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CONCRETE ADMIXTURES

• DEFINITION: CHEMICAL COMPOUNDS,ADDED TO CONCRETE MIX IMMEDIATELY BEFORE OR

DURING MIXING TO MODIFY ONE OR MORE OF THE SPECIIFC PROPERTIES OF CONCRTE IN FRESH OR HARDENED STATE.• FUNCTIONS:

ACCLERATE INITIAL SET OF CONCRETE RETARD INITIAL SET OF CONCRETE ENHANCE THE WORKABILITY / FLOWABILITY /

PUMPABILITY OF CONCRETE. REDUCE SEGREGATION IN GROUT AND IN CONCRETE MIXTURE.

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INCREASE STRENGTH BY REDUCING WATER CONTENT INCREASE DURABILITY BY INCREASING RESISTANCE

TO DIFFERENT EXPOSURE CONDITION. IMPROVE IMPERMEABILITY. INHIBIT CORROSION OF REINFORCEMENT REDUCE HEAT OF HYDRATION IMPROVE BOND BETWEEN OLD AND NEW CONCRETE SURFACES

Cont………

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CLASSIFICATION OF ADMIXTURES

A) GENERAL PURPOSE ADMIXTURES

1. ACCELERATING ADMIXTURES

2. RETARDING ADMIXTURES

3. AIR ENTRAINING ADMIXTURES

4. WATER- REDUCING ADMIXTURES

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B) SPECIAL CATEGORY ADMIXTURES1. GROUTING ADMIXTURES

2. AIR DETRAINING ADMIXTURES

3. GAS FORMING ADMIXTURES

4. CORROSION INHIBITING ADMIXTURES

5. SHRINKAGR REDUCING ADMIXTURES

6. PERMEABILITY REDUCING ADMIXTURES

7. BONDING ADMIXTURES

8. SURFACE HARDENING ADMIXTURES

9. COLOURING ADMIXTURES

10. FUNGICIDAL,GERMICIDAL AND INSECTICIDAL ADMIXTURES

Cont………

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PURPOSE:a) TO INCREASE RATE OF HYDRATION AND HENCE RATE OF

DEVELOPMENT OF STRENGTHHELPS ----------1) EARLY REMOVAL OF FORM WORK2) REDUCTION IN CURING PERIOD3) EARLY PLACEMENT OF STRUCTURE IN SERVICE4) CONCRETE TO BE PLACED AT LOW TEMPERATURESb) TO SHOTEN SETTING TIMEHELPS ----------1) EARLY FINISHING OF SURFACE2) REDUCTION OF PRESSURE ON FORMS3) EFFECTIVE PLUGGING OF LEAKS AGAINST HYDRAULIC

PRESSUREAPPLICATIONS1. UNDER WATER CONCRETING2. BASEMENT WATERPROOFING3. REPAIRS OF WATERFRONT STRUCTURES IN TIDAL ZONES

1. ACCELERATING ADMIXTURES

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o RAPID DISSOLUTION OF C3 S IN WATER -- RAPID HYDRATION o AVAILABLE IN FLAKES - ( 77 % CaCl2 )o FUSED FORM - ( 92 % CaCl2 )o DOSAGEo AS PER IS 7861(Part II) 1981 = 1.5% (IN PRACTICE UP TO 2 %):o TEMP = BELOW 25 O C FOR EFFECTIVE USEo LARGE DOSE = FLASH SETo 2 % DOSE REDUCES SETTING TIME BY 33 % AND RAISES THE 7 DAY STRENGTH BY 3- 8 MPa FLEXURAL STR INCREASES BY 40-80 % OF 1 DAY STR AND 12 % OF 28 DAY STRDOSAGE SELECTION DEPENDS ON 1.TYPE OF CEMENT2.TEMPERATURE OF CONCRETE3.AMBIENT TEMPT

CaCl2 MOST WIDELY USED ACCLERATORMECHANISM

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QUICK SET = SETS WITHIN SECONDS DOSE = NOT GREATER THAN 0.2 % BY MASS OF CEMENT

