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Publication Number: FHWA-HRT-10-069
Date: September 2011

 

Long-Term Effects of Electrochemical Chloride Extraction on Laboratory Specimens and Concrete Bridge Components

APPENDIX A. STATE-OF-THE-ART REPORT

ECE History and Current Status

ECE is a process where chloride ions are extracted from chloride-contaminated reinforced concrete structures by applying an electrical current between the embedded steel and an external anode. The process was originally referred to as ECR. However, due to confusion with the more popular abbreviation for epoxy coated rebar, ECE was adopted, particularly in North America. In Europe, the process is often referred to as desalination or sometimes as ECM. ECE is becoming popular as a rehabilitation option for chloride contaminated reinforced concrete structures to stop/mitigate ongoing corrosion of embedded steel.

The technique of removing chloride ions from contaminated reinforced concrete by electrochemical means was first studied in the mid 1970s by KDOT and Battelle Columbus Laboratories.(1,2) Both studies were funded by FHWA. Some adverse effects were observed in these studies possibly because of the high current densities used during treatment. These included some loss in rebar-to-concrete bonding, increased permeability in the concrete, and some uncertainties in the effects on the concrete of the migration of ions other than chloride and in the rate of remigration of the residual chloride in the concrete after the treatment. Consequently, further research on ECE was not pursued in the United States until the mid to late 1980s.

From 1988 to 1993, SHRP sponsored additional research on the ECE process. There were extensive laboratory investigations, and limited field trials were also conducted.(3,4) Independent studies in Europe during the mid 1980s resulted in a Norwegian patent for ECE in 1985 known as the Norcure process, which became commercially available in 1988.(5) A patent on some ECE process parameters was also granted in the United Kingdom in 1994.(6)

SHRP Laboratory Work

Laboratory work under the SHRP contract addressed several concerns that can arise as a result of the passage of large amounts of current through concrete. Reinforcing steel-concrete bond strength was measured over the full range of current and charge experienced for both chloride removal and cathodic protection. The application of a high current density (464 mA/ft2 (5,000 A/m2)) and/or a high amount of total charge (186 A-h/ft2 (2,000 A-h/m2)) resulted in a reduction of bond strength when compared to controls containing salt. However, the use of either lower current density or lower charge had no adverse effect. Even at the highest current density and charge, bond strength was reduced only to the values equal to those of no-salt control specimens.

Concrete compressive strength was not reduced at lower current densities, but concrete treated at high current (1.9 A/ft2 (20 A/m2)) for long periods of time (464 A-h/ft2 (5,000 A-h/m2)) experienced a softening of the cement paste around the reinforcing steel. This softening is probably also responsible for the loss of bond strength of severely treated specimens. This strong treatment also caused one slab to crack and delaminate. As a result, the current regime used in previous studies (up to 19 A/ft2 (200 A/m2)) was judged to be excessive, and more modest treatment conditions were used for field trials.

The possible hydrogen embrittlement of conventional reinforcing steel was also studied. Although a slight temporary loss of ductility was noted on smooth specimens, this loss was not structurally significant. The generation of chlorine gas from the anode, which could present a safety hazard, was also studied. It was decided that the electrolyte should be maintained at a basic pH to prevent the generation of chlorine gas. Several buffers were studied for this purpose, and a sodium borate buffer was found to be the most effective and practical. Control of the electrolyte pH in this way also prevented any etching or acid attack of the concrete surface. Other studies have shown that this electrochemical treatment of concrete causes an increase in the alkali cation concentration in the vicinity of the reinforcing steel. This study confirmed these results and demonstrated that serious damage could result if the chloride removal process was used on concrete containing alkali-reactive aggregate. It was also found that the presence of lithium ion in the electrolyte could be used to mitigate this problem. Where alkali-sensitive aggregate is present, the use of lithium borate buffer is recommended.

Based on the laboratory and test yard results, a guideline was developed, and a chloride removal treatment process was defined, resulting in the effective removal of chloride without any damage to the concrete or reinforcing steel.(7) Treatment current density was limited to less than 0.5 A/ft2 (5 A/m2) of concrete. System voltage was also limited by the Occupational Safety and Health Administration to less than 50 V for safety reasons. Under these conditions, treatment time for chloride removal can be expected to be 2 to 4 weeks. Typical applied charge will be 74 to 112 A-h/ft2 (800 to 1,200 A-h/m2). Treatment times and charges greater than these will probably yield little additional benefit in terms of chloride removed or corrosion prevented.

