Transcript Regulating Concrete Quality

Regulating Concrete Quality
Ken Day, Consultant
Melbourne, Australia
The Objectives
To achieve suitable regulation it is first
necessary to:
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A) Realise what you are trying to achieve
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B) Realise what you are trying to prevent
Historically:
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Specification was related to an individual
batch of concrete
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Batch quantities were the subject of the
regulation
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Full time inspection was affordable
Strength as a Criterion
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Strength was then recognised as the only
workable basis
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An absolute minimum strength was
specified
Inevitable Variability
recognised
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Strengths of successive deliveries of
supposedly identical concrete were seen
to vary by up to +/- 15MPa, rarely less
than +/- 5MPa
Grouping Results
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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
“Percentage Defective”
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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
“Percentage Defective”
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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
What are You Trying to Stop?
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A low mean strength?
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A high variability?
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Occasional gross errors?
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ALL OF THE ABOVE!
Gross Errors
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Even testing alternate trucks (at
excessive expense) would give only a
50% chance of detection
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You are reliant on the producer’s
equipment and QC system so these need
maximum encouragement/reward
Penalisation
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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
Downturn detection
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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?
Speeding downturn detection
Two techniques make a huge difference:
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Base control on plant rather than project
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Use multigrade basis, i.e. combine results
from possibly hundreds of grades of
concrete in an analysis of situation
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
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
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
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
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
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
Regulation in UK and Europe
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Recent new standard EN206
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Requirements rather than control system
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QSRMC is real control system in UK
QSRMC
Quality Scheme for Ready Mixed Concrete
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Established by the industry, big advance
on world scale
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First to introduce Cusum (dev by RMC)
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Multigrade technique uses transposition
of results to a single grade for analysis
USA
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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
Australia (AS1379)
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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
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
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
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
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
Draft of Desirable Regulations
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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
Quality Implications
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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
Quality Implications
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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
Cost Implications
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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!
Conclusions
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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
Conclusions
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A comparison of practice in different
countries illustrates that failure to apply
these principles inhibits development of
improved technology
Conclusions
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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!