What is an Admixture?
An additive is an element of concrete or mortar that alters one or more characteristics of the concrete in its plastic or solidified state. It is commonly incorporated into the concrete mixture prior to or during the mixing process.
Admixture in Concrete
Concrete is a commonly utilized building material. It consists of four primary components: cement, water, sand, and aggregates. When mixed together, the cement and water create a paste that binds the sand and aggregates, resulting in a durable and sturdy structure capable of supporting buildings of various designs and dimensions. Additionally, an admixture can be incorporated to enhance specific characteristics of the concrete. For more details on the purpose, uses, and variations of admixtures in concrete, refer to available resources.
Uses of Admixtures?
Admixtures find extensive application in the subsequent scenarios:
- Admixtures are utilized in specialized tasks such as pre-stressed concrete bridges and precast concrete elements like beams, columns, floors, walls, etc. These tasks necessitate high early strength and workability retention at elevated temperatures. Once the concrete achieves early strength, it separates from the precast mold, thereby expediting production.
- Admixtures can be employed to enhance the workability of concrete without compromising its compressive strength or increasing the water-cement ratio. This proves advantageous in cases where concrete pores are restricted due to congested reinforcement or intricate formwork.
- In ready-mix concrete, admixtures can be utilized when a retarding action with high workability is required. This proves beneficial when concrete needs to be transported over long distances before placement.
- Concrete used in hydraulic structures, damp-proof courses, swimming pools, retaining walls, kitchen gardens, etc., requires reduced permeability. An admixture can confer permeability reduction properties to the concrete.
- In construction projects that demand high-performance concrete, admixtures may be added to the mix to enhance strength at a later stage and improve the final structure’s resistance to chemical attacks.
- Admixtures are added to concrete that is exposed to corrosion attacks resulting from severe exposure conditions.
- Admixtures that enhance bonding properties can be employed to bond new concrete to old concrete during patching.
The Influence of Admixtures on the Characteristics of Fresh Concrete
- It enhances the workability of concrete by not requiring additional water.
- It can either speed up or slow down the setting time of concrete. The initial setting time is the period between the addition of water to cement and the paste beginning to lose its plasticity. The final setting time is the duration between the addition of water to cement and the paste completely losing its plasticity.
- It alters the rate of bleeding in concrete. Bleeding is the process where water rises to the surface as free water until the concrete hardens.
- It enhances the pumpability of concrete, allowing it to be mobilized under pressure while maintaining its properties.
- It either reduces segregation in concrete or increases its cohesion. Segregation occurs when water and cement separate due to uneven distribution of concrete constituents.
- It aids in slowing down the rate at which concrete loses its workability.
The Influence of Admixtures on the Characteristics of Set Concrete
- It enhances the speed of strength growth in the initial phase.
- It boosts strength and toughness while reducing permeability.
- It aids in managing expansion triggered by the alkali-aggregate reaction.
- It enhances the adhesion of concrete to reinforcement or existing/new concrete.
- It inhibits the corrosion of steel bars embedded in the concrete.
Types of Chemical Admixtures
(IS) code 9103
- Accelerating Admixtures
- Retarding Admixtures
- Water-Reducing Admixtures
- Superplasticising Admixtures
- Air Entraining Admixtures
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