Session 3 Admixtures

8/9/2019 Session 3 Admixtures

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Admixtures are those ingredients in

concrete other than Portland cement,

water, and aggregates that are added to

the mixture immediately before or during

mixing.


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Why Are They Used?

To modify properties of fresh &hardened concrete

To ensure the quality of concreteduring mixing, transporting, placing& curing

To overcome certain unexpectedemergencies during concreteoperations (ie, set retarders


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Efectiveness

!actors e"ecting results of use#Type & amount of cement#

$hemistry e"ects due to Portlandcomponents

%ater content & lump !lowability will e"ect how well admixtures

are activated due to internal agitationactitivity

'ixing time

"ects of admixtures are dependent on timeallowed to react.


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Benecial efects o admixtures on concreteproperties

Concrete

property

Admixture

%or)ability* uper plastici+er*Airentraining agents

etting Accelerators

*-etarders

trength ilica fume*Polymers

urability Airentraining agent*ilica fume*$orrosion inhibitors


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Reasons or usin admixtures

/. To reduce the cost of concreteconstruction

0. To achieve certain properties in concretemore e"ectively than by other means

1. To maintain the quality of concrete duringthe stages of mixing, transporting, placing,and curing in adverse weather conditions

2. To overcome certain emergencies duringconcreting operations


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Classication o admixtures/. Accelerating

0. -etarding1. %ater -educing

2. Air ntraining

3. %aterproo4ng

5. Pumping6. uper plastici+ers

7. $orrosion inhibitors

8. 'iscellaneous such wor)ability, bonding,

damp proo4ng, permeability reducing,grouting, gasforming, and pumpingadmixtures


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7. hrin)age reducers

8. Al)alisilica reactivity inhibitors

/9. $olouring admixtures

//.'iscellaneous admixtures suchwor)ability, bonding, damp proo4ng,permeability reducing, grouting, gasforming, and pumping admixtures

Classication oadmixtures


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Admixtures or concrete


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Admixtures


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Permit early removal of formwor)

reduce time required for curing

reduce the time required to structurebe in use

in emergency repair wor)

Acceleratin Admixtures


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Acceleratin Admixtures

used to accelerate strengthdevelopment of concrete at an earlyage.

Typical 'aterials are# $alcium chloride# most commonly used for

plain concrete.

Triethanolamine.

$alcium formate.

$alcium nitrate.

$alcium nitrite.


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Retardin Admixtures

:sed to retard the rate of setting of concrete at hightemperatures of fresh concrete (19;$ or more.


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Retardin Admixtures

ome reduction in strength at early ages (one to

three days accompanies the use of retarders.

The e"ects of these materials on the other

properties of concrete, such as shrin)age, may

not be predictable.

Thereore! acceptance tests o retarders

should "e made #ith actual $o" materials

under anticipated $o" conditions%


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The typical materials used asretarders are#

=ignin,

>orax,

ugars,

Tartaric acid and salts.

Retardin Admixtures


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-etarders are used to#/. o"set the accelerating e"ect of hotweather on the setting of concrete,

0. delay the initial set of concrete when

di?cult or unusual conditions ofplacement occur,

1. delay the set for special 4nishingprocesses such as an exposed aggregatesurface.

Retardin Admixtures


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Water&Reducin Admixtures

'aterials#

=ignosulfonates.

$arbohydrates.

@ydroxylated carboxylic acids.


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Water&Reducin Admixtures' (lastici)ers *

The e"ectiveness of water reducerson concrete is a function of theirchemical composition, concretetemperature, cement compositionand 4neness, cement content, andthe presence of other admixtures.


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:sed to reduce the quantity ofmixing water required to produceconcrete of a certain slump, reducewatercementing materials ratio,

reduce cement content, or increaseslump.

Typical water reducers reduce the

water content by approximately 3to /9.

Water&Reducin Admixtures' (lastici)ers *


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+ih&Rane Water Reducers ',uperplastici)ers *

These admixtures are added toconcrete with a lowtonormal slumpand watercementing materials ratio

to ma)e highslump Bowing concrete. !lowing concrete is a highly Buid but

wor)able concrete that can be placedwith little or no vibration or

compaction while still remainingessentially free of excessive bleedingor segregation.


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Applications where Bowing concrete isused#

/. thinsection placements,

0. areas of closely spaced and congestedreinforcing steel,

1. pumped concrete to reduce pump pressure,thereby increasing lift and distance capacity,

2. areas where conventional consolidationmethods are impractical or can not be used,and

3. for reducing handling costs.

,uper plastici)ers '+ih&RaneWater Reducers*


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-lo# a"le concrete #ith hih slump


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.s easily placed


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Even in areas o heavy reinorcinsteel conestion


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=ow water tocement ratioconcrete with lowchloridepermeabilityeasily made withhighrange water

reducers is idealfor bridge dec)s


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Plastici+ed, Bowingconcrete is easilyplaced in thin

sections


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,uperplastici)ers '+ih&Rane WaterReducers*

Typical superplastici+ers include#

ulfonated melamine formaldehydecondensates.

ulfonated naphthalene formaldehydecondensate.

=ignosulfonates.

Polycarboxylates.


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bleed signi4cantly less than control concretes ofequally high slump and higher water content.

@ighslump, lowwatercontent, plastici+edconcrete has less drying shrin)age than a high

slump, highwatercontent conventionalconcrete. has similar or higher drying shrin)age than

conventional lowslump, lowwatercontentconcrete.

The e"ectiveness of the plastici+er is increasedwith an increasing amount of cement and 4nesin the concrete.

,uperplastici)ers '+ih&Rane WaterReducers*


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Air&Entrainin Admixtures

:sed to purposely introduce andstabili+e microscopic air bubbles inconcrete. Airentrainment willdramatically improve the durability

of concrete exposed to cycles offree+ing and thawing .ntrained airgreatly improves concreteCs

resistance to surface scaling causedby chemical deicers


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-rost damae at $oints o apavement


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-rost induced crac/in near $oints


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caled concretesurface resulting

from lac) of airentrainment,poor 4nishing

and curingpractices


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The primary ingredients used in airentraining admixtures are

/.salts of wood resin (Dinsol resin,

0. synthetic detergents,1.salts of petroleum acids.


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Corrosion .nhi"itors


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The chlorides can cause corrosion ofsteel reinforcement in concrete.

!errous oxide and ferric oxide form onthe surface of reinforcing steel in

concrete. !errous oxide reacts with chlorides to

form complexes that move away fromthe steel to form rust. The chloride

ions continue to attac) the steel untilthe passivating oxide layer isdestroyed.



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$orrosioninhibiting admixtures chemicallyarrest the corrosion reaction.

$ommercially available corrosion inhibitors

include# calcium nitrite,

sodium nitrite,

dimethyl ethanolamine,

amines, phosphates,

ester amines.



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,hrin/ae&Reducin Admixtures

hrin)age crac)s,such as shown onthis bridge dec),

can be reducedwith the use ofgood concretingpractices and

shrin)age reducingadmixtures.

Documents

Session 3 Admixtures