The materials commonly used as the aggregate for concrete are sand and gravel. The grains of natural sand and particles of gravel are very hard and insoluble in water and can be economically dredged or dug from pits and rivers. The material dug from many pits and river beds Consists of a mixture of sand and particles of gravel and is called ‘ballast’ or ‘all-in aggregate’. The name ballast derives from the use of this material to load empty ships and barges. The term ‘all-in aggregate’ is used to describe the natural mixture of fine grains of sand and larger coarse particles of gravel.