Water-Cement Ratio is a key factor in the design and proportioning of concrete mixes. Simply put, it is the relationship between the weight of water and the weight of cement in the mix. It can be determined by dividing the weight of water in the mix by the weight of cement. Typical values will run between 0.4 and 0.7

Through simple stoichiometric means, it can be seen that the chemical reaction which gives portland cement concrete its strength requires roughly 1 pound of water for every 4 pounds of cement. This would yield a water-cement ratio of 0.25. However, the common values are higher than this, for several reasons:
  1. The aggregate used in the concrete mix will absorb a percentage of its weight in water. This will make some of the water unavailable to chemically combine with the portland cement.
  2. Extra water is needed depending on the angularity of the aggregate to maintain workability. Concrete with a very low water content will not be as easily worked as a high water content concrete, as there isn't as much water available to lubricate the aggregate and allow it to flow over itself.
  3. Similar to the point above, water must be added to modify the consistency (measured by the slump) of the concrete. Using very little water will yield a very stiff concrete, while large amounts of water produce concrete which is very flowing.

Water-cement ratio is also a key indicator of the strength of concrete. As water-cement ratio increases, compressive strength of the hardened concrete decreases. Due to this fact, most construction projects do not allow the addition of water to concrete after it has arrived on site. This is because the water-cement ratio has been carefully measured by the batch plant, and the addition of water can reduce the strength.

Design and Control of Concrete Mixtures, Thirteenth Edition, published by the Portland Cement Association
Knowledge gained from classes in engineering at Iowa State University