admixtures 8

7/28/2019 Admixtures 8

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Admixtures

Additive: Substances which are added at the cement

manufacturing stage (pozzolan, gypsum, etc.

Admixtures: Ingredients in concrete other than Portlandcement, water, and aggregate that are added to the

mixture immediately before or during mixing


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Accelerators

Accelerate the hardening or the development of early

strength of concrete.

Has little effect on setting time.

Most common type is calcium chloride (CaCl2).

Accelerators are used for early form removal, cold

weather and urgent repair.

CaCl2 may be added to different types of Portland

cements.


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Dosage: 1 to 2 percent of cement weight (dissolved in

mixing water).

CaCl2 acts as catalyst in the hydration of C3S and C2S.

Side Effects:

1. Reduces the resistance of cement to sulfate attack.

2. Increase the risk of alkali-aggregate reaction.

3. Increase shrinkage and creep of concrete.4. Increase the potential of steel corrosion in reinforced

and prestressed concrete. ( ASTM and BS limit the used

of CaCl2 in concrete).


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Set-accelerators (quick-setting)

Reduce the setting time.

Used in shotcreting and for urgent repair.

Examples: sodium carbonate(washing soda), potassium

carbonate, aluminum chloride.

Side effect: reduce strength.


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o The increased retardation occurs especially with cements

which have a high C3A content because, once some C3A hashydrated, it does not absorbs the admixtures; it is therefore

available for reaction with calcium silicates.

o The use of retarding admixtures reduces the early strength

but later the rate of strength development is higher, so that

the long-term strength is not much different.

o Retarders tend to increase the plastic shrinkage because

the plastic stage is extended, but drying shrinkage isunaffected.


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Water-reducers (plasticizers):

These admixtures are used for three purposes:

1. Achieving higher strength by decreasing the w/c ratio.

2. Decreasing the cement content as to reduce the heat of

hydration in mass concrete, while maintaining workability

and strength.

3. Increasing the workability so as to ease placing of concretein inaccessible locations, pumping, tremie.


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Classification of Water reducer according to ASTM C494 (see

table 8.2) as follows:

Type A: water-reducing only

Type D: water-reducing and retarder

Type E: water-reducing and accelerator

Type F: high-range water reducing (superplasticizer)

Type G: high-range water reducing and retarder


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How water-reducer works?

The principle active components of water reducing admixtures

are surface-active agents.

The surface-active agents are absorbed on the cement particles,

giving them negative charge, which leads to repulsion betweenthe particles and results in stabilizing their dispersion; air

bubbles are also repelled and cannot attach to the cement

particles.

Lubricate the mix so that the workability is increased


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Water-reducer basic materials:

Hydroxylated carboxylic acid

Lignosulphonic acid

The reduction in water content for these materials rangefrom 5 to 15%, depending on cement content, aggregate

type, pozzolan and air-entraining agent if present.

Typically the dosage rate of Type A water-reducers rangefrom 130 to 390 mL per 100 kg of cementitious materials.

Higher dosages may result in excessive retardation of the

concrete setting time.


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Other Advantages:

1. The dispersing ability of these materials results in a greater

surface area of cement exposed to hydration, for this reason

there is an increase at early strength compared with a

control mix (without water-reducer) with same w/c ratio.

2. Long-term strength may also be improved because of a more

uniform distribution of the dispersed cement throughout the

concrete.

Side effects: possible bleeding (Hydroxylated carboxylic acid

type)


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Superplasticizer are usually used to

1. Produce flowing concrete in situations where placing in

inaccessible locations such as:

Thin sections

Congested reinforcing steel

Tremie Pumped concrete

1. Produce very high strength concrete (strength as high

as 100 MPa), using normal workability but a very low

w/c ratio.

A water reduction of 25 to 35% can be obtained


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The improved workability produced by superplasticizer is of

short duration and thus there is a high rate of slump loss; after30 to 90 min the workability return to normal. Due to this

slump loss, these admixtures are often added to the concrete at

the jobsite.

Dosage:

1 to 2% of cement weight, for example using 2% raises the

slump from 30mm to 200 mm, or reducing the w/c ratiofrom 0.60 to 0.45 for same workability


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Tests have shown that some plasticized concrete bleedmore than do control concretes of equal w/c ratio. But

bleed significantly less than do control concretes of equal

high slump.

Superplasticizers do not significantly affect the setting of

concrete except in the case of cement with a very low C3A

content when there may be excessive retardation.


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AIR-ENTRAINING ADMIXTURES

Air-entraining admixtures are used to purposely introduce

and stabilize microscopic air bubbles in concrete.

Air entrainment will dramatically improve the durability of

concrete exposed to cycles of freezing and thawing.

Entrained air can be produced in concrete by use of an air-

entraining cement, by introduction of an air-entraining

admixture, or by a combination of both methods.

An air-entraining cement is a Portland cement with

an air-entraining addition interground with the clinker

during manufacture..


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Other admixtures and additives

1- Mineral additives/admixtures

Pozzolanic materials: fly ash, silica fume, blast furnace

slag..etc.

Inert materials: hydrated lime, limestone dust as workability

aids for grouts and masonry mortar, Coloring pigments are

also considered as inert admixtures.

2- Gas forming admixtures: such as powdered zinc or

aluminum. Used to produce aerated concrete (gas concrete)

used as lightweight concrete for insulation purposes.


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3- Bonding Admixtures: polymer emulsions (latexes) used

to improve the bond of fresh concrete to old concrete.

4- Water-repellent admixtures: used to reduce the

concrete permeability (water tanks)

5- Corrosion inhibitors (calcium nitrite, sodium nitrite,

6- Shrinkage reducers (propylene glycol )

7- Alkali-silica reactivity inhibitors(barium salts)

8- Colouring admixtures (coloured pigments: blue, red

.etc.)

admixtures 8

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