Q.: Our company has a room in which machines are located directly below the hoppers of some 15-foot-square reinforced concrete bins that hold raw popcorn. Sometime in the past the bins were sprayed on the inside with an epoxy coating. Now small rock-size chunks of concrete are breaking out of the walls of the bins and falling with the popcorn into the machines. The concrete in the walls is very hard, but the exterior walls of the building show rust over the rebars. We think vibration from the machines may be causing the particles to break out. What can we do to stop the concrete particles from getting into the popcorn?
A.: You need to find out for sure what is causing the particles to break out. If there is a cause of moisture buildup behind the coating you might be getting popouts from traces of chert or other expansive aggregate. Vibration from the machines doesn't seem a likely cause of popouts, though perhaps vibration could loosen concrete particles adjacent to any cracks that exist. A concrete petrographer could identify the cause by examining some particles and a sample of the area from which they came.
The kind of repair needed will depend on the cause. Very possibly you will want to recoat the repaired area by applying an epoxy or polyurethane coating. To find the names of manufacturers of such coatings suitable for use on concrete look under "Coatings, Wear Resistant" in the Buyers' Guide in our most recent December issue, or look under the same heading in the Concrete SourceBook (available for from Concrete Construction Publications, 426 South Westgate, Addison, Illinois 60101).