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
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
The Important Properties
- Avoidance of bleeding and segregation
- Low shrinkage
- Low permeability
- Strength as needed
- Less dependent on skill on site
- Safer, quieter sites (no vibration)
- Better appearance
- Better durability
- Strength as needed
- 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 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!
We are going to expect concrete that is a little more expensive per cubic meter to buy
- 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