Chemical Admixtures 2

8/6/2019 Chemical Admixtures 2

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Chemical admixtures is are materials that form offluids or powder.

This materials are added to concrete to give certain

characteristics and ease concrete workAdmixtures are added to the concrete at the time ofbatching or mixing.

The addition of this admixture in the mix must be

correct amount which is must be homogenous anduniformly mix in concrete. Eg : In normal use, admixture dosages are less than 5% by

mass of cement and are added to the concrete at the time ofbatching or mixing.


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Increase workability without increasing water contentor decrease the water content at the same workabilityRetard or accelerate time of initial settingReduce or prevent shrinkage or create slight expansionReduce segregationRetard or reduce heat evolution during early hardening

Accelerate the rate of strength development at earlyages Increase strength (compressive, tensile, or flexural)

Reduce rate of slump loss


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Collapse Shear True

In a collapse slump theconcrete collapses

completely because theindication of too wet a

mix

In a shear slump the topportion of the concrete

shears off and slipssideways is due to

inadequate mix (less

cement)

In a true slump theconcrete simply

subsides, keeping moreor less to shape becauseits cohensive and rich.


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Type and amount of cement

Water content

Mixing time

SlumpTemperatures of the concrete and air.

y Note : Sometimes, by altering the concrete mixture-reducing the water-cement ratio, adding additional

cement, using a different type of cement, or even changingthe aggregate and aggregate gradation can give similareffects to those achieved through the addition of admixtures .


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COMMONTYPE OF

ADMIXTURES

ACCELERATOR

RETARDERS

AIRENTRAINING

WATERREDUCING

PLASTICIZERS


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Accelerators speed up the hydration (hardening) of the

concrete.

Typical materials used are CaCl2, Ca(NO3)2 and NaNO3.

However, use of chlorides may cause corrosion in steel

reinforcing and is prohibited in some countries, so that

nitrates may be favored. Eg: Calcium chloride( CaCl2 )


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FUNCTIONACCELERATOR

Increase the raceofsetting and

hardening

Overdosing mix

causes corrosionto the

reinforcementbar

Used in buildingstructures where earlyremoval of formworkand in cold climate

countries

use in concretework


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CaCl2

NaNO3


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Us ally se i t cli ate c tries t verc eeffect f i a ie t te erat re

Hel s t re ce lee i ryi s ri a e i c crete


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Retarder For OilWell Cement

Gray Gysum Retarder


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y Air entrainments add and entrain tiny air bubbles inthe concrete, which will reduce damage during freeze-thaw cycles

y Increasing the concrete's durability and workability.

y Addition of 1% of this admixture with respect to

weight of cement causes 5% of air being trapped, itreduces 4% to 7% concrete strength.


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y Use in cold climate countries.

y Air-entraining admixtures stabilize air bubbles in

concrete by reducing the surface tension at the air-water interface.

y The shearing action of the mixer and aggregates will

entrain and divide air into finely divided bubblessurrounded by the stabilizing agent.


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LowHeat e e t ; ortlaozzola a e e t ; ir-

trai i

Air- tr i concretecontains inute


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y Increase the workability of plastic or "fresh" concrete

y Allowing it be placed more easily, with less

consolidating effort

y Used to reduce the water content of a concrete whilemaintaining workability (water reducers)


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Superplasticizers Also known as lasticizers or i -range water re ucers

(re ce ter c te t y 12 t 30 erce t)

Be use to increase worka ility ore t an is racticalwit tra itional lasticizers.

e effect of superplasticizers lasts only 30 to 60inutes, epending on t e rand and dosage rate, and

is followed byarapid loss in workability.

Usually added to concrete at t e jobsite


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Figure shows how

superplasticizers isproduced


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A material that changes the colour of reflected ortransmitted light as the result of wavelength-selectiveabsorption.

A pigment must have a high tinting strength relative tothe materials it colours.NOTE : It must be stable in solid form at ambienttemperatures.

It more to aesthetics


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Natural ultramarine pigment in powdered form

Synthetic ultramarinepigment is chemically

identical to naturalultramarine


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Water reducing admixtures is needed to increase theworkability of concrete when the amount of watercement ratio used is reduce to strengthen the concrete.

When the amount water cement used is low, the workability of

concrete reduces, but the strength of concrete increases

Cement Workability Strength

This is when water reducing admixtures is needed toact as dispersing agent in concrete


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Maintain the desired workabilityof the mix even though the water

cement ration reduces

To increses the cohesivinessof the concrete.

Required to make highstrength concrete


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CALCIUM LIGNOSULP ONATE

NATRIUMGLUCONATE


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The admixture is use for ;-

To control the quality of concrete during

mixing, transporting, placing, and curingReduce the cost of concrete construction

Alter the properties of hardened concrete

To overcome certain emergencies duringconcrete operations

Documents

Chemical Admixtures 2