Unchanging concepts - Concrete Mix Design, Quality Control and Specification - page 250

5

90% Condence limits on
penalty levied
Average penalty
cement cost saving
Maximum inequity ≈ $ 1.22 penalty
or
10 kg/m 3 Excess cement

4

3

2

1

0
-4

-3 -2 -1
True Mean Strength Minus Required Mean Strength (MPa)

0 +1 +2

Figure 11.1 Graph of average penalty applied.

Figure 11.1 shows the average penalty that would be applied and the
90% confidence limits on that penalty for strength shortfalls up to 4 MPa
(580 psi). The graph shows there is very little risk of any significant
unmerited penalty and even less chance of the cement saving outweighing
the penalt y.

Effect of k value changes
The effect of an increasing k value would be to increase the required mean
strength. This could be offset by a reduction in the specified strength below
that used in the structural design. The effect of such a compensated increase
in k value would be to provide a greater incentive to attain a low variability
in the concrete strength by imposing a larger safety margin on suppliers
of higher variability concrete. The actual minimum strength (say, the 3
standard deviation limit below which only one in a thousand results would
fall) would be raised by such a specification.
In the authors' view, an increased incentive to reduce variability and
increase security against the occurrence of very low strengths would be
highly desirable. It is suggested to use a k value of 3 and to reduce the speci-
fied strength by 5 MPa (725 psi) in compensation.
For a k value of 1.28 (existing U.S. practice) and a specified strength of
30 MPa (4348 psi), the effect of this would be

2.5 MPa (362 psi) (good control)
• Required mean strength 30 + (1.28 × 2.5) = 33.2 MPa (4811 psi)
• Effective minimum strength 33.2 - (3 × 2.5) = 25.7 MPa (3725 psi)