Admixtures used in Concrete
Admixtures are the substances which are added in the concrete in addition to its ingredients to enhance its performance.
In addition to the main components of concrete like cement, sand, coarse aggregates and water, admixtures are often used to improve concrete performance.
Admixtures are used to obtain following objectives:
- to accelerate or retard setting and hardening.
- to improve workability.
- to increase strength.
- to improve durability.
- to decrease permeability.
- to impart other desired properties.
Following types of admixtures are commonly used:
Chemical admixtures used in concrete
Air entraining agents
These are probably the most commonly used admixtures at the present time. They cause the enterainment of air in the form of small disperse bubbles.
- These improve workability and durability and reduce segregation during placing.
- They decrease concrete strength because of increasing void ratio. However this can be overcome by reduction of mixing water without loss of workability.
These are used to reduce setting time and accelerate early strength development of concrete. Calcium chloride is the most widely used accelerator. Because it is cost effective. But it should not be used in prestressed concrete. Because of its tendency to promote corrosion of steel. Non-chloride, non corrosive accelerating admixtures are also available.
Set retarding admixtures
- These are used to offset the accelerating effect of high ambient temperatures.
- These are used to keep the concrete workable during the entire placing period.
- This helps to eliminate cracking due to form deflection.
- These also keep concrete workable long enough that succeeding lifts can be placed without the development of cold joints.
Plasticizers are organic and inorganic compounds which are used to reduce the water requirements of a concrete mix for a given slump. Reduction in water demand may result in the reduction of water cement ratio. This can also lead to increase in slump for the same water cement ratio and cement content.
Plasticizers work by reducing the interparticle forces that exist between the cement grains. Thus increasing the paste fluidity.
These are high range water reducing admixtures. These are used in high strength concrete with a very low water cement ratio. They can be used at higher dosage rates without slowing hydration process.
The effects of water reducing admixtures vary with:
- different cement.
- change in water cement ratio.
- mixing temperature.
- ambient temperature.
That is why trial batches are generally required to fix the dose of these admixtures.
Finely divided siliceous or siliceous and aluminous material that reacts chemically with slakes lime at ordinary temperature and in the presence of moisture to form a strong slow-hardening cement.
Fly ash and silica fumes fall under this category.
It is precipitated as a by product of exhaust fumes of coal fixed power stations. It is very finely divided and reacts with calcium hydroxide present in the cement in the presence of moisture to form a cementitious material. It tends to increase the strength of cement at ages over 28 days.
It is highly active and combine with calcium hydroxide, the soluble product of cement hydration, to form more calcium slicate hydrate. This is the insoluble product of cement hydration. These are used to replace a part of portland cement in concrete mixes.
Silica fume is a by product resulting from the manufacture of ferro silicon alloys and silicon metal in electric arc furnaces. It is extremely finely divided and is highly cementitious when combined with portland cement.
In contrast to fly ash, silica fume contribute mainly to strength gain at early ages, from 3 to 28 days. Both fly ash and silica fume have been important in the production of high strength cement.
When these are used, then we will refer to water cementitious material ratio rather than water cement ratio.