After one of our customer-contractors poured an integrally colored patio at a new residential home, the homeowner had the carpenters add a railing to the adjacent redwood deck. Afterward, wood debris covered a portion of the patio. During the final walk-through at loan closing, the homeowner noticed a red stain on the tan-colored concrete. How can the contractor remove the stain to satisfy the owner without replacing the concrete?
The stain in this example was caused by wood extracts. These natural oils permeated concrete surface voids, leaving the brown or red stains. The best way to attack the stain is with early treatment that draws the stain up from the concrete surface with a poultice. A poultice is usually made by mixing some essentially inert fine powder, such as ground limestone, hydrated lime, portland cement or fuller's earth, with the solvent or solution. (Shredded pieces of highly absorbent paper would also work.) The selection of the cleaning liquid is based upon the cause of the stain. The two parts are blended to make a smooth paste that is troweled over the stain. The liquid portion of the poultice migrates into the concrete, where it dissolves some staining material. Then the liquid gradually retreats upward from the concrete into the poultice, from which it evaporates, leaving its burden of dissolved staining material in the poultice's inert powder. Once dry, the residue is scraped or brushed away. Usually it takes more than one application to remove all the stain. Whether the concrete surface is colored or not, it is important to first test the poultice in an out-of-the-way small area to check its effect on the concrete's surface. To remove the wood extracts' residue, first use a soft brush to wash the stained area thoroughly with a solution that is 1 part glycerol in 4 parts water. Treat the stained area with a poultice made with a diluted hypochlorite solution, made up of household bleach (which is usually about 5% sodium hypochlorite) and tap water, mixed at a ratio of 1 part bleach with 4 to 6 parts water. Reference "Methods of removing some specific stains from concrete: Oil to Wood," W. Kuenning, Concrete Construction, September 1986, pp 821-826.