Concrete admixtures

Concrete materials and concrete applications have changed and improved in the last 100 years. Whilst concrete admixtures have been around for many years, there has been an extensive amount of development of new admixtures in recent years. It is probable that almost all concrete used around the world today contains some types of admixtures. Admixtures added to the concrete can modify its properties on both the fresh and hardened stages, but are used primarily to modify the properties of fresh or plastic concrete.

Admixture s can be employed to entrain air for free-thaw resistance, to accelerate or retard setting time, to control strength development, to achieve shrinkage compensation and to improve workability. Most of unwanted effects of early admixtures have been overcome (such as the retarding effect of early plasticizers) so that modern admixture do not pacl “unexpected surprises”.

The most dramatic development have been in the area of superplasticizers or high range water reducers. These products now allow high level of water reduction without a loss in workability. Whilst early superplasticizers had a limited time over which they were effective, modern superplasticisers allow sufficient control for the setting in, for example, a large pour, until all the concrete has been placed.

Care should be taken when designing concrete mixes containing admixtures to assess sensitivity to changes of properties with changes in other ingredients and mix designs should be done on the specific materials that will be used.