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 producers 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

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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

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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 months mean strength to the required would
  be sufficient to ensure fair competition

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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!