Q.: Discoloration mars the appearance of exterior concrete on one of our institutional buildings here in the Southeast. What causes such poor looking concrete? Can anything be done to even out the color?
A.: Without knowing more about construction practices used, we can only speculate about the causes. Some of the discoloration appears to be related to formwork. The dark horizontal lines are caused by water leakage at form sheathing joints. Leakage draws cement and other fine particles toward the joint causing a dark discoloration.
Some of the other discoloration may have been caused by differences in form sheathing absorbency. Absorbent forms darken the surface concrete because they absorb water, reducing the water-cement ratio. Variable absorbency of unsealed plywood, for instance, can cause blotchy looking surfaces. Differences in type of form release agent, or in application rate, may also account for the color variations.
Some of the color differences may have originated at the batch plant. Color differences are likely if concretes made with cements from different sources are used in the same structure. Admixtures such as calcium chloride or silica fume may change the concrete color, as will some fly ashes.
Weathering stains also may have caused some of the discoloration, and in your area mildew may be the culprit. Photos of the building immediately after it was built would help to establish if the mottled appearance was caused by weathering or mildew.
Stain removal methods may correct some of the discoloration. Pressure washing with a mixture of water, bleach, and detergent has been used for mildew. Other stain removal techniques are described in Removing Stains from Concrete, a collection of articles reprinted from Concrete Construction. It can be ordered from Concrete Construction Publications, Inc., 426 South Westgate, Addison, Illinois 60101. Price is $10.
Discoloation caused by construction practices can't usually be corrected by stain removal methods. Painting or coating is an alternative method, but you will still have to clean the surface first.