Q: For a concrete plaza surrounding a large sculpture, the engineer specified a wet cure. The contractor used burlap to keep the concrete wet while it cured, but the burlap left some ugly stains. Can you recommend a way for us to get rid of the stains?
A: The problem of staining is one reason manufacturers have developed alternatives to burlap for wet curing. McTech Group’s Ultracure (www.mctechgroup.com), Loganville, Ga., and PNA’s Hydracure (www.pna-inc.com), Atlanta, are two products that hold water to maintain 100% humidity at the concrete surface. They provide for optimal wet curing without leaving stains. That won’t help you with this project, however.One difficulty in trying to remove this type of stain is that you may wind up with a nonuniform appearance; that is, with areas of cleaner, brighter concrete standing out where dark stains used to be. To avoid this, you might try blasting lightly with a medium that’s not too abrasive, such as crushed walnut shells.For surfaces such as walls that aren’t subjected to traffic, a grout cleandown and sack rubbing can help make a more uniform appearance. It may not hold up for long, though, in a busy plaza. If the problem is really severe, another alternative would be to apply some type of overlay. Such products can be spray-applied thin enough to preserve the existing concrete surface texture.It’s also possible that the passage of time and the effects of the elements may adequately fade the burlap stains without any other effort on your part.