Admixtures

1
Admixtures

• Definition
• Proprietary mixture of chemicals added
• at the mixing stage to concrete to alter
• or improve the properties of fresh or
• hardened concrete.

2
Admixtures Uses

• When properties cannot be made by varying the
  composition of basic material
• To produce desired effects more economically
• Unlikely to make a poor concrete better
• Not a substitute for good concrete practice
• Required dose must be carefully determined and
  administered

3
Use of admixtures

• Check job specification
• Use the correct admixture
• never use one from an unmarked container.
• keep containers closed to avoid accidental
  contamination.
• Add the correct dosage.
• avoid adding 'a little bit extra'
• use a dispenser
• wash thoroughly at the end the day
• Best if added to the mixing water
• Manufacturer's recommended dosage is usually
  adequate
• Trial mixes are important to determine most
  effective dosage

4
Use of admixtures

• Check job specification
• Use the correct admixture
• never use one from an unmarked container.
• keep containers closed to avoid accidental
  contamination.
• Add the correct dosage.
• avoid adding 'a little bit extra'
• use a dispenser
• wash thoroughly at the end the day
• Best if added to the mixing water
• Manufacturer's recommended dosage is usually
  adequate
• Trial mixes are important to determine most
  effective dosage

5
Types of admixtures

• Air entraining agents AEA
• Water reducers
• Plasticisers
• Superplasticisers
• Retarders
• Accelerators

6
Air entraining agents

• Foaming agents, gas producing chemicals
• introduces millions of tiny, stable bubbles of
  uniform size that are uniformly distributed
  throughout the mix (usually about 5 of the
  volume).
• Improves properties of fresh concrete
• workability, cohesion
• reduces segregation and bleeding .
• Properties of hardened concrete
• For every 1 of air there is a 4 loss in
  strength.
• Minimised by the reduction in water content
• Improves durability
• frost, de-icing salts

7
Air entraining agents

• Improves durability
• frost, de-icing salts

8
Water reducers plasticisers

• Properties
• increases the dispersion of cement particles and
  a reduction in viscosity
• Improve workability with same water
• Reduce water required for equal slump
• Do not affect shrinkage

9
Water reducers / plasticisers

• Mechanism

10
Water reducers / plasticisers

• Dispersion of cement

11
Water reducers / plasticisers

• Corresponding mixes

12
Superplasticisers

• Properties
• Produce flowing concrete
• no reduction in strength
• Assist placing when heavy reinforcement
• flowing characteristics
• short duration (30 mins) old technology
• New generation can tailor made
• Usually added at site immediately before placing
• Can produce high strength concrete
• reduce w/c ratio
• maintain the same workability

13
Superplasticisers

• Properties
• Amount of fines in mix usually increased
• Take care with rapid pouring in high walls
  (hydraulic pressure)

14
Superplasticisers

• Cement dispersion

15
Superplasticiser

• Normal slump concrete with no superplasticiser

16
Superplasticiser

• Concrete with superplasticiser courtesy of MBT
  admixtures

17
Retarders

• Properties
• Longer setting times
• Slower strength gains
• Prevents the formation of cold joints
• Assist with long transit times
• Main uses
• large concrete pours
• sliding formwork
• hot weather concreting
• Retarding water-reducing admixtures are also
  available
• Overdosing can cause problems
• Excessive retardation
• Excessive bleeding

18
Accelerating agents

• Properties
• Quicker setting times
• Higher early strength development
• Types
• CaCl2 most effective
• Chloride free use in
• Main uses
• winter conditions
• emergency repair work
• early removal of formwork

19
Accelerating agents

• Disadvantages
• increased drying shrinkage
• reduced resistance to sulphate attack
• CaCl2 highrisk of corrosion of steel - not
  permitted in RC
• Cl free
• more expensive
• less effective