Google plywood stains on concrete and you'll get a lot of hits. I found this in the first one: "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."