ADMIXTURES IN CONCRETEADMIXTURES IN CONCRETE

1. 1. ChemicalChemical admixturesadmixtures

2. 2. Pozzolanic Pozzolanic admixturesadmixtures

AdmixturesAdmixtures

A material A material added to plastic concreteadded to plastic concrete or or mortarmortar, other than , other than water, aggregates, water, aggregates, cement, and fibrecement, and fibre TO TO change change one or one or more properties of fresh or hardened more properties of fresh or hardened concreteconcrete

When to use admixture?When to use admixture?

When the desired modification of When the desired modification of properties of fresh or hardened properties of fresh or hardened concrete concrete cannotcannot be achieved by be achieved by changes in the composition of the mix changes in the composition of the mix proportion proportion

ExampleExampleHigh-strength concreteHigh-strength concreteConcrete grade 80Concrete grade 80Cement = 450kg/mCement = 450kg/m33

Water/Cement ratio = Water/Cement ratio = 0.350.35Coarse Aggregate = 1120 Coarse Aggregate = 1120 kg/mkg/m33

Fine Aggregate = 980 Fine Aggregate = 980 kg/mkg/m33

WHAT WILL BE THE WHAT WILL BE THE PROBLEM?PROBLEM?

Form of AdmixturesForm of Admixtures

LiquidLiquid

FlakeFlake

PowderPowder

Chemical AdmixtureChemical Admixture

CHEMICAL ADMIXTURESCHEMICAL ADMIXTURES

Chemical added in Chemical added in very very small amountsmall amount or or quantity to the mixture quantity to the mixture to develop special to develop special properties in properties in fresh or fresh or hardened concretehardened concrete

1)1) AcceleratingAccelerating admixtureadmixture

2)2) RetardingRetarding admixtureadmixture

3)3) Water-reducingWater-reducing/high-/high-range water-reducing range water-reducing admixtureadmixture

4)4) Air-entraining Air-entraining admixtureadmixture

5)5) othersothers

Accelerating admixtureAccelerating admixture

To To accelerateaccelerate the setting and early strength the setting and early strength development of concrete, particularly in development of concrete, particularly in cold weather applicationscold weather applications

E.g. calcium chloride (CaClE.g. calcium chloride (CaCl22))

Early removalEarly removal of formwork of formwork

Compressive strengthCompressive strength at 3 days at least 25% at 3 days at least 25% higher than concrete without admixturehigher than concrete without admixture

The amount of CaClThe amount of CaCl22 should be should be limited to 2%limited to 2%

by weight of cementby weight of cement

Cont.Cont.

Large dosage may cause severe Large dosage may cause severe corrosion and loss of strength at later corrosion and loss of strength at later agesages

CaClCaCl22 is not recommended is not recommended in hot weather in hot weather

concreting and for pre-stressed concrete concreting and for pre-stressed concrete and may be ineffective at temperatures and may be ineffective at temperatures below freezing because concrete can below freezing because concrete can freeze before it reaches the initial setfreeze before it reaches the initial set. .

Problems Associated with Problems Associated with AcceleratorAccelerator

IncreaseIncrease the heat of hydration the heat of hydration

Found to Found to aggravate corrosionaggravate corrosion of of reinforcing and pre-stressing steelreinforcing and pre-stressing steel

IncreaseIncrease creep and shrinkage creep and shrinkage

Retarding AdmixtureRetarding AdmixtureAdded to concrete to Added to concrete to delay its setting and delay its setting and hardeninghardening, especially in hot weather applications , especially in hot weather applications (temperature greater than 32 °C)(temperature greater than 32 °C)

Hot weatherHot weather increases the rate of hardening increases the rate of hardening which makes it difficult to place and finish the which makes it difficult to place and finish the plastic concreteplastic concrete

Recommended in large structuresRecommended in large structures and and difficult difficult situationssituations such as large piers and foundations, to such as large piers and foundations, to delay the initial set as well as to keep concrete delay the initial set as well as to keep concrete workable throughout the entire placing period workable throughout the entire placing period (eliminate cold joint)(eliminate cold joint)

Cont.Cont.

SugarSugar in any form is a in any form is a retarder retarder (coke?)(coke?)

The use of retarder The use of retarder may reduce the may reduce the strength at early agestrength at early age

Water-Reducing AdmixtureWater-Reducing Admixture

Used Used to reduce the quantityto reduce the quantity of mixing of mixing water required, which water required, which increase the increase the compressive strength, and to produce compressive strength, and to produce concrete of desired consistency or high concrete of desired consistency or high slump.slump.

Can reduce water required by 12% or Can reduce water required by 12% or moremore

Its effect is due to increased Its effect is due to increased dispersion of dispersion of cement particlescement particles, causing a reduction in , causing a reduction in the viscosity of the concrete.the viscosity of the concrete.

