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Is there any information on what materials are suitable for use as Hardcore?
The principal uses of hardcore are as a make-up material to provide a level base on which to cast a ground floor slab, to raise levels and to provide a firm base to carry construction traffic. A variety of graded materials can be used and ideally they should be granular and consolidate readily. Gravel and crushed hard rock including quarry waste, generally give good service though waste from gypsum mines should not be used; it frequently contains a mixture of limestone and gypsum which can attack concrete to form the mineral thaumasite. Concrete rubble, if clean and graded is a good hardcore material but if it contains lumps of gypsum plaster there may be a risk of sulfate attack.
Brick and broken tile is useful but unless the bricks are soft there may be a lack of fine material and therefore difficult to compact. Dry rot spores may be present in pieces of timber included in hardcore from demolished buildings and there is a risk of an outbreak occurring. Other less widely used materials include blastfurnace slag, colliery spoil, oil shale residue and pulverized-fuel ash.
Acknowledgement: The Concrete Society
Other references:BRE Good Building Guide 39 Part 2 Simple foundations for low-rise housing: `rule of thumb` design