New Generation Admixtures Presented by: Graeme SMITH, Hennie van - - PowerPoint PPT Presentation

New Generation Admixtures

Presented by:

Graeme SMITH, Hennie van HEERDEN Sephaku Cement

• What are admixtures?
• Why use them and current use
• Admixture types to SANS 50934
• Plasticisers and Superplasticisers
• SCC ; new generation concrete!
• Air-Entrainers
• Set control admixtures
• Sprayed Concrete

Agenda

• Admixtures are Chemicals typically consisting of
• 1 or 2 major constituents
• + 2 or more minor constituents
• Added to the concrete during mixing
• Usually as an aqueous solution, < 40% active chemical
• Quantity is very small
• 0.2 to 5.0% Admixture by weight on cement
• Active chemical usually less than 0.15% on the concrete
• South African Standard – SANS 50934 :1 to 6 (adoption of EN 934);not yet a

compulsory standard

• Modify the properties of the concrete in the Plastic and / or Hardened state

Admixtures

Heading

• Economic - Producer
• Optimised mix design
• Placing - Contractor
• appropriate workability (consistence), compactability, cohesion, setting and

strength development

• Problem Solving – Designer / Engineer
• concrete that will meet special needs for placing or performance
• Durability - Owner
• Ensuring that concrete fulfils its design requirements for the intended life of the

structure

• Sustainability - Everyone
• helping to reduce the health and safety aspects of concrete during placing and

environmental impact during its life cycle

• www.sustainableconcrete.org.uk

Why use Admixtures?

• Admixtures are currently used in over 80% of all Ready Mixed and Precast

Concrete

• Admixtures are now a normal ingredient of concrete, together with Cement,

Aggregate, Additions and Water- the 5th ingredient!

• Admixture sales by type for 2012

Current Admixture Use

Admixture Types to SANS 50934 (EN 934)

Normal Plasticisers (Water reducers) EN 934-2 T2 Super-plasticisers (High Range WR) EN 934-2 T3 Air Entraining EN 934-2 T4 Accelerating EN 934-2 T6 & 7 Retarding EN 934-2 T8 Water Resisting (Waterproofing) EN 934-2 T9 Mortar Admixtures EN 934-3 Grout Admixtures EN 934-4 Sprayed Concrete Admixtures EN 934-5

Other Admixture Types

Corrosion Inhibiting Admixtures BS 8443 T4 Foamed Concrete Admixtures BS 8443 T7 Polymer Dispersion Admixtures Pumping Admixtures BS 8443 T5 Viscosity Modifying (Segregation Reducing) Admixtures BS 8443 T6 Semi-Dry concrete Admixtures BS 8443 Shrinkage Reducing Admixtures BS 8443 T3 Wash-water Recycling Admixtures Anti-Washout Underwater Concrete Admixtures BS 8443 T2

• 80% of all Concrete Admixture Sales
• Both types are essentially Particle Dispersants
• The difference is in the dispersing power
• But also in the secondary effects:
• Retardation
• Air entrainment
• Also known as Water Reducers / High Range Water Reducers

Plasticisers & Superplasticisers

• Dispersing Effect of Plasticisers & Superplasticisers

Particle Dispersion

add a plasticising admixture

cement flocs reduce fluidity cement is uniformly dispersed increasing fluidity

• Plasticiser or Super is adsorbed onto cement particle
• These dispersant polymers repel other cement particles by electrostatic or steric

repulsion

• Repulsion stops cement from flocculating in clumps and trapping water

• Plasticise at = Cement content & W/C
• Increased Consistence
• Water Reduce to = Consistence
• Increased Strength,
• Reduced Permeability
• Cement & Water Reduce to = W/C & Consistence
• Lower Rands, lower water use, lower cementitous
• Improved sustainability, lower embedded CO2

Dispersant Uses

Plasticisers vs Superplasticisers

Plasticiser Superplasticiser Main constituent

Lignosulphonate Selected Carbohydrates Melamine Polymers SMFC Naphthalene Polymers SNFC Polycarboxylate Ethers PCE

Typical dose (40% Solution)

0.3 to 0.5% 0.6 to 1.5%

Cost

Plasticiser x 2 Not economic for cement cost savings

Overdose

Retardation Air Entrainment Little effect till x 2 overdose Without water reduction, mix may segregate

Water reduction

8 to 12% 16 to 30% +

Slump Test

• Add the plasticiser / superplasticiser to the concrete.
• W/C does not change so strength and durability are unchanged, only

consistence increases

• slump increased from 50 to 120 mm for site use
• Normal Plasticiser
• 180 to 220 mm slump for flowing concrete and congested reinforcement
• Mid range or Superplasticiser
• Pump mix to prevent segregation
• Self Compacting Concrete for self levelling concrete, very congested

reinforcement, high quality surfaces, difficult shapes, H&S -vibration free (reduces noise), no vibration white finger and back problems.

• PCE Superplasticiser needed

Increased Workability/Consistence

Low Slump Concrete

High Flow but not quite Self- Compacting

High Flow but not Self-Compacting

• Pre 1970 – most concrete 50 to 120mm slump, occasionally 180mm
• 1970 – 1995 up to 220mm slump
• Based on SMF or SNF Superplasticisers
• required little vibration or compaction
• 1990 – Japan introduces SCC
• 1995 – Europe experiments with SCC
• 2003 to date – SCC use increasing
• Widely used in Precast
• Low noise
• Early strength
• Excellent surface finish
• Used on site mainly for special applications:
• Difficult access
• Difficult shapes
• Improved surface finish

Use of high consistence concrete

• SCC resulted from the development of PolyCarboxylate Ether (PCE)

Superplasticisers.

