Contd. from Part – I
( https://civilconstructionresourcez.wordpress.com/2012/02/07/admixtures-for-concrete-part-i/ )
Admixtures can be classified in several categories depending on the purposes of their use. Some common among them are water reducing admixtures, accelerating admixtures, retarding admixtures, air entraining admixtures etc.
Accelerating admixtures : These admixtures are used in order to quicken or accelerate the setting process of concrete mixes. They are normally useful in cold weather conditions when setting of concrete takes much longer time than usual. An example of a common accelerating admixture is calcium chloride (CaCl2). For CaCl2, a dosage of upto 1.5% of cement content of a mix can be introduced. However, this admixture is not recommended for prestressed concrete. There are many other accelerating admixtures such as Sigunit Powder or Sigunit LN10 etc.
Retarding Admixtures : These admixtures are used in order to slow or retard the setting process of concrete mixes. That means increasing the setting time of concrete. They are normally useful in hot weather condition when setting of concrete takes place much quicker than usual which is harmful to concrete. Dosage of retarders are usually limited to 0.5% of cement content of mix unless a higher value is specifically prescribed by the manufacturer. An example of a commonly available retarding admixture is sugar. Normal dosage is around 0.05% of weight of cement in the mix. A dosage of 0.2% to 1% can prevent the setting process completely. There are many other retarding admixtures such as Plastiment R etc.
Water reducing Admixtures : These admixtures are used in order to reduce the water content and thus the w/c ratio without reducing the workability of the concrete mix. These can also be used to increase the workability of a concrete mix without increasing the water content or the w/c ratio of the mix. As explained in Part – I, these can also be used for modifying other properties of a mix.
Water reducing admixtures are possibly the most widely used admixtures in any construction project. There are huge no of varieties available in the market from virtually all additive manufacturers. These are also commonly found as plasticisers and superplasticisers. As a thumb rule the dosages of plasticisers and superplasticisers should be restricted to 1% and 2% of weight of cement respectively unless higher values are permitted by manufacturers. It is to be noted that many water reducing admixtures also retard setting of concrete mixes by an hour or so to several hours.
Air entraining admixtures : These admixtures are used not only to increase workability but most importantly to increase resistance of concrete against freezing and thawing in extremely cold weather which cannot be achieved by ordinary water reducing or workability enhancing admixtures. These also help in reducing seggregation and bleeding. Resins, fats etc. are examples of these admixtures. FLOMO AEP, Sika AER etc. are few of the many names of air entraining admixtures available in market.
There are several other types of admixtures used for various other purposes. Some admixtures contain soluble chlorides & sulphates which can be harmful if present in excess quantities. While excess chlorides may cause corrosion in steel reinforcement, excess sulphates can cause disintegration of concrete in the long run. Manufacturers must specify their content in admixtures and it is necessary to ensure that the gross chlorid & sulphate content from all sources in any concrete mix donot exceed the maximum values prescribed in specifications.