Workshop on Specifications and Producer QC

1
Workshop on Specifications and Producer QC Ken
Day

• I shall start by spending a few minutes
  describing some provisions of the Australian Code
  before inviting your participation.
• I will then make provocative statements about the
  effects of prescription and performance
  specifications, plant and project control,
  achievable variability, quality of testing, the
  analysis of data and financial benefits
• I enjoy arguments, PLEASE OBLIGE!

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Workshop on Specifications and Producer QC

• Australian Code Requirements
• AS1379 / DR05253www.standards.com.au

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Specification of Concrete

• Concrete shall be specified as either Normal
  class or Special class and by strength grade or
  other readily verifiable parameter
• Standard grades
• 20, 25, 32, 40, 50,/ 65, 80, 100 MPa
• (202,900, 507,250 / 100 14,500psi)
• 32MPa (4,640psi) is the minimum strength allowed
  to be specified for external reinf. concrete
• Also max agg size, slump, method of placement,
  air entrainment are to be specified.

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Other Standard Requirements(wide basic
requirements for all concrete)

• Density 2100-2800kg/m (130-175lb/cu ft)
• Acid soluble chloride and sulphate limits
• Shrinkage (max 1000)
• 7 day strength 50 of grade strength (up to N50)
• Cement complying with AS3972 alone or plus one
  or more supplementary cementitious materials

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Special Class Concrete

• Still preferably uses strength grade can be
• compressive, flexural or indirect tensile
• One of 3 exposure classifications - which imposes
  limits on aggregate durability and class and type
  of cementitious material
• gt50MPa (7,250psi) has to be special class
• Can specify other requirements
• in consultation with the supplier

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Special Class Concrete

• It is certainly allowed to specify tighter limits
  on shrinkage, low permeability, the use of fly
  ash, silica fume, or ggbfs etc.
• This will usually restrict the range of producers
  prepared to supply to those having arrangements
  with particular material suppliers and suitable
  storage arrangements (extra bin?)
• Other special class items could include
  self-compacting concrete, lightweight, very high
  early strengths, colour control, wear resistance,
  heat generation, shotcrete, underwater etc.

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Assessment for Compliance

• Tests at least 1 per 100cu m (133cu yd).
• Assess at selected production interval from 2
  weeks to 3months (1 month usual)
• One grade to be selected as control grade and
  to have at least 10 results per production
  interval (most have many more)
• Variability of production to be assessed on basis
  of this grade

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Compliance Requirements

• The Characteristic Strength is that exceeded
  by 95 of results so
• Mean 1.65SD (90 so -1.28SD in USA)
• However it is recognised that there is an error
  in assessment so k in mean-kSD is varied
  according to number of samples in control grade
  from 3.2 for 4 samples to 1.5 for 10samples and
  1.25 for 15 or more samples (1.25 gives all error
  margin to producer, I do not agree with this)

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Compliance Requirements

• For associated grades the SD is scaled from
  that of the control grade using relative factors
• lt20(2,900psi)-0.9,
• 20-1.0,
• 25-1.1,
• 32-1.2,
• 40-1.3,
• 50 (7,250psi) -1.4
• Note that these are far from assuming the same
  coefficient of variation applies,
  eg 40 _at_ 1.3 not 2x20 _at_1.0

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Dissemination of Information

• A long list of information is to be prepared at
  the end of each production interval and kept
  readily available for inspection
• Customers can require relevant data to be
  submitted to them within 15 days
• The supplier shall notify the relevant customers,
  within 2 working days, if a particular quantity
  of concrete is likely to be below the specified
  strength

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Project Assessment

• At least 1 sample per 50cu m
• Moving average of 3 to exceed fc
• Not much different to US, less likely to find any
  problem than the plant control

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The Purpose of Testing Concrete

• In 1958 I wrote
• The only rational objective for any but 100
  testing is not to discover and reject faulty
  products but to ascertain the minimum quality
  level of the production
• -Some ideas take a while to sink in!

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The Purpose of Testing Concrete

• More recently I have added a second objective of
  quality control
• To detect at the earliest possible moment any
  change in the quality of concrete being supplied
• The factors involved in such a detection are
  
• the frequency of testing,
• the basic variability of the concrete,
• the analysis system in use.

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Plant v Project Control

1. It is far more efficient to ensure that no
   defective concrete is produced at a plant than to
   ensure that no defective concrete is delivered to
   a project because
2. More data is available at less cost per cubic
   yard
3. Problems can be detected and identified earlier
4. Producers can react to a limited amount of early
   age test data but project control usually
   requires a significant number of 28 day results
   to demand action on marginally defective
   concrete.
5. A limited number of standard mixes can be
   accurately maintained with data also on
   shrinkage, durability, fresh properties (slump,
   pumpability, bleeding) etc

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DISCUSSION

1. Prescription specifications provide no incentive
   for producers to know or care anything about
   designing or controlling concrete.
2. As a consequence such specifications have to
   provide a very large safety margin to cover high
   variability and inefficient design and material
   selection.
3. Since there is no incentive for the producer to
   employ competent staff, purchase good materials,
   and have good production facilities, it is
   necessary for all such matters to be specified in
   detail and to employ supervision to ensure
   compliance.
4. Therefore prescription specification concrete is
   inevitably more expensive than producer-controlled
   performance concrete.

