Concrete cleaning should be part of regular maintenance for keeping the exterior of your home looking nice.
Since concrete is porous, dirt, grime, stains, mold, etc. build up over time but can be removed with a good cleaning. It is also a necessity as a first step in enhancing surfaces with concrete stains, paints, and sealers. Cleaning can also help restore the beauty of decorative concrete.
Before cleaning the entire surface, you will more than likely have to do some spot cleaning to remove specific types of stains that otherwise can't be removed with general cleaning. Some of these stains can be stubborn and you may have to resort to cleaning with acid. You still may not be able to remove them completely.
There are several methods for cleaning concrete. The easiest and most effective method is pressure washing combined with a mild soap. Just mop or spray some soap water across the entire surface, let it sit a few minutes and blast it with a pressure washer. Make sure the water fans out a few inches as it strikes the concrete or you could actually leave permanent marks or streaks in the concrete.
If you don't have access to a pressure washer, then you will need to do a little scrubbing with a stiff brush.
Some surfaces will be tougher to clean than others, especially those with a rough or broomed finish. It may require a chemical cleaner/degreaser specifically designed for concrete which will penetrate the pores to loosen dirt and grime. These can be found at most home improvement stores. Just follow the simple instructions on the label.
This kind of cleaning must be done before applying any acid stains or acrylic stains.
A good cleaning will usually lighten the color of the surface. So you will need to clean the entire surface to keep it consistant.
After concrete cleaning, it's a good idea to apply a coat of concrete sealer if doesn't already have one. This keeps contaminants from getting down into the pores which makes cleaning much easier as well as keeping the concrete protected from elements and contaminants.