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Admixtures are ingredients other than water, aggregates, hydraulic cement, and fibres that are added to the concrete batch immediately before or during mixing.
- Improved quality
- Acceleration or retardation of setting time
- Enhanced frost and sulfate resistance
- Control of strength development
- Improved workability
- Enhanced finish ability
Chemical admixtures are added to concrete in very small amounts mainly for the entrainment of air, reduction of water or cement content, plasticization of fresh concrete mixtures, or control of setting time.
Chemical admixtures are material in the form of powder or fluid that is added to the concrete to give certain characteristics not obtainable with plain concrete mixes. Chemical admixtures available are:
Accelerates i.e. speed up the hydration (hardening) of the concrete.
Retarders i.e. slow the hydration of concrete and are used in large or difficult pours where partial setting before the pour is complete is undesirable.
Air entraining agent that add and entrain tiny air bubbles in the concrete, which will reduce damage during freeze-thaw cycles, thereby increasing the concrete's durability.
Plasticizers that increase the workability of plastic or "fresh" concrete, allowing it be placed more easily, with less consolidating effort. They can be used to reduce the water content of a concrete while maintaining workability and are sometimes called water-reducers due to this use. Such treatment improves its strength and durability characteristics.
Super plasticizers (also called high-range water-reducers) are a class of plasticizers and can be used to increase workability more than is practical with traditional plasticizers. They increase the retention time of the concrete and are available in two types of bases:
- 1. Napthalene
- 2. Polycarboxylate
Corrosion inhibitors are used to minimize the corrosion of steel and steel bars in concrete.
Bonding agents are used to create a bond between old and new concrete (typically a type of polymer).
Pumping aids improve pump ability, thicken the paste and reduce separation and bleeding.