Concrete is a construction material that consists of cement (commonly Portland cement), aggregate (generally gravel and sand), water and admixtures.
Engineers usually specify the required compressive strength of concrete, which is normally given as the 28 day compressive strength in megapascals (MPa) or pounds per square inch (psi). Twenty eight days is a long wait to determine if desired strengths are going to be obtained, so three-day and seven-day strengths can be useful to predict the ultimate 28-day compressive strength of the concrete.
Potable water can be used for manufacturing concrete. The w/c ratio (mass ratio of water to cement) is the key factor that determines the strength of concrete. A lower w/c ratio will yield a concrete which is stronger and more durable, while a higher w/c ratio yields a concrete with a larger slump, so it may be placed more easily.
The water and cement paste hardens and develops strength over time. In order to ensure an economical and pract