MATERIALS ENGINEERING CONCEPTS - Materials for Civil and Construction Engineers - page 30

reduce price adjustment cost

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decrease inspection frequency

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provide a basis for altering specification limits

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provide a permanent record of quality

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provide basis for acceptance

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instill quality awareness

■

There are many types of control charts, the simplest of which plots in-
dividual results in chronological order. For example, Figure 1.20 shows a
control chart of the compressive strength of concrete specimens tested at a
ready-mix plant. The control chart can also show the specification tolerance
limits so that the operator can identify when the test results are out of the
specification requirements. Although this type of control chart is useful, it
is based on a sample size of one, and it therefore fails to consider variability
within the sample.
Statistical control charts can be developed, such as the control chart for
means (X-bar chart) and the control chart for the ranges (R chart) in which
the means or the ranges of the test results are chronologically plotted.
Figure 1.21(a) shows a control chart for the moving average of each three
consecutive compressive strength tests. For example, the first point repre-
sents the mean of the first three tests, the second point represents the mean
of tests two through four, and so on. Figure 1.21(b) shows a control chart for
the moving range of each three consecutive compressive strength tests. The
key element in the use of statistical control charts is the proper designation
of the control limits that are set for a given process. These control limits are
not necessarily the same as the tolerance or specification limits and can be
set using probability functions. For example, the control chart for means re-
lies on the fact that, for a normal distribution, essentially all of the values
fall within standard deviations from the mean. Thus, control limits can
be set between standard deviations from the mean. Warning limits to
identify potential problems are sometimes set at

; 3

; 3

; 2 standard deviations

from the mean.
Observing the trend of means or ranges in statistical control charts can
help eliminate production problems and reduce variability. Figure 1.22 shows

50

x 35.3 MPa
s 4.0 MPa

45

40

35

2s

30

3s

25

20

FIGURE 1.20 Control
chart of compressive strength
of concrete specimens.

0 10 20 30 40 50

Test number