Question: Last fall we placed a concrete sidewalk in front of a downtown office building. Because it was relatively cold weather, we used a winter flatwork mix and covered the slab with curing blankets after installing a light broom finish. We left the blankets on for seven days as required by the specifications. When we removed the blankets we noticed surface discolorations, but thought they would go away when the concrete dried. Now, seven months later, the owner is not happy with the strange pattern of discoloration. It’s not very noticeable when the concrete is dry, but it’s extremely visible when wet. What caused the surface discoloration? More importantly, is there anything we can do to remove it?
Answer: Your surface discoloration was caused by nonuniform curing conditions. In your case, the uneven curing was caused by folds or wrinkles in the curing blankets. This form of discoloration is normally associated with clear polyethylene sheeting; however, it can happen with any type of waterproof sheeting.
Folds or wrinkles frequently create water evaporation-condensation cycles that cause light and dark areas to form on flatwork. Light areas occur where the blanket is in full contact with the concrete and a thin film of water forms between the blanket and concrete. Dark areas form where the blanket is not in full contact with the concrete, enabling surface evaporation to take place.
Discolorations can form as early as 16 hours after concrete placement.
Water evaporation and condensation
Water evaporating from the concrete pulls alkali salts from within the concrete to the surface, where they are deposited in the surface pores of the concrete. Because these deposits are relatively transparent and their optical behavior is similar to that of water or clear oil, they darken the color of the concrete surface.
Heat from the sun and the hydration of portland cement causes water to evaporate from the concrete. Water vapor then condenses on the cool, high portions of folds and wrinkles in the curing blanket. The water eventually runs down the sides of the blanket to collect and seal the surface of the concrete where the blanket is in contact with the surface. The thin film of water between the blanket and surface of the concrete does not allow water evaporation from the concrete to occur. Therefore, these areas are essentially free of alkali salt deposits as compared to areas below folds and wrinkles. (See illustration above.)
The use of calcium chloride as a chemical accelerator can exacerbate the problem by increasing the magnitude and permanence of the surface discolorations. Since a winter flatwork mix was used and considering the severity of the discolorations, most likely your concrete contained calcium chloride. As the dosage rate of the calcium chloride increases, the risk and severity of discolorations also increases.