Concrete is permeable. There are tiny pockets all through it, so any moisture in the ground can still come through.
Also, if the concrete is cooler than the air, with humidity, will "sweat" unless there is air movement to help dry it.

 

they got them big car ziplock bags you can park it in on dirt

 

Hey Lariat,
Being "open air", you'll never be able to totally eliminate moisture, but if you lay gravel, the opportunity for evaporation is greater with all of the extra surface areas and air pockets.
If you're up to it, install 2-3" of golf ball sized gravel ( what we cal 57 up here) over the thickest poly you can find. 9mil or better. Then a 1-2" layer of "crush-n-run" ( marble sized chips and a bunch of dust in the mix). Make sure the dirt under the poly is as smooth as possible and has a slight slope for drainage.

 

The part I'm referring to is in the third pane with a heading of Location, this is what is displayed on each of your posts.

 

I got it covered now. Thanks, Chief!

 

what about drain or septic rock the. plastic on top of that along with crush run on top plastic?

 

About the sweating issie of concrete, I have a very similar addition to my garage where I park my truck and here in central NC I have never had it sweat. I think it is because when it was built a vapor barrier was put under the pad and I think that is effective in controlling sweating.

I have seen some older carports in this area that would sweat under the right
weather conditions. I don't think vapor barriers were used much years ago.

 

"Sweating" is not caused by moisture migrating up from under the slab.
It is caused by the surface being below the dew point of the surrounding air..
Relative humidity cannot be controlled in an open structure.

Is moisture migrating through that glass of iced tea?
Of course not, but the glass' surface temperature is below the dew point and moisture condenses on the outside.