Concrete Fresh Properties (3)

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Marciaho
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269830
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Concrete Fresh Properties (3)
Updated:
2014-04-08 18:51:37
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Concrete Fresh Properties
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Concrete Fresh Properties (3)
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Concrete Fresh Properties (3)
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  1. Consistency of concrete definition 
    and 2 tests
    • Measure of the stiffness of sloppiness of a mix and is influenced by the fluidity of the cement paste phase
    • Slump test 
    • Flow spread test
  2. Slump test range
    Moderate to low stiffness
    Moderate to low stiffness 30-150mm
  3. low/spread
    Flow-able concrete
    • Flowable concrete 
    • 550-800mm diameter
  4. Workability of concrete
    Determines how much energy is require to place and compact concrete
  5. Workability is influenced by (4)
    • Stone size
    • Fines content of sand
    • Cement content and type
    • Admixtures and mix proportions
  6. Water demand
    Is the amount of water required to produce specified level of consistency (normally 100mm slump)
  7. Water demand should be kept as low as possible to: (3)
    • Reduce cost
    • Minimise shrinkage and creep
    • Control bleeding and segregation
  8. Typical range of water demands
    • 145L for well rounded smooth agg
    • 160L for normal alluvial gravels
    • 175 for crushed, angular agg
  9. Water demand influences (2)
    • Mix Constituents (cement and aggregates)
    • Consistency required ( dry mix - SCC)
  10. Constituents

    4 applications 
    Kerb/pavers
    Floor slabs
    Structural
    SCC Mix
    • Aggregate characteristics
    • Fines content of mix
    • Binder characteristics
    • Admixture type and dosage

    • Applications
    • Kerb Pavers 145L
    • Floor Slabs 155L
    • Structural 160L
    • SCC mix 205L
  11. Aggregate effect on water demand
    Angular
    Chunky
    Elongated
    Rounded
    • Angular 190
    • Chunky 175
    • Elongated 165
    • Rounded 150
  12. Bleeding and settlement
    Under normal conditions and good mix design:

    Under extreme conditions and poor mix design:
    Good mix design: Bleed water rises toward surface while solids settle downwards

    Poor mix design: Bleed water forms channels and lenses and segregation may occur
  13. Effect of bleeding on concrete
    Beneficial
    Harmful
    • Beneficial:
    • Removes excess bleed water
    • Improves surface finishing
    • Indicates when setting occurs
    • Helps prevent plastic shrinkage cracking

    • Harmful:
    • Causes plastic settlement cracking 
    • Forms bleed lenses under bars and aggregates
    • Topbar effect
    • Segregation
    • Less bond between steel and concrete
  14. Plastic shrinkage cracking occurs when :
    • Drying exceeds bleed rate leading to contraction of concrete
    • During periods of hot dry and windy conditions
    • When setting is disrupted by changes in section of horizontal reinforcing bars
  15. How to avoid plastic shrinkage cracking
    • Pre wet all surfaces before pouring concrete
    • Protect concrete from rapid drying
    • Avoid casting in extreme weather conditions
    • Ensure concrete has reasonable bleed
    • Pour smaller sections
    • Revibrate critical sections
    • Improve mix designs
  16. Slump loss occurs as consistency is lost with time
    Reasons and control
    • Reasons: 
    • Cement hydration
    • Evaporation of water
    • Water absorption by agg
    • Absorption by dry surfaces

    • Control:
    • Batch at higher slump
    • Reduce concrete temperature
    • Add super plasticiser
    • Use retarder
  17. 5 Site Practices C's 
    • Concrete 
    • Cover
    • Casting
    • Compaction
    • Curing
  18. When placing concrete ensure:
    • required consistency for workability
    • at a rate so no cold joints
    • manner that doesnt displace reinforcing
    • no segregation of concrete
    • no contamination from soil, groundwater
  19. Compaction. Entrapped air needs to be removed because it:
    • Reduces strength 1% air = 5%decrease in strength
    • Reduces durability
    • Reduces bond between bond reinforcement
    • Increase in permeability
    • Produces visual defects
  20. Finishing of concrete 
    Floating and Trowelling
    • Floating: Embeds stones beneath surface
    • Compacts surface of concrete
    • Done as soon bleeding stops

    • Trowelling: Polishing of concrete surface
    • Draws cement upwards
    • Produces hard, dense surface
  21. Curing of concrete
    • Curing compounds
    • Ponding of water
    • Spraying water
    • Wet hessian sacking
  22. Curing effect of durability/strength
    Cover concrete 
    Bulk/interior concrete
    Cover concrete: Protects reinforcement 

    • Bulk Interior concrete: Provides most strength 
    • not affected by initial curing

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