When we bought the house, we had hoped a French drain would fix the extreme moisture issues in the shop section of the detached garage.  However, when that didn't do the trick, the assumption was that either the drain was overwhelmed, or the garage was built atop a micro wetland.  Two 4' deep test pits were dug in 2 areas to check the soil and determine if more aggressive drainage prevention measures were needed.  The findings were surprising.  The soil was in fact dry, but the concrete was loaded with some type of fibrous additive that turned it into a giant sponge, pulling moisture from the air.  Initially I was dubious of that explanation, but after a minor 10 minute rainstorm left the slab visibly moist, a quick inspection showed the soil beneath in the test pits was still effectively dry.  So...out with the old slab, in with the new one complete with 10 mil vapor barrier.

Click on the first photo to launch the annotated slide show.