Unchanging concepts - Concrete Mix Design, Quality Control and Specification - page 255

(who are ever optimistic that the 7- to 28-day strength gain will improve
on current production). Suppliers complain that precise enforcement is
unrealistic, yet without strict controls, deficiencies would no doubt tend
to gradually increase. A cash penalty as proposed would avoid all need
for such argument. The deficiencies would be acceptable with the penalty
paid, but it is suspected that deficiencies would rapidly disappear in such
circumstances.
There have been suggestions that, in fairness, penalty clauses should
be balanced by bonus clauses. This is not recommended because excess
strength beyond that specified is of little benefit to the owner and may be
detrimental. The type of cash penalty clause advocated here is a real benefit
to the good concrete supplier. He can aim at the mean strength truly needed
without restriction. If he slightly miscalculates, the penalty is very moder-
ate and involves no cost of delays or further investigation. He is defended
from unfair competition by less competent or less scrupulous competitors.
Finally, he can include his own bonus in his pricing if he wishes.
Conclusions
It is concluded that a cash penalty of twice the cost of the cement or admix-
ture deficiency can be accurately established by the analysis of a group of
30 consecutive test results. Such a penalty would be capable of regulating
concrete strength with fairness. The system would result not only in an
improved degree of contractual compliance but also in a cooperative attitude
in day-to-day control between the contractor and the supervising engineer.
It would provide an effective incentive to improve control, which would,
over a period, produce significant improvements in concrete production
techniques.
11.2 WHAT IS ECONOMICAL CONCRETE?
This section appeared in Concrete International (Day, 1982a). It is quoted
verbatim as Day's views have not changed. Permission granted by the
American Concrete Institute to reproduce it here is gratefully acknowledged.
The question “What is economical concrete?” may seem a ridiculous
question, but consider the example of the Rialto project in Melbourne. This
project is very unusual in that the concrete supplier, the builder, and the
eventual owner were one and the same. It involved 6000 m 3 of a 60 MPa
(8700 psi) grade, which was the highest grade of concrete specified for
such a project in Australia at that time. This was only 6% total concrete
quantity. Accordingly construction started with a very conservative mix
that actually provided a mean strength of over 80 MPa (11600 psi) and a
characteristic strength of approximately 75 MPa (10875 psi). Considerable