An Overview of Admixtures – Functions and Classifications

Concrete Must Possess Workability To Be Mixed, Transported, Placed, Compacted And Till Finished To Meet The Specifications And Strength Without Compromising The Structural Strength , Capacity Or Durability.To Achieve This, It Is Often Necessary To Add Some Chemical Compounds Or Mixes To Modify Or enhance Their Properties, And These Additional Substances So Added Are What Referred To as “ADMIXTURE”.

This Is How We Define Admixture? Admixtures Are Artificial Or Natural Material Added To The Concrete Besides Cement, Water & Aggregate To Improve Certain Property Of Concrete During Casting Or Its Service Stage.It mainly Modifies The Qualities Like Workability, Durability and Setting Time. Their Usage And Dosage Must Be Followed According To The Guidelines To Achieve The Desired Specification.

Functions Of Admixtures :-

  1. It retards or accelerates the setting time of concrete
  2. It imparts colour to the concrete
  3. Reduce bleeding
  4. Increase in strength (compressive, tensile, flexural)
  5. Increase bond of concrete to steel reinforcement
  6. Reduce heat of hydration
  7. To make light weight concrete
  8. Reduce permeability
  9. Control alkali- aggregate expansion
  10. Controls corrosion
  11. Making It Favourable In Weather Conditions
  12. It Improves Workability Of Concrete

Now as we read about the  above mentioned functions of admixtures , there are some terms like WORKABILITY, SETTING TIME, BLEEDING, lets get to know  a quick brief about one of each.

  • Workabilityit can be defined as the ease with which the concrete is mixed, batched, transported, placed, compacted and finished, is called its workability.
  • Setting Time  – it is the time taken to harden the concrete and set at its place. it is generally from 30 minutes to 10 hours. Now one can easily correlated the fact that why the admixtures are being used in concrete, i.e., to accelerate or retard the setting time.
  • BleedingPhenomenon in which free water in the mix rises up to the surface and and form a paste or slurry known as “laitance”, it takes place when course aggregate  settle down and water rises up.

Now as we move further, i hope that you can easily connect with the terms and imaginations.  Lets discuss about the classification of admixtures.

Classification Of Admixtures-  as per Is 1903-1999, Admixtures are Classified into following types given below :-

  1. Accelerating Admixtures
  2. Retarding Admixtures
  3. Water reducing (PLASTICIZERS)
  4. Super plasticizer
  5. Shrinkage reducing Admixture
  6. Mineral Admixture
  • Accelerating admixtures –  These are used to speed up the setting time of concrete. As when mixed, the time taken for concrete to sets up get accelerated and concrete is sets up earlier. in other words, to shorten the setting time of concrete or to rapid setting of concrete , accelerating admixtures are used .it also gain strength.  common examples are – Calcium Chloride, Sodium  Chloride, Formaldehyde, Ammonium flouride, silicates, Sodium and potassium hydroxide.              IT is majorly used in cold weather concreting, pavement construction and emergency repair works.
  • Retarding admixtures  – These are used to slow down or prolonged the setting time of concrete. It can delay the hydration by  hours. In other words it is used to slow down the setting time of concrete. It is majorly used in ready mix concrete (plant to site transportation to maintain the workability) and specifically in hot weather conditions (because the water will evaporates fast and concrete will get hardened) to cure this. Some examples are –  unrefined calcium, sodium hydrocarboxylic acid, phosphate, magnesium salt, calcium sulphate (gypsum), tartaric acid, starch, sugar and cellulose.
  • Water Reducing “Plasticizers”These are organic or combination of organic and inorganic substances that reduce the water content from concrete. It reduce the content of water for a certain degree of workability, when added in  a mix. It act as a “deflloculating” agent.  It is usually mixed in 0.1 to 0.4% and it reduces 5% of water  and thereby an increase in Slump by 30mm – 150mm. one can obtain specified strength at low cement content. Some examples are – Lignosulfonate, carbohydrates, hydroxylated carboxylic acid.
  • Super PlasticizerHelp to achieve reduction in water content without affecting the workability . these are similar to plasticizers but are chemically different. it eliminates concrete segregation, increase the strength and also help in removal of framework earlier.

Some common examples are – modified lignosulfonate, SFM and SNF.

  • Shrinkage Reducing Admixture  – Help to reduce shrinkage that occurs on hardened concrete.
  • Mineral Admixture – These are natural or pozzolanic or industrial byproducts commonly used in concrete to replace part of cement or sand which helps to save cost and energy.They possess certain characteristics through which they influence the property of concrete.                                    examples – Fly ash (FA) , Silica fume (SF) , blast furnace slag , ground granulated  blast furnace slag (GGBS) , Metakaolin (MK), rice husk ash.

ONE another is as important as above

  • Air Entraining Admixture – Admixture that emits air bubbles in between voids of aggregate which then act as a flexible bearing and modify the properties of concrete, like workability, resistance to frost action and durability. 1% reduces 5% of strength. examples are – Resin, Steric Acid, Oelic Acid, Hydrogen Peroxide and Aluminium Powder.

This was about the admixture, its functions and the classifications .


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