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
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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
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