Black spots are a common sight on sidewalks, varying in appearance and composition. These marks can be biological growths or residues from human activity. Understanding their origins helps identify them.
Identifying Common Sidewalk Stains
Black spots on sidewalks include biological growths like lichen, mold, and mildew. Lichens appear as crusty, irregular patches, often black or greenish-black, adhering closely to concrete. They are symbiotic organisms of fungi and algae. Mold and mildew typically manifest as fuzzy or slimy black films, spreading across damp surfaces, thriving in humid, shaded conditions.
Other common black spots are inorganic. Oil stains from vehicles appear as dark, irregular splotches with a distinct sheen and greasy texture. They may show iridescent patterns. Tar, a petroleum byproduct, creates dense, sticky, uniformly dark patches, often remnants from road repairs or construction, adhering firmly to the sidewalk surface.
Why These Spots Appear
Biological black spots like mold, mildew, and lichen are linked to environmental factors. Ample moisture and limited sunlight create ideal conditions for these organisms. Shaded areas under trees or buildings, and locations with poor drainage, accumulate moisture. Organic debris, such as fallen leaves or dirt, also provides nutrients that support their development.
Inorganic black spots result from different processes. Oil stains form from vehicle leaks, where fluids drip onto the pavement and seep into its porous surface. Over time, these oil residues can oxidize and darken. Tar spots are deposited through direct contact with asphalt or petroleum products, often occurring during road maintenance, roofing work, or when vehicles track fresh tar.
Removing and Preventing Them
Removing black spots from sidewalks depends on their type. For biological growths like mold and mildew, a mixture of water and mild detergent or outdoor cleaner, followed by scrubbing, is effective. Pressure washing can also clean large areas of mold, mildew, or superficial lichen, using a fan tip to avoid concrete damage. Lichen, being deeply embedded, may require specific fungicidal solutions or extended soaking before scrubbing.
For oil stains, absorbent materials like cat litter or sand can initially soak up fresh spills. Older, set-in oil stains respond to degreasing agents or specialized concrete cleaners designed to break down petroleum products. These cleaners are applied, allowed to penetrate, and then scrubbed or pressure washed away.
Preventing reoccurrence involves addressing root causes. Improving sidewalk drainage reduces moisture, making the environment less hospitable for mold and mildew. Trimming overhanging vegetation increases sunlight exposure, deterring biological growth. For oil stains, regular vehicle maintenance prevents leaks. For tar, caution during construction or repair minimizes accidental deposition.