It is first necessary to remove all topsoil and vegetable matter from the site. Topsoil is easily compressed and, if laid on such a surface, the floor slab would undoubtedly settle.
Once the foundations are in position and the loadbearing walls are built to DPC level, this type of floor construction can commence. To prevent loose earth from contaminating the wet concrete and to both provide a suitable base for the slab and to spread its load evenly, a layer of hardcore at least 150mm thick is spread over the ground. Hardcore is usually obtained from a quarry and consists of graded stone, i.e. it contains a wide range of particle sizes so that it can be firmly compacted. Compaction is usually assisted by a small vibrating plate or roller.
If the hardcore is deep, because some of the original subsoil is unsuitable and requires excavation, or because the site is on a slope, it is important to compact the hardcore in layers (about 200mm thick). Failure to do this will almost certainly lead to problems in the future as the hardcore consolidates. In situations where the hardcore is likely to be over 500–600mm deep, a suspended concrete floor would usually be specified. There are other reasons why these have become more commonly specified and these are explained later.
There are alternatives to quarry-based hardcores, these have included broken bricks, builder’s rubble and colliery shale. The use of such materials has, on occasion, caused a number of defects due to chemical reaction and expansion between materials. More recently, a number of recycled materials, such as crushed concrete, have proven to be suitable substitutes for traditional hardcores.
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