Chances are it is not concrete.
Most likely it is flowable fill, or low strength concrete, with an accellerator admixture added to it.
Flowable fill flows like poorly graded sand, requires no compaction, sets up like concrete, but can be easly removed with most ground engaging equipment. - A skidsteer with a bucket could take it out.
It works great for backfilling conduit runs, abandoning manholes and subsurface vaults, and I would guess, temporary sidewalk extensions.
To quote Max Klinger "Trust me, I know concrete, and it's not that hard."
-dave