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The effective depth (d) of a reinforced concrete floor slab is the distance from the compression face to the centre of the tensile steel when an element is subjected to a bending moment.

In calculations regarding concrete slabs, the effective
depth is important as it is used to calculate the required level of
reinforcement for an element of a certain thickness to resist a certain bending
moment.

The effective depth is also used to find the critical punching perimeter and is used in the shear capacity calculations. In this instance the effective depth of a slab with fibre reinforcement or no reinforcement should be taken as 0.75h, where h is the overall depth.

The Concrete Society: Technical Report 34, 6.4.

Eurocode 2,

The Concrete Society: Visual Concrete: Weathering, Stains and Efflorescence.

http://www.concreteconstruction.net/concrete/what-causes-efflorescence-and-how-do-you-remove-it.aspx