Sodium Hydroxide is the best way to remove grime from concrete. You have to be very careful with it though. It is very dangerous in its raw form. It will burn you so use proper protection. Mix 3/4 pound and a good squirt of Joy, dawn or other surfactant to a gallon of water and you have an awesome concrete cleaner. Down stream this or x-Jet which you prefer. Wait 5 min then wash.....