Incipient anodes in reinforced concrete repairs: A cause or a consequence?

Patch repairs are a common repair technique for corrosion-damaged reinforced concrete structures. However, this repair method is sometimes associated with limited durability and in many cases further corrosion damage has been noted around the repaired patches, a phenomenon known as the “incipient anode” effect. The diagnosis of this problem is widely reported to be macrocell activity. It is deemed that the cause of incipient anodes is the loss of the natural cathodic protection provided by the corroding steel to the steel in the parent concrete adjacent to the patch repair. This diagnosis is based on very limited data. Indeed potential measurements on field structures repaired with proprietary materials have provided data that suggest that macrocell activity is not a cause of incipient anode formation but it is a consequence. Alternative mechanisms that may cause incipient anode activity include repair/parent material interface effects, residual chloride contamination within the parent concrete, and/or vibration damage to the steel/parent concrete interface during repair area preparation. The aim of the work presented here was to assess the impact of macrocell activity on the formation of incipient anodes around the perimeter of repairs in patch-repaired reinforced concrete structures. This was examined based on a major multi-storey car park and a bridge structure both located in the UK. The analysis challenges the view that macrocell activity is a cause of incipient anode formation. Indeed this work shows that the data supporting the existing diagnosis is not convincing and suggests that macrocell activity is primarily a consequence of incipient anode formation and the cause probably, results from other factors.