Types of admixtures in concrete and its properties .

Admixture :-

Sometime some material are added in concrete to improve few properties and to get required result these material added are known as admixture .

These are either added at the time of grinding of cement or added during mixing of concrete or different substances used to alter the properties are known as admixtures .

Admixture are material other than the basics ingredients of concrete -cement , water and aggregates added to the concrete mix immedietly before or during mixing to modify one or more of the specific properties of concrete in the fresh or hardened concrete .

Trades name of admixture :-

1) Muraplast -Fk-61
2) Vinsol resin darox
3) Conplast
4) Dr. Fixit

Function of admixture or purpose of Admixture :-

1) It helps to reduce required quality of concrete .
2) It also helps to resist chemical action like corrosion of steel etc .
3) It helps in improving durability .
4) It helps in improving various strength such as compressive ,flexural etc.
5) Admixture such as accelerators accelerate setting of concrete and retardors retard setting of concrete.


Classification of Admixture :-

1) Accelerating admixture.
2) Retarding admixture .
3) Air enterning admixture .
4) Super plasticizer .
5) water proofers .
6) pozzolana .
7) Pigments .
8) plasticizer.

1) Accelerating admixture :-

These admixture accelerate the rate of hydration reaction and hence accelerate the rate the rate of development of strength .
  By using these admixture all the Concreting operation can be speeded up.

Advantages of accelerating admixture :-

1) Early finishing of surface .
2) Reduction of pressure on forms or reduction of time during which the formwork is subjected to hydraulics pressure .
3) More effective plugging of leaks against hydraulics pressure .
4) Earlier removal of formwork .
5) Reduction in time period of curing .
6) Earlier placement of structure in service .

Properties of accelerating admixture :- 

1) Quick setting time .
2) water reducer up to 8 %.
3) More earlier strength development in concrete .
4) Less resistance to sulphate attack .
5) More drying shrinkage .

fig .1 Accelerating admixture .

2) Retarding admixture :- 

The retarding admixture delay or prolong the setting of the cement paste in the fresh concrete .
  These are used when more workability is required in initial stage of placing .Due to these the complete achievement of strength takes place but only it is delayed .

Calcium sulphate or Gypsum is commonly used retarder used retarder Lignosulphonic  acids and their salts are also used .

Retarding admixture is also known as water reducing admixture .

Advantage of retarding admixture :-

1) Increase the setting time concrete and give more time for handling and placing of concrete .
2) prolong the setting of cement in the concrete hence useful in hot -weather  concreting where setting is accelerated due to hot weather .
3) Reduce shrinkage due to quick drying .
4) Increase workability during initial stages without affecting the strength in the final stages .

Properties of water reducing admixture :

1) It increase workability .
2) It increase strength .
3) Due to reduction in W/C ratio durability increase .
4) It reduces segregation and bleeding .

fig .2 Retarding admixture .

3) Air enterning admixture :-

Air enterning agents acts in two ways .
They either react with water to form same natural gas or CO2 which evolves in the mixtures in the form of thousand of tiny microscopic bubbles .

     Thus air enterning agent are also useful in production of cellular concrete or light weight concrete.

    Thus , they help to incorporate a controlled amount of air during mixture , in the form of millions of small bubbles . It results in :

1) Improved workability .
2) Easier placing .
3) Increased durability
4) Better resistance to frost action .
5) Reduction in bleeding and water - gain.

Properties of air enterning admixture :-

1) Ability of formation of tiny , stable , air ,bubbles in concrete .

2) Easy to disperse in concrete .

3) Resistance to segregation and bleeding .

4) Good bonding between aggregates particles .

fig.3 Air enterning admixture 

4) Super plasticizer Admixture :- 

 Plasticizer and super plasticizer work by the principle of adsorption .
  The main difference between the air - enterning agent and plasticizer is that plasticizer do not create any bubble and thus retain the density and permeability of the original concrete and act by adsorbing on the surface of cement particles by forming a thin molecular film surrounding the cement particle .
   Beacause this film is of negatively charged ion , the particle repel each other further , and flocculation of cement particles and their uniform distribution is promoted , which is greater durability .
    Super plasticizer permits 30 % water reduction without affecting the workability and with concrete of highly increased strength .
  Super plasticizer is a new category . It was developed during 1960  to 1970 in japan and Germany .
     Use of super plasticizer has advanced concrete technology to a new stage . Now a days , acrylic polymer based new generation plasticizer give even better results than super plasticizer .

fig .4 Super plasticizer admixture .

5) Water proofers admixture :- 

Water proofers are mainly used to resist absorption of water and make structure impermeable under water pressure .
   These are obtained in powder , paste or liquid form . These are not only helps in making structure impermeable but also helps in accelerating the setting time .
    Water  proofers may be chemically active or inactive , chemically active water proofers are silicate of soda aluminum and zinc sulphate and aluminum chlorides etc. Chemically inactive water proofers are chalk , further , earth etc .
   Chemically active water proofers accelerate setting time of concrete and make it more in previously at an initial stage which chemically in active water proofers improves workability and improve density of concrete .

fig .5 waterproofing admixture 

6) Pozzolan's Admixture :- 

Pozzolana are siliceous or aluminous material which posses very little or no cementation value but are finely devided form and in presence of water react with lime at normal temperature to form compound having cementation properties .

Classification of pozzolana :-

A) Natural pozzolana 
B) Artificial pozzolana 

Advantages of pozzolana :- 

1) Reduces heat of hydration
2) Prevent leaching .
3) Improves workability .
4) Protect from salts and sulphates .
5) In case of large dams it save cement and
     thus reduces cost .

fig .6 Pozzolana admixture 

7) Pigments admixture :- 

These are admixtured use to coloured cement .
Pigment are ground with cement in ball mill.
The basic  requirements of this pigment is that there should not have any adverse effect on properties of concrete .

8) Plasticizers admixture :- 

Plasticizer are used to improve workability without using excess water .
 Generally plasticizer get adsorbed on cement particle due to this there is repulsion between particles due to which cement particle get dispered .




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