The FIP Manual of Lightweight Aggregate Concrete, References 25, suggests that the residual-absolute-volume/cement ratio may be used to estimate concrete strength.
The residual absolute volume is defined as
Wwater x (Vwater + Vair)/Vwater
Where
Wwater = weight of free water per unit volume of concrete.
Vwater = volume of Free Water per unit volume of concrete.
Vair = volume of air per unit volume of concrete.