2.8 admixtures

7/29/2019 2.8 Admixtures

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2.8 ADMIXTURES FOR

CONCRETE 2.8.1 Introduction

2.8.2 Plasticisers and superplasticisers

2.8.3 Air entrainment

2.8.4 Retarders

2.8.5 Accelerators

2.8.6 Other admixtures


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Demonstration

500 g Cement

150 ml Water

5 ml Superplasticiser


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Fosroc Literature


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Primary admixture propertiesPLASTICISERS

Dispersion of cement particles increases fluidity

Water reduction increases strength Water reduction reduces permeability, increases durability

Cement reduction reduces cost

AIR ENTRAINERS

Increase cohesion, Reduce bleed and segregation Easier to pump

Impart freeze thaw resistance

RETARDERS

Prolong period over which concrete may be placed Reduce problems with cold joints

ACCELERATORS

Reduce the time to reach initial set

Increase the early age strength of the concrete


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2.8 ADMIXTURES FOR




2.8.4 Retarders

2.8.5 Accelerators



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Effects of water reducers

Cement 250kg


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Increasing strength


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Plasticiser and superplasticiser


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Examples of

superplasticiser

use


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Reducing

temperature

rise with sp


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Methods

ofaddition


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(lack of) effect on strength


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Effect of water reducer on heat

evolution


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Effect of plasticiser on rheology


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Slump vs flow table


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Self

compacting

concrete


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Self

Compacting

Concrete


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Self Compacting Concrete


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2.8 ADMIXTURES FOR




2.8.4 Retarders

2.8.5 Accelerators



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Air entrainers


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Effect of pozzolans on air entraining


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Effect of fine aggregate on air

Th k i d l d f


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These cracks in dolos sea defence

unit prevented with air entrained

concrete


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2.8 ADMIXTURES FOR




2.8.4 Retarders

2.8.5 Accelerators



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Workability loss


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Retarders


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The retarder therefore helps to:-

Hold the workability of initially high workabilitysuperplasticised concrete at a medium workability by

delaying the onset of hydration.

Assist with the removal of the casing after introduction of

concrete into deep piles by holding some workability in themix.

Give monolithic concrete between batches placed with a

time interval between them.

Reduce the incidence and intensity of surface lift marks,surface blemishes and colour changes due to poor

intermixing between pours.

Reduce the incidence of cold joints between pours.


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2.8 ADMIXTURES FOR




2.8.4 Retarders

2.8.5 Accelerators



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Accelerator


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Superplasticiser for strength gain


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2.8 ADMIXTURES FOR




2.8.4 Retarders

2.8.5 Accelerators



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Other Admixtures Viscosity modifiers for use with superplasticisers

when making self compacting concrete

Foaming agents "Foamed concrete" is a non-

structural void filler which can be dug out withexcavators.

Shrinkage compensators.

Corrosion Inhibitors. Calcium Nitrite is used as

an anode inhibitor. Other admixtures can inhibitthe cathode.

Alkali Aggregate Reaction Inhibitors. Lithiumbased compounds are used to reduce AAR.

C i I hibit


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Corrosion Inhibitors


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Other Admixtures Viscosity modifiers for use with superplasticisers

when making self compacting concrete

Foaming agents

"Foamed concrete" is a non-structural void fillerwhich can be dug out with excavators.

Shrinkage compensators.

Corrosion Inhibitors. Calcium Nitrite is used as

an anode inhibitor. Other admixtures can inhibitthe cathode.

Alkali Aggregate Reaction Inhibitors. Lithiumbased compounds are used to reduce AAR.


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When using more than one

admixture Never pre-mix admixtures before adding

them to the concrete

The order and timing of admixture additioncan be critical

2.8 admixtures

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