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.

Issue link: http://mp.epubxp.com/i/889936

Contents of this Issue

Navigation

Page 41 of 84

39 NACE INTERNATIONAL: VOL. 56, NO. 11 MATERIALS PERFORMANCE NOVEMBER 2017 returned to the laboratory for regeneration. No major problems were experienced during the experi- ment. No cracks in the concrete occurred as a result of the treatment as evidenced by a careful post-treatment visual examination. Post-Treatment Measurements The results of the electrochemical treatment were moni- tored in terms of post-treatment measurements of chloride content, electro chemical potential measurements, and LP elec- trode measurements. Chloride Analyses. Post-treatment cores [75 mm (3 inch diameter)] were taken from the treated area as identified in Figure 4. (Cores 12, 13, 15, 16, 17, 19, 20, 21, 22, 24, 25, and 26.) Cores 16 and 20 were taken 127 mm (5 inches) deep. All other cores were taken mostly to the level of the top reinforcing steel (about 2 inches). Most of the cores were then sectioned into 2.5 cm (1 inch) thick slices, although Cores 15, 16, and 17 were sectioned in 12 mm (1/2 inch) slices to permit a better discrimination of the chloride concentration profile. In several cases, the concrete immediately above the rein- forcing rod [about 6 mm (1/4 inch) radius] was analyzed separately. Table 2 presents a summary of the pre- and post-treatment chloride concentrations. Although there was some variation in measured chloride from specimen to specimen, a reasonable assessment of effect of treatment parameters on chloride removal could be made. 0.0 to 1.0 Inch Level. The average pretreatment chloride content in the top inch of the treated area of the deck was 0.41% (15.5 lbs/yd 3 ) as measured on Cores 7, 8, 9, and 10. Using this value as the initial chloride content, the reduction in chlo- ride effected by the treatments is shown in Table 3 for the vari- ous treatment areas. A general correlation existed between maximum current, charge passed, and the amount of chloride removed. Certain treatment areas did not conform to this generalization (Areas 2A and 2B). The average chloride removed in the top 2.5 cm (1.0 inch) was 31% in 12 hours and 51% in 24 hours. 1.0 to 2.0 Inch Level. The average chloride content in this depth of concrete in the treated area was 0.20% (7.6 lbs/yd 3 ) as measured in Cores 7 and 9. The reduction in chloride in the various test areas is shown in Table 4. The average chloride removed in 12 hours was 59% while 70% was removed in 24 hours. A comparison of Tables 3 and 4 shows that a significantly greater percentage of chloride was removed at the 1 to 2 inch level than at the 0 to 1 inch level. As for the top inch, there was a general correlation between maxi- mum current and charge passed and the amount of chloride removed. Concrete Adjacent to Reinforcing Steel. The pretreatment chloride contents were obtained on Cores 2 and 4 taken from the berm section. The post-treatment data were obtained on the remnants of Cores 26 (Area 1 A), 25 (Area 2A), 22 (Area 4A), and 21 (Area 5A). These results are shown in Table 5. The average reduction in chloride content in the concrete immedi- ately adjacent to the reinforcing steel for a 24 hour treatment was 73%. Significantly, the post-treatment cores were from some areas which did not show exceptionally high chloride extraction at the 0 to 1 and 1 to 2 inch levels of concrete (Tables 3 and 4). Potential Scans. Potential scans were taken on the deck 24 hours after the final section was treated, and then 1 week, 1 month, and 3 months after treatment. The scan after 24 hours (Figure 7) includes numerical val- ues of the potential to show the effects of treatment on the potential. The Roman Numerals show chronological order of treatment. The first section treated (5 days prior to scan) exhib- its passive potentials. This is in direct contrast to the active values shown by this section prior to treatment. The second section treated shows a range of potentials from extremely active to moderately passive. This indicates that the extreme cathodic polarization applied to the steel during treatments can take days to decay. Sections III and IV show similar effects. The last section treated, Section V, shows potentials which were all well within the cathodic protection range (more active than -0.85 volt) for steel. The scan obtained 1 week after completion of treatment is shown in Figure 8. The obvious difference is the decay of the cathodic polarization in Sections II, IV, and V, and the mainte- nance of passive potentials in these areas. The exception to this is the band of polarization along the midsection of treat- ment Sections II and IV. The results from the 1 month scan are shown in Figure 9. At this time, the vast majority of the deck exhibited a potential well within the passive range, with no readings on the treated surface being more active than -0.30 volt. The most passive potentials were exhibited by steel in treatment Sections II and V. The 3 month scan (Figure 10) shows that all potentials within the treated area were within this passive range. A check of potentials outside the treated area, however, revealed that acti ve corrosion of the steel was continuing, possibly enhanced by the large adjacent passive area generated by the electrochemical treatment. Linear Polarization Readings. Corrosion rates of the rein- forcing steel as deduced from linear polarization electrodes were measured prior to 1 week, 1 month, and 3 months after treatment. The results are shown in Table 6. Thus, 1 week after treatment, the linear polarization elec- trodes were showing a corrosion rate almost twice as high as before treatment. However, 1 month after treatment, the cor- rosion rate had dropped to that observed before treatment. The reasons for this unexpected behavior may be due to the destruction of the passive oxide film on the rebar by the cathodic treatment, and its subsequent growth following treatment. Discussion In principle of using electromigration methods to extract chloride ion from salt containing reinforced concrete has been shown effective in laboratory experiments, and on a bridge deck in the field. For the concrete situated immediately [6 mm (1/4 inch)] above the reinforcing steel, an average of 31% of the chloride was removed after 12 hours, and 51% after 24 hours of treatment. For concrete at the 1 to 2 inch level, these values were 59% and 71%. Peak extraction values of better than 90% were obtained. Since the chloride content of the concrete immediately adjacent to the reinforcing steel is the determining factor in the corrosion initiation and continuation, the reduction in chlo- ride level at this location to below 0.02% is extremely signifi- cant. The literature suggests that this is close to the lower limit for initiation of corrosion. The potential scan before the treat- ment showed more than 55% of the treatment area rebar was actively corroding. Post-treatment scans (up to 3 months after treatment) showed that all active corrosion had ceased, and Electrochemical Removal of Chlorides From Concrete Bridge Decks

Articles in this issue

Archives of this issue

view archives of Materials Performance - NOV 2017