I wondered about that too. Two reasons were given to me: Quote from the site manager: Brick remnants "rot" together over time and cause driveways to become uneven. The civil engineer had to remove that again. When building the terrace, I also asked if we really had to remove it again, since the civil engineer had put it in there on purpose. One of my children-in-law (construction worker) took a handful of it, squeezed the brownish stuff together and it held together. Then he did the same with a handful of mineral concrete, which remained crumbly. I assume he wanted to tell me that one can drain water and the other cannot. We laid granite slabs in a dry bed. Presumably, a floor slab supports itself and that is why a certain amount of RC-"debris" can be concealed beneath it. It may be different with a fine-grained covering. Our experience is that civil engineers like to conceal something and it is certainly not what the client wanted to keep. Best regards Gabriele