What would be the best way to clean the concrete on the entrance and exit of the carwash. Also, is there any way to clean the block work in this area? We have tried degreaser and clorox and pressure washed. These areas remain black.
A strong degreaser, a scrub brush and a pressure washer should work. If not you may need to use Muriatic Acid, however, this is nasty stuff and should be used with extreme caution and care. Protective equipment is required, google it and educate yourself before messing with it.
I'm in complete accord with John. You might also try a full strength chlorine soaking because the stain sounds like mold. Household bleach may not be strong enough.
If that doesn't work, muriatic acid has been used for decades for cleaning cement from tanker & truck bodies that haul the stuff. The acid actually reacts with concrete and the result is etching and neutralizing the alkaline in cement. Be sure to dilute the acid and if you plan to use a lot of it for a large area, I suggest wearing a respirator that protects your face, mouth & nose. Low-pressure spray the acid on the affected area... and let it sit for a few minutes, until it turns a brownish foam. Then rinse it thoroughly. Once dry, it is wise to seal the concrete against future discoloration.
If the acid splashes on your skin or clothing, flush rinse immediately.
Forgot one thing. When applying chlorine... or even acid, be sure to apply it to a dry surface. Once the surface gets wet, any chemical applied on a wet surface will be significantly diluted, making it seem to not work. The same is true with any cleaning activity that relies on a chemical reaction.
For example, if you seek to clean a badly stained alloy wheel... like an older BMW. Apply a buffered cleaner on the dry surface. After a few moments, apply more chemical to reestablish the proper pH. Do not get the wheel wet until the chemical cleaning has given you acceptable results. Once wet with water, the chemical reaction you're seeking will be greatly minimized because the chemical will not effectively reach the surface, but instead will be shielded by the water.
Depending on where you are located, you may want to consider hiring a professional service to do the work.
Here in my part of Florida, business is so bad that carpet cleaners are quoting retail work for as low as $0.10 per SF. With banks, shopping centers and strip centers cutting back on maintenance, you may find professional surface cleaning relatively inexpensive.
We use Kleen Wall here at our wash. Work great on the walls, floor, etc. Using this you have to have all the protective gear, and exterme caution. Have a good day
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