Abstract
The use of sacrificial anodes consisting of sprayed zinc on the surface of concrete containing corroding reinforcing steel bars has been examined by means of field tests and laboratory experiments. The anodes are sprayed directly on the external surface of spalled regions of marine substructure elements. Electrical contact between the steel and zinc can be achieved efficiently by directly spraying over exposed steel. Field tests were made in the substructure of bridges in the Florida Keys and in Tampa Bay. The installations were performed economically and were functional after as much as 5 years following placement. Protective current densities reaching 1 mA/ft2 (1 µA/cm2) of the reinforcing steel were achieved routinely. Tests with reinforcing steel probes and with entire reinforcement assemblies showed steel polarization decays (upon temporary disconnection) typically exceeding 100 mV even after several years of service. The laboratory tests revealed that in the marine substructure conditions of interest concrete resistivity does not represent a main limiting factor in the performance of the galvanic anodes. However, absence of direct wetting of the anode surface can result in long-term loss of adequate current delivery, even when the concrete is in contact with air of 85% relative humidity. Periodic water contact (as caused by seawater mist, splash or weather exposure) is considered a necessary factor for long-term anode performance. The method is considered as a competitive alternative to impressed-current cathodic protection systems, and a considerable improvement over simple gunite repair of corrosion damaged substructure concrete.
