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Expert: - 6/3/2009


Question

Concrete Discoloration
My house is 12 yrs old I bought it just less than 2 yrs ago.  The concrete driveway and sidewalk were fine.  Last fall some varied sized black spots started showing up on the sidewalk in an area near the front door stoop and also some smaller rusty spots there and out on the driveway. When I  look closly I see tiny black spots clustered in the dark areas.  The rusty spots are covering more and more area but staying separated.  What is causing this and what can I do to stop it, and clean up what is there.  I have tried stiff brush and soapy water and it doesn't change.

I always sweep any fertilizer off the concrete immediately; and some of the areas affected are not exposed to fertilizers.  I don't know what else might be getting on it.

Answer
Lorraine, the black stuff looks like some version of mold. I suspect the area doesn't get much direct sun.  Add bleach to your cleaning efforts to kill and remove the mold.  There are zinc strips available which prevent the mold from establishing itself.

Rust spots are a different issue.  CLR or something similar should remove it.  Treating the concrete with a sealer should prevent it from coming back.  Prosoco, Dayton Superior, and others make some excellent sealers which are clear and fairly easy to apply.

Dan Griffin

Expertise

I can answer almost all questions related to the total construction process. My expertise is in commercial construction, though I can field most any residential question. I have hands on experience in concrete, heavy equipment, masonry, all phases of carpentry, interior finishes, and I am fairly strong in mechanical and electrical.

Experience

I have over 20 years experience as a commercial carpenter and commercial construction superintendent. I have another 20 years experience in facility management for a major school district.

Organizations
My favorite hobby for he past 12 years has been singing bass in a The OkChorale men's barbershop chorus and the Mature Moments quartet.

Education/Credentials
I hold a Bachelor's degree in English and Math. I have completed many continuing education hours in the building trades. I hold a Master Carpenter card from the AGC, Associated General Contractors.

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