The ratio of water to concrete is important because it determines the strength of the concrete. Abram's Law demonstrates the importance of the ratio, stating that the strength of concrete is directly proportional to the balance of cement, water and air.
The chemical reaction of hydration is what causes concrete to harden. The process requires 0.25 pounds of water for every one pound of cement in order to complete the reaction. However, depending on weather conditions and the type of aggregate being used with the cement it may be necessary to add more water for the mixture to be workable.
The aggregate is the gravel mixture in the cement base. The larger the gravel pieces or higher concentration of aggregate directly affects the amount of water need to harden the concrete properly. More commonly, water ratios of 0.4 to 0.6 per pound of cement are necessary. In this case, you use less water and aggregate, and add a plasticizer to make high-strength concrete with good flowability.
A mixture with too much water causes uneven hardening throughout the concrete. As the water evaporates, it leaves fissures and gaps in the concrete, which causes cracking and reduces the overall strength of the cement. Building codes usually allow for a maximum of 0.45 to 0.50 water-to-cement ratios depending on the temperature at which you are pouring the concrete.