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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