Severe corrosion recently documented in Florida post-tensioned bridges were related to grout segregation that created deficient grout with characteristics of having low chloride content, high sulfate concentration, high pore water pH, and high moisture content. The role of elevated sulfate concentrations in the development of steel corrosion in deficient grout has not been elucidated. As first approach to determine the corrosion mechanism of steel strand in deficient grout, the objective of the research, described here, was to evaluate the role of sulfate ions on the corrosion of steel in alkaline solutions. Steel coupons were exposed to two different alkaline solutions (SPS) simulating pore solutions with pH 12.6 (SPS1) or 13.3 (SPS2) with varying sodium sulfate levels (early fixed content and later increments). Electrochemical experiments included the steady-state condition and potentiodynamic polarization tests. Solution pH was found as an important parameter in the corrosion development of steel in sulfate SPS solutions. In the highly SPS2 solution (pH 13), sulfate ions did not impair passive film growth or stability. In the moderate SPS1 solution (pH 12.6), early fixed sulfate presence could be aggressive by impairing passive film development; however, even high levels of later increments of sulfates cannot depassivate steel. The instability of the passive film in alkaline sulfate solutions resulted in pitting corrosion of steel.

Corrosion failure of post-tensioning (PT) tendons with pre-packaged thixotropic grout has been documented in Florida bridges. Analysis of the deficient grout in Florida PT bridges, where severe corrosion developed, indicated elevated sulfate ion concentration, high pore water pH, low chloride ion content, and enhanced moisture content. Limited information is available on the corrosion behavior of PT strand in grout materials with elevated sulfate ion content. In this work, the corrosion performance of steel strands embedded in deficient grout was evaluated. The deficient grout was cast in large scale test assemblies that would represent geometry and material conditions found in Florida bridges, where PT strand corrosion occurred. Large scale mock up tendons (developed by the modified inclined tube [MIT] test) of about 4.57 m long were cast using expired grout materials, excess of 15% water, and sulfate ion concentrations to create deficiencies and corrosion conditions in the grout environment. Corrosion measurements such as corrosion potential (Ecorr) and corrosion current density (icorr), as well as potentiodynamic polarization tests were performed on the steel probes located along the MIT samples in contact with the deficient grout. Grout characterization parameters included the determination of the grout resistance (Rs), as well as the moisture and sulfate ion content. Also, visual inspection of the MIT samples (steel strand and steel corrosion probes) after a time was performed, as well as the corrosion products characterization. Testing confirmed that enhanced corrosion can occur in the deficient grout with high free sulfate ion accumulation.

Steel piles in a Florida bridge showed severe localised corrosion uncharacteristic of the general corrosion typically found in marine environments. Sampling from the water and the steel pile surface showed presence of bacteria associated with microbiologically influenced corrosion, MIC. Coincident heavy marine growth was observed. The interactions of marine macro- and microorganisms set up conditions where the severe corrosion occurred. The initiation of macrofouling require initial biofilm presence, but the subsequent fouling crevice environments would sequentially promote enhanced microbe growth (such as sulphate reducing bacteria, SRB) to support MIC. Antifouling and antibacterial coatings that have been developed to manage macrofouling may be applicable to prevent MIC. However, long-term durability of the mitigation coating technology can be compromised. The research objective was to identify the efficacy of biocides in an antifouling coating to mitigate microbial activity that are associated with fouling and MIC. Research included laboratory and field examination of a commercially-available antifouling coating in presence of SRB. It was confirmed that degradation of the coating would result in reduced antibacterial and antifouling efficacy and facilitation of SRB growth and marine fouling.

Severe localized corrosion of submerged steel bridge piles in Florida was associated with fouling and microbial-influenced-corrosion (MIC). The research in this study evaluated the efficacy of cathodic protection (CP) in those environments. Field tests were conducted in two natural rivers with different marine fouling morphologies. The test steel arrays were coupled to a zinc-sacrificial anode, and system potentials of approximately $-923\mathrm{mV}$ with respect to the saturated-calomel electrode (SCE) developed. Global CP currents to the steel exceeded $3\mathrm{\mu A}/{\mathrm{cm}}^2$ and generally reduced the overall corrosion rate. The CP current was related to cathode surface availability under fouling, and portions of the array did not receive sufficient cathodic polarization. Laboratory tests were conducted using specimens with different surface crevice morphologies that were cathodically polarized at $-850$ and $-950{\mathrm{mV}}_{\mathrm{SCE}}$ and immersed in solutions inoculated with sulfate-reducing bacteria. Tests confirmed that the presence of crevices reduce CP effectiveness. Fouling environments caused insufficient levels of CP and supported the growth of bacteria associated with MIC. CP alone may not be adequate to mitigate crevice corrosion and MIC under heavy fouling.