1.8 AdmixtureBETTER

8/7/2019 1.8 AdmixtureBETTER

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ADMIXTURES???? ADMIXTURES????

Materials added to the concrete besidesMaterials added to the concrete besidescement, water and aggregate.cement, water and aggregate.

To improve the properties of the concreteTo improve the properties of the concreterequired.required.

Admixtures can be divided in 2 groups that is: Admixtures can be divided in 2 groups that is:a) Chemical admixturesa) Chemical admixturesb) Mineral admixturesb) Mineral admixtures



FUNCTION OF ADMIXTUREFUNCTION OF ADMIXTURE

To improve workability of fresh concreteTo improve workability of fresh concrete

To improve durability by entrainment of To improve durability by entrainment of

airair

To reduce the water requiredTo reduce the water required

To accelerate setting & hardening & thusTo accelerate setting & hardening & thus

to produce high early strengthto produce high early strength

To aid curingTo aid curing

To impart water repellent / waterTo impart water repellent / water

proofing propertyproofing property



To cause dispersion of the cement To cause dispersion of the cement

particles when mixed with waterparticles when mixed with waterTo retard settingTo retard setting

To improve wear resistance (hardness)To improve wear resistance (hardness)

To offset / reduce shrinkage duringTo offset / reduce shrinkage duringsetting & hardeningsetting & hardening

To cause expansion of concrete andTo cause expansion of concrete andautomatic prestressing of steelautomatic prestressing of steel

To aerate mortar / concrete to produce aTo aerate mortar / concrete to produce alight light--weight product weight product



To impart colour to concreteTo impart colour to concrete

To offset or reduce some chemicalTo offset or reduce some chemicalreactionreaction

To reduce bleedingTo reduce bleeding

To reduce the evolution of heat To reduce the evolution of heat



Among the type of chemical admixture Among the type of chemical admixture

used are:used are:a) Acceleratora) Acceleratorb) Water reducing Admixtureb) Water reducing Admixturec) Super plasticizerc) Super plasticizerd) Air Entraining Admixturesd) Air Entraining Admixturese) Retarding Admixturese) Retarding Admixturesf) Corrosion Inhibitorsf) Corrosion Inhibitors

g) Alkalig) Alkali--Aggregate Reaction Inhibiting Aggregate Reaction Inhibiting Admixtures Admixtures

h) Shrinkage Reducing Admixturesh) Shrinkage Reducing Admixtures



ACCELARATOR ACCELARATOR

Added to increase the rate of hydration of Added to increase the rate of hydration of concrete mix which then lead to theconcrete mix which then lead to theincreases in the rate of development of increases in the rate of development of

strength and greater heat evolution.strength and greater heat evolution. And to shorten the setting time And to shorten the setting time

More rapid gain of strength & rapidMore rapid gain of strength & rapid

settingsettingDisadvantages is possible cracking due toDisadvantages is possible cracking due to

heat evolution & possibility of corrosionheat evolution & possibility of corrosion

of embedded reinforcement of embedded reinforcement



WATER REDUCING ADMIXTUREWATER REDUCING ADMIXTURE

/ PLASTICISERS / PLASTICISERS Used to reduce the amount of Used to reduce the amount of water necessary to produce awater necessary to produce aconcrete of a given consistencyconcrete of a given consistency

To increase the slump for a givenTo increase the slump for a givenwater content water content

To obtain specified strength at To obtain specified strength at lower cement content lower cement content

Increases workability with fasterIncreases workability with fastergain of strengthgain of strength

Increase the slump, reduce waterIncrease the slump, reduce water

content, save cement content, save cement Disadvantages, it has the risk of Disadvantages, it has the risk of

corrosioncorrosion



SUPERPLASTICIZERSUPERPLASTICIZER

By adding to a hydraulic binder, givesBy adding to a hydraulic binder, givesvery high workability and allows a largevery high workability and allows a largedecrease in water content for a givendecrease in water content for a given

workabilityworkability Allows the particles to be more workable Allows the particles to be more workable

where it enable working with low w/cwhere it enable working with low w/cratioratio

Enhances hydration process, increasesEnhances hydration process, increasesstrengthstrength

Eliminate concrete segregation & allowEliminate concrete segregation & allow

good dispersion of cement particles ingood dispersion of cement particles inwater acceleratin the rate f h drati n.water acceleratin the rate f h drati n.



