8/9/2019 Use of Chemical and Mineral Admixtures in Concrete Ppt

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Use of Chemical and Mineral

Admixtures in Concrete

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What is admixtures ?

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Admixtures are the chemical compounds in

concrete other than Portland cement, water,and aggregates that are added to the mixture

immediately before or during mixing .

Admixtures have formulated chemical

composition and special chemical action, and

are used to modify certain properties ofconcrete.

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FUNCTIONS OF

ADMIXTU!S

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Admixtures can "e classi#ed "$function as follo%s&

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The general action of accelerators is to cause more

rapid dissolution of compounds of cement , particularly

Tricalcium silicate , in water and hence facilitate morerapid hydration of these compounds.

The generally used accelerators consist of NaCl,

Na2Co, !2"#$ ,!#%.

Acceleratin' admixturesor accelerator

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An admixture used to speed up the initial set of the

concrete is called ac accelerator.

These are added in concrete either

&'( To increase the rate of hydration of cement ,and

hence to increase the rate of development of strength .

&2( To shorten the setting time

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An increase in the rate of early strength development

may help in

&'( )arlier removal of forms,

&2( *eduction of re+uired period of curing

&( )arly placement of structure in service.

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Air (entrainin' admixtures

Air entering admixtures help to incorporate a controlled

amount of air , in the form of million of minute non

coalescing bubbles distributed throughout the body ofconcrete ,during mixing, without significantly altering

the setting or the rate of hardening of concrete.

n air/entrained concrete ,the air bubbles provide

space to relieve the pressure of expanding ice as

shown fig.&'(

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Water reducin' admixturesor )lastici*ers

Water reducin' admixtures ena"le a'i+en fresh concrete mix to ha+e hi'her

,o%a"ilit$ -%or.a"ilit$/ %ithoutincreasin' the %ater content increasin'the %ater content %hich results in fasterrate of concrete 0lacement 1 eas$

0lacement in relati+el$ 0oorl$ accessi"lelocation %ithout +i"ration 1true shutter#nish for hi'hl$ reinforced mem"ers andreduction in cement content 2

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Wor.in' of su0er0lastici*ers

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T)C%NCA P)*3#*4ANC)

The performance of concrete is normally optimi0ed in

one of the following four ways 5

'. To produce flowing concrete 2. To produce concrete with varying low water cement

ratio

. To produce high performance concrete $. To produce concrete mix with reduced cement

content.