Characterization of Chloride Thresholds in Florida Coastal Concrete Bridge Substructures

Sea water induced reinforcing steel corrosion often results in high maintenance costs and can be service life limiting for concrete bridge substructure elements in marine environments. In the present research, a novel piling type specimen assembly and test protocol were developed to simulate performance of actual substructure elements undergoing marine exposure. Specimen mix design was based on the mortar component of a Florida Department of Transportation (FDOT) Class V high performance concrete, both with and without fly ash. The relatively low diffusion coefficient for such mixes and the long time that normally would be required for corrosion initiation was offset by employing covers of 12 mm for the non-fly ash mortar and 8 mm for the fly ash one. Exposures involved partial submergence in 15 wt% NaCl and in some cases periodic spraying of the above waterline zone to simulate splash. The rebar of some sprayed specimens was connected to submerged bare steel such that the lower portion of the simulated piling rebar was cathodically polarized, and it is demonstrated that the resultant potential profile was similar to that of actual marine pilings for which reinforcement below the waterline also exhibits a relatively negative potential because of oxygen concentration polarization. Consequently, the above waterline (splash) zone of the present specimens was cathodically polarized similar to what occurs in actual structures. Times to corrosion for the specimens were approximately the same for sprayed and unsprayed specimens but were greater for sprayed and polarized ones. In some cases, corrosion initiated above the waterline for sprayed and sprayed and polarized specimens, as typically occurs in actual bridge substructure elements. Threshold chloride ion concentrations to initiate corrosion, C sub T, were measured on the rebar mortar trace subsequent to corrosion initiation and specimen dissection using Energy Dispersive X-Ray Analysis and reported for various test conditions. Values for C sub T for corrosion initiation in both the submerged and above waterline zones are projected for both mix designs; however, these were based in some cases on limited data. It is recommended that the specimen design and test protocol investigated here be studied further.