What is Admixture/Classification of Admixture/Use of Admixture.

 Admixture

Admixtures are the ingredients other than basic ingredients cement, water and aggregates that are added to concrete batch immediately before or during mixing to modify one or more of the specific properties of concrete in fresh and hardened state. It is added in small quantity either in powder form or liquid form. Combination is added when more than one property to be altered.

                                                Purpose

1. To modify fresh property

• Increase the workability without increasing the water cement ratio or decrease the water content at the same workability.
• Retard or accelerate initial setting time.
• Reduce or prevent the settlement or create slight expansion. Modify the rate of capacity of bleeding.

2. To Modify hardened Property

• To reduce the heat of evolution.
• Accelerates the rate of strength development at early stages.
• Increase the durability.
• Decrease the permeability of concrete.

3. To ensure the quality of concrete during mixing, transporting, placing and curing.

Classification of Admixtures

1. Chemical Admixture:

These are the materials in the form of powder or fluids that are added to the concrete to give certain characteristics. Dose: less than 5% added at the time of mixing.

1. Plasticizers and Super Plasticizers

Increase workability of fresh concrete by reducing water content by 12%-30%.Dose: 1% and 2% by weight of cementitious material respectively according to IS456-2000. Eg. Calcium lignosulphonate.

    2. Retarders:

Slow down the rate of hydration of concrete which results; delaying in setting time of concrete so used in placing the concrete at high temperature. Eg. Sodium tartrate, Sugar, Tartaric acid. Dose: 0.5%

    3. Accelerators:

Increase the rate of hydration which results shorten the setting time of concrete of so used in cold weather below 5 degree centigrade. Eg, NaCl, CaCl2, Fluco silicates, Tri-ethanolamine fluorides, Nitrides  etc.

    4. Air-entraining admixture:

Microscopic air bubbles in the intentionally incorporated in mortar or concrete during mixing, usually by use of a surface active agent; Eg. Aluminum powder, Neutralized vinsol resin, Animal and vegetables fats and oils, Water soluble soaps of resin which helps by increasing resistance to freezing and thawing, increase in durability, reduced segregation and bleeding effect.

2. Mineral Admixtures:

These are added to the concrete mix in order to improve the properties of concrete. Need large amount of volume in comparison with chemical admixture. Examples

• Cementitious
• Pozzolanic
• Ground granulated blast furnace slag
• Fly ash
• Silica fume, Rice Husk, surkhi.

Use of Mineral Admixtures:

• To increase the strength development early age.
• To increase the water tightness
• To retard the initial setting time
• To increase workability
• To improve the extensibility
• To decrease heat evolution
• To increase the resistance to freezing and thawing.
• To increase the resistance capacity of Sulfate attack.