What is Admixture?
It is an optional ingredient of concrete which is added to modify the properties of fresh and hard concrete and grout materials as per some specific requirements. Addition of admixture may alter workability, pumping qualities, strength development, appearance etc in fresh concrete and permeability, strength, durability etc in hardened concrete. But use of chemical admixture is a must for producing high grade concrete.
What are Categories of Chemical Admixtures?
Various categories of chemical admixtures are available in the market as given below.
- Water reducing admixtures
- Retarding admixtures
- Air entraining admixtures
- Accelerating admixtures
Water Reducing Admixtures
These admixtures reduce the requirement of water for a given workability. For full hydration a water-cement ratio of 0.23 is sufficient but generally much higher water-cement ratio is adopted due to the requirement of workability. Workability is an equally important design parameter in addition to strength because inadequate workability leads to honeycombing and non uniform strength. Excess water is used to overcome the internal friction between solid particles of concrete and facilitate mixing, placing, transportation and compaction of concrete. There are two categories of water reducing admixtures.
- Plasticizers
- Super-plasticizers
Super plasticizers are improvised version of conventional plasticizers. They reduce water requirement significantly. These are, therefore, also called ‘High range water reducers’. Plasticizers reduce water requirement up to 15% whereas super plasticizers can reduce this requirement even up to 30%.
There are four types of super plasticizers which are generally used for concrete as given below.
- Sulphonated melamine – It is suitable in low temperature areas
- Sulphonated naphthalene – It is more suitable in high temperature areas
- Ligno sulphates – It is suitable for Indian conditions where temperature variation is high
- Carboxylated admixture – It is suitable where workability is required to be retained for large duration.
Retarding Admixtures
These admixtures are used where setting time of concrete need to be delayed. Retarder delays the hydration process but doesn’t affect the eventual process. Initial setting time can be delayed by more than 3 hours. The main application of retarding admixtures is in eliminating the cold joints and controlling the setting time of concrete.
Air Entraining Admixtures
These are also surface active agents that form stable air bubbles of very small size ranging from 5 micron to 80 micron. The main function of air bubbles is to break capillary structure within the concrete and to act as roller ball bearings so that the particles in the mix move freely against each other, thus improving the workability of concrete without adding more water.
Accelerating Admixtures
These are used to accelerate the setting time and hardening process of cement in concrete. These are used in concreting under flowing water and in road repair works so that work can be put to use as earliest as possible.
Among all above admixtures, water reducing agents or super plasticizers are the most commonly used admixture for high performance concrete.