Protection of Reinforced Concrete Bridge Substructures Using Submerged Bulk Anodes

Reinforced concrete bridge substructures in Florida coastal waters have historically experienced deterioration as a consequence of embedded steel corrosion and resultant concrete cracking and spalling. Ultimately, this deterioration leads to added maintenance costs, reduced service life, and unsafe conditions unless intervention measures are instituted. Galvanic anode cathodic protection (CP), as affected by thermally sprayed zinc for cast-in-place substructure components and zinc mesh jackets for precast ones, is being employed to control this corrosion and extend useful service life. For both types of systems, a submerged bulk zinc anode (SBA) is included to polarize the reinforcement below the waterline and thereby reduce current drain from the lower portion of the thermal spray or zinc mesh. The objective of this research was to determine the extent to which SBAs alone provide protection to the above waterline region of bridge substructures. If adequate or even partial protection is afforded by this means, then considerable cost savings could be realized because of the reduced expense and ease of installation of SBAs compared to zinc thermal spray and CP jackets. To investigate this, two substructure piers on the Bahia Honda Bridge and two on the Niles Channel Bridge, both in the Florida Keys, that were comprised of cast-in-place footers and columns were instrumented with SBAs alone and monitored to determine the level of protection that resulted. For the former bridge, zinc was the SBA type and for the latter magnesium. It was determined that the entire footer reinforcement received protection and that polarization extended up the columns to a certain degree, more so for magnesium than zinc anodes. In addition, the substructures and resultant polarization from the SBAs were modeled using Boundary Element Analysis and two numerical methods. The model results were compared with the field measurements, and the two data sets were determined to be in general agreement. It was concluded that SBAs have utility for extending the useful service life of reinforced concrete bridge substructure elements.