Admixtures

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DefinitionAdmixtures are

those ingredients in concrete other than Portland cement, water, and aggregates that are added to the mixture immediately before or during mixing.

Reason for Use of Admixture

To reduce the cost of concrete construction

To achieve certain properties in concrete more effectively than by other means

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

To overcome certain emergencies during concreting operations

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Admixture EffectivenessThe effectiveness of an

admixture depends upon:◦ Type, brand, and amount of

cementing materials◦water content◦aggregate shape, gradation and

proportions◦mixing time◦Slump◦Temperature of the concrete.

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Admixture’s Classification Air-entraining

admixtures Water-reducing

admixtures Plasticizers Accelerating admixtures Retarding admixtures Hydration-control

admixtures Corrosion inhibitors Shrinkage reducers Alkali-silica reactivity

inhibitors Coloring admixtures

Miscellaneous admixtures such as ◦ workability,◦ bonding, ◦ dampproofing, ◦ permeability

reducing,◦ grouting, ◦ gas-forming, ◦ anti washout,◦ foaming, ◦ pumping admixtures

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Brief History of Admixture Use

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Plasticizer (Water Reducer) Admixtures

A material, which either increases workability of freshly mixed concrete without increasing water cement ratio or maintains workability with a reduced amount of water, is termed as water reducing admixture.

Reduction in water depends on dose of◦Admixtures◦cement content◦type of aggregate◦ ratio of cement ,fine and coarse aggregate

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Plasticizers - Chemical Composition:

Lignosulfonic acids, derivatives and their salts.Hydroxylated carboxylic acids, their salts and

derivatives.Naphthalene sulphonic acid basedSulfonated melamine polycondensation

productsBlends of aboveInorganic material like borates, phosphates,

amines and their derivatives, carbohydrates, sugar and certain polymeric compounds like cellulose, ethers etc.

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Plasticizers - Advantages

Increase the workability of the concrete without reducing the compressive strength

High strength can be obtained with the same cement content by reducing water cement ratio.

Saving in the quantity of cement (approx. upto 10%) can be achieved keeping the same water/ cement ratio and workability.

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Effect on Cement Particles

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Description Dosage. Cement W/C Slump Compressive strength N/mm2 of mix % cement wt kg/m3 Ratio cm 1 day 3 days 7 days 28 days Reference -- 300 0.6 7 7 18 26 34

Plasticizes 0.2% 300 0.6 10 7 18 28 37

0.3% 300 0.6 12 6 17 27 35

Strength increase 0.2% 300 0.56 7 7 21 32 41 0.3% 300 0.54 7 8 23 33 44

Cement saving 0.2% 280 0.6 7 7 19 28 36

0.3% 270 0.6 7 6 19 27 35

Use of Plasticizers:

Rate of Slump Loss - Plasticizer

Setting Time Retardation by Plasticizers

Effects of Plasticizer• Fluidize the mortar or concrete.• Reduces water requirement by 5-15% .• Improves workability. Increase in slump

from 30-150 mm.• May entrain air at a small percentage(1-

2%).• Likely to retard setting and hardening.• Increase in concrete strength 10 – 20%.• Rapid slump loss.• Small effect on drying shrinkage.

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Super Plasticizer

Super Plasticizers are water reducers, chemically different from the plasticizers or normal water reducer and capable of reducing water content by about 30% or increasing slump upto 200mm without causing segregation or bleeding in concrete.

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Sulfonated melamine formaldehyde condensates

Sulfonated naphthalene formaldehyde condensates

Modified lignosulfonatesOthers such as sulfonic acid esters

and carbohydrate esters

Super Plasticizer – Chemical Composition

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AdvantagesCement content can be reduced to a

greater extent keeping the same water/cement ratio. This will lead to economy.

Water-cement ratio can be reduced significantly keeping same cement content and workability. This will lead to increase in strength.

Higher workability at very low water cement ratio like casting concrete with heavy reinforcement..

Reduction in permeabilityWhere early strength development is

required in prestressed concrete or casting of floor, where early access for finishing equipment is required.

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SN Dosage in Lt/50 W/C ration Cement content Slump in mm kg of cement I Control Mix 0.55 350 50 II 0.2 0.55 350 80 III 0.4 0.55 350 150 IV 0.6 0.55 350 200

kg/m3

Effect of use of Super Plasticizers

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Slump Loss Due to Super Plasticizer

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Slump loss at 32°C (90°F) in flowing concretes (TN, TM, TB, and TX) compared with control mixture (TC)

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Retardation in Setting Time

Retardation of set in flowing concrete with Super plasticizers(N, M, B, and X) relative to control mixture

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Compressive strength development in flowingconcretes. C is the control mixture. Mixtures FN, FM, andFX contain super plasticizers

Strength Development

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Effect on Air Content

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Bleeding of flowing concretes with plasticizers (N,M, B, and X) compared to control (C)

Effects on Bleeding of Concrete

Retarding Admixtures• Retard setting time of concrete.

