Factors that Influence the Efficiency of Electrochemical Chloride Extraction During Corrosion Mitigation in Reinforced Concrete Structures

About the report:

Electrochemical chloride extraction (ECE) is an electrochemical bridge restoration method for mitigating corrosion in reinforced concrete structures. ECE does this by moving chlorides away from the reinforcement and out of the concrete while simultaneously increasing the alkalinity of the electrolyte near the reinforcing steel. Despite its proven success, ECE is not used extensively in part because of an incomplete understanding of the following three issues: (1) the time required for ECE with varying water-to-cement ratios (w/c) and cover depths; (2) the cause of the decrease in current flow and, therefore, chloride removal rate during ECE; (3) the additional service life that can be expected following ECE when the treated member is subjected to chlorides. This study addressed the first two issues. Plain carbon steel reinforcing bars were embedded in portland cement concrete slabs of varying w/c and cover depths and then exposed to sodium chloride solutions. A fraction of the slabs contained sodium chloride as an admixture. All slabs were subjected to cyclical ponding with a saturated solution of sodium chloride. ECE was then used to remove the chlorides from the slabs while electrical measurements were made in the different layers between the reinforcing bar (cathode) and the titanium mat (anode) to follow the progress of the ECE process. The resistance of the outer concrete surface layer increased during ECE, inevitably restricting current flow, and the resistance of the underlying concrete either decreased or remained constant. During ECE, a white residue, or surface film, formed on the surface of the concrete. The residue contained calcium carbonate, calcium chloride, and other yet unidentified minor components when calcium hydroxide was used as the electrolyte. The surface film can be removed mechanically or, to some extent, inhibited chemically. There was no obvious relationship among cover depth, w/c, and chloride extraction efficiency, although cover depth did influence the current density. The investigators recommend that the Virginia Department of Transportation's Structure & Bridge Division (1) require that contractors mechanically remove the latent surface layer of concrete prior to treatment using ECE and (2) discuss with corrosion consultants the potential for using a scale inhibitor during ECE to increase the efficiency of chloride removal. The benefits and costs assessment of treating a structure using ECE cannot currently be determined, but research currently underway will provide the necessary information for the assessment.

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