2High Performance Concrete Concrete may be regarded as high performance for several different reasons:high strength,high workabilityhigh durability– and perhaps also improved visual appearance.
3High 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).
4Ultra High Strength Concrete How High?Strengths of MPa 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
5Ultra 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
9The Important Properties Self-compactabilityAvoidance of bleeding and segregationLow shrinkageLow permeabilityStrength as needed
10The Benefits Less dependent on skill on site Safer, quieter sites (no vibration)Better appearanceBetter durabilityStrength as needed
11What’s Different?Segregation resistance from mortar viscosity, not aggregate gradingWorkability through admixtures, not water content
12What’s The Snag? Higher cost – especially if high strength not needed Plant control has to be better
13Will it happen?My guess is that 50% of concrete will be self-compacting within 10 yearsIn USA a very large proportion of precast concrete is already SCC
14What is the secret? Paste Viscosity! Attained by one of three means: High cement contentHigh content of Fly Ash, Silica Fume etcUse of Viscosity Modifying AdmixturePlus low water content using HRWR
15How to Check Suitability? There are many new tests: V-funnel, L-box, U-box, Fill-box, Orimet, GTM Screen + Slump Flow and J-ringWhile several of these may be used in mix development, only the latter will find site use
16What is Slump Flow Test? Measuring spread rather than height plus: Speed of flow outwardsAbility to pass through J-ringObservation of edge during flowInteresting to note that it works better with the cone upside down!
18Why Does It Look Better?Perfect compactionNo bleeding
19ConclusionWe are going to expect concrete that is a little more expensive per cubic metre to buyBUTIt will require less skill and effort to placeIt will look better with no appearance defectsIt will be more durableIt will require more skill at the batching plant