Pumping ConcretePumping Concrete

High-Range Water ReducerHigh-Range Water Reducer

Also called SuperplasticizerAlso called Superplasticizer

Added in Added in small dosagesmall dosage with mixing with mixing water (typically 0.2% by weight of water (typically 0.2% by weight of cement) to increase slumpcement) to increase slump

Flowing concreteFlowing concrete – slump greater than – slump greater than 180 mm180 mm

Dosage rateDosage rate may vary from brand to may vary from brand to brand for different types of cement.brand for different types of cement.

Without SP

With SP

Cont.Cont.

By using superplasticizer, it is possible By using superplasticizer, it is possible to have to have w/c ratio in the range of 0.3 to w/c ratio in the range of 0.3 to 0.45 which0.45 which would be impossible would be impossible otherwise.otherwise.

Addition of superplasticizerAddition of superplasticizer increases increases cost by 5% cost by 5% BUTBUT savings in labour can savings in labour can be as high as 33%be as high as 33%

Effect of SuperplasticizerEffect of Superplasticizer

IncreaseIncrease in workability in workability

IncreaseIncrease in compressive strength in compressive strength

EliminatesEliminates segregation segregation

Allow Allow good dispersiongood dispersion of cement of cement particles in waterparticles in water

Superplasticizer – Superplasticizer – how it works?how it works?

Concrete with SuperplasticizerConcrete with Superplasticizer

Has smooth surfaceHas smooth surfaceMuch less likely to Much less likely to chip and spallchip and spallHas fewer plastic Has fewer plastic shrinkage cracksshrinkage cracksHigher compressive Higher compressive strengthstrengthbecause of rapid because of rapid gain in strengthgain in strengthEarly removal of Early removal of formworkformwork

Less shrinkageLess shrinkage

Low permeabilityLow permeability

Less time to place Less time to place and finish the and finish the concreteconcrete

An accelerating An accelerating curing periodcuring period

Air-Entraining AdmixtureAir-Entraining Admixture

Added to entrain air in concreteAdded to entrain air in concrete

Air entrainmentAir entrainment is a process by which a is a process by which a large number of microscopic air bubbles large number of microscopic air bubbles are dispersed throughout the concreteare dispersed throughout the concrete

DiameterDiameter of air bubbles in the range of of air bubbles in the range of 20 to 200 µm20 to 200 µm

Air BubblesAir Bubbles

AdvantagesAdvantages

It causes It causes the disruption of the continuity the disruption of the continuity of capillary poresof capillary pores result in reduction in result in reduction in permeability and reduces internal permeability and reduces internal stresses caused by expansion of the pore stresses caused by expansion of the pore water on freezing.water on freezing.

Foaming agents are added in small Foaming agents are added in small amount (typically about 0.05% by weight amount (typically about 0.05% by weight of cement) during mixing and can be used of cement) during mixing and can be used to to entrain 3 to 10% air.entrain 3 to 10% air.

Cont.Cont.

The use of air-entraining admixture can:The use of air-entraining admixture can:– Improve workabilityImprove workability– Reduce bleedingReduce bleeding– Reduce segregationReduce segregation

However:However:– Lower compressive strengthLower compressive strength

Chemical AdmixtureChemical AdmixtureDescriptionDescription ApplicationsApplications

Water-reducing Water-reducing

AdmixtureAdmixture

To get dense concrete, to improve To get dense concrete, to improve workabilityworkability

Retarding admixtureRetarding admixture To delay setting and hardening, To delay setting and hardening, hot weather concreting, large hot weather concreting, large structuresstructures

Accelerating admixtureAccelerating admixture To accelerate setting and early To accelerate setting and early strength development, cold strength development, cold weather concretingweather concreting

Air-entraining admixtureAir-entraining admixture To improve durability and To improve durability and workabilityworkability

POZZOLANIC / MINERAL POZZOLANIC / MINERAL ADMIXTURESADMIXTURES

Mineral AdmixturesMineral Admixtures

Are Are natural pozzolanicnatural pozzolanic materials materials oror industrial by-productsindustrial by-products that are used in that are used in concrete to concrete to replace part of cementreplace part of cementIt is known as It is known as supplementary supplementary cementing materialcementing materialAre generally added in relatively Are generally added in relatively large large quantities in comparison with chemical quantities in comparison with chemical admixturesadmixtures

Contains Contains reactive silica (SiOreactive silica (SiO22))

PozzolansPozzolans

Are Are siliceous or siliceous and aluminoussiliceous or siliceous and aluminous materials which in themselves possess little materials which in themselves possess little or no cementitious propertyor no cementitious propertyBUTBUT in finely divided form and in the in finely divided form and in the presence of moisture, these materials can presence of moisture, these materials can react with calcium hydroxidereact with calcium hydroxide of concrete at of concrete at ordinary temperatures to form compounds ordinary temperatures to form compounds of cementitious properties.of cementitious properties.This reaction is called This reaction is called POZZOLANIC POZZOLANIC REACTIONREACTION

