and decrease the yield stress, which can be very beneficial in concrete where the breaking efficiency is very important. It also helps to prevent plastic shrinkage during curing and can make it easier for concrete to be placed, compacted, and finished quickly on site.
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