Concrete is not only a big chunk of heavy mix of cement, rock and sand.
There are many installation that defer somehow from a foundation or slab. We have installed several isolated foundations where machinery are so sensitive to the surrounding noise and vibration that the footings cannot have any direct contact with the surrounding surface.
Sometime we need to have surfaces that conform to a very specific pattern like on a “Belgian Road” where every 4”x4” block is set at a pre-established elevation to create a very specific pattern.
In particular harsh environments the existing concrete structures are so deteriorated that major support and reconstruction is necessary to maintain the integrity of the structure itself.