Admixtures | Civil Engineering SSC JE (Technical) - Civil Engineering (CE) PDF Download

Anything added in the concrete other than water, cement, sand and coarse aggregate in order to modify its property is termed as admixture. Admixtures are generally of following types:–
(1) Plasticizers
(2) Superplasticizers 
(3) Retarders
(4) Accelerators
(5) Air entraining admixtures.
(1) Plasticizers: 
-These are organic substances or mixture of organic and inorganic substances and are primarily used for their water reducing capacity; maintaining the desired workability of a concrete mix. In other words it has a property of enhancing workability at a given water-cement ratio in compare to concrete mix without Plasticizers.
-These are primarily used for their water reducing capacity; maintaining the desired workability of a concrete mix. In other words it has a property of enhancing workability at a given water-cement ratio in compare to concrete mix without Plasticizers.
-These are primarily used for their water reducing capacity; maintaining the desired workability of a concrete mix. In other words it has a property of enhancing workability at a given water-cement ratio in compare to concrete mix without Plasticizers.

Ex. Lignosulphonates and their derivatives, hydroxylated carboxylic acids and carbohydrates. These admixtures are generally added in the preparation of concrete to be used in heavily reinforced structure.
 

(2) Super plasticizers :
These are the modified versions of plasticizers that allow the reduction of water upto 30% unlike plasticizers in which it can be only reduced upto 15%.
Chemicals: Modified Ligno- sulphates, Sulphonated Melamine Formaldehyde Condensate (SMF) ,Sulphonated Napthalene Formaldehyde Condensate (SNF)

(3) Retarders:
Retarding admixtures slow down the rate of hydration of cement in its initial stage and increase the initial setting time of concrete. These are also called as retarders and used especially in high temperature zones where concrete will set quickly.
The quick setting in some situations may lead to discontinuities in structure, poor bond between the surfaces, creates unnecessary voids in concrete etc. Retarders are useful to
eliminate this type of problems.
Commonly used retarding admixture is calcium sulphate or gypsum. Starch, cellulose products, common sugar, salts of acids are some other retarders. Most of water reducing admixtures are also acts as retarding admixtures and they are called as retarding plasticizers.eliminate this type of problems.

(4) Accelerators:
Accelerating admixtures are used to reduce the initial setting time of concrete. They speed up the process of initial stage of hardening of concrete hence they are also called as accelerators. These accelerators also improves the strength of concrete in it early stage by increasing the rate of hydration.

Earlier hardening of concrete is useful in several situations such as early removal of formwork, less period of curing, emergency repair works, for constructions in low temperature regions etc.
Some of the accelerating admixtures are triethenolamine, calcium formate, silica fume, calcium chloride, finely divided silica gel etc. Calcium chloride is the cheap and commonly used accelerating admixture.

(5) Air entraining Admixtures:
-These are type of admixtures that entrap million of air bubble in between the voids of the cement particles that act as flexible ball bearing slipping parts one over each other, thereby helps in modifying the properties of concrete w.r.t. plasticity, workability, bleeding, segregation, permeability and frost action.

Example :
(1) Natural wood resins
(2) Animals and plants oil and fats
(3) Hydrogen peroxide
(4) Aluminum powder