Materials Performance

NOV 2017

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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35 NACE INTERNATIONAL: VOL. 56, NO. 11 MATERIALS PERFORMANCE NOVEMBER 2017 Laboratory Experiments The electrochemical removal concept involves the migra- tion of chloride ions in concrete under the influence of an elec- trical potential gradient through the bridge deck concrete and into an electrolyte contained above it. The potential gradient is produced by applying a direct current source between the reinforcing steel and an electrode contained in the electrolyte above the bridge deck. An ion exchange resin contained in the electrolyte captures the chloride ions prior to their reaching the anode, thus preventing the evolution of chlorine gas and minimizing corrosion of the anode. Initial laboratory investigations were conducted to deter- mine: (1) identification of a suitable electrolyte, (2) identifica- tion of a suitable anode material, and (3) identification of a suitable ion exchange resin. Studies showed that calcium hydroxide solution (0.1 N) was a suitable surface electrolyte, being noncorrosive and innocuous to concrete. The require- ments of an anode material are good corrosion resistance, and good electrocatalytic activity. These parameters were studied for candidate materials by conducting potentiodynamic sweeps in 0.1 N calcium hydroxide solution with and without additions of NaCl. Results are shown in Figure 1 for titanium, graphite, and platinized titanium. While titanium shows excel- lent corrosion resistance, its electrocatalytic activity is poor. Graphite showed good electrocatalytic activity but tended to disintegrate at high current densities. Platinized titanium per- formed satisfactorily in all categories and was thus selected as the anode material for extraction procedures. Two candidate ion exchange materials were investigated: (1) Dowex 1-X8 and (2) Dowex 2-X8. The selection was made on the best selectivity: regeneration ease ratio, and on this basis Dowex 2-X8 was chosen to capture the chloride as it emerged from the concrete. Following this work, effort was directed to a study of the effect of concrete and process variables on the electrochemi- cal removal of chloride. The variables studied included: (1) The magnitude of the applied electrical potential gradient; (2) the duration of treatment; and (3) the initial chloride content of the concrete. Pre- and post-treatment measurements of chloride content at various depths in the concrete were made using techniques described by Berman. 1 This phase of the program involved studies on 7.5 x 15 cm (3 x 6 inch) concrete cylinders and 117 x 152 x 23 cm (46 x 60 x 9 inch) concrete slabs. The specimens were prepared with con- cretes to which preselected quantities of chloride had been added to the mix water. Experiments on the small cylinders showed that the amount of chloride removed for a given treat- ment time increased with applied dc voltage. At 100 volts dc (the maximum voltage used in the program), treatment times of 16 to 48 hours resulted in significant reductions in the chlo- ride content, depending on the initial chloride level and distri- bution. Treatment at 50 volts was not sufficient to provide sig- nificant levels of chloride removal within a reasonable time. The treatment time necessary for chloride removal increased as the initial chloride content of the concrete was increased. Substantial reductions in total chloride, however, were achieved in a high chloride content concrete, [0.18% (7 lb/yd 3 )] in 24 hours. Although the efficiency of the chloride removal technique was quite low, it was possible to reduce the chloride content of the concrete to below 0.020%, which is now considered a threshold value for the corrosion of the reinforcing steel. Attempts to improve the efficiency of the treatment through changes in composition of the electrolyte solution were not successful. The temperature of the concrete (laboratory specimens) during electromigration treatment at 100 volts increased from 24 to about 52 C (75 to about 125 F). The elevated temperature exposure had no obvious adverse affect on the integrity of the concrete. For the laboratory specimens, there appeared to be a threshold value of chloride which was the limit of removal. This residual chloride content was 0.02% or about 0.8 lb chlo- ride/yd 3 of concrete. A portion or all of this residual chloride is present in an insoluble form and hence will not be amenable to easy removal by the electro chemical technique. The positive results obtained in the initial experiments prompted the decision to continue the electrochemical removal investigations on the large simulated bridge deck slabs. The experimental arrangement used is shown in Figure 2. The slabs were constructed in several lifts, with concrete containing chloride (added as NaCl to the mix water) above the top rebar mat. Concrete composition and rebar placement and size was the same as in the actual bridge deck selected for study in the program. Electrical power for the large slab work was supplied by a 5kW portable generator. Voltage was controlled by a variable trans former, and a full wave rectifier was used to convert ac to dc. Electrical connections were made to the reinforcing steel in the concrete and to a 61 x 76 cm (24 x 30 inch) platinized tita- nium electrode (in expanded metal form) on the upper surface of the slab. A wooden dike, sealed with silicone sealant, served to contain the ion exchange resin and the electrolyte solution (0.1 N calcium hydroxide solution). Electrochemical Removal of Chlorides From Concrete Bridge Decks FIGURE 2 — Experimental setup for the electrochemical removal of chloride ion from 4 x 5 foot simulated bridge deck slabs.

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