OTHER EXAMPLESNaCl Na 2 SO 4 K 2 SO 4 KOH Na OH Na2CO3

NOT TO USED IF 1.IN PRESTRESSED CONCRTE: HAS SOLUBLE CHLORIDES AND POSSIBILITY OF STRESS COROSIONSUBJECTED TO ALKALI - AGGRGATE REACTION,EXPOSED TO SOIL , WATER CONTAINING SULPHATESTEMPT GREATER THAN 25 0 C(2% DOSE CAUSES ACCELERATION OF HYDRATION AND RISE IN TEMPT OF ABOUT 11 0 C)

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RETARDERS• SLOWS DOWN THE INITIAL RATE OF HYDRATION• PROLONGS SETTING OF CEMENT PASTE IN CONCRETE,OFFSTS ACCLERATING AND DAMAGING EFFECTS OF HIGH TEMPT• KEEPS CONCRETE WORKABLE DURING T/ P # USE IN RMC• SUCEEDING LIFTS CAN BE PLACED WITHOUT DEVELOPMENT OF COLD JOINTS/ DISCONTINUITIES IN STR UNIT• FIXING EXPOSED AGGREGATES TO HAVEARCHITECTURAL FINISH(REMOVE FORMWORKAND BRUSH OFF CEMENTEXPOSED AGGREGATES.

ACTION DELAYS SETTING BY FORMING THIN COATING SLOWS DOWN DISSOLUTION IN AND REACTION WITH WATER INCREASES INTRAMOLECULAR DISTANCE OF REACTING SILICATES AND ALUMINATES FROM WATER MOLECULES BY FORMING TRANSIENT COMPOUNDS IN THE SYSTEMFINALLY ADM ARE INCORPORATED INTO HYDRATED MATERIAL.

CEMENT

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Ex: 1. SOLUBLE CASRBOHYDRATE DERIVATIVES: SUGAR SOLUBLE STARCH DEXTRIN 2.INORGANIC RETARDERS HYDROXIDE OF ZINC AND LEAD ALKALI BI-CARBONATES CALCIUM BORATE HYDROXYLATED CARBOXYLIC ACIDSDOSAGE :0.05 To 0.1 % BY MASS OF CEMENT# 0.05 % OF SUGAR DELAYS SETTING BY 4 HrWARNING MAY PROMOTE BLEEDING.

HOLDS BACK HYDRATION PROCESS, LEAVING MORE WATER FOR WORKABILITY AND ALLOWING CONCRETE TO BE FINISHED & PROTECTED BEFORE DRYING OUT.SOME RETARDERS ALSO REDUCES WATER REQUIREMENT # LESS W/C RATIODO NOT AFFECT 1.FINAL SETTING TIME OF CEMENT 2.STR AT 28 DAYS

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CAUTION:CORRECT DOSE IS MUSTSTORYCEMENT BAGS USED FOR SHIPMENT OF AGGTREGATE SAMPLES TO LABORATORY.MOLASSES BAGS WERE USED FOR TRANSPORT OF FRESHLY MIXED CONCRETE .GAVE CONFLICTING RESULTS.

Retarder Content0.2%

0.4% 0.8%0.6%

Initial setting Time.(Hr)

Tem

pt o c

0%

Smaller effect at high Tempt35

2530

20

10

15

10 20 3040

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AIR ENTRAINING ADMIXTURES:Controlled amt of air in the form of non –coalescing bubbles.without affecting setting / hardeningHelps: Improves workability/Durability Easy placing/finishing Better resistance to Frost Action Reduction in Bleeding and Segregation.used for : Cellular /Light weight Concrete.Chemistry:

Anionic surfactants, which are adsorbed onto the cement particles, forming a sheath of limited solubility.

Bubbles are formed on agitation, Which stabilizes as microscopic spheres. (Dia 0.05- 0.25mm and 0.003 mm apart)

Entrained Air Entrapped AirIntentionally incorporated in the form of minute spherical bubbles

Voids occurring in concrete due to insufficient or poor compaction.

