During large concrete pours, pains are taken to ensure no foreign debris enters the mix, like a scrap of wood, for instance.
You may be right, but I do know it is not an ideal situation for the integrity of a structure to have something so large as a dead body buried within its concrete.
Lots of care, apart from when the Mob wants to bury a body where no one will ever be able to look during the length of a human lifetime.