Concrete mix proportioning with fly ash can be carry out by any one following method

1) Direct partial replacement by mass or volume. This result lower strength up 3 months even when moist cured and it will give higher strength after 6 month.

2) Fly ash is added as fine aggregate without reducing the cement. This result in higher strength at all ages.

3) Modified partial replacement of cement , fine aggregate and water , part replacement of cement by fly ash in excess quantity than cement and adjustment in sand and water content is made so that strength and workability remain same as that of controlled concrete.

Example of using efficiency factor

Take and example of designing M35 grade of concrete using only ordinary portland cement. Assume that W/C ratio 0.45. water content of 163kg/m3 and cement content of 360 kg/m3 are the parameter work out.assuming efficiency factor 0.53 for fly ash.

Equivalent w/c ratio = $\frac{W}{C+K F}$

For 15% replacement of fly ash

0.45= 163/(0.85Cm+0.53 X 0.15Cm)

0.45= 163/(0.85Cm +0.0795 Cm)

0.45 = 163/ 0.9295 Cm

Cm=163/0.45 X 0.9295

Cm= 163/ 0.4183

= 389.6 or say 390 $kg/m^3$

OPC content = 390 X (85/100) = $332 kg/m^3$

Fly ash content = 390 X (15/100) = $58kg/m^3$