Rough texture sidewalk and slab outside of garage in GA/SC area have DARK appearance. Mold is suspect but moving here only a year ago doesn't provide much if any corporate memory. Similar appearance on steps down to the pool and around block wall 'tween pool and flower bed. Remediation steps would be great! Expect bleach solution for correction but concentration needed. Additionally is abrasion with brush needed when whatever solution is applied?
Hard to be certain but you are probably correct. If the dark areas are even greater in shaded areas then a form of mold is probably the cause (along with normal buildup of airborn contaminates). I had similar situation in Houston. I also had occasional buildup of green algee and moss in heavily shaded areas. I had a heavily landscaped yard with a lot of moss rock borders and accents so I used to scrape the moss and transfer it to the rock.
I used common household bleach in a pressure sprayer. I would wet down the driveway first, (or do on a day when you have light rain) then spray the bleach over those areas with the highest concentration of stain buildup. Follow that with stiff broom to loosen the buildup. Finally, use a pressure washer to complete the cleaning. The pressure washer is going to do the majority of the work but the bleach will make it easier. You may need to do a bit of extra work on areas where the stains are stuborn. Finally, after you are done, allow to dry and spray on Thompsons Water Seal. This will help retard the stains from returning quite so quickly. They will return though and the process will need to be repeated in a few years.
PS: D> Rough texture sidewalk and slab outside of garage in GA/SC area have
1:10 bleach:water is plenty strong - I haven't bothered with sidewalks, only stucco walls. Took off a lot of paint chalk as well. Be sure you don't get it on metal (exterior hardware, car, window frames, etc.). I used a brush, but a hose-end sprayer probably would work fine on flat surfaces. Do it when the sun won't dry it out right away. Rinse well. If in doubt about delicate plants, avoid them. Didn't hurt mine.
Power washer is the easiest and fastest removal and will take it right off. I wouldn't screw around with a brush, etc for a sizable area. I'd use one of the detergent solutions made for power washers, availabe at the home center stores in paint dept. You could add bleach, but I don't thick it's necessary or will make any difference in getting it clean. The power washer will remove what's there and any bleach/detergent, etc is going to be washed away. Maybe finishing with a rinse of bleach solution might assist in preventing it from coming back for awhile.
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Had a similar situation here, with open carport (used to have a structure overhead, no longer there. Now it's just a wide expanse of concrete with somewhat rough texture.) My problem was algae, which I fixed with a couple gallons (one case of two gallon jugs) of 30% hydrogen peroxide in the form of swimming pool clarifier (sold by some swimming pool supply stores, ask for Baquacil, or something similar. Mine is brand-name Softswim "a BIOGARD pool care product" with a red circle containing a white "C" on the white plastic jug.) Poured it on and scrubbed it with a steel-bristle brush I bought at Home Depot for this purpose. Wear junk shoes, and expect them to be bleached by the solution. Let it set for an hour or two when you are through scrubbing, then rinse the sludgy remains away with high-pressure nozzle on regular water hose. This is a biologically friendly way of getting rid of the offending vegetable matter, as the peroxide turns into carbondioxide and water as it reacts with the organic material on top of the concrete (C + H2O2 -> CO2 + H2O). If you have the time, a little goes a long way. I used 2 gallons on my nearly 300 sqft carport. And letting it set is as important as scrubbing a little, to let it get down to the bottom of the algae layer. This product is also a bleaching agent, so expect your feet to be white if it gets on them. Won't hurt them though, as long as it is not poured on them and allowed to set there.
Much more freindly to garden wildlife than chlorine bleach. Try it on a small patch with the standard 3% that you buy at Walgreens or CVS if you want proof if it's effectiveness, just don't expect miracles with that weak of a solution. And you *will* have to scrub that small patch with something, but should see results. A case of Softswim clarifier cost me $30.00, which might seem rich, but it is indeed biologically friendly.
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