Admixtures are the chemical compounds in concrete other than hydraulic cement (OPC), water, cement and aggregates, and mineral additives that are added to the concrete mix immediately before or during mix to modify many of the specific properties of concrete in the newly prepared (fresh) or hardened state. 


The use of admixture should offer an improvement not economically attainable by adjusting the proportion of water, cement, and aggregate, and should not adversely affect the performance of the concrete. The admixture is no substitute for good concreting practice. 


An admixture should be employed only after appropriate evaluation of its effects on the performance of concrete under the conditions in which the concrete is intended to be used.

Different types of admixtures.
Admixture

The function of admixture:-

  1. To produce non-skid wearing surfaces.
  2. To produce cellular concrete.
  3. To make the weight of concrete low per cubic meter.
  4. To increase the bond between old & new concrete surface.
  5. To enhance the bond of concrete to the steel reinforcement.
  6. To increase the durability of concrete, i.e. its resistance to special conditions of exposure.
  7. To inhibit the corrosion of reinforcement in concrete.
  8. To increase the resistance to chemical attack.
  9. To reduce the heat of hydration.
  10. To accelerate the initial set of concrete, i.e. to the speed of the rate of development of strength at early ages.
  11. To retard the initial setting time i.e. to make concretely workable for a longer time for placement.
  12. To enhance the workability.
  13. To reduce the segregation in grout and concrete mixtures.
  14. To increase the hardness or strength of concrete by reducing the water content.

Types of admixture:-

The admixture is broadly classified into two categories.

A) General Purpose Admixture:- 

1. Accelerating admixture or Accelerators.

2. Retarding admixture or         Retarders

3. Air-entraining admixture.

4. Water reducing admixture

B) Specialty category Admixture:-

1. Grouting admixtures

2. Air detraining admixtures

3. Gas-forming admixtures

4. Corrosion inhibiting admixtures.

5. Shrinkage reducing admixtures

6. Water or damp proofing & permeability-reducing admixtures

7. Bonding admixtures

8.Coloring admixtures pigments

9.Concrete surface hardening admixtures

10. Fungicidal, germicidal, and insecticidal admixtures

A)

1.Accelerating admixtures:-

  • It allows earlier finishing of concrete surface and application of insulation for protection and 
  • also putting the structure into service earlier.
  • These are added when concrete is to be placed at a temperature (2-4°e).
  • Most widely used accelerator are CaCl, Nael, NaSo, NaOH, NaCl, K, so, & KOH etc.
  • An admixture which speeds up the initial set of concrete, thereby early strength development of
  • Concrete.
  • These are used where a rapid removal of formwork is desirable in urgent repair work.
  • Conversely, at high temperatures, accelerators may result in too high rate of development of
  • the heat of hydration and shrinkage cracking.

2. Retarding admixtures:-

  • It delays the setting of cement & hardening hence used in hot weather when the normal setting time in shorten by higher temperatures
  • It does opposite function to accelerating admixtures.
  • High temperatures, low humidity and wind cause rapid evaporation of water from mix during summer. This drying of concrete leads to the cracking of the surface which controls by the addition of retarder.
  • Most commonly used retarders are sugar, carbohydrate derivatives, soluble zinc salts, soluble
  • borates and some other salts.

3. Air entraining admixtures:- 

  • Using admixture should offer an improvement not economically attainable by adjusting the proportion of mix, and should not affect the performance of the concrete.
  •  It is made by mixing a small quantity of air entraining agents or by using air-entraining cement.
  • It helps to incorporate a controlled amount of air, in the form of millions of noncoalescing bubbles distributed throughout the body of concrete, during mixing, without significantly altering the setting or the rate of hardening of concrete.
  • It modifies the elastic properties of concrete regarding workability, segregation, bleeding and finishing quality and increased durability of concrete.
  • It increases the permeability.
  • It improves place ability and early finishing.
  • It reduces the cement content, the heat of hydration.
  • it reduces the alkali-aggregate reaction.
  • It reduces the unit weight of concrete.
  • Air entraining agents are anionic surfactants, natural wood resins, animals and vegetables fats and oils.

4. Water reducing admixtures:-

  • Water reducing admixtures enables a given fresh concrete mix to have higher workability without increasing the water content which results in faster rate concrete placement, easy placement in relatively poorly accessible locations without vibration and reduction in cement content.
  • When water is added to mortar or plaster or concrete mix the cement and other fines in the mix as fly ash, silica fume, rice husk ash, and the stone dust flocculate together which increase the viscosity.
  • To reduce the viscosity ( a functional effect of workability) to the desired level, it may necessary to add more water but beyond the limit it affect the workability and hardened physical properties so to achieve the desired workability and hardened physical properties it is often necessary to add a water reducer to disperse or deflocculate the system and reduce the quantity  of water to be added during mixing.
  • The organic or combinations of organic and inorganic substances to achieve these objectives are termed as plasticizing admixtures.
  • Advantages of water reduction in the hardened state of concrete are increased strength, density, durability, volume stability, abrasion resistance, reduced permeability, and cracking.
  • The specific effect of water reducing admixtures varies with the type of cement, water-cement ratio, mixing temperature and other job conditions.
  • Most widely used water reducing admixtures are Ca, Na, NH4 salts and hydroxylated carboxylic acids and their salts, formaldehyde, carboxylated acrylic easter co-polymers etc.

