Water-cement ratio is the ratio of weight of water to the weight of cement used in a concrete mix. It has an important influence on the quality of concrete produced. A lower water-cement ratio leads to higher strength and durability, but may make the mix more difficult to place. Placement difficulties can be resolved by using plasticizer. The water-cement ratio is independent of the total cement content (and the total water content) of a concrete mix.
Often, the water to cement ratio is characterized as the water to cement plus pozzolan ratio, w/(c+p). The pozzolan is typically a fly ash, or blast furnace slag. It can include a number of other materials, such as silica fume, rice hull ash or natural pozzolans. The addition of pozzolans will influence the strength gain of the concrete.
The concept of water-cement ratio was developed by Duff A. Abrams and first published in 1918.
Concrete hardens as a result of the chemical reaction between cement and water (known as hydration). For every 4 lbs of cement, 1 lb of water is needed to fully complete the reaction. This results in a water-cement ratio of 1:4 or 20% (1/1+4). In reality, a mix formed with 20% water is too dry and does not flow well enough to be placed, and some of the water is taken up by the sand and stone and is not available to participate in the hydration reaction. So more water is used than is technically necessary to react with the cement. More typical Water-Cement ratios of 35% to 40% are used, along with a plasticizer.
Too much water will result in settling and segregation of the sand/stone components (more sand in the top layers because the stone will settle at the bottom). Also, any water that is not consumed by the hydration reaction will eventually leave the concrete as it hardens, resulting in microscopic pores or holes that will reduce the final strength of the concrete (but interestingly - microscopic pores or holes are desired in outdoor concrete that gets wet while experiencing low temperatures resulting in freeze-thaw cycles. For flatwork (driveways, steps, sidewalks, etc) an air-entrainment agent is added to create small bubbles in the concrete. This helps the finished, hardened concrete resist fracturing caused by freeze-thaw temperature cycling. Finished, hardened concrete can be as much as 6% air (by volume) because of this.
And finally, a mix with too much water will experience more shrinkage as the excess water leaves, resulting in internal cracks and visible fractures (particularly around inside corners) which again will reduce the final strength. Anyone who has set wooden fence posts in concrete piers will probably notice cracks radiating from the corners of the post. This can be reduced by keeping the water to an absolute minimum.