Admixtures are defined as the chemicals which are added to the concrete other than cement, aggregate, and water to modify its properties during or before the mixing of concrete.
The properties which are commonly modified are the rate of hydration, setting time, and workability of concrete.
Admixtures help to make concrete suitable for the required situation on the construction site.
Importance of Admixtures
Every Construction site has different site conditions and requirements. A concrete with conventional properties cannot be used in a special situation. We have to modify the properties of concrete as per the site.
Concreting Operations are done at different weather conditions. The concrete has a tendency to change its properties in extreme weather conditions. Admixtures help to improve its properties and allow it to do concreting in Hot and Cold weather without any kind of risk.
Purpose of using Admixtures in concrete
- It helps to produce the required quality of concreting operations.
- It helps to improve the durability of concrete.
- It helps in increasing the strength of concrete such as compressive and flexural.
- It helps in corrosion as well as abrasion resistance.
- It reduces the water consumption of concrete.
- Reduces segregation and bleeding in concrete.
Classification of Admixtures
Accelerating admixture:-
This type of admixtures is used for increasing the rate of setting time of concrete and helps to gain early strength.
Example:- Calcium Chloride (CaCl2 ), Calcium Nitrate, Sigunit Powder.
Retarding admixture:-
Retarding admixtures are used to delay the setting time of concrete.
Example:- Calcium Sulphate (gypsum), Starch and cellulose products.
Difference between Accelerating and Retarding admixture
|
Accelerating Admixtures |
Retarding Admixtures |
| It accelerates the rate of hydration. | It delays the setting time of concrete. |
| It accelerates the rate of hydration. | It retards the rate of hydration. |
| Earlier removal of formwork is possible. | Earlier removal of formwork is not possible. |
| It is suitable for cold weather conditions. | It is suitable for hot weather conditions. |
Air entraining admixture:-
The use of air-entraining admixture in concrete helps in resistance to freeze-thaw degradation, increased cohesion (resulting in less bleed and segregation), and improved compaction in low-workability mixes.
Example:- vinsol resin, Alkyl sulphonates.
Plasticizers:-
This is used for improving the workability of concrete without adding any excess of water in the concrete.
Example:- Ligno sulphate, hydroxylated carboxylic acid.
Superplasticizers:-
These are the chemical compounds that are used to reduce the water consumption of concrete. This is also used for making High strength concrete.
Example: sulfonated melamine formaldehyde (SMF), sulfonated naphthalene formaldehyde (SNF).
Waterproofing admixture:-
These admixtures are used to resist the entry of water into concrete. This is added to make concrete impermeable.
Example:- Calcium hydroxide, Aluminium stearate and Calcium stearate.
Nowadays Admixtures are an integral part of concrete. Admixtures prove very useful in the concreting Operations in extreme Hot and Cold weather. We can now construct structures at any weather conditions with the help of admixture without compromising with the quality of construction.
