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 mega pascals (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.