Water-cement ratio
The weight of water divided by the weight of cementitious material in a mix. It is the single strongest predictor of concrete strength and durability.
What does water-cement ratio mean?
Lowering the ratio makes concrete stronger, denser, and less permeable, but also stiffer and harder to place. Raising it makes concrete easier to work with and weaker in direct proportion, because the extra water leaves behind voids when it eventually leaves the paste. Modern admixtures exist largely to break this tradeoff, delivering workability without adding water.
In practice
Every gallon a driver adds at the chute to make the mud easier to pull raises this ratio permanently, which is why suppliers document water added on the ticket.