Unexpected effects on the corrosion behaviour of a local concrete repair in a marine environment

The case study is about the repair of a bridge column in the Baltic Sea which had been constructed with low concrete cover in some areas and also exhibited signs of ASR. Therefore, a repair mortar had been chosen which is designed for use both on structures suffering ASR and a marine environment. During a test repair on one of the damage-/breakout areas the possibility of incipient anode formation had to be in mind, because the original concrete had a generally increased chloride content. Initial potential surveys on and around the repair spot ca. 3 weeks after the concrete placement/ spraying showed no changes in the original concrete close to the repair area, and the potential values in the repair area were about 200 mV more positive. Enhanced corrosion measurements resulted in a sound passive behaviour of the rebar in the original concrete. Very unexpectedly, the reinforcement embedded in the repair mortar was in an active state at low polarisation and electrolyte resistances – despite the more positive potentials, which dropped by ca. 100 mV after a minute of wetting the surface. Dust samples of the repair mortar were analysed for pH value, which were about 12.3 and possibly not sufficiently basic to establish passivity on the reinforcement which had a slight, covering layer of corrosion products from the recent air exposure over the whole area. It was agreed to thoroughly clean the rebar in other repair spots before spraying the repair mortar there and to watch for the corrosion behaviour on all repair spots. Apparently, the passivation of the reinforcement in the repaired areas is retarded at the observed low pH values and can take several week or months. This seems to be beneficial for avoiding the formation of incipient anodes around the repair area. At the time of writing the manuscript final measurements to prove this are scheduled.