Concrete admixtures are chemicals or minerals that modifies the properties of concrete. Concrete admixtures have very wide range of application and they come under various brand names. Most commonly used admixtures are listed and explained below.
- Plasticizers and Superplasticizers
- Set retarders
- Set accelerators
- Air entraining admixtures
- Pozzolanic or Mineral Admixtures
Plasticizers and superplasticizerrs
Plasticizers are used to improve workability while keeping water cement ratio constant or in other words say plasticizers improve workability without increasing water cement ratio. They are also called water reducers.
Most commonly used plasticizer is Styrene Butadiene Rubber (SBR).
Superplasticizers are high range water reducers. They are more advance than plasticizer.
Here are some important types of superplasticizers.
- Polycarboxylate ester
- Modified lignosulphonates
- Cross linked acrylic polymer
These are used to reduce the rate of hydration of cement. Set retarders are most commonly used with ready mix concrete. Common sugar is one of the most commonly used set retarder. Other admixtures used as set retarder are Ligno-sulphonic acids and their salts.
Set accelerators are used to increase the rate of hydration. They have very wide range of application as explained below.
- It permits earlier removal of form work
- Reduce the require period of curing
- Advance the time that a structure can be placed in service
- Set accelerators are highly used in emergency repair work
Most commonly used material as set accelerator, till 20th century was Calcium Chloride, but now with advancement in technology its use has become obsolete.
At present, some soluble carbonates are used as set accelerators.
One of the most important evolution in concrete technology is invention of air entraining admixtures. There are two types of air voids in the concrete
- Entrapped air (not a desirable one)
- Entrained air It is minute spherical bubbles introduced intentionally ranging in size from 5 microns to 80 microns
The following types of air entraining agents are used at today’s age
- Natural wood resins
- Animal and vegetable fats and oils
- Water soluble soaps of resin acids
The major alteration, air entraining admixtures made to the properties of concrete are-
- Reduction of strength
- Improved workability
- Very high resistance to frost action
Pozzolanic or mineral admixtures
These are mainly used to improve durability and decrease permeability. Silica fume, Fly ash, Rice husk ash, Surkhi, blast furnace slag are commonly used pozzolanic admixtures.
They directly do not contribute to strength but affects permeability.
Other interesting facts about pozzolanic admixtures are
- It reduces the alkali aggregate reaction
- Lower costs of concrete for the same strength
- Reduces the alkali aggregate reaction