I am turning part of my carport into a new living room. I am going to pour a concrete slab about 4 inches thick over the existing carport floor which is a concrete slab. The existing slab has a few cracks, the largest of which is about an 1/8in. Wide and raised about an 1/8in. What would be the best way to go about putting in this new slab? Should I cover the old slab with plastic to prevent the two slabs from bonding, or should I grind down the raised crack and fill all the cracks in? Or is there a better approach? Any help would be greatly appreciated, thank you!

Im goning to say not to run rebar from one slab to the other on this one, I would focus on making the new slab more self supported, try using some fiberglass reinforcement in the mix, or they make some metal wire sheets you can set in the concrete to give it more strength,

How long has the crack been there on the existing slab and has it gotten bigger?

I'm not sure how long the crack has been there. I bought the house 2 years ago, the crack was there when I bought the house and it has been the same (as far as I can remember) since I moved in. The house was built in the '50s though. One other thing I forgot to ask, the slab is going to be about 17x14, so should I cut a joint or something else? The reason I ask is because it's going to be covered with tile or hardwood/laminate flooring, I've never installed either with any kind of joints in the slab. Thanks!

No I wouldn't cut a joint in the floor, we do that for driveways for the reason, we know its going to crack and by doing a joint there it makes the crack go in a straight line. It just looks better than a crooked crack.

As far as your flooring goes, if your doing tile is one thing but hardwood flooring especially glue down , you need to make sure your floor is flat with no dips or humps, and get it smooth, not a broom finish. The floating floor isn't as bad to notice dips and humps unless there excessive. So what id do is again focus on the strengh of the new slab. Definitely add some type of additive because of the crack you have on the existing slab

Ok, sounds reasonable, thank you. You wouldn't recommend filling in the old crack? And I don't have to worry about whether the new slab bonds to the old one, is that the reason for focusing on the strength of the new slab?