Many tall apartment buildings are constructed using cast-in-place concrete. The floor thickness is usually around 200 mm. Walls between apartments may also be cast-in-place concrete but there are other possibilities.
The table below gives representative STC values and mass per square metre for some concrete slabs. Simple concrete slabs with density around 2300 kg/m3 and thickness in the range 150 to 200 mm provide STCs from about 53 to 58. It does not matter whether the slab is a wall or a floor, the airborne sound transmission is the same.
For thicknesses not in the table but within the range of the table thicknesses, STC can be estimated using this expression:STC = 37.5 log10(thickness, mm) - 28.5.
The concrete should not contain any cracks or fissures that would allow sound to pass through. If any such exist, they should be plastered to seal them. Covering the exposed surface of a properly sealed concrete slab with a coat of plaster or a layer of gypsum board has no significant effect on the sound tranmission.