putting carpet directly onto sub floor, especially concrete slab is a commercial application. Its called direct glue down. You can do it in a residence, but its not done because its not comfortable. Residential broadloom carpets should be laid on carpet pad and its for comfort. Tile is laid on thin set directly on sub floor. Carpet tiles are another story. They are vinyl backed and designed to go directly on sub floor. They are for commercial use. Carpet tiles were designed so that office cubical systems would not have to be disassembled and you could replace high traffic areas without disrupting the whole business. Often electrical runs under a raised floor in a commercial building and carpet tiles allow electrical access for each cubical. One person recommended a concrete sealer and in addition, for residential, I'd use a carpet pad, which is for cushion not a moisture barrier. Carpet pads come in a variety of thicknesses. As an interior designer, I have never seen residential carpet tiles, and i hope I never do.