If  you want to accelerate the corrosion of the reinforcement bars in the concrete, you simply reverse the polarity of the impressed current and make the bars the anode.  When you do this, you may want to change the material that is normally the anode to make it larger so that you will reduce it's resistance to ground.  That will allow you to reduce the operating voltage of your impressed current system now that the relative sizes of the new anode and cathode are different than before.

The other option would be to change the material that was the anode before and now use something (such as more rebar) as the new cathode.  You need to do this because some anode materials (such as metal oxide coated titanium) do not function as effective cathodes due to overvoltage.