ReactionsReactions

Hydration process:Hydration process:

Cement + HCement + H22O → C-S-H gel + Ca(OH)O → C-S-H gel + Ca(OH)22

Pozzolanic reaction:Pozzolanic reaction:

SiOSiO22 + Ca(OH) + Ca(OH)22 → C-S-H gel → C-S-H gel

Effect of Pozolanic AdmixtureEffect of Pozolanic Admixture

Example of PozzolansExample of Pozzolans

Ground Granulated Blastfurnace SlagGround Granulated Blastfurnace Slag

Pulverised Fuel AshPulverised Fuel Ash

Silica FumeSilica Fume

Rice Husk AshRice Husk Ash

Palm Oil Fuel AshPalm Oil Fuel Ash

Method of ReplacementMethod of Replacement

Replacing Replacing part ofpart of the cement the cement content (by weight of cement)content (by weight of cement)

Can be addedCan be added durning mixing durning mixing process or process or grindgrind together with together with cement (pozzolan cement)cement (pozzolan cement)

Effect of pozzolanic admixturesEffect of pozzolanic admixtures

LowerLower early strength early strength

Lower Lower heat of hydrationheat of hydration

Higher Higher ultimate strengthultimate strength

LessLess permeable permeable

ReduceReduce cost cost

IncreaseIncrease workability (some) workability (some)

SlagSlagIs a Is a non-metallic productnon-metallic product, consisting , consisting essentially of silicates and aluminates of essentially of silicates and aluminates of calcium basescalcium bases

Not only Not only pozzolanic but also cementitiouspozzolanic but also cementitious

It is develop in a molten condition It is develop in a molten condition simultaneously with iron in a blastfurnacesimultaneously with iron in a blastfurnace

Is the Is the glassy granular materialglassy granular material formed formed when molten blastfurnace slag is rapidly when molten blastfurnace slag is rapidly chilled as by immersion in waterchilled as by immersion in water

Amount replacement : Amount replacement : up to 80%up to 80%

SlagSlag

Fly Ash (PFA)Fly Ash (PFA)

Is a by-product of Is a by-product of burning powdered coalburning powdered coal in in electric generating power plantelectric generating power plantGenerally finer than Portland cement and Generally finer than Portland cement and consists mainly of consists mainly of small spheres of glasssmall spheres of glass involving silica, alumina, and ferric oxideinvolving silica, alumina, and ferric oxideAmount of replacement : Amount of replacement : 20 to 70%20 to 70%Two types: Two types: – Class F Fly AshClass F Fly Ash – low calcium fly ash. Has no – low calcium fly ash. Has no

cementitious properties cementitious properties butbut can react with calcium can react with calcium hydroxidehydroxide

– Class C Fly AshClass C Fly Ash – high-calcium fly ash. Has – high-calcium fly ash. Has cementitious properties cementitious properties in additionin addition to pozzolanic to pozzolanic propertiesproperties

Fly AshFly Ash

Microstructure of Fly AshMicrostructure of Fly Ash

Silica Fume (SF)Silica Fume (SF)Is a Is a by-productby-product of the electric arc furnaces of the electric arc furnaces in the silicon metal and ferrosilicon alloy in the silicon metal and ferrosilicon alloy industriesindustries

Consist of non-crystalline silica (Consist of non-crystalline silica (85 to 90% 85 to 90% silicon oxide)silicon oxide)

Very fine particles – less than 0.1um Very fine particles – less than 0.1um (100 (100 times finer than cement)times finer than cement)

Highly pozzolanicHighly pozzolanic and very reactive and very reactive

Amount replacement : Amount replacement : 10 to 20%10 to 20%

Rice Husk Ash (RHA)Rice Husk Ash (RHA)

Is an Is an agricultural fly ashagricultural fly ash which is which is obtained from burning and obtained from burning and grinding of rice huskgrinding of rice husk

Contains Contains silicasilica

Amount replacement : Amount replacement : 10 to 30%10 to 30%

Palm Oil Fuel Ash (POFA)Palm Oil Fuel Ash (POFA)

Obtained from the burning of Obtained from the burning of palm oil clinkerpalm oil clinker

Amount replacement : Amount replacement : 10 to 10 to 30%30%

Factors Affecting the Suitability and Factors Affecting the Suitability and Performance of Admixtures in ConcretePerformance of Admixtures in Concrete

Type Type of cementof cement

Mix proportionsMix proportions and method of and method of mixingmixing

Temperature and timeTemperature and time when the when the admixture is added into the mixadmixture is added into the mix

Chemical compositionChemical composition of the of the admixtureadmixture

Monitoring and PrecautionMonitoring and Precaution

Selection of materialSelection of material

Trial mixTrial mix

Short-term and long-term effectShort-term and long-term effect

Eliminate overdosing – on siteEliminate overdosing – on site

Manufacturer’s adviceManufacturer’s advice

Thank YouThank You

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