Grouting agents

Admixtures used for grouting are:

• Accelerators
• Retarders
• Gag forming agents 
• Workability agents

Workability agents

Workability is one of the most important characteristics of concrete especially under the following circumstances:

1. If the concrete is to be placed around closely placed reinforcement or in difficulty accessible sections.
2. Where special means of placement are required such as tremie, chute or pumping methods.
3. If the concrete is harsh because of aggregate characteristics or grading

The materials used as workability agents are:

1. Finely divided material
2. Water reducing agents 
3. Air entraining agents

• Common materials added as workability agents are bentonite clay, diatomaceous earth, fly ash, finely divided silica, hydrated lime and talc
• When they are added and if the quantity of water is not reduced they greatly increase the workability.

Retarders

A retarder is an admixture that slows down the chemical process of hydration so that concrete remains plastic and workable for longer time than concrete without the retarder

Retarders are used to overcome the accelerating effect effect of high temperature on setting properties of concrete in hot weather concreting.

Common materials added as retarding agents:

1. Most commonly known retarder is calcium sulphate.It is interground to retard the setting of cement.
2. Starches, cellulose products, sugars, acids or salts of acids.
3. Common sugars are one of the most effective retarding agents.
4. Other admixtures, which have been successfully used as retarding agents are Ligno sulphonic acids and their salts, hydroxylated carboxylic acids and their salts.

Accelerators

Accelerating admixtures are added to concrete to increase the rate of early strength development in concrete to

• Permit earlier removal of formwork
• reduce the required period of curing
• Advance the time that a structure can be placed in service
• Partially compensate for the retarding effect of low temperature during cold weather concreting
• In the emergency repair works

 Calcium chloride is the material very commonly used as accelerator.

The action of calcium chloride is assisting concreting at low temperature is three fold as-

• It lowers the temperature at which freezing of the wet mixture takes place
• It helps to keep the mixture warm by accelerating the generation of heat by chemical action 
• It increases the ability of the concrete to resist frost action by speeding up the rate of gain of strength

Effects of calcium chloride on concrete:

• Increases volume changes in concrete 
• Increases the alkali-aggregate reaction 
• Reduces the resistance of concrete  to sulphate attack

 

Pozzolanic admixture

Best pozzolans in optimum proportions mixed with Portland cement improves many qualities of concrete such as

1. Lower the heat of hydration and thermal shrinkage
2. Increase the water tightness
3. Reduce the alkali-aggregate reaction
4. Improves resistance to attack by sulphate soils and sea water
5. Improve extensibility
6. Lower susceptibility to dissolution and leaching
7. Improve werkability
8. Lower costs

Pzzolanic materials are-

• Clay and shales
• opaline shales 
• Diatomaceous earth
• Volcanic tuffs and pumicites

Artificial pzzolans are-

• Fly ash
• Surkhi

Air-Entraining Agents

The following types of air-entraining agents are used for making air entrained concrete:

•  Natural wood resins
• Animal and vegetable fats and oils
• Alkali salts or sulphated and sulphonated organic compounds
• Water-soluble soaps of resin acids and animal and vegetable fatty acids
• Miscellaneous materials such as the sodium salts of petroleum sulphonic acids, hydrogen peroxide and aluminum powder etc.

The most important air entraining agents which at one time enjoyed worldwide market are vinsol resin and darex.

Factors affecting air entrainment

1. The type and quantity of air entraining agents used
2. Water and cement ratio of the mix
3. Type and grading of aggregate
4. Mixing time
5. The temperature
6. Type of cement
7. Influence of compaction
8. Admixture other than air entraining agents used

The effect of air entrainment on the properties of concrete:

Air entrainment will affect directly the following three properties of concrete

• Increase resistance to freezing and thawing
• Improvement of workability
• Reduction in strength

Incidentally air entrainment also affect the following properties of concrete

1. Reduce the tendencies of segregation
2. Reduces the bleeding and laitance
3. Decreases the permeability
4. Increases the resistance to chemical attack
5. Permits reduction in sand content
6. Improves placeability and early finishing
7. Reduces the cement content, cost, and heat of hydration 
8. Reduce the unit weight
9. Permits reduction in water content
10. Reduces the alkali-aggregate reaction 
11. Reduce the modulus of elasticity

Admixture

Admixtures are those which are added to concrete to give some desired quality to the concrete.

As per the report of ACI committee 212 admixtures have been classified into 15 groups according to type of materials constituting the admixture or characteristic affect of the use. The following are 15 groups:

1.Air-entraining agents
2.Pozzalonas
3.Accelerators
4.Retarders
5.Gas forming agents
6.Air-detraining agents
7.Alkali-aggregate expansion inhibitors
8.Damp-proofing and permeability reducing agents
9.Workability agents
10.Grouting agents
11.Corrosion inhabiting agents
12.Bonding agents
13.Fungicidal, germicidal and insecticidal agents
14.Coloring agents
15.Miscellaneous agents