High Performance Concrete

Ken Daywww.kenday.id.au

High Performance Concrete

Concrete may be regarded as high performance for several different reasons: high strength, high workability high durability

– and perhaps also improved visual appearance.

High Performance Concrete

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 symposiumHow 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

Ultra High Strength Concrete

In what kind of structures?Thin shell roofing (2cm thick) and “bulb” double and single tees were reportedBoth insitu and precast applicationsFlexural and tensile strengths also high, allowing omission of secondary reinft.Concrete in tees was generally self- compacting

Self-Compacting Concrete

Ken Daywww.kenday.id.au

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?

Segregation resistance from mortar viscosity, not aggregate grading

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 contentHigh content of Fly Ash, Silica Fume etcUse 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!

Why Does It Look Better?

Perfect compaction

No bleeding

Conclusion

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

I leave you with a look at my website:

www.kenday.id.au