Buckling concrete floor ? ? ?

The concrete basement of our 80-year-old six-unit apartment building is generally in pretty good shape, except in one of the corridors the concrete has buckled up badly.
It's almost as if there were some roots growing underneath, but that cannot possibly be the case.
There seems to be an air-pocket beneath the buckle.
I can break up the top layer easily, but my question is, just what type of concrete do I use to replace it? It's smooth-finish, of course -- not like a sidewalk.
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check for broken leaking water or sewer lines...
I would be more interested in the cause of the undermining.
to replace floor sakrete concret bags if you dont need a lot.
if you need 4 yards or more then your better off getting a concrete truck..
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Ray wrote:

The slab repair should not be difficult. As noted by others, the cause is most important. When placing the new concrete, consider dowels drilled into the remaining slab and poured into the new. This wil tie old and new together and reduce differential settlement. TB
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