Q: We had a slab placed in a plaza to surround a large concrete sculpture. The engineer specifi ed a wet cure, so the contractor used burlap, which left unsightly stains. We're cutting grooves in the slab and inserting metal strips to create a grid that is part of the look. How can we get rid of the stains?
A: The problem you've described is one of the reasons manufacturers have come up with alternatives to burlap for wet curing. Products such as McTech Group's Ultracure (www.mctechgroup.com) and PNA's Hydracure (www.pna-inc.com) hold water to keep 100% humidity at the concrete surface for optimal wet curing without leaving stains. But that won't help you with this project.
One problem with trying to remove this type of stain is that you may find yourself left with a nonuniform appearance--the areas of cleaner, brighter concrete standing out where dark stains used to be. You might try blasting lightly using blast media that's not too abrasive, such as crushed walnut shells.
For nontraffic surfaces, such as walls, a grout cleandown and sack rubbing can help make to appearance more uniform. (See our Web site for a link to an article describing this process.) However, it is hard work and may not hold up over time in a busy plaza.
Another alternative would be some type of overlay. Manufacturers continue to improve the performance of such products, spurred on in part by the rising interest in decorative concrete. One example is Spray-top, offered by Concrete Solutions (www.concretesolutions.com), which can be applied with a hand-pumped sprayer and thin enough to preserve the existing concrete surface texture.
If left alone, of course, the passage of time and the effects of the elements may do an adequate job of dispersing the stains left by the burlap.