Good ol grandma! That woman knew a remedy for everything, and often they involved little to no pain at all! The first thing you should do, go down to the local gas station, or in to your garage, and get yourself a nice Coke. Take it home, and set it aside while you go find yourself a nice, stiff, wire bristle brush. Go out to your concrete in question, crack open the coke, take a swig, and then pour it on the stain, careful to not get it in the flowerbeds. Let it sit for about two minutes, and when you come back, attack it with that wire bristle brush, giving it a good scrubbing. After that, wash it gently with a spray hose, and the rust should be gone. For an even more stubborn rust stain, spend a few more dollars, and buy a commercial solvent.