Unchanging concepts - Concrete Mix Design, Quality Control and Specification - page 248

Typical concrete specifications around the world continue to levy one
penalty of rejection and continue to base judgment on criteria that
are known to be inefficient at distinguishing the actual quality of the con-
crete assessed (Chung, 1978). The result of this ostrich-like attitude is to
leave supervising engineers in untenable positions, to subject concrete sup-
pliers to gross unfairness on occasions, frequently to allow unsatisfactory
concrete to be supplied with impunity, and worst of all, to fail to encour-
age responsible producers of low-variability concrete.
The proposed system
The quality of concrete is assumed to be represented by the mean and stan-
dard deviation of strength. Quality should be specified by the requirement:
Any deficiency in strength can be readily assessed in terms of inadequate
mean strength. The cost of remedying that deficiency can be readily assessed
in terms of cement content or admixture to achieve the required water to
cementitious materials (w/cm) ratio.
For a limited extent of deficiency, a penalty of twice the cost of remedying
the deficiency could be imposed. This penalty is negligible for small
deficiencies, but if the criterion is sufficiently accurate, the penalty will be
sufficient to ensure that no concrete supplier can make additional profit by
supplying understrength concrete. This penalty system benefits producers
of low-variability concrete and encourages improved quality control.
The key to this system is the determination of the values of mean strength
and standard deviation with sufficient accuracy, and the selection of a
suitable value for k . It is immaterial whether the cement-content change
required to provide a given strength change is truly a constant for all
concrete, providing the change is never more than twice the assumed value.
Accuracy of assessment
The gross inaccuracy of assessment encountered under most specifications
arises from an inadequate number of test results (Chung, 1978) and from
attempting to assess the quality of an amount of concrete sufficiently small
to accept or reject as a whole. There is no such requirement in a cash-
penalty specification.
A secondary reason for basing a criterion on a small number of results
is to enable a judgment to be made quickly, thus limiting the amount of
defective concrete supplied before a halt is called. This pious intention may
become a joke when the results are obtained at 28 days.
The solution to this dilemma is to separate the functions of (1) acceptance/
penalisation and (2) detection and arrest of adverse quality.
An interesting and valuable result of operating under a cash-penalty
scheme is that the interests of the supervisor and the concrete supplier