Improves Durability and PlasticityReduces Strength

Can be of any shape and sizeNon uniform distribution, Size large (0.01 – 1.0 mm or more), Reduces strength and Durability

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• Each 1% increase in Air Content ,reduces strength by 1.4 MPa However:

•Reduction in W/C ratio and Sand Content for Given Workability Compensates loss of Strength.The Beneficial Amt of entrained Air depends on

• Type and Quantity of air entraining agent.

• W/C ratio of mix• Type /Grading /Max size of aggregates• Mixing time

• Extent of compaction of concrete.

• Tempt and type of cement. OPTIMAL AIR CONTENTMax Size ,

mm % By volume

Natural optimal

10 3 8+- 1.520 2 7+-1.530 1 4.5+-1.5

COMPOUNDSi) Natural wood Resins.ii) Sulphonated

Compoundsiii) Animal and Vegetable

fatsiv) Oils Olive oil and

their acids (Stearic and Oleic acids)

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Factors Affecting Air Content:

1. W/C ratio: Low: No adequate foaming Action.

0.4-0.6 : Abundant Air Bubbles. High: Abundant, However many are lost

progressively with time.2. Entrained air content increases with mixing time up

to certain limit, then gets reduced

3. Entrained air content decreases with increase in tempt of concrete.

4. Vibration for compaction 2.5 min loss of bubbles 50 %

10 min loss of bubbles 80 %

5. Use of Calcium Chloride: has tendency to limit Air

entrainment.

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• CEMENT/FLY ASH/SILICA FUME/RICE HUSK ASH/STONE DUST+ WATER = FLOCULATES

INCREASES VISCOSITY BY ENTRAPPING PART OF WATER AND PHYSICALLY RESISTING THE FLOW ADD MORE WATER BUT NOT BEYOUND CERTAIN POINTHence ADD WATER REDUCER TO DISPERSE OR DEFLOCULATE THE SYSTEM. THE ORGANIC/INORGANIC(COMBINATION) SUBSTANCES TO ACHIEVE THESE OBJCTIVES ARE “PLASICIZING ADMIXTURES”

WATER REDUCING ADMIXTURES:

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HIGHER FLOWABILITYFASTER RATE OF CONCRETE PLACEMENTEASY PLACEMENT IN POORLY ACCESSIBLE LOCATIONS WITHOUT VIBRATIONTRUE SHUTTER FINISHREDUCTION IN CEMENT CONTENTINCRESED STRENGTH / DENSITY / DURABILITY / STABILITY/ ABRASION RESISTANCE DUE TO REDUCTION IN WATER

Benefits:

Benefits will depend upon:1.Type of Cement 2.W/C ratio3.Mixing Tempt 4.Ambient Tempt5.Job Condition

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Mechanism of Plasticizers:1.Dispersion:1.Avoids flocculation: as plasticizers get adsorbed

on cement particles, which creates repulsive force between particles., overcoming attractive forces. (Gives negative charge.)

Repulsive Force= Zeta Potential.Cement particles are deflocculated and dispersed.water trapped inside the flocks gets released and

becomes available to fluidify the mix.Also1.Inter flock and inter particle friction is reduced.2.More surface area of cement particle for

hydration.

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2. Retarding effect :

Plasticizers form thin sheath around cement particle , which inhibits hydration as long as sufficient plasticizer molecules are available at particle solution interface.

The sheath also prevents close approach of particle to one another , Therefore the mobility

The quantity of available plasticizers progressively decreases as polymers gets entrapped in hydration product.

