Mix design - Concrete Mix Design, Quality Control and Specification - page 214

Given that the cement is a 42.5, the coefficient k for fly ash is equal
0.4. Hence, a first equation:

C + 0.4 FA = 308 kg/m³

The second equation corresponds to the optimal dosage in microfines.
For D = 20 mm, the optimal microfines volume is 135 l/m³. Hence, we
can obtain the second equation:

C A
3,15

+= 135

2, 2

The solution of these two equations gives C = 265 kg/m³ and
FA = 114 kg/m³.
It remains to be seen whether the ratio FA /( C + FA ) is less than 0.33.
In fact, it is slightly higher. For this reason, the calculation must be
repeated by replacing the second equation by the ratio specified by the
standard, namely,

C + 0.4 FA = 308 kg/m³

FA /( C + FA ) = 0.33

The solution of these two equations gives C = 257 kg/m³ and
FA = 127 kg/m³.