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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