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The history of concrete admixtures is as old as the concrete. Concrete admixtures are the part of concrete technology. In olden days, sometimes, site engineer used to suggest (as per their experience) adding chemicals before or during mixing of concrete ingredients. Concrete admixtures are an important component in concrete with cement, water, and aggregates. Nowadays concrete admixtures are an important and increasingly widespread component in the world. Concrete without admixtures is nowadays an exception. They are not harmful to concrete structures however they ultimately improve the properties of the fresh and hardened concrete.
These days concrete is being used for various purposes to make it suitable for different conditions. Ordinary concrete may not give us the required quality performance such as durability, strength, workability and finishing quality of concrete. In such situations, concrete admixtures are used to modify or improve the ordinary concrete properties. i.e. mass concreting, concreting at narrow spaces, at remote places where water is not available easily, where formwork need to be removed early, concreting work near coastal area, etc.
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Importance of Concrete Admixtures:
However, note that admixture must not be used for the replacement of poor quality of ingredient or poor workmanship. 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.
Admixtures are primarily used for:
- 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.
- To control the shrinkage cracks.
- To achieve specific concrete properties more effectively.
- To ensure quality of concrete during mixing, transporting, placing, and curing in adverse weather condition.
- Modify the rate or capacity for bleeding.
- Improve pumpability
- Decrease permeability
- Increase bond between existing and new concrete.
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:
01. Chemical Admixtures for Concrete:
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 enhances the properties of hardened concrete.
Chemical Admixtures are classified as below.
(a) Air Entertainers:
Air entertainers are used to distribute tiny air bubbles in the concrete. Adding this admixture reduces the bleeding and segregation of fresh concrete. It also improves the resistance of concrete to severe frost action that reduce the damage during freeze-thaw cycles, there by increasing the durability of concrete. They also help to reduce the density of concrete.
(b) Water Reducers:
Water reducer admixtures are used to reduce the quantity of mixing water required to produce concrete of a desired slump. They reduce water-cement ratio, reduce cement content and increase slump and we can obtain specific strength of concrete.
(c) Set Retarders:
A retarder is an admixture that slows down or to delay the chemical process of hydration so that concrete remains plastic and workable for a longer time than concrete without the retarder. These types of concrete admixtures are widely used where concreting is carried out at high temperature. As high temperature could produce a faster initial setting of concrete.
(d) 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. Calcium chloride is the most common accelerator; however, it could promote corrosion activity. But, best concrete practices like proper mix design, consolidation, adequate cover prevent the corrosion issues. These types of concrete admixtures are especially useful for modifying the concrete properties in cold weather.
(e) Corrosion Inhibitors:
Corrosion inhibitors are used to reduce the corrosion in steel and steel bars in concrete. We can reduce the maintenance cost of structure by using corrosion inhibitors concrete admixtures. They can reduce maintenance cost of RCC structures throughout a typical service life of 30-40 years.
02. Mineral Admixtures for 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 mineral admixtures are used.
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 “pozzolana,” reacting chemically to impart far greater strength and durability.
A mineral admixture makes mixtures more economical, reduce permeability, influence other concrete properties and increase the 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.