Hardcore
/ ˈhɑːdkɔː /
Also known as: sub-base, Type 1 MOT, granular fill, crushed stone fill
Definition
Hardcore is a layer of compacted granular material - typically Type 1 MOT crushed stone, crushed concrete, or clean brick rubble - laid on prepared sub-grade beneath a concrete slab, floor, driveway, or path. It provides a stable, load-spreading base that resists settlement, allows drainage of groundwater, and brings the formation level up to the underside of the blinding and slab. Adequate compaction is essential to prevent future settlement.
In practice
For a typical domestic ground-bearing slab the sequence is: excavate topsoil to formation level; compact sub-grade; lay hardcore in compacted layers of 150mm maximum; compact each layer with a vibrating plate compactor; then lay a blinding layer of sand or weak concrete to provide a smooth surface for the DPM. The total depth of hardcore under a domestic oversite slab is usually 150-200mm, though this increases if fill has been placed to raise levels.
Recycled crushed concrete is widely used as hardcore and is acceptable under Part C provided it does not contain soluble sulfates (from gypsum plasterboard) which can attack concrete. Type 1 MOT is the preferred material where a precisely graded and tested sub-base is required. Hardcore must never include clay, soft material, or biodegradable content such as topsoil, timber, or vegetation.
Building Regulations
Approved Document C requires ground floors to resist moisture from the ground. The hardcore sub-base forms an important part of the drainage layer beneath the DPM. Where sulfate-bearing soils or fill materials are present, the sulfate exposure class must be determined in accordance with BRE Special Digest SD1, as this affects concrete mix specification for the slab above.
Full Building Regulations guidanceSee also