Precast composite floors require a concrete topping for structural purposes, levelling purposes and a finished surface. Usually steel fabric reinforcement (mesh) will be required in the concrete topping. The structural drawings will state a minimum thickness for the concrete topping, perhaps 40mm.
However, most precast concrete planks will have a built-in upwards camber. The amount of camber will usually depend on the length of the plank. Where the topping thickness is detailed to be 40mm at the supports, at mid-span the topping thickness will be only be 15mm, if there is a 25mm camber on the planks. Thus to achieve a minimum topping thickness of 40mm at mid-span the topping thickness at the supports needs to be the minimum thickness plus the maximum camber, e.g. 65mm.
If reinforcement is present the minimum thickness at mid-span will need to allow for adequate cover to the reinforcement at laps. For example for a 25mm maximum camber, nominal top reinforcement cover of 25mm, a nominal bottom reinforcement cover of 20mm, and two layers of A142 fabric (6mm bars) the topping thickness at mid-span would need to be 20 + (4 x 6) + 25 = 69mm. This means that the topping thickness at the supports would be 69 + 25 = 94mm.