Admixtures are substances added to concrete mixes in order to modify one or more properties of the mixes to render them suitable for specific needs or requirements. For example, if a concrete mix is to be rendered suitable for easy pouring and compacting in a structure congested with reinforcement, an admixture that enhances workability is a much better option than adding further water to the mix. Admixtures are also used in grout and similar other mixes for the same purpose as mentioned above.
Some, admixtures are capable of modifying more than one properties of the concrete or grout mix even though they are normally used to modify a single one. In other words, such an admixture used for a particular purpose can also be introduced in a mix for one or more other purposes.
For example, a water reducing admixture can also be used to enhance the workability of a concrete mix as it is capable of modifying both the properties i.e. water to cement ratio (w/c ratio) as well as workability of the mix. That means, if higher workability is to be achieved without adding additional water to the mix, ie without changing the w/c ratio of the mix, a water reducing admixture can be introduced to the mix which otherwise is used to reduce the w/c ratio without reducing workability. In the first case, workability gets increased without addition of any further water to the mix and hence it is as good as reducing water. In the second case, the admixture enables reduction of water content (or the w/c ratio) of the concrete mix without decreasing it’s workability. In other words, the desired workability has been kept intact and the w/c ratio too has been reduced by introducing the admixture.
Now, the same admixture can also modify a third property which is the strength of the concrete when the mix hardens. Actually, the basic aim of modifying the property called the w/c ratio is only to modify the property named strength eventually. The basic purpose of reducing the water content, and thus the w/c ratio, of a concrete mix is to increase the strength of the resulting concrete.
Interestingly, the same water reducing admixture can also be used to modify a fourth parameter of the mix – the cement content. Suppose, a concrete mix has been designed to achieve a desired characteristic strength (f”) at a desired workability (Wk) and w/c ratio without introducing any admixture. Now, if the aim is only to reduce the cement content of the mix without modifying other properties, then addition of a suitable dose of the same water reducing admixture can serve the purpose. That’s because, due to the introduction of the admixture water content will get reduced for the same workability Wk. Since, no change in f” is wanted, the cement content now can be reduced proportionately so that the same old w/c ratio w is maintained. In this case, the same old admixture has enabled reduction of the cement content, and thus more economy, without affecting other parameters at all.
The optimum doses of admixtures are usually prescribed clearly by their manufacturers. The same should be considered during the process of designing the mix itself in order to to fix the exact quantities to be introduced.
Contd. in Part – II
( https://civilconstructionresourcez.wordpress.com/2012/02/07/admixtures-for-concrete-part-ii/ )