One rule for mixing concrete is 1 part cement and 2 parts sand to 3 parts aggregates by volume or about 10 to 15 percent cement, 60 to 75 percent aggregate and 15 to 20 percent water by weight. Entrained air in many concrete mixes may also take up another 5 to 8 percent of the mixture.
Concrete is a mixture of paste and aggregates, or rocks. The paste is composed of portland cement and water and coats the surface of the fine and coarse aggregates. The dry ingredients are mixed and water is added slowly until the concrete is workable. Through a chemical reaction called hydration the paste hardens and gains strength to form the rock-like mass known as concrete. To strengthen and to promote hydration, the concrete is soaked in water after setting.
The physical properties of density and strength of concrete are determined, in part, by the proportions of the three main ingredients: water, cement and aggregate. Water is the key ingredient. Too much water results in weak concrete. Too little water results in a concrete that is unworkable. High-quality concrete is produced by lowering the water-cement ratio as much as possible without sacrificing the workability of fresh concrete, allowing it to be properly placed, consolidated and cured. The mixture should not be too stiff or too sloppy. A properly designed mixture possesses the desired workability for the fresh concrete and the required durability and strength for the hardened concrete.