Transcript Multigrade,………………… Multivariable,……. Cusum…… Quality

Multigrade,…………………
Multivariable,…….
Cusum……
Quality Control
Ken W Day
Multigrade, Multivariable, Cusum QC
In October last year I gave a paper
‘Concrete in the 22nd Century’ to the
Concrete Institute of Australia Biennial,
trying to look as far as possible into the
future.
• Now however I have chosen to look into
the past because my largest contribution
to concrete technology is evidently still not
universally understood.
Multigrade, Multivariable, Cusum QC
• It took 25 years from the early 1950s to the late
1970s to perfect my technique for analysing
concrete compression test data.
• In the 25 years since then, the technique has
spread widely as part of the ConAd QC program
• but still not fully incorporated in any national or
international code of practice.
• So I want to take this opportunity to present its
exact basic concepts.
Multigrade, Multivariable, Cusum QC
I need to present each of the three major
components:
• Multigrade,
• Multivariable
• Cusum
and to add to them one further item:
• Prediction from early age tests.
I will take the three major components in reverse order:
Cusum cumulative sum analysis
I did not invent cusum,
• It was developed in the UK chemical
industry (Woodward and Goldsmith, 1964)
• first used for concrete QC in the UK in the
1970s (Testing Services, 1970).
• I started to apply it some six months later
(independently of the Testing Services development).
Cusum cumulative sum analysis
• Cusum involves subtracting a target value from
each result and maintaining a cumulative sum of
the remainders.
• Its main value is that it detects a change in a
string of results about three times as quickly as a
normal Shewhart Chart.
• Detection can be by mathematical analysis of
the string of cumulative sums, or by graphing the
cumulative sums.
• The graphical method is preferable because it is
easier to detect and eliminate false changes due
to testing error or abnormal circumstances.
Cusum (cumulative sum analysis)
• My major contribution is to use the
continuously updated current average of a
variable as the target.
• This focuses the cusum on detection of
change rather than adherence to a
selected target.
• It also has huge significance for the ease
of combining very large numbers of grades
of concrete in a single cusum analysis.
Cusum chart
Use of V mask to detect change
Cusum graph exhibiting false change
Multivariable
• Multivariable relates to including graphs of other
variables such as:
• density,
• workability,
• temperature,
• tests on constituent materials such as cement
strength and sand grading,
• also average pair difference of 28day results
(to detect any deterioration in testing quality)
all on the same sheet as concrete strength.
Multivariable
• I started to do this in my first year in the concrete
business (1952)
• but the idea still does not seem to have been
fully accepted by the UK QSRMC (Quality
Scheme for Ready Mix Concrete), even though I
have presented papers there on two occasions.
• The concept is that changes in concrete strength
will be mirrored in, and so confirmed and
explained by, changes in one or more of the
other variables.
Multigrade
• Multigrade relates to combining the results
of several (or many) grades of concrete in
a single analysis.
• If done effectively, this gives an equivalent
effect to increasing the frequency of
testing many times over.
Multigrade
• EN206, the European quality scheme, does this
to a limited extent but makes hard work of it and
has to limit it to a few similar grades of concrete.
• EN206 combines grades by adjusting or
converting the results of other grades so that
they can be analysed as though from a selected
control grade.
• This requires initial and continued effort to set up
and continually adjust the conversion process to
be fully effective.
Multigrade
• I have been happily applying my version to combining
the results from hundreds of grades of widely different
character with no human effort required (the computer
does it all) for more than 20 years.
• My technique is to cusum departures from
the various current average values as though
these were all from the same average value.
• While I was initially dubious of this, I have found that it
works beautifully for strengths of 20MPa or less to
strengths of 100MPa or more, and including normal
dense and structural lightweight concrete in the same
analysis.
Multigrade, Multivarible Cusum
Early Age Prediction
• Normally ‘early age’ means something of the
order of 3 to 7days
• My contribution for normal results has been to
recognize that a more useful prediction is
obtained by adding the average gain to the early
result than by assuming a percentage increase.
• This is because the early age result tends to be
of a single specimen and to be subject to more
than usual error. Applying a percentage increase
multiplies the effect of any error.
