CLASSIFICATION OF CONCRETE ADMIXTURES





Admixtures are the construction materials other than water aggregate and cement .that are added to the concrete before or during mixing of concrete. Admixtures are used to improve the properties of concrete the uses of concrete should offer an improvement not economically attainable by adjusting the proportions of cement and aggregates and should not adversely affect any properties of the concrete

very small amount of admixture is added to the concrete. excess amount of admixture can change the properties of concrete



FUNCTION OF ADMIXTURE



1.to accelerate the initial setting time of concrete

2.to retard the initial setting time

3.to improve the strength of concrete

4.to improve the strength of concrete

5.to inhibit the corrosion of concrete

6.to increase the durability of concrete

7.to increase the bond between concrete to the steel

8.to increase chemical residence to the concrete

9.to reduce the segregation in grout mixtures

10 to decrease the weight of concrete

11. to increase bond bw old and new concrete surface

CLASSIFICATION OF ADMIXTURES

1.air entraining agents

2.retarders

3.accelerators

4.water proofers

5.pozzolana

6.pigments

7.Workability agents



1.Air entraining agents

Air entraining agents are used to increase the workability of concrete to improve resistance to weather and action of frost.

2.retarders

Retarders are used to delay the setting time of concrete. Calcium sulphate in the form of gypsum is generally added during the manufacture of cement to retard the setting Time. some other materials which are effectively used to retard the rate of hydration are ammonium chloride, ferrous and ferrous and ferric chlorides

3.accelerators

Accelerators are used to decrease the setting time of concrete without affecting strength of concrete. the most commonly used accelerator is calcium chloride .sodium chloride may used to acceleration of concrete

4.water proofers

Water proofers are used in the form of powder liquid and paste and consists of pore filling materials or water repelling materials they can be chemically active or inactive .alkaline silicates notably silica of soda aluminum and zinc sulphates . aluminium and calcium chlorides are the chief pore filling materials these are all chemically active and hence may accelerate the setting time of the concrete thus making it more impervious at an early age

5.pozzolana

A pozzolana is a silicious materials. which its self don’t have cementititous properties will either be processed or unprocessed and its finally divided form. React in the presence of water with lime normal temperatures to form compounds of low solubility having cementitious properties.

Advantages

Following advantages are achieved to use of the pozzolanas

1.obtain maximum workability with using small amount of water

2.attacks from salt arid sulphates is reduced

3.calcium hydroxide leaching is reduced

4.heat of hydration is reduced

Disadvantages

1.strength development is slowed

2.durability may be decreased

3.shrinkage may be increased

6.pigments

Pigments are the substances are used to produce the coloured cement .the main pigments used in concrete are brown, black ,red, green, blue, and yellow .to produce the colouring effect , the pigment should be ground with cement in a ball mill.

7.Workability agents
Workability agents are used to increase the workability of concrete. Workability and water reducing agents are same .many materials appear to perform all these function

Workability agents some times may be in the form of powder.some times much finer than the cement.

Workability agent chemicals are classified into three types

Organic acid ,lignosulfonates and carbohydrates. Excessive use of workability agents some times may cause shrinkage of concrete

FUNCTIONS
1.Its is used to spreading of cement over the fine aggregates

2.its wets the surface of the aggregates

3.its used to increase the binding properties of concrete

ALSO READ------ PROCESS INVOLVED IN MANUFACTURING OF CONCRETE

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