Cut rebar in concrete
A Dremel Tool with a cut off wheel is going to be your tool of choice here. Here is a picture of what that looks like:



When approaching a job like this I make sure I have plenty of the cutoff wheels. I also have a half dozen of the mandrels so that I can have all of them with cutoff wheels premounted so it is quick change in the tool when one wears down or breaks. 

The cutoff wheels come in thin normal thickness and thicker heavy duty style as shown above. The heavy duty ones have to grind away more to perform a cutoff bit are less prone to breakage. 

You will have to cut the re-bar at an angle as shown on the below picture. It may even be necessary to break a small amount of the concrete on either side to permit the tool to have a better angle of attack on the re-bar.