What are admixtures in Concrete? And its Type
There are Various types of admixtures are used in concrete to enhance the performance of concrete. Chemical admixtures are the ingredients in concrete ,which are added to the mix immediately before or during mixing.
Admixtures primarily used :-
- To reduce the cost of concrete construction
- To modify the properties of hardened concrete
- To ensure the quality of concrete during mixing, transporting, placing, and curing
- To overcome certain emergencies during concrete operations.
Types of Concrete Admixtures
- Air en-training concrete admixture
- Retarding admixtures
- Water Reducing Admixtures
- Accelerating Admixtures
- Damp-proofing Admixtures
- Bonding Admixtures
- Super plasticize-rs
- Air detraining Admixtures
- Corrosion-inhibiting admixtures
Air en-training concrete admixture
Air entraining admixtures are one of the most important inventions in concrete technology. Air entrainment in concrete will also improve the work ability of concrete.
Durability , prevents segregation and bleeding, lower the unit weight and modulus of elasticity of concrete, improves the chemical resistance of concrete .Increased resistance to deterioration from cyclic freezing and thawing .
Retarding admixtures
Retarding admixtures slow down the rate of hydration of cement in its initial stage and increase the initial setting time of concrete .
Commonly used retarding admixture is calcium sulphate or gypsum. Starch, cellulose products, common sugar, salts of acids are some other retarders.
Retarders keep concrete workable during placement and delay the initial set of concrete. Most retarders also function as water reducers and may en-train some air in concrete.
Water Reducing Admixtures
Water-reducing admixtures usually reduce the required water content for a concrete mixture by about 5 to 10 % . Concrete containing a water-reducing admixture needs less water to reach a required slump than untreated concrete.
Accelerating Admixtures
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.
Accelerating admixtures are especially useful for modifying the properties of concrete in cold weather
Damp-proofing Admixtures
These are used to make the concrete structure impermeable against water and to prevent dampness on concrete surface.
Bonding Admixtures
Bonding admixtures are used to create a bond between old and fresh concrete surface . When fresh concrete is poured over a hardened concrete surface, there is a chance of failure of fresh concrete surface due to weak bond with old surface.
To make the bond stronger, bonding admixtures are added to cement or mortar grout before 1 hour of concreting
Super plasticize-rs
It reduce water content by 10 to 30% and can be added to concrete with a low-to-normal slump and water-cement ratio to make high-slump flowing concrete. The effect of super plasticizers lasts only 30 to 60 minutes, depending on the brand and dosage rate, and is followed by a rapid loss in workability.
As a result of the slump loss, super plasticizers are usually added to concrete at the job site.
Corrosion-inhibiting admixtures
To prevent or to slow down the process of corrosion preventing admixtures are used.