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A method of assessing flatness was given in CP 204: 1965 In-situ floor finishes (metric edition 1970) which used a 10ft (~3m) straight edge that was not set directly on the slab but on feet. No apparatus description was given, nor methodology to cover its use or sampling criteria. The idea was to assess unevenness below the temporary datum of the straight edge as a +/- measurement.
CP 204 was superseded by BS 8204 Screeds, bases and in situ floorings in 1987. This introduced a 3m straightedge in direct contact with the slab and surface regularity classes SR1, 2, 3, with corresponding permitted deviations below the straight edge of 3, 5 and 10mm. (A +/- value is sometimes still erroneously seen specified.)
For a proposed revision of BS 8204 in 2003, a straight edge with feet was again suggested, which was intended to be more portable than the 3m straight edge. This consisted of a collapsible 3m long frame with a wire stretched to standard tension from which to measure the deviation was proposed. This method was rejected in favour of a 2m straight edge set on the slab and is the method is adopted in the current BS 8204, BS EN 13670 Execution of concrete structures, the National Structural Concrete Specification and National Building Specification.
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.”
Acknowledgement: The Concrete Society
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