Q.: We do a lot of concrete restoration work and need to know what problems are associated with using epoxies for coating partial lengths of reinforcing steel. When corrosion of in-place rebar has started but hasn't yet significantly reduced bar diameter, our crews remove damaged concrete, clean the bar, and field coat it with epoxy. Then they patch with a cementitious repair material.

I've heard that when only part of a bar is coated, corrosion is accelerated at each end of the coated area. And that even if the bar isn't coated, differences in the chloride content of the existing concrete and repair concrete cause faster corrosion in the unrepaired areas. Has this been proven in practice? Is it cause for concern?

A.: For an answer, we contacted several sources including the Concrete Reinforcing Steel Institute (CRSI) and W. R. Grace & Co. There doesn't appear to be a well-documented answer to this question. Theory suggests that a corrosion-causing galvanic cell may be created if a rebar is partly covered by chloride-contaminated concrete and partly by uncontaminated concrete. Galvanic cells can also develop because of differences in oxygen and moisture concentrations between the existing and repair concretes. CRSI, however, says its field experience doesn't substantiate this. The organization has no data indicating that coating partial lengths of rebar has harmful effects or accelerates the corrosion process.

Neal Berke of W. R. Grace & Co. recommends that repair crews remove all concrete with a chloride content greater than 1.5 pounds of chloride ion per cubic yard of concrete. He cites a state highway department presentation on bridge decks where spot repairs had been made. After repairs were made, the decks continued to deteriorate in originally undamaged portions. Berke suggests advising the owner that replacing deteriorated concrete doesn't guarantee that other parts of the structure won't start to deteriorate. But he doesn't believe that using epoxy on partial lengths of rebar accelerates the corrosion process.