Nowadays concrete is being used for various purposes to make it suitable in different conditions. In these conditions ordinary concrete may fail to exhibit the required quality performance such as durability, strength, workability and finishing quality of concrete. In such cases, admixtures are used to change the properties of the ordinary concrete so as to make it more suitable for any situation.
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Please however note that admixture should not be used for replacement of poor quality of ingredient or poor workmanship.
Concrete admixtures are the component in concrete other than cement, water, and aggregate that is added to the mix immediately before or during mixing.
Concrete Admixtures are used to improve the behaviour of concrete under various conditions and ensure the quality of concrete during mixing/transporting/placing/curing, both as a fresh concrete and hardened concrete.
Cement is a most common and widely used as binding material in construction. For concreting cement is mixed with crushed rock, sand and water in precise proportion to produce concrete. For getting better results, better workability, more strength, and finishing, cement or mortar admixtures are used. Admixtures are primarily used to reduce water content in concrete i.e keep water cement ratio as less as possible.
- To increase compressive strength and durability parameter of concrete.
- To enhance the workability and give more transportation and placing time.
- To impart specific property of concrete or to create favourable condition during placing.
Use of admixtures in concrete successfully depends on the use of appropriate methods of concreting and batching. Most admixtures are supplied in ready-to-use liquid form and are added to the concrete at the plant or on the project site.
Concrete admixtures are of two main types:
Chemical admixtures are water-soluble chemicals that are added in small quantity to concrete in order to change certain properties. It reduces the cost of construction and changes the properties of hardened concrete.
Chemical Admixtures are classified as below.
- bubbles in the concrete that reduce the damage during freeze-thaw cycles, thereby increasing the durability of concrete. They also help to reduce the density of concrete.
- Water Reducers-Water reducers are used to reduce the quantity of mixing water required to produce concrete of a certain slump, reduce water-cement ratio, reduce cement content and increase sluAir Entertainers- Air entertainers are used to distribute tiny air mp.
Two type of water reducers available in concrete admixture such as plasticizers and Superplastizers
- Set Retarders- A retarder is an admixture that slows down the chemical process of hydration so that concrete remains plastic and workable for a longer time than concrete without the retarder.
- Set Accelerators- Accelerators are added to concrete to increase the rate of early strength in concrete to permit earlier removal of formwork and reduce the required period of curing. They are useful when concreting on walls etc.
- Corrosion inhibitors- Corrosion inhibitors are used to reduce the corrosion in steel and steel bars in concrete.
A substance added to concrete both as a filler i.e. improving the physical structure by occupying the spaces between the cement particles, and as a “pozzolan,” reacting chemically to impart far greater strength and durability.
A mineral admixture makes mixtures more economical, reduce permeability, influence other concrete properties and increase strength.
Mineral admixtures affect the nature of the hardened concrete though pozzolanic activity. Pozzolans are cementitious materials and include pozzolan such as fly ash and silica fume.With advent of new chemical and research on modern chemistry there is a flood of concrete admixtures in market .one should use it carefully, possibly in consultation with architect / Engineer. Sub-Standard brand may not help to achieve desired result leading to grave problems.