Retardation of up to 10 hours is possible.

• Help obtain architectural finishes.• Reduce slump loss.• Offset effects of hot weather.• Reduction in early strength

expected.• Affects on shrinkage unpredictable.

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Chemical Composition

Lignosulphonic acids and their salts. e.g. Na, Ca or NH4,

Hydro-carboxylic acids and their salts.

Carbohydrates including sugar.Inorganic salts based on

flourates, phosphates, oxides, borax and magnesium salts.

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Advantages of Retarders

Improves workability, cohesion and extends setting time, provides protection against delays and stoppages and facilitates keeping workable concrete for extended period.

In the large construction, good workability of the concrete throughout the placing period and prevention of cold joints is ensured by adding retarders in the concrete.

Extended setting time minimise risks of long distance delivery in hot weather, improves pumpability of concrete by extended setting period and improved workability of concrete.

Reduces bleeding and segregation where poor sand grading are unavoidable.

Reduces adverse environmental effects of various nature on concrete and embedded steel by considerable reduction in permeability.

Materials - Retarders

Gypsum.Starches.Cellulose.Sugar.Acids and salts.Lignosulphonic acids and theirs salts.Hydroxylated carboxylic acids and

their salts.Calcium Acetate

AcceleratorsAccelerators are admixtures which

increase the rate of early strength development in concrete to:◦Permit early removal of formwork.◦Reduce curing period.◦Early use of structure.◦Compensate effects of low temperature.◦Emergency repair work.

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Material AcceleratorsAlkali Hydroxides,SilicatesFluoro-SilicatesOrganic Compounds,Calcium Formates,Calcium NitratesCalcium ThioSulphatesAluminium ChloridesPotassium CarbonatesSodium ChloridesCalcium Chlorides.

Mechanism AcceleratorsNot explicitly

understood.Set accelerate formation

of ettringite. Inorganic accelerate rate

of dissolution of tricalcium silicate resulting in better hydration.

Hardening accelerators reduce distance between CSH to interaction and better strength.

Accelerators

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Advantages

Shortens the setting time of cement and therefore increases the rate of gain of strength.

Enables earlier release from precast moulds thus speeding production.

Reduces segregation and increase density and compressive strength.

Cures concrete faster and therefore uniform curing in winter and summer can be achieved.

Early use of concrete floors by accelerating the setting of concrete.

Reduces water requirements, bleeding, shrinkage and time required for initial set.

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Calcium ChlorideCalcium chloride (CaCl2) is the chemical

most commonly used in accelerating admixtures, especially for non reinforced concrete.

Should not exceed 2% by mass of cementing material.

An overdose can result in placement problems and can be detrimental to concrete. It may cause: rapid stiffening, a large increase in drying shrinkage, corrosion of reinforcement, and loss of strength at later ages

Air Entraining Admixtures

Allow introuction of fine air bubbles in concrete .

Entrapped air remains even after hardening.

May result in loss of strength. Require good control of dosage being

effective.

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Chemical Composition

Abietic and pimeric acid saltsFatty Acid saltsAlkyl - aryl sulphonatesAlkyl sulphonatesPhenol ethoxylates

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Advantages

Durability Reduced bleeding and segregation because of :

◦ Attachment of air bubbles with cement particle and

linking them.◦ Increase in inter particle attraction caused by

adsorption of air entrainer.◦ Bubble acting as extra fine filler and increased total

surface area of constituents relative to water volume.◦ Water flow between cement particles is restricted.

Workability increased due to action of air bubbles as ball bearing which assist movement of particles each other.

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Other Admixtures HYDRATION-CONTROL ADMIXTURE . CORROSION INHIBITORS. CHEMICAL ADMIXTURES TO REDUCE ALKALI-AGGREGATE

REACTIVITY (ASR INHIBITORS) SHRINKAGE-REDUCING ADMIXTURES. COLORING ADMIXTURES (PIGMENTS) PUMPING AIDS DAMPPROOFING ADMIXTURES BONDING ADMIXTURES AND BONDING AGENTS PERMEABILITY-REDUCING ADMIXTURES GROUTING ADMIXTURES FUNGICIDAL, GERMICIDAL, AND INSECTICIDAL

ADMIXTURES ANTIWASHOUT ADMIXTURES GAS-FORMING ADMIXTURES