Wednesday, February 08, 2017

Admixtures (as per IS :456-2000)

Admixtures (as per IS :456-2000)

The physical requirement for the main types of admixtures are given in IS 9103. Admixtures are of two types:

a) Mineral admixtures:

Mineral admixtures are finely divided siliceous materials, which are added to concrete in relatively large amounts. The IS 456:2000 permits the use of the following mineral admixtures: flyash, silica fume, rice husk ash, metakaolin, ground granulated blast- furnance slag, etc. They can be broadly divided into two groups:

1. Reactive mineral admixtures

It could be either pozzolonic (for example, low calcium flyash, silica fume), or cementitious (for example ground granulated blast furnance slag), or both pozzolonic & cementitious. When they are used to partially replace cement, they react with the calcium hydroxide in the hydrated cement paste to form complex compounds, which result in a reduction in permeability, improvement in the ultimate strength, water tightness and durability, besides imparting economy to the mix.

2. Inert mineral admixtures:

They do not have any cementitious or pozzolonic value and are added as a filter material ( for example, limestone powder).

b) Chemical admixtures:

They are inorganic or organic materials- solid or liquid- which when added to the normal components of a mix ( either concrete, mortar or paste), interact with the cementitious system through chemical, physical or physico - chemical means, modifying one or more properties of the mix in the fresh, setting, hardening or hardened state. They can increase the workability without increasing the water content, reduce or prevent settlement, modify the rate and / or capacity of bleeding, reduce seggregation and reduce slump loss, retard or accelerate the time of initial/ final setting. They can also retard or reduce heat evolution during early hardening, accelerate the rate of strength development at early ages, increase the compressive/ flexural / tensile strength of concrete, improve durability, control alkali aggregate reactivity, produce aerated concrete, inhibit corrosion of reinforcement, produce coloured concrete/ mortar etc. Commonly used admixtures are water - reducing/ plasticizing admixtures, set controlling admixtures, or retarders, air- entraining admixtures, accelerating admixtures, etc. The effectiveness of an admixture depends upon such factors as type, brand and amount of cement, water content, aggregate shape, grading and proportions, mixing time, slump, and temperatures of concrete and air.