A cleanliness layer (or: underlay concrete) is a layer of concrete that is applied to the bottom of the excavation pit to create a flat, dry and clean work surface. A cleanliness layer is usually 5-10 cm thick and made of unreinforced concrete of low strength class (e.g. C8/10).
The withdrawn DIN 1045-3:2008-08 required a cleanliness layer before the installation of foundation slabs and foundations in order to ensure the necessary accuracy in the installation of the spacers for the reinforcement and thus the necessary concrete cover.
The European standard DIN EN 13670, which applies in Germany in conjunction with DIN 1045-3:2012-03, does not require a cleanliness layer, so that there is currently no normative basis for the installation of a cleanliness layer (the informative Annex F of DIN EN 13670, which provides for a cleanliness layer with a thickness of at least 50 mm, does not apply in Germany according to section no. 2.16 of DIN 1045-3:2012-03).
In this respect, the planner (usually the architect in coordination with the structural engineer) must decide how a flat and clean base can be ensured that ensures the necessary cover dimensions during the concreting process.
If a gravel or sand base is to be produced instead of a cleanliness layer of lean concrete, the greater inaccuracy of the subsoil must be taken into account when covering the concrete reinforcement. This means that a floor slab or foundation must be planned and built thicker by the extent of the expected deviations. This is the only way to ensure that the required coverage is guaranteed in all areas. It is therefore necessary to define a tolerance measure in the planning, which must also be monitored during the construction of the excavation pit floor.
The correspondingly dimensionally accurate and careful laying of perimeter insulation boards under foundation boards can also replace a cleanliness layer under the above-mentioned conditions.
Before concreting floor slabs or foundations, a separation layer must be provided for all the options mentioned, e.g. a PE film. This ensures that the floor slab has no connection to the underlying layer, which would hinder the free sliding of the floor slab as a result of shrinkage during drying and could thus lead to cracks in the floor slab. In the case of increased requirements for freedom from cracks (e.g. in the case of waterproof concrete), the separation layer is often even designed in two layers. In the case of perimeter insulation or a gravel or sand underlay, it also prevents the fresh concrete from running into the insulation joints or into the gravel/sand.
Source: bauwion
