Hardcore is the name given to the infill of materials such as broken bricks, stone or concrete, which are hard and do not readily absorb water or deteriorate. This hardcore is spread over the site within the external walls of the building to such thickness as required to raise the finished surface of the site concrete. The hardcore should be spread until it is roughly level and rammed until it forms a compact bed for the oversite concrete. This hardcore bed is usually from 100 to 300 mm thick.
The hardcore bed serves as a solid working base for building and as a bed for the concrete oversite. If the materials of the hardcore are hard and irregular in shape they will not be a ready path for moisture to rise by capillarity. Materials for hardcore should, therefore, be clean and free from old plaster or clay which in contact with broken brick or gravel would present a ready narrow capillary path for moisture to rise.
The materials used for hardcore should be chemically inert and not appreciably affected by water. Some materials used for hardcore, for example colliery spoil, contain soluble sulphate that in combination with water combine with cement and cause concrete to disintegrate. Other materials such as shale may expand and cause lifting and cracking of concrete. A method of testing materials for soluble sulphate is described in Building Research Station (BRS) Digest 174.
The materials used for hardcore are: