Admixtures

1
Admixtures

• proprietary mixture of chemicals
• added to concrete to alter or improve the
  properties of fresh or hardened concrete.
• Should be used if
• properties cannot be made by varying the
  composition of basic material
• 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

2
Use of admixtures

• Permission for use - 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
• add to the mixing water
• manufacturer's recommended dosage is usually
  adequate
• the ideal dosage depends on the materials
  temperature
• Trial mixes are important.

3
Air entraining admixtures

• 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

4
Accelerating agents

• Increases setting and early strength development
• CaCl2 is the most effective
• use in
• winter conditions
• emergency repair work
• early removal of formwork
• increased drying shrinkage
• reduced resistance to sulphate attack
• risk of corrosion of steel
• not permitted in RC
• Chloride free accelerators are available for RC
• more expensive
• less effective

5
Retarders

• Slow down the setting and hardening times
• prevents the formation of cold joints
• compensates for time lost in transit
• Useful in
• large concrete pours
• sliding formwork
• hot weather concreting
• Retarding water-reducing admixtures are gaining
  popularity

6
Water reducers / plasticisers

• Normal water reducing admixtures
• increases the dispersion of cement particles and
  a reduction in viscosity
• ? workability for same w/c ratio
• ? strength without adding cement
• ? cement
• Superplasticisers
• flowing concrete
• no change in composition
• no reduction in strength
• heavy reinforcement / inaccessible
• rapid placement
• flowing characteristics last for only a short
  time (30 mins)
• added at site immediately before placing
• high strength concrete
• reduce w/c ratio
• maintain the same workability

7
Superplasticiser

• Concrete with no superplasticiser

8
Superplasticiser

• Concrete with superplasticiser courtesy of MBT
  admixtures