Acidification at rebar-concrete interface induced by accelerated corrosion test in aggressive chloride environment

The rebar-concrete interface plays a critical role in initiating corrosion of steel reinforcing bars embedded in concrete. The intimate physical contact between a steel rebar and the Ca(OH)(2)-rich interfacial transition zone of concrete provides physical protection and buffering action against corrosion. However, the mechanism of the buffering action is not well understood and a systematic study has not yet been conducted. This paper investigates acidification at the rebar-concrete interface, buffering capabilities of concrete made with 100% Portland cement and with 50% fly ash or ground-granulated slag as cement replacement, and corrosion behavior of the embedded rebar. It was found that a simple technique with filter paper wrapped around the steel rebar under a high impressed anodic current and a severe chloride environment allowed for interface solution to be accumulated, squeezed out, and collected for analysis. The solutions had pH values less than 2, thus proving acidification at the interface during the accelerated corrosion test. With a high content of Ca(OH)(2) in the matrix, the concrete with 100% cement as binder was shown to have a higher buffering capacity as compared to the matrix with 50% fly ash or slag as cement replacement. This acidification phenomenon promoted steel dissolution and subsequent corrosion activity, although the presence of cracks in the concrete cover could also play a dominant role in controlling the subsequent corrosion rates. Acidification was also shown to occur under natural corrosion of lower corrosion activity.