The strength of a concrete mixture depends on the water cement ratio. The water and cement form a paste. If the paste is made with more water, k becomes weaker. Traditionally, concrete mixes have been identified in terms of the ratio of cement to fine aggregate to coarse aggregate. For example, the ratio 1:2:4 refers to a mix which consists of 1 cu. ft. of cement, 2 cu. ft. of sand and 4 cu. ft. of gravel. Cement and water are the two chemically active elements in concrete and when combined form a paste or glue which coats and surrounds the particles of aggregate and upon hardening binds the entire mass together.