The single most important indicator of strength is the ratio of the water used compared to the amount of cement (water/cement ratio).
Basically, the lower this ratio is, the higher the final concrete strength will be.
This concept was developed by Duff Abrams of The Portland Cement Association in the early 1920's and is in worldwide use today.
A minimum w/c ratio (water-to-cement ratio) of about 0.3 by weight is necessary to ensure that the water comes into contact with all cement particles (thus assuring complete hydration).
Typical values are in the 0.4 to 0.6
Advantages of low water/cement ratio
* Increased strength.
* Lower permeability.
* Increased resistance to weathering.
* Better bond between concrete and reinforcement.
* Reduced drying shrinkage and cracking.
* Less volume change from wetting and drying.