Disadvantages Of Chemical Admixtures

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3.6.1 Chemical Admixtures They reduce the cost of construction, modify the properties of concrete and improve the quality of concrete during mixing, transportation, placing and curing.
Some of the chemicals admixtures are:
1. Air-entrainment
2. Water-reducing
3. Set-retarding
4. Accelerating
5. Super-plasticizers
6. Corrosion-inhibitors

3.6.2 Mineral Admixtures These are inorganic materials that also have pozzolanic or latent hydraulic properties. These very fine-grained materials are added to the concrete mix to improve the properties of concrete (mineral admixtures), or as a replacement for Portland cement (blended cement).
 Fly ash: A by-product of coal-fired electric generating plants which is used to partially replace Portland …show more content…

Workability depends on water content, aggregate (shape and size distribution), cement content and age (level of hydration) and can be modified by adding chemical admixtures, like super plasticizer. Raising the water content or adding chemical admixtures will increase concrete workability. Excessive water will lead to increase bleeding (surface water) and segregation of aggregates (when the cement and aggregates start to separate), with the resulting concrete having reduced quality. Workability of fresh concrete is determined by following …show more content…

The slump test result is a measure of the behavior of a self-compacted inverted cone of concrete under the action of gravity. It is a measure of the concrete's workability or the dampness of concrete. Slump test as per IS: 1199 – 1959 is followed. The apparatus used for doing slump test are Slump cone and tamping rod.

Procedure to determine workability of fresh concrete by slump test:
• The internal surface of the mould is thoroughly cleaned and applied with a light coat of oil.
• The mould is placed on a smooth, horizontal, rigid and non absorbent surface.
• The mould is then filled in four layers with freshly mixed concrete, each approximately to one-fourth of the height of the mould.
• Each layer is tamped 25 times by the rounded end of the tamping rod (strokes are distributed evenly over the cross section).
• After the top layer is tamped, the concrete is struck off the level with a trowel.
• The mould is removed from the concrete immediately by raising it slowly in the vertical direction.
• The difference in level between the height of the mould and that of the highest point of the subsided concrete is measured.
• This difference in height in mm is the slump of the

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