Q.: We were very pleased with our architectural concrete office building until ugly rust stains appeared on several surfaces. Did the contractor make a mistake, or is this unavoidable? What can be done?
A.: There are several possible causes. Occasionally iron sulfide particles in the aggregate are at fault. Loose.iron or steel particles left in inadequately cleaned forms can also cause trouble, as can reinforcing bars too close to the concrete surface. Bar supports that weren't protected against corrosion can produce small rust stains in a recognizable pattern. At times, a reinforcing bar is incorrectly positioned too close to the concrete surface and starts to rust.
Removal of the stains depends on where and how deep they are. Both chemical and mechanical means have been used. Detailed instructions for removal of iron rust stains are given in the August 1986 issue of Concrete Construction, page 742.
On future work, consider preventive measures such as careful form cleanup and use of enough noncorroding bar supports to keep reinforcement the specified distance away from the concrete surface. Plastic-tipped or stainless steel bar chairs are available. All-plastic spacers like those shown in the photograph also work well.