Effect of Usage of Admixture in Concrete

Mr. Hasan Rizvi, Asst. General Manager–Business Development, CICO Technologies Limited, New Delhi

Concrete consists of cement, sand, aggregate and water. Anything other than these if added in concrete either before or during mixing to alter the properties to our desired requirement are termed as admixtures. The use of admixtures offers certain beneficial effects to concrete like improved workability, acceleration or retardation of setting time, reduce water cement ratio, and so on.

There are two basic types of admixtures available: chemical & mineral. Admixtures like flyash, silicate fume, slag comes in the category of mineral admixtures. They are added to concrete to enhance the workability, improve resistance to thermal cracking and alkali–aggregate reaction and to enable reduction in cement content.

Flyash is fine residue left after combustion of ground or powdered coal. They are all generally finer than cement and consist mainly of glassy–spherical particles as well as residues of hematite and magnetite, char and some crystalline phases formed during cooling. The use of flyash in concrete makes the mix economical, and improves the workability, reduces segregation, bleeding and reduced heat of hydration but also provides ecological benefits.

Silica fume, which is also known as microsilica. It is obtained as a byproduct during the production of silicon and ferrosilicon alloys. The particle size of silica fume is 100 times smaller than cement particles i.e. its fine as cigarette smoke. Its a highly effective pozzolanic material, which improves the properties of concrete such as improved compressive strength, bond strength, abrasion resistance, dense concrete that results in protection of reinforcement against corrosion.

Chemical admixtures are added to concrete in very small amounts mainly for air entrainment, reduction of water or cement content, plasticizing of fresh concrete mixtures or to control the setting time of concrete. These admixtures can be broadly catagorised as superplasticizers, accelerators, retarders, water reducers and air entraining admixtures.

Superplasticizers are added to reduce the water requirement by 15 to 20% without affecting the workability leading to a high strength and dense concrete. Superplasticizers are liner polymers containing sulfonic acid groups attached to the polymer at regular intervals. The commercial formulation can be sulfonated melamine–formaldehyde conden- sates, sulfonated naphthalene formaldehyde condensates, and modified lignosulfonates, polycar- boxylate derivatives. The main purpose of superplasticizers is to produce a flowing concrete with very high slump 175 to 200 mm which can be used effectively in densely reinforced structures, the increased slump of concrete depends upon dosage, type & time of super– plasticizers (it's better to add it before concrete is placed.), water cement ratio, nature and amount of cement.

Accelerators are added to reduce the setting time of concrete thus helping early removal of forms and are also used in cold weather concreting. Calcium chloride is the most commonly used accelerator for concreting. The use of calcium chloride in reinforced concrete can promote corrosion activity of steel reinforcement. As people are getting aware so there is a growing interest in using chloride free accelerator.

Retarders are added to increase the setting time by slowing down the hydration of cement. They are preferred in places of high temperature concreting. Retarders consist of organic & inorganic agents. Organic retarders include unrefined calcium, sodium & ammonia salts lignosulfonic acids, hydrocarboxylic acids & carbohydrates. Inorganic retardants include oxides of lead, zinc, phosphate and magnesium salts. Most retarders also act as water reducers. They are called water-reducing retarders. Thus resulting in greater compressive strength due to low water cement ratio.

Water reducing admixtures are added to concrete to achieve certain workability (slump) at low water cement ratio. A concrete with specified strength at lower cement content thus saving on the cement. Water reducers are mostly used in hot weather concreting and to aid pumping. Water reducer plasticizers are hygroscopic powder, which can entrain air into concrete.

Air entraining admixtures entrain small air bubbles in concrete. These air bubbles act as rollers thus improving the workability and are also very effective in freeze-thaw cycles as they provide a cushioning effect on the expanding water in the concreting in cold climate.

Air entraining admixtures are compatible with most admixtures, care should be taken to prevent them from coming in contact during mixing.

Generally, the effectiveness of both the types of plasticisers are dependent on the ambient temperature condition and thus in summer the amount of plasticiser to be used to cater for the same degree of increase in plasticity can be more than the quantity to be used in winter.

Change in normal setting time within some fixed requirement also makes the production dependent on others chemicals and as such plasticisers with different nomenclatures are available in the market.

CICO Technologies Limited, an Indian ISO 9001: 2000 Company with 75 years backing produces a range of plasticising admixtures for concrete.

A number of RMC companies are using CICO admixtures. some modifications are required at the time of trials. modification in the Plasticisers can fulfill the requirements of any particular client.

Conclusion

It can be seen that proper use of admixtures offers certain beneficial effects to concrete including improved quality, acceleration or retardation of setting time, enhanced frost & sulphate resistance improves workability.
NBM&CW December 2008
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