Question, does it look like professional job, or it is a improper proper technique.
Laterally pored Concrete slab damage, as in a walk or driveway, are due to several factors. Erosion being the underlying factor, correct?
How did you determine in a non destructive manner to the concrete and observe what lay underneath the old concrete.
More to the point, was the substructure, soil, sloped in the right direction,away from a foundation, pier or footing and was there 2 to 4" inches of clean aggregate underneath the old slab?
Lets look at two concrete erosion probabilities. Uplift and torsion.
A flat rigid surface being lifted up and not in a uniform manner. It is twisted.
Question, Will a 1.5" inches thick slab of concrete not bonded properly have the same rigidity as a 4" slab when it is placed under stress.
I would be looking for; What was in the proximity of the slab? Trees, what type and how big. Any bushes or plantings. Automatic sprinkler system,. Is the any potability of soil heaving. Was the old slab sinking and that is why a new slab was placed on top?
Any downspouts or other water sources that could allow a stream of water to penetrate the slab at grade.
Again I will ask you a question. Did it look like a professional job or was it the proper technique?
If the old slab was broken, then adding more wight on top will make it do what?
Await a reply.