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Classification:Adm reducing water requirements by > 5 % = Water Reducers

(WR) / Plasticizers

1. Normal WR = (5 to 10 %) CaSO4 / Sugar 2. Mid Range WR = (10 to 15 %)

reduces setting time by 2-4 Hrs e.g. i) When Cement is < 350 Kg/m3 ( 0.1 To 0.5 % of

mass of cement)Lignosulphonic acids and their salts. (Ca / Na/NH4 salts) ii) When Cement is > 350 Kg/m3 ( 0.1 To 0.2 % of

mass of cement) Hydroxylated carboxylic acids and their salts

iii) Processed carboxylates

3. High Range WR = (20 to 40 %) *(Super Plasticizers) effect lasts only for 30 to 60 min followed by rapid loss of workability (Add at job sites

only)• Used for HPC with low w/c ratio (up to 25 %) and

Water reduction up to (30-40 %)

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Classification of Super plasticizers:

Cat A: Sulphonated melamine Formaldehyde Condensates (SMF) or Poly melamine Sulphonates (PMS) {Sulphonated Super Plasticizers }

Molecular Weight : 20 000 eg : Sodium Salts : Easily soluble in water ; Do not

interfere with hydration of cement ; very littlie effect on set at high dosage; Dosage up to 3% by mass of cement.

rapid loss of workability : set time accelerates by 30-40 min over normal mix of equal workability.

Mix will be more prone to segregation and bleeding hence higher sand content is desired.use min vibration.

Used for low tempt concreting where high early str is required.

Workability may fall to 70 mm slump in less than 15 min at 40oC Therefore add at job sites.

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Cat B: Sulphonated Nepthalene - Formaldehyde Condensates (SNF) or Poly – B- Nepthalene Sulphonates

Molecular Wt @ 2000: Significant effect on surface tension ;

therefore use with defoaming materials. Eg Sodium salts ; Gives slightly greater

level of set retardation (20-40 min) and air entrainment than Cat A.

Retains workability for longer period therefore dosage at RMC. Prior to Trucking to the site.

High sand content is desirable. Effective for Precast Concrete.

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Cat C: Modified Lignosulphates (MLS) Naturally accuring macromolecular organic

compound Crude lignosulphates: Waste liquid product

obtained during process of production of paper making pulp, from wood.

Refined by removing sugar and undesirable imurities, which causes excessive set.

Molecular Wt varies from 100-100000 Creates workability retention therefore very

effective at high ambient tempt/ long trucking. Mix may become over cohesive therefore

prevents water reducing potential of adm from being realised therefore sand content of the mix can be reduced by 3 to 5 %.

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Cat D : Carboxylated Acrylic Ester Co- Polymers (CAEC) It overcomes placement

drawbacks. Acrylic Polymers are active

ingredients of this category of Superplasticizers.

Comparatively Expensive.*Higher the Molecular Mass Higher the Efficiency.*

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Site Problems in the use of Superplasticizers. No information on slump of ref (without

plasticizer) mix.Zero slump can not be improved.

Inefficient lab mixers for premix trials(use of pan mixer is suggested)

Use of crusher dust (Dust interferes with plasticized properties of mix.

Compatibility with Cement. Selection of proper plasticizer/super

plasticizer Proper Dosage. Slump Loss. ……Contd

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……Contd Slump Loss with time is bound to be existing; Therefore, Maintain intial slump level. Use retarders (at RMC Plant) and apply dose at

discharging point. Use retarding plasticizers / Superplasticizers. Apply repetitive dose. Dose at Final Points. Keep tempt low.(reards Hydration)AIR CONTENT: May decrease with addition of SNF/SMF for air entrained

concretes. It can increase with MLS based Adm.SEGREGATION AND BLEEDING: IF APPLIED IN EXCESS BEYOUND SATURATION POINT

CEMENT PASTE BECOMES TOO FLUID.COMPATIBILITY PROBLEMMS: Compatibilty with cement is a must therefore go for

“ marsh Cone Test” and determine optimum dosage.

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As per ASTMC- 49492 Only Water Reducers Type A WR + Retarders Type D WR + Acclerators Type E Super plasticizers Type F Super plasticizers + Retarders Type G

General Classification:

Dosage: 1 to 3 lit / m3; 5 to 20 lit / m3 if used as water reducing adm.Time : Immediately after cement + water comes in contact (within 20 sec) In repeated dosage 2-3 tims before being delivered at site.

Normally retains workability upto 60-120 min For use of Super plasticizers “ Design mix for hardened physical properties and then add

sufficient super plasticizers to achieve required workability”

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