2.8 ADMIXTURES FOR

CONCRETE

• 2.8.1 Introduction

• 2.8.2 Plasticisers, superplasticisers and

viscosity modifiers

• 2.8.3 Air entrainment

• 2.8.4 Retarders

• 2.8.5 Accelerators

• 2.8.6 Other admixtures

Demonstration

• 500 g Cement

• 150 ml Water

• 5 ml Superplasticiser

Fosroc Literature

Primary admixture properties PLASTICISERS

• 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

2.8 ADMIXTURES FOR

CONCRETE

• 2.8.1 Introduction

• 2.8.2 Plasticisers, superplasticisers and

viscosity modifiers

• 2.8.3 Air entrainment

• 2.8.4 Retarders

• 2.8.5 Accelerators

• 2.8.6 Other admixtures

Effects of water reducers

Cement 250kg

Increasing strength

Plasticiser and superplasticiser

Examples of

superplasticiser

use

Reducing

temperature

rise with sp

Methods

of

addition

(lack of) effect on strength

Effect of water reducer on heat

evolution

Viscosity

(see section 1.6 of the notes)

Shear Stress τ

yield

stress

γ

Newtonian

Fluid

Bingham

Fluid

Shear

Stress τ

δy Fluid velocity V1

Fluid velocity V2

Gradient of

this line =

viscosity (e)

The viscosity e is

defined as:

e = τ δy Pa s

v1 - v2

where τ is the shear

stress

Shear rate

Rheometers

See section 2.6 of the

notes on concrete testing

Results from viscometer test

Torque

Shear Rate

See section 2.6 of the

notes on concrete testing

Rheology of Concrete Mixes

Viscosity

Modifyer

Plasticiser or

vibration

Water

yield

stress g

gradient h of shear stress vs shear rate

(see fig 1.6.1)

Effect of increasing:

%fines

PFA,

GGBS

See section 2.6 of the

notes on concrete testing

Effect of plasticiser on rheology

Self

compacting

concrete

Self

Compacting

Concrete

Tests for Self Compacting Concrete

L Box test Normal concrete, SCC

V Funnel test, Normal concrete, SCC

2.8 ADMIXTURES FOR

CONCRETE

• 2.8.1 Introduction

• 2.8.2 Plasticisers, superplasticisers and

viscosity modifiers

• 2.8.3 Air entrainment

• 2.8.4 Retarders

• 2.8.5 Accelerators

• 2.8.6 Other admixtures

Air entrainers

Effect of pozzolans on air entraining

Effect of fine aggregate on air

These cracks in “dolos” sea defence

unit prevented with air entrained

concrete

2.8 ADMIXTURES FOR

CONCRETE

• 2.8.1 Introduction

• 2.8.2 Plasticisers, superplasticisers and

viscosity modifiers

• 2.8.3 Air entrainment

• 2.8.4 Retarders

• 2.8.5 Accelerators

• 2.8.6 Other admixtures

Workability loss

Retarders

The retarder therefore helps to:-

• Hold the workability of initially high workability

superplasticised 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 the

mix.

• 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.

2.8 ADMIXTURES FOR

CONCRETE

• 2.8.1 Introduction

• 2.8.2 Plasticisers, superplasticisers and

viscosity modifiers

• 2.8.3 Air entrainment

• 2.8.4 Retarders

• 2.8.5 Accelerators

• 2.8.6 Other admixtures

Accelerator

Superplasticiser for strength gain

2.8 ADMIXTURES FOR

CONCRETE

• 2.8.1 Introduction

• 2.8.2 Plasticisers, superplasticisers and

viscosity modifiers

• 2.8.3 Air entrainment

• 2.8.4 Retarders

• 2.8.5 Accelerators

• 2.8.6 Other admixtures

Other Admixtures • Foaming agents "Foamed concrete" is a non-

structural void filler which can be dug out with excavators.

• Shrinkage compensators.

• Corrosion Inhibitors. Calcium Nitrite is used as an anode inhibitor. Other admixtures can inhibit the cathode.

• Alkali Aggregate Reaction Inhibitors. Lithium based compounds are used to reduce AAR.

Corrosion Inhibitors

Other Admixtures • Foaming agents. "Foamed concrete" is a non-

structural void filler which can be dug out with excavators.

• Shrinkage compensators.

• Corrosion Inhibitors. Calcium Nitrite is used as an anode inhibitor. Other admixtures can inhibit the cathode.

• Alkali Aggregate Reaction Inhibitors. Lithium based compounds are used to reduce AAR.

When using more than one

admixture

• Never pre-mix admixtures before adding

them to the concrete

• The order and timing of admixture addition

can be critical