• SCC requires a combination of:
• High fluidity
• Resistance to segregation
• Resistance to blocking in congested reinforcement
• Earlier Superplasticisers could not consistently produce concrete that

exhibited all these properties.

• SCC requires greater control on constituent quality and on production

control so is not appropriate for all concrete.

Self-Compacting Concrete

• High powder content
• PC + ggbs, fly ash, limestone fines etc. 450 to 600 kg/m3
• PCE Superplasticiser
• Sand 45 to 55% of aggregate
• Coarse aggregate normally <16mm
• Viscosity Modifying Admixture may be needed
• Mix design must 100% consistently achieve:
• Good flow
• No segregation
• Good passing ability through reinforcement /no blocking
• Appropriate viscosity / speed of flow
• SCC mix design requires an in depth knowledge of concrete mix design and materials

as well as consistently good constituents

• SCC should only be purchased from companies with a proven track record for

consistent and high quality supply of this type of concrete.

SCC mix design

• Most SCC will have a slump flow
• f 660 – 750 mm
• A J-Ring or L-Box should not

significantly reduce the flow and there should be no height step from inside to outside the bars.

• Coarse aggregate should be

visible at the surface and right up to the outer edges

• There should be no indication of

bleed or grout separation even if left to stand.

Consistence by Slump Flow

Self-Compacting Concrete

Precast – High flow & No Vibration

Site – High flow & No Compaction

High flow and good passing ability

Precast – Surface Detail

Surface Quality

• Air Entrainers
• Set Control
• Accelerators
• Retarders
• Water Resistance – (Waterproofers)

Other SANS 50934-2 Admixtures

• 1% Air reduces strength by

about 5%

• Air entrainment:
• Provides Freeze Thaw
• resistance. Is a

requirement for highway pavements

• Air increases the apparent

paste volume, increasing cohesion, reducing settlement and can significantly reduce bleed

• In low fines mixes and

semi dry concrete it lubricates the mix, improving compaction

Air Entertainment

• Occurs when saturated concrete is subjected to Freeze Thaw action.
• Water in the capillaries freezes, generating pressure and causing the surface

to spall

• Aggravated if de-icing salts are used
• De-icing salts can increase the rate of freezing within the concrete
• Can result in osmotic pressure due to concentration gradients
• Entrained Air:
• Does not fill with water in saturated concrete due to capillary action
• Relieves the pressure if water in the capillaries start to freeze
• Bubble spacing needs to be small (< 0.2 mm spacing factor)
• Bubble size typically less than 0.3 mm

Freeze Thaw Resistance

Freeze-Thaw

Cohesion/Bleed reduction

• Acceleration of Set or Strength?
• Most types do one or other not both.
• Sprayed Concrete accelerators
• very fast set, used in Mining/Tunnelling applications
• Calcium Chloride
• accelerates both set and strength.
• very cost effective
• enhances corrosion of embedded steel
• Other Accelerators
• Less effective than Calcium chloride
• Usually accelerate Set or Strength but not both

Accelerating Admixtures

• Superplasticisers have generally replaced strength accelerators
• Accelerators are most effective at low temperatures
• Use accelerators for very early strength, < 12 hours
• > 12 hours strength, better to use HRWR
• 24 hour strength can be more than 200% of a control mix
• HRWR can be used with a set accelerator
• Accelerating HRWR admixtures are also available
• Use set acceleration for:
• Early finishing of floors
• Early strength for access (Pavement repairs - Roads/Airports)

Use of Accelerating Admixtures

• Most retarders do not maintain the workability at the initial level
• Some special workability retention admixtures are available but usually need a

special accelerator to initiate setting.

• Retarders delay the setting
• Setting equates to the time when two layers of concrete can no longer be vibrated

to fully remove a joint between concrete pours.

• Retarders are often incorporated in plasticisers or superplasticisers to give higher

initial workability and hence longer workability = Retarding Plasticisers etc.

Retarding Admixtures

Workability retention and Set retardation 1

Workability retention

Retardation

Workability retention and Set retardation 2

Slump 80 mm 0 mm Set

0 slump but not set NO cold Joint 0 slump & setting Cold Joint Formed

• Function:
• Reduce water absorption into the concrete surface and / or
• Reduce passage of water through the concrete
• Action:
• Reduce number and continuity of the capillary pore structure with a water

reducer AND:

• Block the capillary pores

OR:

• Line capillaries with hydrophobic material to prevent capillary suction into the

concrete

Water Resisting (waterproofing)

• Hydrophobic types line the

capillary wall but are only effective against rain and low water pressure

• Capillary blockers are needed if

there is water pressure

• Many waterproofing admixtures

also contain a water reducer to reduce the capillary continuity

Water Resisting Admixture Selection

• Corrosion Inhibiting Admixtures
• Viscosity Modifying
• Shrinkage Reducing
• Sprayed Concrete

Other Admixtures outside SANS 50934

Spraying Concrete

• Mainly used in Mining/Tunnelling Applications.
• The accelerating admixture is added at the spray nozzle to give very fast stiffening
• f the concrete mix.
• Used to be based on silicates and aluminates which were very alkaline and

therefore a health hazard.

• Modern Sprayed Concrete Accelerators are non corrosive, give good stiffening but

slower strength gain. However longer term durability is better.

Sprayed Concrete Accelerators

Questions?