Admixtures are chemicals which are added to concrete at the mixing stage to modify some of the properties of the mix. Admixtures should never be regarded as a substitute for good mix design, good workmanship, or use of good materials.
Types of admixtures
Admixtures are normally categorized according to their effect:
- Plasticizers (water-reducing agents)
- Superplasticizers (high range water reducers)
- Air entrainers
- Accelerators
- Retarders
- Others
Many admixtures provide combinations of properties such as plasticizer/retarders or plasticizer/air entrainers.
Uses of admixtures
The most common reasons for using admixtures in concrete are:
- To increase workability without changing water content.
- To reduce water content without changing workability.
- To effect a combination of the above.
- To adjust setting time.
- To reduce segregation and/or bleeding.
- To improve pumpability.
- To accelerate the rate of strength development at early ages.
- To increase strength.
- To improve potential durability and reduce permeability.
- To reduce the total cost of the materials used in the concrete.
- To compensate for poor aggregate properties.
