Concrete Discoloration – Dark Spots, Stripes, and Blotchy Concrete

Concrete discoloration comes in the form of dark spots, stripes, blotchy areas, and color differences. There are many causes of discoloration on a concrete surfaces and the solution will vary depending on the cause.

Discoloration Issues on New Concrete

Discoloration on the surface of new concrete can be from several factors including inconsistent mixtures, too much or not enough water, low quality materials, poor workmanship, the use of calcium chloride, environmental issues, or issues created during the pore or during the curing process. In some cases, the cause is quite evident, but in others, it is a pure guessing game. It is important to check with the concrete contractor, or concrete manufacturer, before attempting to remove dark spots from the surface.

The most common ways to remove dark spots include repeatedly flushing and scrubbing the surface with warm water or acid washing the concrete surface. Once the concrete is full cured, if these methods have been attempted and the discoloration is still present, it may be time to consider applying a concrete coating to hide the discoloration. If you don’t want to hide the discoloration and simply want to get rid of it, you can consider continuous cleanings, or applying a concrete overlay (cement coating over the entire surface).

If cleaning methods have failed and you want to hid the discoloration, you want to use a sealer. Penetrating sealers won’t change the look of the concrete, but acid stains and acrylic sealers will. Acid stains will create a marble-like finish, where acrylic sealers will darken the surface (provide a consistent wet look) and leave behind a low to high gloss finish. If you are going to apply an acrylic sealer, apply a solvent based acrylic and start with first applying the product to a test area to make sure you get the consistent finish you are going after. Colors can also be added to acrylic sealers to completely hide the discoloration below.

Discoloration Caused by Water

If your concrete is fully cured and you haven’t had any previous issues with discoloration, you may be experiencing discoloration caused by water, poor drainage, or irrigation systems. If water has a high iron or mineral content, or if the water is carrying leaves or dirt, it can leave behind iron or rust colored stains, or dark spots on the concrete.

The most common way to remove discoloration caused by water is to use a heated pressure washer, or acid rinse. Once the stains have been removed, it is important to seal the concrete to reduce future staining. Water repellent sealers, such as the Armor SX5000 or Armor SC25, can be used to reduce the absorption of the water on the surface, or a coating can be used, such as the Armor AR350 or Armor AR500, to provide a protective film over the entire surface.

Discoloration Caused by Efflorescence

Efflorescence looks like white powder, or white crystals. It continues to appear even after cleaning, and seems like a problem that never goes away. It is caused when water or moisture carries it from deep within the pores to the surface.

Efflorescence is water soluble so it often disappears when rinsed away with water. The efflorescence however really isn’t gone and will re-form again quickly after. The best way to remove efflorescence is with an efflorescence cleaner, such as the Foundation Armor ER100. While this method requires a bit of elbow grease, it will remove efflorescence on the surface and from within the surface pores.

Once the efflorescence has been removed, it is important to seal the concrete to reduce the movement of water and moisture. Densifiers and water repellents are the two most popular types of sealers that are used. Densifiers, such as the Armor S2000 and Armor L3000, reduce the movement of water through the pores and increase the strength of the surface by up to 45%. Water repellents, like the Armor SX5000, reduce the movement of moisture and stop up to 95% of water absorption on the surface. When used as part of a system (densifier first followed by water repellent 7-10 days later), efflorescence can be significantly reduced.

While sealers prove to be very effective, they can’t always stop efflorescence. In some cases, the amount of efflorescence present, or the amount of water and moisture present, is extreme. If this is the case, a coating or overlay can be attempted, but only after a concrete professional has had a chance to examine the concrete.

Discoloration Caused by De-Icing Salts

De-icing salts and pellets can cause discoloration. This isn’t always made known by manufacturers, but you can trust that consumers will always include it in reviews. Discoloration isn’t always caused by the pellets. In some cases, it is an issue with the manufacturing process. Oil or grease from the manufacturing equipment can leach into the pellets during the manufacturing or packaging process. Before applying de-icers or pellets to the surface, always apply to a test area.

The cause of the discoloration will determine the best way to fix the issues. First, consider contacting the manufacturer of the pellets. If they are unable to help, consider trying a heated pressure washer, degreaser, or concrete surface cleaner. There is no way to tell how permanent the stains are until a few different cleaning methods have been used. If all cleaning methods fail, a coating may be the only solution to hide the discoloration.