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Admixtures of Concrete (Concrete Technology)

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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 superplasticizerrs

Plasticizers are used to improve workability while keeping the water cement ratio constant or in other words say plasticizers improve workability without increasing the water cement ratio. They are also called water reducers.
The most commonly used plasticizer is Styrene Butadiene Rubber (SBR).
Superplasticizers are high range water reducers. They are more advance than plasticizers.
Here are some important types of superplasticizers.

  1. Polycarboxylate ester
  2. Modified lignosulphonates
  3. Cross linked acrylic polymer

Set retarders

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

Set accelerators are used to increase the rate of hydration. They have very wide range of application as explained below.

  1. It permits earlier removal of form work
  2. Reduce the require period of curing
  3. Advance the time that a structure can be placed in service
  4. Set accelerators are highly used in emergency repair work

The most commonly used material, as set accelerators, till the 20th century was Calcium Chloride, but now with advancement in technology its use has become obsolete.
Some soluble carbonates are used as set accelerators.

Air-entraining admixtures

One of the most important evolution in concrete technology is the 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

Air-Entraining agents

The following types of air entraining agents are used in 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, and blast furnace slag are commonly used pozzolanic admixtures.
They directly do not contribute to strength but affect 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

Further reading
Concrete Repair Products By Sika In UK (United Kingdom)
Shape tests on coarse aggregates
Ready mix concrete advantages, disadvantages and other aspects

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