When concrete hydrates, it hardens and gains strength. This hydration process continues over a long period of time. It occurs quickly at the outset and slows down as time passes. It would require a wait of several years before the strength of concrete can really be measured. Since this is impractical, a time period of 28 days was introduced by specification-writing authorities as the time where all concrete should be tested. In this period, a substantial percentage of the hydration has already taken place.