I have a 20x20 floor that needs to be replaced. Originally was a wood floor built on 4x4 with pressure treated ply. Got a moisture problem. Floor gave out and had giant mushrooms growing. Anyway, it needs replaced.

Thought about doing it in wood again, but looking to concrete so as not to run into a moisture issue again. Problem is I know nothing about concrete.

Was thinking of getting (renting or buying a small mixer and doing it myself. Figured I could cut the existing floor up in 5 to 10 ft sections, frame it with a 2x4, then do the pour. Repeat as necessary. I know it needs expansion joints, so figured it would be okay to do it that way.

Does my logic seem okay?  Also, how thick would I need to go to support a kit car and my fat rear end?  Was thinking 4" should be enough?  How long does concrete need to cure for it to be load bearing?

Thanks for the help.

ETA: What else prep wise is needed for under the concrete?  Believe there is crushed stone under there now.

I am not a concrete guy but I put in a cement patio in my back yard and a cement retaining wall.

First off a 20'x20'x4" is about 5 yards of cement that would be 225 80 lbs of cement.

You want to do a monolithic pour and then notch in the expansion joints after you get it all spread out, the cement would be hard by the time you get to the end if you use a mixer even with a few people helping you and pouring 225 bags is a lot of work. I mixed 1 yard of cement by hand and was almost dead after I did it.

I am not sure how strong 4"is but I think it would hold, my parking pad is 5" I put in a metal mesh but I do that in all my cement pours even little ones under a grill or something

One thing you could use is a pier pad which will elevate you off the ground a bit and frame it out but you said you want to put a car on it so I don’t think that would be strong enough.

I never understood why people want to try and mix their own for things like this just call the concrete plant give them your measurements and order it  to be delivered. You will get a more consistent concrete and save your self a butt load of work and wont end up with any cold joints. Also ordering it from the plant gives you the  option to have fiber added for more strength or retarder to slow its cure to make it workable longer.

A consistent 4" depth would be okay, but 5" depth would be best.  Turn down the edges of the slab (i.e. make the first 6" of the slab inward from the perimeter a few inches deeper than the 4"-5" depth).  Compact (hand tamp) the stone base.  Put down a vapor barrier on top of the stone base.  Use WWF (welded wire fabric) across the entire slab for reinforcement and raise it up off of the base (use rocks) so that it is centered vertically with 2" of concrete minimum above and below it.  Join the sections of WWF using metal tie wire.

Hand-mixing a yard of concrete is VERY difficult.