Vinegar did the trick for me. I had two wheel wells full and all down the side of the door. I just thought it was a mud splash and it sat for a few days. I discovered it was concrete when I went to wash it.
I was about to strangle someone at the concrete plant down the street from where I work. There are still little splashes of concrete that dripped out of my wheel well right where I park at work.
Anyway, vinegar stinks, but it does the trick and seems to be fairly harmless.
Rinse well, and be prepared to stink.
