Quality control - Concrete Mix Design, Quality Control and Specification - page 237

Changes in
mean strength
(change points)

Basic
variability

Isolated high
slump sample

Sample Number

Figure 10.5 Change points and basic variability.

the two methods of calculation should give the same result, but if there
have been one or more changes in mean strength, as envisaged earlier, the
traditional way of calculating SD will give an increased value but the basic
method will be largely unaffected. This is valuable when assessing the situ-
ation with a monthly printout table of what may be a large number of
grades of concrete, since it directs attention immediately to any grades that
have experienced a change during the month (or other period). Of course
such a change should have been detected from early age results when it
occurred and quickly rectified.
It is likely that the basic variability is due to such items as batching accu-
racy, moisture/slump regulation, waiting times, and so forth, whereas the
“infrequent occurrences” may relate to changes in fine aggregate grading,
cement or admixture properties, a failure to allow for seasonal temperature
change, and so on.
In a well-run plant with low basic variability, the QC system may be
primarily aimed at the early detection and identification of the infrequent
occurrences and the strong advocation of cusum in this chapter has this
objective. However arriving at the condition of low basic variability is
obviously also important and will, in addition to cusum graphing, involve
truck-to-truck variation better revealed by direct plots.
The purpose of a QC system is to ensure the provision of satisfactory
concrete at a minimum cost. To do this it needs to be able to detect any
change in the quality of concrete being produced at the earliest possible
moment commensurate with realistic expense and time frame.
This involves a judgment of the relative cost of providing a higher margin
between mean and specified strengths (or other criteria such as permeabil-
ity affecting long-term durability) or spending more on control.