What are Admixtures in Concrete?
Admixtures are components that are added to concrete before or during mixing to change one or more of the properties of the concrete in its plastic or hardened form. Water-soluble chlorides and sulphates are likely to be present in the admixtures. If these are present in large numbers, they may cause damage to concrete structures over time.
- The chlorides have the potential to corrode the steel reinforcing.
- Sulphates may induce concrete degradation by producing sulpho-aluminates.
Most admixtures for concrete have the drawback of being difficult to measure the behavior of the concrete under diverse scenarios quantitatively. As a result, the performance of an admixture is assessed by contrasting the properties of concrete, including the admixture under test, with those of concrete containing no admixture or a reference admixture.
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Types of Admixtures in Concrete
Admixtures are divided into different categories depending upon the functions they serve. The amount of water necessary to achieve the required slump of 5%-10% can be reduced. Retarding slows the concrete setting, keeping it workable, and is frequently used to combat the accelerating effect of hot weather. Accelerating increases the rate of early strength development and shortens the curing time. Admixtures can lower water content in concrete by 12-30%, resulting in a highly fluid but usable form known as flowing concrete. Admixtures form small air bubbles uniformly throughout the concrete mix to improve cohesiveness and freeze-thaw resistance.
- Mineral Admixtures: Fly ash, rice husk ash, silica fume, ground granulated blast furnace slag.
- Chemical Admixtures: Accelerating admixture, retarding admixture, water-reducing admixture/plasticizer, air-entraining admixture, super plasticizing admixture.
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Uses of Admixtures in Concrete
Workable, finishable concrete should be robust, durable, watertight, and wear-resistant. Rather than using admixtures, these properties can frequently be acquired quickly and economically by selecting appropriate components (except air-entraining admixtures when needed). Hydrated cement paste shrinks when moisture evaporates from its tiny pores. The surface tension of the remaining water tries to pull the pores together as moisture is lost in these microscopic pores, resulting in a loss of volume over time. Shrinkage-reducing admixtures (SRAs) reduce pore surface tension and the effects of drying shrinkage.
The following are the main reasons for employing admixtures:
- To reduce the unit weight of concrete.
- To more efficiently obtain particular qualities in concrete than by other techniques.
- To maintain the quality of concrete during the mixing, transportation, placement, and curing stages in unfavourable weather.
- To improve the resistance against frost action.
- To improve the resistance against the action of acids.
- To reduce the permeability of concrete.
- To increase the strength of concrete.
- To improve corrosion resistance.
- To improve resistance against segregation and bleeding.
- To increase the durability.
Chemical Admixtures in Concrete
Chemical admixtures are used to minimize construction costs, adjust the properties of hardened concrete, ensure concrete quality during mixing, shipping, placing, and curing, and solve certain problems that arise during concrete operations.
- Accelerating admixtures: When added to concrete, mortar, or grout, an accelerating admixture accelerates the hydration of hydraulic cement, reduces the time it takes to set, or accelerates the hardening or strength development. The uses of an accelerator are to shorten the set time of concrete, allowing a cold-weather concreting, early removal of formwork, early surface finishing, and early load application. Examples include calcium chloride, sodium chloride, sodium thiocyanate, fluorosilicates, calcium nitrate, and nitrate.
- Retarding admixtures: An admixture that delays the setting of cement paste, and hence of mixtures, such as mortar or concrete cement. Retarding admixtures increase the set time of concrete and are used in hot weather when the normal setting time of cement gets reduced due to high temperatures. Examples are sugar, carbohydrate derivatives, soluble zinc salts, soluble borates, calcium sulphate, starch, ferrous and ferric chloride, lignosulphonate acids, etc. A small quantity of sugar ( 0.05% by mass of cement) delays the setting time of concrete by about 4 hours.
- Water reducing admixtures/Plasticizer: An admixture that increases the workability of concrete with the same water and cement contents or reduces the water content in the concrete mix when the cement content is unaltered, which in turn increases the compressive strength of concrete without changing the workability or saves the cement content by the reduction in both the cement and water contents in the mix while maintaining the same workability and compressive strength. These admixtures increase the setting time by about 2 to 6 hours, during which concrete can be vibrated, revibrated, and finished. Examples are lignosulphonate and its salts, hydroxylated carboxylic acids and salts, etc.
- Air entraining admixtures: An admixture for concrete or mortar which causes air to be incorporated as minute bubbles in the concrete or mortar during mixing. It helps to increase the workability, resistance to freezing and thawing, etc. It also helps in the disruptive action of de-icing salts. Examples are oleic and stearic acid, aluminium powder, hydrogen peroxide, animal or vegetable fats and oils, water-soluble soaps of resin acids, etc.
- Super plasticizing admixture: An admixture for mortar or concrete which imparts very high workability or allows a large decrease in water content for a given workability. Superplasticizers are the same as that plasticizers in terms of action but are chemically different from plasticizers. They may reduce the mixing water requirement by about 30%, whereas plasticizers may reduce about 15% only. Examples are modified lignosulphonate, sulfonated melamine formaldehyde, naphthalene sulphonate formaldehyde, etc.
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