Coping with Variability of Input Materials for Readymix Concrete

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Coping with Variability of Input Materials for
Readymix Concrete

• Ken W. Day
• Consultant, Melbourne, Australia

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Objective

• To produce concrete of uniform
• properties from materials that vary from
• time to time

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Ways of Detecting Variation

• Tests on incoming materials
• Analysis of concrete test data

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Tests on incoming materials

• Theoretically the way to go
• Need system capable of instant mix adjustment
• In practice up-to-minute test data difficult to
  arrange

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Analysis of Concrete Test Data

• Function of testing NOT to establish whether
  satisfactory but to DETECT CHANGE
• Multigrade, Multivariable, Cusum Graphing supreme
  in detection of change

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CUSUM Graphing

• CUSUM Cumulative Sum of differences from a
  target
• Best target for variability detection is previous
  average mean

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MULTIGRADE

• Multigrade means combining data from many grades
  on a single graph
• Two techniques available
• Adjust results of other grades so they can be
  analysed as from basic grade
• Cusum differences from individual grade targets
  as though from same target

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MULTIVARIABLE

• Cusum graphs of other variables eg
• Density
• Slump
• Temperature
• Cement content
• Sand specific surface
• To see if change points coincide

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Cusum Graph
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SandTimeline of Grading
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Selection of Grades to Include

• Start with all grades in a single analysis
• Select groups of grades excluding one particular
  ingredient at a time
• If change point disappears when a particular
  ingredient is not present, that ingredient is the
  cause of the change

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Tabulated results(1)

• QC Program should display table of all grades in
  the multigrade analysis
• Each grade should show at least a dozen
  properties and it should be possible to display
  them in order of difference between target
  strength and actual/predicted strength
• This allows concentration on few extreme grades
  and avoids any grade with problems but few
  results being overlooked

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Factors other than strength

• This paper does not cover durability aspects
• Workability usually covered by specific surface
  but not necessarily. Authors system offers two
  alternative additional criteria
• Gap Index squared sum of differences between
  retained on each sieve and 7
• Ability to input a desired grading with warning
  and action limits

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Summary Overall Procedure

• Test incoming materials and adjust for change
• Cusum results for all grades together
• Detect adjust for overall factors eg
  temperature
• Examine tabulated results for problem grades
• Separate results into groups excluding each
  material in turn
• Use Multivariable graphs to examine influence of
  change in materials