SAB2112 Admixtures L6

8/6/2019 SAB2112 Admixtures L6

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CONCRETE ADMIXTURESCONCRETE ADMIXTURES

Lecture 6Lecture 6


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TEST 1

Monday 23rd August 2010 (Week 7)

M47 212 & 213

8.30 9.30 p.m. Two questions up to concrete

Q1. a)

b)

c)


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Admixtures

A material added to plastic concrete or

mortar, other than water, aggregates,

cement, and fibre TO change one ormore properties of fresh or hardened

concrete


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When to use admixture?

When the desired modification of

properties of fresh or hardened

concrete cannot be achieved bychanges in the composition of the mix

proportion


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Functions of Admixtures

Pg. 61

To accelerate or retard initial setting

To enhance workability

To increase durability of concrete

Etc.


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Example

High-strength concrete

Concrete grade 80

Cement = 450kg/m3

Water/Cement ratio =0.35

Coarse Aggregate = 1120kg/m3

Fine Aggregate = 980

kg/m3

WHAT WILL BE THEPROBLEM?


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Form of Admixtures

Liquid

F

lake Powder


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Chemical Admixture


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CHEMICAL ADMIXTURES

Chemical added in very

small amount or

quantity to the mixture

to develop special

properties in fresh or

hardened concrete

1) Accelerating

admixture

2) Retarding admixture

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

admixture

4) Air-entraining

admixture

5) others


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Accelerating admixture

To accelerate the setting and early strength development

of concrete, particularly in cold weather applications

E.g. calcium chloride (CaCl2)

Early removal of formwork

Compressive strength at 3 days at least 25% higher than

concrete without admixture

The amount of CaCl2 should be limited to 2% by weight of

cement

Read page 63-64


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Cont.

Large dosage may cause severe

corrosion and loss of strength at later

ages

CaCl2 is not recommended in hot weather

concreting and for pre-stressed concrete

and may be ineffective at temperatures

below freezing because concrete canfreeze before it reaches the initial set.


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Problems Associated with Accelerator

Increase the heat of hydration

Found to aggravate corrosion ofreinforcing and pre-stressing steel

Increase creep and shrinkage


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Retarding Admixture

Added to concrete to delay its setting and hardening,

especially in hot weather applications (temperature

greater than 32r

C) Hot weather increases the rate of hardening which makes

it difficult to place and finish the plastic concrete

Recommended in large structures and difficult situations

such as large piers and foundations, to delay the initial

set as well as to keep concrete workable throughout the

entire placing period (eliminate cold joint)


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Cont.

Sugar in any form is a

retarder (coke?)

The use of retardermay reduce the

strength at early age

Read pg. 64-65


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Water-Reducing Admixture

Used to reduce the quantity of mixing water

required, which increase the compressive

strength, and to produce concrete of desired

consistency or high slump. Can reduce water required by 12% or more

Its effect is due to increased dispersion of

cement particles, causing a reduction in the

viscosity of the concrete.

Read pg. 70


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Pumping Concrete


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High-Range Water Reducer

Also called Superplasticizer

Added in small dosage with mixing

water (typically 0.2% by weight ofcement) to increase slump

Flowing concrete slump greater than

180 mm

Dosage rate may vary from brand to

brand for different types of cement.


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Without SP

With SP


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Cont.

By using superplasticizer, it is possible

to have w/c ratio in the range of 0.3 to

0.45 which would be impossibleotherwise.

Addition of superplasticizer increases

cost by 5% BUT savings in labour can

be as high as 33%


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Effect of Superplasticizer

Increase in workability

Increase in compressive strength Eliminates segregation

Allow good dispersion of cement

particles in water


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Superplasticizer how it works?


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Concrete with Superplasticizer

Has smooth surface

Much less likely tochip and spall

Has fewer plasticshrinkage cracks

Higher compressivestrength

because of rapidgain in strength

Early removal offormwork

Less shrinkage

Low permeability

Less time to place

and finish the

concrete

An accelerating

curing period


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Air-Entraining Admixture

Added to entrain air in concrete

Air entrainment is a process by which a

large number of microscopic air bubblesare dispersed throughout the concrete

Diameter of air bubbles in the range of

20 to 200 m


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Air Bubbles


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Advantages

It causes the disruption of the continuity

of capillary pores result in reduction in

permeability and reduces internal

stresses caused by expansion of the porewater on freezing.

Foaming agents are added in small

amount (typically about 0.05% by weightof cement) during mixing and can be used

to entrain 3 to 10% air.


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Cont.

The use of air-entraining admixture can:

Improve workability

Reduce bleeding Reduce segregation

However:

Lower compressive strength


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Chemical Admixture

DescriptionDescription ApplicationsApplications

WaterWater--reducingreducing

AdmixtureAdmixture

To get dense concrete, to improveTo get dense concrete, to improve

workabilityworkability

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

structuresstructures

Accelerating admixtureAccelerating admixture To accelerate setting and earlyTo accelerate setting and early

strength development, coldstrength development, cold

weather concretingweather concreting

AirAir--entraining admixtureentraining admixture To improve durability andTo improve durability and

workabilityworkability


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POZZOLANIC / MINERAL

ADMIXTURES


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Mineral Admixtures

Are natural pozzolanic materials orindustrial by-products that are used inconcrete to replace part of cement

It is known as supplementarycementing material

Are generally added in relatively large

quantities in comparison with chemicaladmixtures

Contains reactive silica (SiO2)


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Pozzolans

Are siliceous or siliceous and aluminousmaterials which in themselves possess littleor no cementitious property

BUT in finely divided form and in thepresence of moisture, these materials canreact with calcium hydroxide of concrete atordinary temperatures to form compoundsof cementitious properties.

This reaction is called POZZOLANICREACTION


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Reactions

Hydration process:

Cement + H2O C-S-H gel + Ca(OH)2

Pozzolanic reaction:

SiO2

+ Ca(OH)2

C-S-H gel


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Effect of Pozolanic Admixture


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Example of Pozzolans

Ground Granulated Blastfurnace Slag

PulverisedF

uel Ash Silica Fume

Rice Husk Ash

Palm Oil Fuel Ash


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Method of Replacement

Replacing part of the cement

content (by weight of cement)

Can be added durning mixingprocess or grind together with

cement (pozzolan cement)


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Effect of pozzolanic admixtures

Lower early strength

Lower heat of hydration

Higher ultimate strength

Less permeable

Reduce cost Increase workability (some)


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Slag

Is a non-metallic product, consisting essentially

of silicates and aluminates of calcium bases

Not only pozzolanic but also cementitious

It is develop in a molten condition simultaneously

with iron in a blastfurnace

Is the glassy granular material formed when

molten blastfurnace slag is rapidly chilled as byimmersion in water

Amount replacement : up to 80%


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Slag


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Fly Ash


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Microstructure ofFly Ash


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Rice Husk Ash (RHA)

Is an agricultural fly ash which is

obtained from burning and

grinding of rice husk

Contains silica

Amount replacement : 10 to 30%


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Factors Affecting the Suitability and

Performance of Admixtures in Concrete

Type of cement Mix proportions and method of mixing

Temperature and time when the admixture is

added into the mix

Chemical composition of the admixture


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Monitoring and Precaution

Selection of material

Trial mix

Short-term and long-term effect Eliminate overdosing on site

Manufacturers advice


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Documents

SAB2112 Admixtures L6