Causes of Concrete Storm Pipe Discoloration

Because concrete is such a porous material, it absorbs materials moving across its surface and stains easily. Stains in storm drains are not seen by most people, but discoloration of concrete storm pipes may indicate structural problems; however, discoloration often indicates nothing more than that the pipes are being for the purpose for which they were designed.

Oil Stains

• The most common cause of oil stains on the walls of concrete storm pipes is motor oil leaked from cars, which washes off the road and into storm drains when it rains. Other oils in storm drains come from cooking oil leaked from the grease collection dumpsters used by restaurants or oils that are accidentally or purposely spilled. Oil will not damage concrete, and the presence of oil staining does not indicate that the storm pipes are damaged in any way. If there is an excessive buildup of grease, the pipes can become clogged; however, this is more of a problem in sewer pipes that carry household waste than in large storm drain pipes.

Rust

• Most concrete sewer pipes do not have iron reinforcement bars, so the appearance of rust is simply staining deposited on the surface of the pipe by rusty debris or rusty water. However, some very large storm pipes are made with rebar; if the surface of the concrete is chipping away to reveal rusty spots, this indicates that the rebar beneath the concrete is degrading. In this case, the stain indicates damage to the pipe, and repair or replacement may be needed.

Efflorescence

• Concrete that is exposed to water and later dries often has surface stains that look chalky. These stains, called efflorescence, are either the result of water-dissolving salts present in the concrete or of water soaking into the concrete in which such salts are carried. As the water slowly evaporates, the salts are carried to the surface of the concrete and left behind. The accumulation of these salts looks like a chalky dust on the surface of the pipes. Efflorescence is a normal result of concrete being exposed to water and does not indicate damage. If the efflorescence needs to be removed, the majority of it can be brushed off with a wire brush and any remaining chalkiness can be dissolved with a weak acid solution or a cleaner formulated to remove soap scum and lime scale.

Fungal Growth

• The damp environment of storm pipes and the many materials that move through them provide excellent growth conditions for mold and mildew. When fungus grows on concrete storm pipes, the growth and its waste products will stain the concrete and can slowly degrade the material. However, it is nearly impossible to prevent fungal growth in this environment; if the fungal growth needs to be removed, it can be manually scraped off and the surface can be treated with an anti-fungal solution.