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Variability of Concrete (COMMENTS?)

1. Under good quality control, the SD of concrete
   strength should not exceed 450psi and can be as
   low as 300psi (achieved on concrete of 14,000psi
   mean strength on Petronas Towers)
2. Under the UK QSRMC system a figure of 600psi is
   regarded as normal
3. Without formal QC, 800 to 1000psi would not be
   surprising (what is yours?)
4. Even under the relaxed US criterion of 10
   defective, each extra 100psi of SD requires a
   mean strength increase of 128psi and so at least
   20lb/cu yd additional cement

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Testing of Concrete

1. In a paper Bad Concrete or Bad Testing to ACI
   San Diego in 1989 I showed that individual tests
   can be inaccurate to the extent of over 1,000psi
   but that an analysed pattern of results was very
   reliable.
2. A single test result is not an invariably
   accurate assessment of the quality of concrete in
   a single truck
3. ALSO we should show as much concern for those
   trucks we did not test as for those we did

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Testing of Concrete

1. Best criterion of testing quality is average pair
   difference of 28day results.
2. 75psi is best attainable, 150psi just OK,
   200psi is POOR
3. Average strength of a pair is depressed by at
   least half the pair difference (more likely twice
   this)
4. So sub-standard testing costs you money even
   without considering rejections and penalties
   (Prof Jurans Gold in the Mine)

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Testing of Concrete

1. Who does the best job of testing?
2. In Australia a lab is required to be NATA
   registered. An independent lab must achieve
   registration but derived no benefit from
   additional quality since the lab was chosen by
   the main contractor in the 1970s
3. Since higher testing quality meant a higher mean
   and lower variability, suppliers labs had more
   incentive to achieve a higher standard
4. So, as an independent analyst and a NATA
   assessor, I found that suppliers labs generally
   achieved at least a slightly higher standard

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Testing of Concrete Duplicate Testing

1. When concrete producers began doing the main
   testing, specifiers were initially keen to have a
   proportion of check tests by independent labs
2. It proved very inconclusive to test different
   trucks
3. Duplicating tests by two lab teams on the same
   truck was more enlightening, but expensive
4. Requiring producers sampling personnel to take a
   proportion of double samples and deliver half to
   an independent lab worked well
5. Now, after years of experience of such checks,
   very few purchasers require independent checks

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Conclusion on Producer Testing

1. After 20 years of experience it is clear that it
   is worthwhile for any substantial producer to
   establish a high standard, officially certified,
   laboratory
2. The laboratory and its test data and analysis
   records must be open to inspection by customers
   representatives at all times
3. In these circumstances, plant control with
   producer testing is substantially more economical
   and reliable than project control

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Physical and Financial Benefits

1. It is clear that producer operated plant control
   results in better controlled, lower variability
   concrete at a lower real cost of production. So
   there is scope for everyone to benefit
2. The question is how to get started specifiers
   probably happy to use plant control once it is
   seen to exist but producers have to provide it
   first?

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Physical and Financial Benefits

• 3) I suggest specifiers include an alternative
  clause waiving requirements such as minimum
  cement content and aggregate gradings where an
  approved system of plant control is in
  operation. They could initially be generous as
  to what is required for approval.
• 4) Some US producers are already using plant
  control and will immediately benefit.
  Competition will ensure others follow.

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Physical and Financial Benefits

1. The initial benefit will be to those producers
   who first achieve low variability and so are more
   competitive
2. As other producers catch up, the benefit is to
   the customers as competition reduces price
3. There may be some tendency for takeovers as some
   small producers are unable to compete
4. However small producers headed by technically
   competent persons can compete and may be able to
   react quicker than larger producers and gain an
   initial advantage (eg Don Bain at
   Maricopa)

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Analysis of Data

1. The efficient analysis of data is of as much
   significance as quality of testing
2. It is to be judged by the rapidity of corrective
   action in terms of number of results needed to
   reach a decision
3. Multigrade, Multivariable, Cusum analysis is 3 to
   10 times as fast as normal Shewhart graphing
4. A semi-automatic mix correction system is also
   required for rapid response to give low
   variability

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Analysis of Data

1. Such analysis can be undertaken using the free
   programs on my website www.kenday.id.au
2. There are far more elaborate systems (such as
   ConAd) giving greater insight into the causes of
   problems and more precise mix correction
3. Such programs may also be of substantial value in
   production engineering matters, locating batching
   plant and constituent material problems and also
   monitoring the performance of individual testing
   personnel and truck drivers