Hardcore

The term 'hardcore' refers to the mass of solid materials used as a make up, formation material to raise levels, fill irregularities in excavations and create a firm and level working base onto which heavy load-bearing surfaces such as stone or concrete can be laid. The hardcore then helps evenly spread imposed loads.

Hardcore may be used in the formation of roads, paving, driveways, foundations, ground floor slabs, and so on. It may also be used on its own to form a temporary road surface on construction sites.

A variety of graded materials can be used to make up a hardcore layer:

Construction waste such as brick and broken tiles.
Gravel.
Quarry waste.
Crushed rock.
Clean, graded concrete rubble.
Blast furnace slag.
Colliery spoil.
Oil shale residue.
Pulverized-fuel ash.

Materials need to be sufficiently hard as well as being capable of being compressed (rammed to form a compact base) before the upper layer is added. This removes gaps or voids which could otherwise threaten the supportive properties of the hardcore layer.

They should also be resistant to deterioration, chemically inert, should not be absorbent and should not be affected by the presence of water (for example, colliery shale expands when moist).

Quarry waste is a good source of hardcore material, although care should be taken not to include waste from gypsum mines. This is because such waste often contains a mixture of limestone and gypsum which can attack concrete. Similarly, concrete rubble may lead to a risk of sulphate attack if it contains gypsum plaster. Construction waste can contain timber which may deteriorate and can spread rot. Substances such as colliery spoil may contain soluble sulphate which, if it reacts with water, can infiltrate and damage cement.

The type of construction, expected load and probable stresses will determine the appropriate thickness of the hardcore layer that must be used. Typically it is laid in well-compacted layers of 100-150 mm.