High Performance Concrete & Self-Compacting Concrete

High Performance Concrete

High Performance Concrete

High Performance Concrete

Concrete may be regarded as high performance for several different reasons:

  • high strength,
  • high workability
  • high durability
  • perhaps also improved visual appearance.

High strength concrete (HSC) might be regarded as concrete with a strength in excess of 60MPa and such concrete can be produced as relatively normal concrete with a higher cement content and a normal water-reducing admixture. However ultra high performance concrete (UHPC) will more usually contain cement replacement materials and a high-range water-reducer (HRWR) or superplasticiser(SP) (different names for the same thing).

Ultra High Strength Concrete

How High?

Strengths of 150-200MPa were reported in several papers at a recent symposium

How is it done?

Using only fine sand as an aggregate, a high content of cement and silica fume, a high dosage of HRWR admixture plus steel fibres

In what kind of structures?

  • Thin shell roofing (2cm thick) and “bulb” double and single tees were reported
  • Both insitu and precast applications
  • Flexural and tensile strengths also high, allowing omission of secondary reinft.
  • Concrete in tees was generally self- compacting

Self-Compacting Concrete

The Important Properties

  • Self-compactability
  • Avoidance of bleeding and segregation
  • Low shrinkage
  • Low permeability
  • Strength as needed

The Benefits

  • Less dependent on skill on site
  • Safer, quieter sites (no vibration)
  • Better appearance
  • Better durability
  • Strength as needed

What’s Different?

  1. Segregation resistance from mortar viscosity, not aggregate grading
  2. Workability through admixtures, not water content

What’s The Snag?

Higher cost – especially if high strength not needed

Plant control has to be better

Will it happen?

My guess is that 50% of concrete will be self-compacting within 10 years

In USA a very large proportion of precast concrete is already SCC

What is the secret?

Paste Viscosity! Attained by one of three means:

  • High cement content
  • High content of Fly Ash, Silica Fume etc
  • Use of Viscosity Modifying Admixture

Plus low water content using HRWR

How to Check Suitability?

  • There are many new tests: V-funnel, L-box, U-box, Fill-box, Orimet, GTM Screen + Slump Flow and J-ring
  • While several of these may be used in mix development, only the latter will find site use

What is Slump Flow Test?

Measuring spread rather than height plus:

  • Speed of flow outwards
  • Ability to pass through J-ring
  • Observation of edge during flow

Interesting to note that it works better with the cone upside down!

Conclusion

We are going to expect concrete that is a little more expensive per cubic meter to buy

BUT

  • It will require less skill and effort to place
  • It will look better with no appearance defects
  • It will be more durable
  • It will require more skill at the batching plant
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