Howdy all, I live in Washington State and have a 35x50 unheated detached garage that has heavy areas of dampness (wet to the touch and dark gray) across the 4” concrete slab. The reason, no vapor barrier was installed prior to pouring. The garage is set back into a slight incline, with the back grade being 2.5 ft higher than the front. The three sides are sloped away and gutters have been installed. The soil conditions are very rocky/gravelly and provide very good drainage during rain. Because of these conditions my concrete contractor felt it was not necessary for the vapor barrier or drain tile. I have since contacted him and we are discussing options to fix the issue.
My question to you all, what is the solution.
Option 1 - tear out the entire floor, remove 4” of earth, and replace with drain rock, put a vapor barrier down (6 mil plastic), and possibly some foam board to fight off condensation then repour. Or, Option 2 put down a 6 mil vapor barrier and pour 2 more inches of concrete (existing concrete would have to be removed around openings). Or option 3, just install drain tile and rock around the outside. Or….any other ideas??

A garage of 35x50 in size is enormous, and it suggests that it might house heavy vehicles. If so, I don't think a 2-inch slab will hold up, even if it is over a 4-inch slab.

A french drain around the three below-grade sides MIGHT help, or it might not.

The only sure remedy seems to be to remove the slab and install 4 inches of gravel and a properly-installed vapor retarder. I don't think foam board will do anything much to solve the problem.

Thank you for the reply. No the garage is used for standard autos and a home shop, nothing heavy.
The sub came up the idea of adding the 2 inches on top of the existing concrete with the vapor barrier in between (less cost). My issue is it does nothing in removing the water from underneath.
The foam board would be used to insulate the floor. Thanks again

If the space is unheated, there is no need for the insulation. I have my doubts that two inches of concrete on top of a vapor retarder will hold up well, because it will be impossible to place wire mesh or any other reinforcing in it. The vapor retarder will keep it from bonding to the original slab.