Specification - Concrete Mix Design, Quality Control and Specification - page 129

Militating against detailed mix submissions is the desirability of using
standard, well-proven mixes from the viewpoint of quality control and a
proper degree of confidentiality from competitors. Also the producer needs
to be entitled to vary his mixes within limits from day to day to maintain
control.
6.13 CASH PENALTIES
It has been pointed out that the producer must be allowed to regulate the
mix on the basis of early age data and compliance with requirements should
be assessed on the statistical analysis of a substantial number of later age
results, for example, the last month's or the last 30 results, whichever is
larger. The important point is that a distinction be made between results
that are below specification requirements and concrete that is unacceptable
and must be replaced, strengthened, or treated in some way. If any concrete
is unacceptable, this cannot be assumed to be unique and all concrete of
the same grade in the same period must be examined. It is not satisfactory
to attempt to locate and core concrete that has given a low result assuming
that untested truckloads will be acceptable.
Strength results are assumed to be normally distributed and their mean
strength is required to be 1.65 SD or 1.28 SD above the specified strength
according to whether a 5% or 10% defective criterion has been specified.
Only 1/1000 results are theoretically expected to be more than 3.09 SD
below the mean and only 1/100 below 2.33 SD. Essentially this means
that there is a negligible likelihood of a result more than 1 SD below the
specified strength if the concrete is acceptable, but concrete up to 1 SD, or
say 5 MPa, below the specified strength cannot be considered to be unsafe.
So the proposal is that concrete failing the specified limit but with
(statistically) no more than 5% more than 5 MPa below the specified
strength be accepted with a cash penalty of say 1% of X truck price per
0.1 MPa shortfall (so concrete marginally inside the acceptance with
penalty limit will incur a penalty of almost 5/0.1 = 50%). This should be
based on a statistical analysis of the last month's or the last 30 results, and
apply to all the concrete of the grade in question supplied in the period.