B) 

1.Grouting admixtures:-

  • The grouting has become one of the most important operations in civil engineering construction.
  • Grouting below the base plate or machine foundations, grouting of foundation bolt holes early and Industrial structures grouting of curtain wall, grouting of oil wells are the few some situation
  • where grouting is extensively used.
  • Grouting under different condition and for different purposes, grout material should have high early and ultimate strength should be free-flowing even at low water content, should develop a good bond with previously set or hardened concrete.
  • Essentially it should be non-shrink in nature.
  • Grouting under different condition and for different purposes grout material should have high early and ultimate strength, should be free-flowing even at low water content, should develop a good bond with previously set or hardened concrete.
  • The grouting materials can be broadly classified into two categories. One is free for grout for use in machine foundation, foundation bolts and fixing crane rails etc. and the second type of grout is meant for injection grouting to fill up small cracks, and is normally accomplished under pressure.
  • Admixtures are used where hot water or high temperature encountered underground to prevent the loss of water from cement paste.
  • Some of the grouting admixtures are gels, clays, pregelatinized starch, and methylcellulose.

2. Air detraining admixtures.

  • These admixtures are used to dissipate air or other gases. These are used to remove a part of entrained air from the concrete mixture.
  • Some of the air detraining admixtures are tributylphosphate, dibutyl phosphate, water-insoluble alcohol, and silicones.

3. Gas-forming admixtures:-

  • This admixture, when mixed with mortar or concrete, react chemically with hydro oxide
  • present in cement and forms minute bubbles of hydrogen gas of size ranging 0.1 to 1 mm
  • throughout the concrete mass.
  • When properly control, cause slight expansion in plastic concrete or mortar and thus reduces eliminates voids caused by normal settlement which occur during the placement of concrete is also prevent bleeding.
  • Aluminum powder is widely used gas forming admixture. Dose =0.005-0.02% by mass of
  • cement. Zinc, magnesium powder and bleaching powder may also be used according to the condition.

4. Corrosion inhibiting admixtures:-

  • These are used to slow down corrosion of steel reinforcement in concrete.
  • They are used as a defensive strategy for concrete structures constructed in marine facilities, highway bridges and in an industrial environment where reinforced concrete is exposed to high
  • the concentration of chloride.
  • Some corrosion inhibiting admixtures are sodium benzoate, sodium nitrate etc. 2% benzoate solution in mixing water may be used to prevent corrosion of steel reinforcement.

5. Shrinkage reducing ádmixtures:-

  • These are also called expansion-producing admixtures either expand themselves or react with
  • other constituents of concrete resulting in expansion.
  • The expansion-producing compound is mixed with cement in appropriate proportion to get the desired expansion.
  • A high proportion of expansion producing admixture is employed to produce self-stressing cement. Shrinkage-compensating type expansive cement is capable of developing 0.03 to 0.1% restrained concrete expansion.
  • Some examples of shrinkage reducing admixtures are iron and chemicals, and anhydrous
  • sulphoaluminate, etc.

6. Water or damp-proofing and permeability-reducing admixtures:-

  • Water may pass from one surface to another either by under pressure called permeability and absorption of surface water by capillary action. Hence permeability reducers are used for first damp proofers are used for the second.
  • These admixtures prevent the water penetration of dry concrete or stop the passage of water through unsaturated concrete.
  • Concrete having a proper mix proportion, low water-cement ratio, and sound aggregate will be impervious and needs no additives. However, the resistance of concrete to the penetration of moisture can be improved by chemically active water repelling agents like soda & potash soaps to which are sometimes added lime or calcium chloride.
  • Some examples of water reducing admixtures are some vegetables and animal fats, calcium-stearate and aluminum stearate, pozzolanas and silica fume etc.

7. Bonding admixtures:-

  • This admixture increases the bond strength between the old and new concrete surface.
  • When fresh concrete is placed over a concrete surface already set, the fresh concrete shrinks
  • while setting which makes the new concrete pull away from the old surface. Due to this reason
  • bonding admixtures are used to overcome such effects.
  • Cement paste lurry is often applied to the prepared surface immediately prior to pouring new
  • concrete to increase the amount of paste available at the surface for bonding purposes.
  • Major application include overlay on an existing pavement, provision of screed over the roof for
  • waterproofing repair works etc.
  • Some examples of bonding admixtures are synthetic lates, polymer, metallic aggregates etc.

8. Concrete surface hardening admixtures:-

  • To protect the damages due to heavy traffic on the concrete surface, continuous wear and tear on the floor of buildings, a certain type of admixtures are used called hardener.
  • The hardener commonly used to prevent the destruction of surfaces can be divided into two
  • groups, namely the chemical hardeners and fine metallic aggregates.
  • The liquid chemical hardeners consist of silicofluorides or fluosilicates and a wetting agent.
  • On the other hand, the metallic hardeners consisting of specially processed grade iron particle
  • are dry mixed with Portland cement?

9.Coloring admixtures:-

  • Colour pigments are used to produce color in the finished concrete.
  • To obtain good coloring effect, the pigment should be ground with the cement in a ball mill.
  • Some pigments are black, blue, brown, green, red, yellow etc and these have different sources.

10.Fungicidal, Germicidal, and Insecticidal admixtures:-

  • Certain material like used as fungicidal, germicidal, and insecticidal admixtures polyhalogenated phenols, dieldrin emulsions, and copper compounds are used as fungicidal, germicidal, and insecticidal admixtures.
  • These admixtures act against fungi, germs, and insects.

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