Regulating Concrete Quality

1. Regulating Concrete Quality Ken Day, Consultant Melbourne, Australia

2. The Objectives To achieve suitable regulation it is first necessary to: • A) Realise what you are trying to achieve • B) Realise what you are trying to prevent

3. Historically: • Specification was related to an individual batch of concrete • Batch quantities were the subject of the regulation • Full time inspection was affordable

4. Strength as a Criterion • Strength was then recognised as the only workable basis • An absolute minimum strength was specified

5. Inevitable Variability recognised • Strengths of successive deliveries of supposedly identical concrete were seen to vary by up to +/- 15MPa, rarely less than +/- 5MPa

6. Grouping Results • Small groups of 3, 4 or 6 results were tried by various countries • Even groups of 6 did not provide an accurate mean strength and variability • Even groups of 3 represented too much concrete to reject as a unit

7. “Percentage Defective” • A “Normal Distribution” was found to be applicable so that results could be analysed for mean strength, standard deviation, and % below any given strength • About 30 results were needed to give good accuracy

8. “Percentage Defective” • Percentage defectives of 1, 5 and 10% have been used, multiplying the SD by 2.33, 1.645 and 1.28 respectively • Decision based on “what is a reasonable margin” • I would suggest it should be based on the value placed on low variability

9. What are You Trying to Stop? • A low mean strength? • A high variability? • Occasional gross errors? • ALL OF THE ABOVE!

10. Gross Errors • Even testing alternate trucks (at excessive expense) would give only a 50% chance of detection • You are reliant on the producer’s equipment and QC system so these need maximum encouragement/reward

11. Penalisation • Marginal underperformance cannot be fairly dealt with any other way than financial penalisation (marginal is grey, not black or white!) Failure to penalise underperformers places good producers at a disadvantage

12. Downturn detection • Even with appropriate financial compensation, purchaser (and producer!) will be keen to avoid defective concrete. This raises two questions: • How to predict eventual strength from early result? • How to get enough results quickly at acceptable cost?

13. Speeding downturn detection Two techniques make a huge difference: • Base control on plant rather than project • Use multigrade basis, i.e. combine results from possibly hundreds of grades of concrete in an analysis of situation

14. Speeding downturn detection The combination of these techniques can increase a hundredfold the number of results available and drastically reduce time to detection of a downturn A downturn in a particular grade at a particular project may be detected before any results are available on that project, or even on that grade

15. Speeding downturn detection Further improvement in detection time possible using advanced analysis system Cusum analysis has been shown to be approximately three times as effective as Shewhart charting – which is still better than normal graphing

16. Speeding downturn detection Better Prediction: Early results not usually % of later results, adding average gain better • Needs continuous feedback of true gain which can change abruptly

17. Speeding downturn detection Multivariable Analysis Cusum graphs of many items – density, slump, temperature, cement tests, sand specific surface etc etc can give instant explanation of strength changes • Cusums are Cumulative Sums of difference between current value and previous mean – can include LW and dense on same density graph, high and low strength grades on strength graph

18. Speeding downturn detection The purchaser is not in as good a position as the producer to detect downturns early If a later penalty is inevitable, the producer will be just as keen as the purchaser to detect and rectify downturns early

19. Conclusion What is needed is a type of regulation that will encourage producers to expend every effort to establish a system and physical facilities that will: • Produce low variability concrete • Correctly target mean strength • React quickly to any downturn

20. Regulation in UK and Europe • Recent new standard EN206 • Requirements rather than control system • QSRMC is real control system in UK

21. QSRMCQuality Scheme for Ready Mixed Concrete • Established by the industry, big advance on world scale • First to introduce Cusum (dev by RMC) • Multigrade technique uses transposition of results to a single grade for analysis

22. USA • Strangely resistant to innovation • Perhaps partly due to fragmented industry but prime example of specification-driven barrier to progress • Prescription mixes still common • Mix adjustment actually prohibited • Producer designs abused if permitted

23. Australia (AS1379) • Regulations are by Aust. Standards Assn. • Production mainly by few large producers • Producers required to undertake own testing and report monthly to purchasers • Not perfect, but best example of suitable regulation leading to good control – could be better early reporting, penalties

24. Draft of Desirable Regulations The concrete producer shall have in operation an effective QC system with at least the following features: 1) Plant to produce, preserve, and link to QC system, complete record of actual and intended batch quantities of every batch

25. Draft of Desirable Regulations 2) Batch records to be analysed to show any systematic trend to error or any significant individual error and any such to be reported to purchasers 3) Mixes may be collected into multigrade groups and each such group shall have a minimum rate of testing each month

26. Draft of Desirable Regulations 4) All data shall be entered in control system within 24hrs of obtaining and analysed daily to detect change using graphical, multigrade, cusum analysis or proven equally effective alternative 5) All purchasers of concrete PREDICTED to be sub-standard shall be immediately informed

27. Draft of Desirable Regulations 6) A monthly report detailing for each mix in production, at least: number of results, early age and predicted and actual mean strength, standard deviation minimum strength, No & % of results below specified strength

28. Draft of Desirable Regulations • Note emphasis on early detection of any problem and ready availability of data to establish cause • A usually trivial cost penalty of twice the cost of the amount of cement that would have raised the month’s mean strength to the required would be sufficient to ensure fair competition

29. Quality Implications • W/C ratio basic factor and directly related to strength – at a given strength the mix with the LOWEST cement content is the best (lower water) • Pozzolanic materials reduce cost, improve durability and environment • More uniform concrete likely to be easier to place, better appearance

30. Quality Implications • Important to understand that this paper does not pass any judgement on desirable strength margins in structural design, or for durability considerations • Author believes extra cost of higher margin often worthwhile but should not be by requiring higher mean regardless

31. Cost Implications • Difficult to quantify savings by proposals • Avoiding costs of further testing, negotiations, rejections, due to poor control (or poor testing!)? • Better mix design, wider material choice? • Reduced expenditure on control testing? • Reduced mean strength due lower SD!

32. Conclusions • Paper is concerned with best way to ensure a selected strength obtained with max certainty and min cost • A key factor is that regulations must not inhibit progress and must provide a fair basis for competition

33. Conclusions • A comparison of practice in different countries illustrates that failure to apply these principles inhibits development of improved technology

34. Conclusions • It may never be possible to completely eliminate problems but if they can be largely foreseen and the rest detected and resolved in minutes or hours instead of days or weeks, the economic benefits could be substantial • The main losers are likely to be the legal profession and the physical investigators of defective concrete!