Types of admixture to be used in concrete depend on the structure’s intended use, design strength, pouring conditions, and other factors. The most commonly used material in construction is concrete. Admixture is a material that is added to concrete along with cement, sand, water, and aggregates to modify or enhance the properties of the concrete and make it more suitable for a specific environment.
Concrete is used in many applications and is exposed to extreme environmental conditions that can lead to severe performance and quality issues. Admixture and their types are important ingredients in today’s high-performance, long-lasting, durable, and beautiful concrete. Admixtures are broadly classified into chemical admixtures and Mineral Admixtures. This article discusses the most common types of admixture used in concrete.
Definition of Admixture and its types used in concrete
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, Admixture reduces construction costs and makes the concrete more economical.
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Functions of different types of admixture
There are many applications for different types of admixture, including but not limited to the following ones:
- 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 improves Resistance to 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 admixture
- Mineral admixtures
- Chemical admixtures
Types of Mineral Admixture
Mineral admixtures are siliceous and insoluble materials other than cement and aggregate that are added to concrete in concentrations ranging from 20 to 70% by mass of cement. These are fine materials that have an impact on concrete via hydraulic and pozzolanic activity. Natural materials, processed natural materials, and artificial materials are all examples of mineral admixtures. The following are some types of mineral admixture commonly used in concrete.
- Blast Furnace Slag
- Silica Fume
- Rice Husk
Types of Chemical Admixture
Chemical admixtures are chemical additives blended with concrete to modify their properties. It imparts special properties to concrete. The most common types of chemical admixture are listed below
- Super Plasticizers
- Set Retarders
Types of concrete admixture based on applications
Admixture is classified into various types 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
Plasticizers are another name for water-reducing admixtures. Basically, by lowering the water-cement ratio, they assist in reducing the water content of the concrete mix by 5 to 20%, resulting in high-strength concrete. Workability is increased by water-reducing admixtures because they can even maintain a high slump without adding more water. Examples include polycarboxylates, multicarbovyl ethers, and acrylic polymers. 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. However, the fast setting rate of concrete affects its strength and durability. Generally, retarding admixtures are widely used to overcome this problem. 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 the concrete that increases durability, workability of the concrete and cohesion. Similarly, 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. Accordingly, It increases the strength, and 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
Dampproofing admixtures make the concrete impervious and durable. It also helps in attaining the early stage of concrete hardening. Some examples of 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 admixtures are Aluminum powder, activated carbon, hydrogen peroxide, etc.
Air detraining admixture
During the mixing of concrete, the air gets entrapped in the concrete. Furthermore, 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 admixture are tributyl phosphate, silicones, water-insoluble alcohols, etc.
Generally, the main application of anti-wash-out admixtures is in underwater construction. Some examples are natural or synthetic rubbers, thickeners based on cellulose, etc. Similarly, 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. Basically, corrosion-inhibiting admixtures help in decreasing the corrosion rate and delaying the corrosion. Some examples of corrosion-inhibiting admixtures are sodium benzoate, sodium nitrate, sodium nitrite, etc.
This type of admixture helps to the bond between the new and old concrete surface. Basically, It is commonly used in floor overlays, screed over roofing, repair work, etc. Some examples of bonding admixtures are natural rubber, synthetic rubbers, and polymers such as polyvinyl chloride, polyvinyl acetate, etc.