Uniform distribution of cement particles isUniform distribution of cement particles is

partly responsible for the highly earlypartly responsible for the highly earlystrength in concrete made withstrength in concrete made withsuperplasticizer.superplasticizer.

Advantages of using Superplasticizers are Advantages of using Superplasticizers area) Decreased time to place and finisha) Decreased time to place and finishb) Accelerated curing periodb) Accelerated curing periodc) Early removal of formworkc) Early removal of formwork

Excessive dosage may render concreteExcessive dosage may render concretetoo fluid, causing severe segregationtoo fluid, causing severe segregation



AIR ENTRAINING ADMIXTURES AIR ENTRAINING ADMIXTURES

An entraining concrete containing An entraining concrete containingair in a rather special form of aair in a rather special form of abubble that trapped in concretebubble that trapped in concrete

Workability improved, increase inWorkability improved, increase inslump, easier placing, increasedslump, easier placing, increased

durability, better resistance todurability, better resistance tofrost actionfrost action

Able to reduce segregation Able to reduce segregationtendency and control of bleedingtendency and control of bleeding

It is necessary to have a carefulIt is necessary to have a careful

control of air content and mixingcontrol of air content and mixingtimetime



Wetter mixes tend to result in higher airWetter mixes tend to result in higher air

content while mixes rich in sand entraincontent while mixes rich in sand entrainless air as fine material competes forless air as fine material competes foravailable waterIf air entrainment is to beavailable waterIf air entrainment is to beused, sand content need to be reducedused, sand content need to be reduced

Bubbles produced by air entrainment areBubbles produced by air entrainment arequite different from entrapped airquite different from entrapped airbecause:because:

They are sealed and wouldn't be filledThey are sealed and wouldn't be filledwith water during normal saturation of with water during normal saturation of the concretethe concrete

They are very small and well distributed.They are very small and well distributed.



Used to increase concrete durability byUsed to increase concrete durability by

protecting it against freezeprotecting it against freeze--thaw cyclethaw cycledamage.damage.

By entraining air in concrete to form aBy entraining air in concrete to form amicroscopic airmicroscopic air--void system , thevoid system , theexpansion is provided a relief valveexpansion is provided a relief valvesystem.system.

The air void system in the hardenedThe air void system in the hardened

concrete paste allows water to freeze, withconcrete paste allows water to freeze, withthe empty air voids providing room for thethe empty air voids providing room for theexpansion that occurs as water changes toexpansion that occurs as water changes to

ice.ice.



RETARDING ADMIXTURERETARDING ADMIXTURE

Prolong or delay the setting time of Prolong or delay the setting time of cement paste in concretecement paste in concrete

Used in hot weather to reduce anyUsed in hot weather to reduce anypremature stiffening of the concrete andpremature stiffening of the concrete andconsequent loss of workabilityconsequent loss of workability

Often carried by ready mixed concreteOften carried by ready mixed concretevehicles to prevent the concrete settingvehicles to prevent the concrete settingin the drum in the event of brakdownin the drum in the event of brakdown

Disadvantage is, it may promote bleedingDisadvantage is, it may promote bleeding



CORROSION INHIBITORSCORROSION INHIBITORS

Able to reduce the rate of corrosion to a level Able to reduce the rate of corrosion to a levelthat major damage to concrete will bethat major damage to concrete will beavoided or at least greatly reduced.avoided or at least greatly reduced.



ALKALI AGGREGATE ALKALI AGGREGATE

REACTION INHIBITINGREACTION INHIBITING ADMIXTURES ADMIXTURES

Lithium and Barium salts can reduce theLithium and Barium salts can reduce the

expansion and cracking associated with alkaliexpansion and cracking associated with alkali--silica reactionsilica reaction



SHRINKAGESHRINKAGE--REDUCINGREDUCING

ADMIXTURES ADMIXTURES Able to reduce the amount of shrinkage that Able to reduce the amount of shrinkage that

occurs as hardened concrete dries.occurs as hardened concrete dries.



MINERAL ADMIXTURESMINERAL ADMIXTURES

Are natural pozzolanic materials or industrial Are natural pozzolanic materials or industrialbyby--products that are commonly used inproducts that are commonly used inconcrete to replace part of the cement orconcrete to replace part of the cement orsand.sand.

Types of mineral admixtures are:Types of mineral admixtures are:a) Fly asha) Fly ash

b) Silica Fumeb) Silica Fumec) Blended hydraulic cement c) Blended hydraulic cement

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1.8 AdmixtureBETTER