Effect of Impressed Current on Bond Strength Between Steel Rebar and Concrete

The deterioration of bond strength between reinforcing steel and concrete as a result of impressed cathodic current with respect to the exposure time was investigated. A current in the magnitude of 3ma/ft2 (based on reinforcing rebar area) was used for a period of five years. Chemical analyses for sodium (Na+), potassium (K+) and chloride (Cl-) ions in concrete cylinders were performed as a function of time and distance from the rebar. Bond strength and chemical analyses for cylinders exposed to the current flow were compared to those determined for the cylinders with no impressed current (control). After forty two months of experiment, it was found that the impressed current had weakened the bond between the rebar and concrete significantly. This weakening in some cases was a factor of two lower compared to the control cylinders of the same age. The concentration of sodium and potassium ions near the rebar for the test cylinders increased with exposure time to current. The chloride ions migrated toward the concrete surface as expected. The decrease in bond strength was probably due to the accumulation of sodium ions near the rebar. It is well known that high sodium concentration will attack the portland cement matrix. These results indicate that there should be some concern about the long term effects of cathodic protection for reinforcing concrete structures.