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
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
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
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
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
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
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
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.
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.