Coping with Variability of Input Materials for Readymix Concrete Ken W. Day Consultant, Melbourne, Australia

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

Ways of Detecting Variation Tests on incoming materials Tests on incoming materials Analysis of concrete test data Analysis of concrete test data

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

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

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

MULTIGRADE Multigrade means combining data from many grades on a single graph 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 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 Cusum differences from individual grade targets as though from same target

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

Cusum Graph

Sand:Timeline of Grading

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

Tabulated results(1) QC Program should display table of all grades in the multigrade analysis 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 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 This allows concentration on few extreme grades and avoids any grade with problems but few results being overlooked

Factors other than strength This paper does not cover durability aspects This paper does not cover durability aspects Workability usually covered by specific surface but not necessarily. Author’s system offers two alternative additional criteria: Workability usually covered by specific surface but not necessarily. Author’s system offers two alternative additional criteria: “Gap Index” – squared sum of differences between % retained on each sieve and 7% “Gap Index” – squared sum of differences between % retained on each sieve and 7% Ability to input a desired grading with warning and action limits Ability to input a desired grading with warning and action limits

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