Concrete is a mixture of cement, sand, gravel, crushed rock, and water. Water reacts with cement in a chemical reaction known as hydration that sets the cement with other ingredients into a solid mass, high in compression strength. The strength of concrete depends on the proportion of the ingredients. The most important factor for concrete strength is the water-cement ratio. More water results in a weaker concrete. However, an adequate amount is needed for concrete to be workable and easy to mix. An adequate ratio is about 0.25 by weight. The process of selecting the relative amounts of ingredients for concrete to achieve a required strength at the hardened state and to be workable in the plastic (mixed) state is known as concrete mix design. The specification of concrete in terms of the proportions of cement, fine (sand) aggregate, and coarse (gravel and rocks) aggregate is called the nominal mix. For example, a 1:2:4 nominal mix has one part cement, two parts sand, and four parts gravel and rocks by volume. Nominal mixes having the same proportions could vary in strength. For this reason, another expression for specification known as the standard mix uses the minimum compression strength of concrete as a basis. The procedure for designing a concrete mix is a trial-and-error method. The first step is to fix the water-cement ratio for the desired concrete strength using an empirical relationship between the compressive strength and the water-cement ratio. Then, based on the characteristics of the aggregates and the proportioning desired, the quantities of the other materials comprising cement, fine aggregate, and coarse aggregate are determined.