Unchanging concepts - Concrete Mix Design, Quality Control and Specification - page 249

coincide in their joint desire to detect and eliminate adverse trends at
the earliest possible moment. This cooperative type of relationship is in
contrast to the traditional requirement to establish with legal precision that
concrete strength is inadequate and then require the unwilling supplier to
rectify the matter.
The suppliers generally recognise that rapid reaction to warnings of low
strength from the quality control engineer can save the supplier money.
A graphing system can provide such information based on a few early-age
test results and will enable the supplier not only to avoid extensive periods
of low strength but also to reduce the overall variability (a double saving in
potential penalties) (Day, 1981).
The expression gives a standard error of approximately 0.74 MPa (107
psi). This means that 90% of assessments will be within 1.22 MPa (177 psi)
of the correct value.
If it is further assumed that a 1 MPa (145 psi) strength change requires
7-8 kg/m 3 (11-14 lb/yd 3 ) of cement change (the actual value could range
from 5 to 10 kg/m3 [8-17 lb/yd 3 )] for different concretes) or equivalent
water reduction, then the inaccuracy amounts to a maximum of ±10 kg/m 3
(±17 lb/yd 3 ) in cement content, or a cost of around $1.80 (Australian)/m 3 (in
U.S. dollars approximately $1.50/yd 3 ).
Operation of the system
The specification might then read as follows.
The specified strength of the concrete shall be X MPa and for every
1 MPa (145 psi) that the mean strength of any 30 consecutive samples
minus 1.28 times the standard deviation of strength of those samples
falls below X MPa, the contractor shall pay a penalty of AU$2.70/m 3 or
US$2.25/yd 3 ) of the whole of the concrete represented by the 30 results
in question.
($1 equals twice the cost of the 7.5 kg [16.5 lb] of cement assumed to be
required to increase the concrete strength by 1 MPa [145 psi]).
To avoid occasional unmerited penalties under such a specification, the
concrete supplier would have to incorporate 10 kg/m 3 (17 lb/yd 3 ) excess
cement content, increasing the cost of concrete by $1.80 (Australian)/m 3
($1.50 (US)/yd 3 ) above the cost strictly required, with the idea that this
increase in cost is justified by the quality control benefits of the entire system.
On the other hand, a concrete supplier would occasionally escape
penalisation when actually supplying concrete as much as 1.22 MPa (177 psi)
under strength. If the supplier decided not to add additional cement or admix-
ture, on average, he would be paying a penalty of AU$3.30/m 3 US$2.75/yd 3 )
or 75% more than the cost of virtually eliminating the risk of a penalty.