Concrete is a manufactured product that has two main phase components: aggregates and paste. Aggregates are normally of two types, fine and coarse, and occupy about 60 to 80 percent of the volume of the concrete. The paste is typically composed of cement, water, and entrained air. The paste ordinarily constitutes about 20 to 40 percent of the total volume of the concrete.
In properly manufactured concrete, the aggregate phase should consist of particles having adequate strength and weather resistance and should not contain materials having injurious effects. Well graded aggregates have a uniform size distribution and a low void content. Well graded aggregates allow for an efficient use of the paste fraction since, in quality concrete production, the paste must completely coat each aggregate particle and fill the space between them.
The quality of the concrete is greatly dependent upon the quality of the paste. In turn, the quality of the paste is dictated by the quality and control imposed over of the paste main components (cement, water, air), and by the extent of curing provided after the concrete mixture is in place. The use of proper quantities of water and cement in a concrete mixture can be monitored through the use of the water to cement ratio concept (quantity of water / quantity of cement). The water/cement ratio has the greatest effect on strength, durability, and water tightness of concrete. For a given quantity of cement, the strength and durability of the concrete will decrease with increased amounts of water. All other things remaining equal, the strength of the concrete will decrease with increased amounts of entrained air. Cement and water combine chemically in a reaction, called hydration. The hydration process can continue for a long period of time in favorable moisture conditions. The quantity of water used in a typical concrete mixture is greater than the quantity required for the hydration to occur, and is necessary to provide plasticity and workability to the concrete mixture. Strength gain ends when hydration stops.
