Chemical Admixtures Chemical Admixtures In ConcreteIn Concrete

What Are They?What Are They?

Ingredients other than:Ingredients other than:– CementCement– WaterWater– AggregatesAggregates

Added before or during mixing.Added before or during mixing.

Why Are They Used?Why Are They Used?

To modify properties of fresh & To modify properties of fresh & hardened concretehardened concrete

To ensure the quality of concrete To ensure the quality of concrete during mixing, transporting, placing & during mixing, transporting, placing & curingcuring

To overcome certain unexpected To overcome certain unexpected emergencies during concrete emergencies during concrete operations (ie, set retarders)operations (ie, set retarders)

How Applied?How Applied?

Most admixtures are supplied in a Most admixtures are supplied in a ready-to-use form and added at plant ready-to-use form and added at plant or jobsite.or jobsite.

Pigments and pumping aids are Pigments and pumping aids are batched by hand in VERY small batched by hand in VERY small amountsamounts

EffectivenessEffectiveness

Factors effecting results of use:Factors effecting results of use:– Type & amount of cement:Type & amount of cement:

Chemistry effects due to portland componentsChemistry effects due to portland components

– Water content & SlumpWater content & Slump Flowability will effect how well admixtures are Flowability will effect how well admixtures are

activated due to internal agitation actitivityactivated due to internal agitation actitivity

– Mixing timeMixing time Effects of admixtures are dependent on time Effects of admixtures are dependent on time

allowed to react.allowed to react.

5 Typical Functions of 5 Typical Functions of Admixtures:Admixtures:

Air entrainingAir entraining Water-reducingWater-reducing RetardersRetarders Set AcceleratorsSet Accelerators Plasticizers (superplasticizers)Plasticizers (superplasticizers)

Some other “specialty” types exist:Some other “specialty” types exist:– Color, corrosion inhibitors, etc.Color, corrosion inhibitors, etc.

Air EntrainmentAir Entrainment

Developed in 1930sDeveloped in 1930s Today, recommended for all concretes Today, recommended for all concretes

exposed to freeze/thaw cyclesexposed to freeze/thaw cycles Imposes microscopic air cells that Imposes microscopic air cells that

relieve internal pressure of freezing relieve internal pressure of freezing waterwater

Typical target air values are 5%-8%Typical target air values are 5%-8% Will also increase slump (workability)Will also increase slump (workability)

Master Builders Micro AirMaster Builders Micro Air

Performance:Performance:– Improved air void systemImproved air void system– Improved workability & plasticityImproved workability & plasticity– Reduced permeabilityReduced permeability

Dosage 1/8 to 1-1/2 oz per cwt of Dosage 1/8 to 1-1/2 oz per cwt of cementcement– Trial batch required to target air.Trial batch required to target air.

Water ReducersWater Reducers

Internal lubricantInternal lubricant Allows for reduction of water/cement ratio Allows for reduction of water/cement ratio

while maintaining workability (increased while maintaining workability (increased strength)strength)

Can reduce water requirement at least 5-10%Can reduce water requirement at least 5-10% Obtain higher slump without adding waterObtain higher slump without adding water Mid-rangeMid-range water reducers result in at least 8% water reducers result in at least 8%

– Mid-range Mid-range water reducers provide more consistent water reducers provide more consistent setting times than standard water reducers.setting times than standard water reducers.

Water Reducer: PozzolithWater Reducer: Pozzolith

Performance:Performance:– Improves workability at low w/c ratioImproves workability at low w/c ratio– Normal to retarded set timesNormal to retarded set times– Helps eliminate “cold joints”Helps eliminate “cold joints”

Dosage:Dosage:– 4-10 oz per cwt of cement4-10 oz per cwt of cement

Set RetardersSet Retarders

Slows curing rateSlows curing rate

Used to counteract hot weather Used to counteract hot weather conditions that cause increased rate of conditions that cause increased rate of hardening.hardening.– This makes placing and finishing difficultThis makes placing and finishing difficult

Pozzolith 961Pozzolith 961– Performance & dosage (see handout)Performance & dosage (see handout)

Set AcceleratorsSet Accelerators

Increase curing rate for achievement of Increase curing rate for achievement of high early strengthhigh early strength

Speeds up start of finishing operationsSpeeds up start of finishing operations

Used for speeding curing rate in cold Used for speeding curing rate in cold weather concretingweather concreting

Pozzolith 122Pozzolith 122– Performance & Dosage (see handout)Performance & Dosage (see handout)

Master Builders PozzolithMaster Builders Pozzolith

Performance: Performance: – High early strengthHigh early strength– Accelerated setting time characteristicsAccelerated setting time characteristics

Dosage: 16-64 fl oz/cwt (100 wt) Dosage: 16-64 fl oz/cwt (100 wt) cementcement

Higher dosages increase acceleration Higher dosages increase acceleration raterate

SuperplasticizersSuperplasticizers Also known as Also known as high-range high-range water reducers (HRWR)water reducers (HRWR)

Reduce water requirement by 12-30%Reduce water requirement by 12-30%

Can make low slump concrete flow like high-slump mixCan make low slump concrete flow like high-slump mix

Makes mix highly fluid and can be placed with little or no Makes mix highly fluid and can be placed with little or no vibration or compactionvibration or compaction

Effect lasts only 30-60 minutes and is followed by rapid loss Effect lasts only 30-60 minutes and is followed by rapid loss of workabilityof workability

Usually added at jobsiteUsually added at jobsite