Rot. Use MEK (Methyl Ethyl Keytone). It will remove the paint but will not eat any of the concrete. Use gloves, you won't believe how this stuff can dry out your skin. Put the MEK on a cloth and scrub the paint off. As the old paint gets on the cloth, put more MEK on a fresh area and do it again. Keep "turning" the cloth or you will just be smearing the old paint around.