The Concrete Network

I live in northeastern New Jersey, and in June we had weeks of rain and humid conditions. Some areas of my concrete driveway developed black spots, and other areas under pine trees have turned green. What is this stuff, and how do I remove it?


The wet conditions you experienced caused mold and mildew growth on the concrete. This growth usually occurs in damp or shaded areas that don't receive a lot of sun or heat. Concrete, with all its nooks, crannies and pores, is actually a great incubator for mold. Wet, humid weather with warm temperatures only improves the growing conditions

The best solution is to scrub and clean the affected areas with a mold-killing detergent or a simple solution of household bleach and water. Be sure to scrub aggressively to get down into the pores of the concrete. Let the detergent or bleach solution soak in for a while to ensure that all the mold is killed. After scrubbing, rinse with a power washer to get all the solid organic matter out of the concrete.

Next, you should seal the concrete a good-quality acrylic sealer designed for exterior use. Let the concrete dry at least 2 or 3 days in rain-free, sunny conditions before sealing. I recommend using a low-solids solvent-based sealer to allow for better breathability, especially considering the warm, humid exposure conditions. Read my article Driveway Sealer for Concrete to learn more about choosing and applying sealer.

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Author Chris Sullivan, technical expert and vice president of sales and marketing for ChemSystems Inc.

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