Concrete over partial concrete

I am wanting to get a 20x20 stamped concrete patio installed in a back corner of our house. We had a flagstone patio when we moved in and after removing all of the rocks and sand I found a partial patio under it. The old patio is an L-shape adjacent to the two sides of the house and comes out about 4 feet. The edges are jagged due to some partial demo I assume. The old patio is low enough that 4 inches of the new could be put on top of it and get it level with the bottom of the door and siding. I dug around the outside of the concrete and where I did it goes down about 12 inches. Is 4 inches new enough to make it ok to install over the old? Maybe extra rebar/mesh? I want to do this right but also don't want to do the demo if it isn't needed. If the old was only 4 inches or so I would do the demo but seeing how thick it is I am not sure.

 

Chances are good that the old slab is only a foot thick around the perimeter, and much thinner in the interior. Drilling a hole or two through it would tell you what the interior thickness is.

You run the risk of poor concrete performance if the new slab is placed on top of the old, particularly if your area gets hard freezes in winter. Water collecting between the two placements would expand when frozen, lifting the stamped concrete out of position (and possibly cracking it). That problem could be avoided by specifying the new slab be bonded to the existing. Just make sure whoever does the work has experience installing bonded concrete overlays, as I've found over the years that it's a skill that very few concrete contractors have.

 

Thanks.
Is it a best and better-than-nothing situation or are both of them acceptable?

 

Not sure I understand--are both of "what" acceptable?

 

Sorry about that.
So I get that removing the old slab is the best.
If I leave the old, by bonding the old to the new (where they overlap) is that almost as good?

 

Too many unknowns for me to guess which option is "best." Such as the thickness of slab to be removed, or the skill-level of the crew doing the bonded overlay. If the entire slab is indeed 12" thick, and you lack the means of effectively removing and disposing of 30 tons of waste concrete, that could immediately put a damper on any "best" for that option. Likewise, using a crew not skilled or experienced with bonded concrete overlays could quickly take the "best" out of that option.

 

Understandable.
Decided that pouring concrete on top of the old would put me right up against the siding/sheathing and thus cause future rot just like some I am replacing now due to that.
So had a guy come take a stab at getting the concrete up. You were right - it was thinner in the center. About 2 feet on the edges and 6 inches in the center. Basically looks like they just poured it up against the foundation and let it settle in rather than backfilling some first. What's left in the picture is on me to get because we couldn't get any closer to the corner due to some pipes just off camera. The plan is to rent a jackhammer and get rid of the 6 inch shelf completely and take the 2ft section down about 6-8 inches. Then pour concrete to get it back to the original level. Since this left part of my foundation exposed what material should I use to backfill? I was planning on rubber coating the exposed foundation and then putting 2 inches of foam board down there prior to backfilling. Good idea? The pipes I mentioned are for the cleanout of a french drain I recently had install so hopefully I won't have that much water up against the foundation but want to make sure I backfill it properly.

 

Residential foundation backfill material is usually whatever soil happens to be available. Many contractors will use overburden they removed to excavate for the foundation, along with construction debris, old boots, empty beer bottles, etc. If you're pouring concrete over it, compacted gravel would be best, of course.

Not sure I'd go through the expense of adding foam board insulation to such a small area of the house, as there's a lot more perimeter that won't be insulated--unless you plan to do the entire foundation.