The free QC program
Free Programs
• I should warn you that the free programs
are not likely to remain free permanently
(other than on a demonstration basis)
owing to a recent partnership between
Contek, Shilstone and myself:
Early Age Prediction
• For very early results it is necessary to have a
temperature history record and to express the
age as an Arrhenius Equivalent Age (EA) rather
than a physical age.
• The concept (and a competing but less accurate
concept of temperature x time maturity) is well
known but the usual technique is to construct a
strength v maturity (or EA) graph, measure the
maturity (or EA), and read off the strength.
Early Age Prediction
• Construction of the calibration graph
requires substantial effort
• A more serious fault is that this technique
assumes that the concrete is the same as
that used to construct the graph and so
cannot react to changes in concrete
quality and cannot be used for QC.
Early Age Prediction
I have devised a simple program that continuously
and automatically feeds back and corrects the
Arrhenius constants so that predictions of 28day
strength (and any other desired age) based on
the actual concrete in question can be obtained
within a few hours.
Such results can be used for QC where the
additional expense of temperature monitoring is
considered justified
Combined Effect
• The real power of my overall system lies in the
way that these separate elements combine
together.
• It is the way I do cusum that enables such
widespread multigrading,
• the use of cusum that links multivariables, and
• the use of multivariables to confirm and explain
the detection of change.
• these features enable the detection and cause
of change to be established several weeks
earlier than most (all?) other control systems.
Combined Effect
• The earlier detection and trend rectification
itself reduces the overall variability of the
concrete being produced –
• and I have demonstrated that the number
of results required to detect a given
change is directly proportional to their
basic variability.
Availability of the Technique
• The technique was made substantially available
as a Lotus spreadsheet in a series of 10 articles
appearing bi-monthly in Concrete International in
1988-89.
• Apart from this it has only been available as part
of the ConAd computer program, marketed by
the author’s company, Concrete Advice Pty. Ltd.
in the 1990s, and now, since the sale of that
company, by Command Alkon Inc.
Availability of the Technique
• To enable the basic technique to become a
standard item, available to all, there is now a
free program available on my website
www.kenday.id.au.
• This program falls far short of ConAd in many
respects but it does enable all the features
presented here to be employed,
• with the exception that the ‘multivariables’
available are limited to strength, density, slump,
and temperature and only eight cusum graphs
can be drawn.
Specification of Concrete
• While not the subject of the current address, a few words
on this subject may be helpful. Although it has taken 50
years in some cases, it seems that the whole world is
coming to accept a view I have been presenting since
the early 1950s. 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’.
Specification of Concrete
• The article went on to assert that: ‘the only
really fair and effective basis for quality
regulation is the imposition of a cash penalty
for marginally defective concrete based on a
statistical analysis of test data’.
• I am still of that opinion, even though (so far!) it
does not appear to be shared by anyone else in
the world.
– maybe another 50 years?
Specification of Concrete
An effective control system must have two quite
separate and to some extent opposing features.
One is to form a very accurate view of the mean
strength and variability of the concrete supplied
to date (no hurry).
The other is to detect as quickly as possible when
the quality of the concrete being supplied
changes (no requirement for accuracy or
infallibility).
Any attempt to combine the two is likely to fail to
accomplish either.
Summary and Conclusion
1) Test data (including specimen
density) should be entered in the
system on the day it is obtained and
visually assessed daily using
automatically generated multigrade,
multivariable, cusum graphs.
Summary and Conclusion
2) Cusum analyses should use the
constantly updated average values
of all variables as a target rather
than a specified target.
Summary and Conclusion
3) Data should ‘multigraded’ by
cusuming these differences as
though from the same mean rather
than transforming results to a control
grade.
Summary and Conclusion
4) Data should include strength at
7days or earlier transformed into a
28day strength prediction by adding
the current average gain. Prediction
should NOT involve any assumption
that a low early age result is likely to
also show a lower subsequent gain.
Summary and Conclusion
5) The daily assessment should include at
least the first few rows of a table ranking
all grades in order of departure of current
and predicted mean strengths from their
target values.
6) Cusums should include average pair
difference of 28day results as an
indication of testing quality.