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The current method is described in BS 8204 Screeds, bases and in situ floorings, which uses a 2m long straightedge placed directly in contact with the surface and the maximum deviations below the straightedge recorded. It assesses the local variability in surface plane in a 2m gauge length. There are in theory an infinite number of locations the straight edge could be placed on the floor.
Although this can assess the localised variation in plane in a 2m length it does not assess the rate of change in variation or ‘waviness’ of the surface, i.e. does not differentiate between long, medium and short wavelength variations. For example a corrugated surface could meet a straight edge tolerance but clearly the surface would not be flat.
Note that BS 8204: part 1 Annex C.2 Methods of assessment of levels and surface regularity states “Where the straight edge basis for specification is used it is advisable for the various interested parties in a contract to agree a sampling rate for testing the concrete base … before the concrete base …is constructed. Such agreement should include the number of positions where the straight edge will be placed to check conformity.”
The straight edge method can be used to assess the surface regularity of a non-level surface, e.g. one laid to falls. It does not measure the texture of a surface.
Surface regularity classes are given in BS 8204 as SR1, SR2 and SR3 relating to 3mm, 5mm and 10mm respectively. BS EN 13670 Execution of concrete structures, the National Structural Concrete Specifciation and National Building Specifciation follow the same test method (although not described) but specify different tollerances.
Acknowledgement: The Concrete Society
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