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It is impossible to construct a ground supported slab to the exact dimensions as detailed on the construction drawings, as is the case with all construction, dimensional tolerances need to be allowed for in the design and construction processes. The tolerances that are relevant to ground supported slabs are:
1. Tolerances on the level of the sub-base or blinding below the slab. TR 34 suggests values for this tolerance band. There is also guidance on sub-base level tolerances in the Specification For Highway Works.
2. Tolerances on the level of the top of the slab. Recommended values for top-level tolerances are given in TR34 and BS 8204.
3. A slab thickness tolerance. There is a paucity of guidance available on suggested values for this tolerance. Thickness variations can be very important for the performance of a slab, particularly if the design is a rigorous plastic type design. The thinner a slab the greater the relative effect of variations in thickness. Ideally the slab thickness tolerance should be determined by combining the bottom and upper level tolerances. Guidance on how to combine tolerances is given in BS 5606. The floor designers should consider the effect of variations in slab thickness, due to thickness tolerances, on the slab performance.
4. Surface regularity tolerances. Tolerances should be appropriate for the use of the slab and should be achievable. Unrealistically onerous and unachievable tolerances should not be specified.
Acknowledgement: The Concrete Society
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