Acceptable crack widths in floors
There is little specific guidance on acceptable crack widths for power trowelled concrete wearing surfaces.
For reinforced concrete sections a maximum design crack width of 0.3mm is recommended in BS 8110 (now withdrawn and superceded by Eurocode 2 EN BS 1992), mainly for aesthetic reasons; in general the crack width has little effect on durability (see Concrete Society Technical Report 44). BS 5950: Part 4 covers the design of composite slabs which recommends the use of ‘anti-crack’ reinforcement, but does not give a maximum design crack width. Concrete Society Technical Report No 34, which covers the design of industrial floors, does discuss the significance of cracking to the serviceability of power trowelled floor slabs, but does not give any guidance on acceptable crack widths.
In practice ‘acceptable’ crack widths in a particular floor slab will depend on the potential of the floor traffic to damage the crack edges and upon structural; in-service durability; and aesthetic considerations.
According to the Concrete Paving Maintenance Manual (Highways Agency/BRITPAVE), unspalled crack widths are classified as narrow (<0.5mm), medium (0.5-1.5mm) and wide (>1.5) corresponding to full, partial or no aggregate interlock/load transfer respectively. The presence of steel fibres or macro-synthetic fibres across the whole section of any discontinuity probably contributes to the load transfer.
(note that from 1 April 2015, Highways Agency has changed name to Highways England)