how do you know there is concrete underneath normally slabs laid on a hardcore base blinded with sand
The longer the pole the easier it is for any movement at the top of the pole to disturb or lift the slab the pole is fixed to
You mentioned the sandy concrete. How deep was it?
Did it have stones in it?
If it didn't have stones it was probably a weak sharp sand/cement mix. This is a usual way of laying slabs that you don't want to move.