I recently purchased this house which has a raised concrete patio in the rear that has a large depression in the middle probably 4' wide by 6' long. You can see the cracks in the concrete where the water settles (picture below). I assume this is due to the material that is underneath from the original construction of the house breaking down and the concrete settling. So I am looking for options other than replacing the whole thing. Possibly filling the area with some type of self leveling top coat or something. Any thoughts would be appreciated.
A self leveling cement may reduce the standing water but if the pad has no wire mesh or rebar it may continue to settle. I would acid etch the surface and rinse then use the self leveling cement . after that I would probably epoxy coat the entire surface to try and seal it. the more water that gets into the cracks the more it will settle. I use a 10-30% Muriatic acid and water solution. Remember to always add acid to water. Muriatic acid can be purchased at most home stores and pool supply stores.
You can get a Mud Jacking company to pump in their foam. This would fill any voids beneath & also raise the concrete back to level. Or, drill holes every 2-feet & pour in a cement slurry to stabilize everything below. To then, apply a Concrete Resurfacer to the entire patio, first leveling off the depression. Resurfacer is applied in 1/4-inch layers for this type of correction.