We connected our medium sized planetary grinder to a 600 V plug provided by the customer. We ground the soft concrete thoroughly to discover that a joint between 2 slabs had been moving .
One side was 1″ higher & over the years of the 2 slabs compressing against each other ,the concrete was “soft” on the joint.
Cutting out the slab 6″ to 1ft wide along the lenghth of the joint and refilling with concrete would have been the best and most durable option but since the tenant was moving in there was not time. We patched the joint with epoxy mortar and I hope that the slabs will not shift too much in the future.