We recently placed and finished a concrete sidewalk. After curing the concrete for one day, we covered it with plywood to protect the surface. It rained that evening and when we removed the plywood the following day, the concrete had brown stains on it. How can we remove the stains?
Extracts from wood may cause yellow or brown stains in concrete. The stains can be removed by treating the concrete with a hypochlorite solution. Household bleach, which is usually about 5 percent sodium hypochlorite, is satisfactory after diluting one part of bleach with four to six parts of water. The stain disappears if the surface is first scrubbed thoroughly with a solution that is one part glycerol in four parts water. Glycerol is flammable and bleach is corrosive to skin. As always, read the warning labels on the chemicals before use.