Concrete is the most commonly used material in the construction industry. Concrete is used for various applications and is subjected to adverse conditions that may cause serious performance and quality issues. Admixture is an ingredient added in concrete along with cement, sand, water, and aggregates to modify/enhance the properties of concrete and make it more desirable for a particular environment. Admixtures are important ingredients in high performance, long-lasting, durable and beautiful concrete produced now a days. This article is about Admixtures and their types.
What are Admixtures?
Admixtures are materials that enhance performance and alter the properties of concrete. Concrete admixtures are natural or manufactured chemicals or additives added during concrete mixing to enhance specific properties of the fresh or hardened concrete, such as workability, durability, or early and final strength and make it desirable for a certain condition. Also, it lessens construction costs and makes the concrete more economical.
Functions of admixtures
Admixtures are used in a wide range of applications which include but not limited to the the following area
- Reduces the water-cement ratio.
- Increases the strength and durability of concrete.
- Improves the workability of concrete.
- Provides particular properties to concrete to make it suitable under different weather conditions
- Limits the shrinkage cracks.
- Avoids bleeding and segregation of concrete
- Accelerates or retards the setting time of concrete
- Reduces corrosion of reinforcement in concrete or improve Resistance to the chemical attack.
- Decreases the heat of hydration.
- Strengthens the bond between old and new concrete surfaces.
Types of Admixture
There are mainly two types of admixtures
- Mineral admixtures
- Chemical admixtures
Mineral admixtures are siliceous and insoluble material other than cement and aggregate that are added at a range of 20 to 70 percentage by the mass of cement. These are fine materials that affect the concrete by hydraulic and pozzolanic activity. Mineral admixtures include natural materials, processed natural materials, and artificial materials.
- Blast Furnace Slag
- Silica Fume
- Rice Husk
Chemical admixtures are chemical additives blended with concrete to modify their properties. It imparts special properties to concrete. This includes
- Super Plasticizers
- Set Retarders
Types of concrete admixtures based on applications
Admixtures are classified based on various applications
- Water-reducing admixture
- Retarding admixture
- Accelerating admixture
- Air entraining admixture
- Pozzolanic admixture
- Damp-proofing admixture
- Gas forming admixture
- Air detraining admixture
- Anti-washout admixture
- Corrosion inhibiting admixture
- Bonding admixture
Water reducing admixtures are also known as plasticizers. They help in reducing the water content of the concrete mix by 5 to 20 % and thereby assists in achieving high strength concrete by reducing the water-cement ratio. Water reducing admixtures can even maintain a high slump without increasing water content and thereby increases workability. Some examples are acrylic polymers, polycarboxylates, multicarbovyl ethers, etc.
Retarding admixtures or retarders decreases the setting rate of concrete. They are suitable in hot weather conditions where the high temperature drastically increases the setting rate of concrete. The fast setting rate of concrete affects its strength and durability. To overcome this problem retarding admixtures are widely used. Some examples of retarders are Starch, cellulose products, common sugar, acid salts, etc.
Accelerating admixture decreases the initial hardening time of concrete. As a result the rate of hydration of cement increases. There are two types of accelerating admixtures.
- Set accelerating admixture
- Hardening accelerators
Accelerating admixture improves the concrete strength by increasing the rate of hydration. This type of admixture is suitable for early formwork removal, emergency repairs, buildings in low-temperature regions, etc. Some examples of accelerators are triethanolamine, calcium formate, active silica, calcium chloride, finely divided silica gel, etc.
Air entraining admixtures
During the mixing of concrete air bubbles are formed.
Air-entraining admixtures facilitate the development of a stable air-void system within concrete that increases the durability ,workability of the concrete and cohesion. These admixtures impart air entrainment that results in:
- Increased resistance to deterioration from cyclic freezing and thawing
- Improved workability and cohesiveness of concrete placement
- Reduced segregation and bleeding
These admixtures are suitable for hydraulic structures such as dams, reservoirs, etc. Pozzolana is a cementitious material that helps in preparing high dense concrete mixes. It increases the strength, reduces the cost of concrete and thermal shrinkage. Some examples of pozzolanic admixtures are fly ash, silica fume, rice husk ash, metakaolin, etc.
Damp proofing admixtures
Damp proofing admixtures makes the concrete impervious and durable. It also helps in attaining the early stage of concrete hardening. Some examples for dam proofing admixtures are hot bitumen, mastic asphalt, bituminous felt, etc.
Gas forming admixtures
During the hydration process of cement, we get hydroxide. The gas-forming admixture reacts with the hydroxides and forms hydrogen gas bubbles. This bubble helps in avoiding settlement and bleeding of concrete. Some examples of gas-forming admixture are Aluminum powder, activated carbon, hydrogen peroxide, etc.
Air detraining admixture
During the mixing of concrete, the air gets entrapped in the concrete. This air reduces the strength of the concrete. So to avoid this air content we use air detraining admixtures. Some examples of this type of admixtures are tributyl phosphate, silicones, water-insoluble alcohols, etc.
The main application of anti wash out admixtures is in underwater construction. Some examples are natural or synthetic rubbers, thickeners based on cellulose, etc. It makes the concrete more cohesive and avoids washing out of concrete mixes underwater.
Corrosion inhibiting admixture
Corrosion of reinforcement is one of the common problems in construction. So to decrease the corrosion this type of admixture is used. Corrosion inhibiting admixtures helps in decreasing the corrosion rate and delays the corrosion. Some examples of corrosion inhibiting admixtures are sodium benzoate, sodium nitrate, sodium nitrite, etc.
This type of admixtures helps to the bond between the new and old concrete surface. It is commonly used in floor overlays, screed over roofing, repair work, etc. Some examples of bonding admixtures are natural rubber, synthetic rubbers, polymers such as polyvinyl chloride, polyvinyl acetate, etc.