Reinforced concrete bridge substructures in Florida coastal waters have historically experienced deterioration as a consequence of embedded steel corrosion followed by concrete cracking and spalling. Galvanic anode cathodic protection (CP), as affected by thermally sprayed zinc for cast-in-place components and zinc mesh jackets for precast ones, has been employed to control this corrosion and extend useful service life. In either case, the CP systems include a submerged bulk zinc anode (SBA) to reduce current drain from the lower portion of the thermal spray or jacket components. To investigate the extent of any contribution of SBAs in protecting the above-waterline zone, two substructures were instrumented with SBAs only and monitored. In addition, the substructures and SBAs were modeled using Boundary Element Analysis, the results compared with the field measurements, and the utility of cp using SBAs alone projected.
Cathodic protection (CP) has been recognized for some 25-plus years as the only method with long-term service experience whereby active corrosion of reinforcing steel in chloride contaminated concrete can be controlled. For the most part, such CP has been of the impressed current (IC), as opposed to galvanic anode (GA), type because of the relatively high resistivity of concrete and, consequently, the high driving voltage necessary for adequate cathode current density and polarization. Even with ICCP, anodes must normally be distributed as "throwing power;" that is, the distance beyond an anode to which current can be projected, is limited. However, ICCP has shortcomings, which include the following:
Electrical power is required.
Systems are susceptible to rectifier failure.
Excessive polarization may cause hydrogen embrittlement and brittle fracture in the case of prestressed components.
System reliability is such that periodic monitoring by trained personnel is required.
These factors either do not apply or are of less significance in the case of GACP systems; however, the fact that driving voltage for these is limited to the native potential difference between anode and cathode can result in underprotection, particularly for high concrete resistivity applications.1,2 Also important are, first, a tendency for resistive corrosion products and for moisture depletion to occur at the anode-concrete interface and, second, limited anode service life. Specific anode types that have been investigated include 1) thermally sprayed Zn with or without a humectant,1,2,3,4,5,6,7 2) thermally sprayed Al-Zn-In alloy,8 3) Zn strip with hydrogel adhesive,9 4) surface mounted penny and expanded Zn sheet,4,10,11 5) Zn mesh with fiberglass jacket,12 6) Zn anode in chemically enhanced mortar.13
While there have been studies and applications of GACP for bridge decks,14 the greater utility has been in marine substructure applications. Here, corrosion and concrete cracking and spalling are most advanced in the splash zone where the concrete remains wet and resistivity relatively low. In this regard, the various research and development efforts pertaining to the six options listed above have at this time converged to the system of choice for cast-in-place substructure components being thermally sprayed Zn and for prestressed pilings Zn mesh fiberglass jackets.