The treatment process described above will remove about 20 to 50 percent of the chloride present in the concrete. The amount of chloride removed depends on several factors including the amount of chloride present, the distribution of chloride in the concrete, and the reinforcing steel design. Typically, after treatment is complete, sufficient chlorides remain in the structure to reinitiate corrosion. However, the remaining chloride is usually distributed away from the steel, and sufficient time is required for redistribution to take place. The return to corrosive conditions is further delayed by the buildup of alkalinity that occurs at the steel surface.

Corrosion may also be caused by factors other than chloride such as carbonation of the concrete cover due to reaction with atmospheric carbon dioxide or damage due to other acidic gases. The chloride extraction process is not designed for such cases. However, there is a similar patented process called realkalization that utilizes sodium carbonate as the electrolyte.(8) In such an application, the objective is to restore the alkalinity of the concrete surrounding the reinforcing

SHRP Field Studies

Four field validation trials were conducted between fall 1991 and fall 1992. Chloride removal was conducted on an Ohio bridge deck and on bridge substructures in Florida, New York, and Ontario. Based on the laboratory and test yard studies, a current less than 0.5 A/ft2 (5 A/m2) and a voltage less than 50 V was used. The treatment was applied until a total charge of 56 to 125 A-h/ft2 (600 to 1,350 A-h/m2) of concrete was accumulated. The pH of the electrolyte was maintained neutral or basic to prevent etching of the concrete surface and the evolution of chlorine gas. Each field site was selected based on criteria established by the laboratory studies and the SHRP ETG. Active corrosion was occurring on a substantial portion of each selected structure, and chloride contamination was well above the threshold levels. The absence of alkali reactive aggregate was a factor until the final trial when a structure with alkali reactive aggregate was selected as a test case.

The first field trial was conducted on an Ohio bridge deck in fall 1991. No physical deterioration of the deck was evident. The treatment was conducted by constructing a pond on the deck and placing an inert catalyzed titanium anode in the pond with a sodium borate buffer electrolyte. Current density for this trial was low because of cold temperatures and resistive concrete; therefore, the treatment time was 61 days. Chloride analyses of the ponded electrolyte indicated that about 0.36 oz/ft2 (110 g/m2) of chloride of concrete was removed during the treatment at a current efficiency of about 20 percent. Problems encountered included vandalism and overflow of the pond due to excessive rainfall.

The second field trial was conducted in spring 1992 on pilings underneath the B.B. McCormick Bridge near Jacksonville, FL. Prefabricated anode/blanket composites were strapped on each pile, and seawater was used as the electrolyte. The system operated for 18 days at an average current density of 0.31 A/ft2 (3.3 A/m2) and accumulated a total charge of 125 A-h/ft2 (1,350 A-h/m2). The success of the trial was difficult to judge since the electrolyte could not be analyzed for an increase in chloride concentration. Although steel in the treatment area was strongly polarized and chloride was removed, the efficiency and amount of chloride removed could not be determined.

The third field trial was conducted in June 1992 on the substructure of a bridge in Albany, NY. Prefabricated anode/blanket composites were strapped on the columns, and a sodium borate electrolyte was continuously circulated through a closed system. Localized high current densities were reported, which was probably a result of the inhomogeneous nature of the structure due to patching. Current densities ranged from 0.09 to 0.3 A/ft2 (1 to 3 A/m2), and two zones accumulated 74 and 86 A-h/ft2 (800 and 930 A-h/m2) in 17 and 24 days, respectively. Based on concrete analyses, chloride removed was 0.29 to 0.49 oz/ft2 (90 to 150 g/m2) at a current efficiency of 7 to 13 percent. Problems at this site included difficulty in preventing electrolyte leakage through small vertical cracks that extended below the removal zone on two columns, and electrolyte dilutions due to rainwater runoff.

The last chloride removal field trial was conducted in August 1992 on abutments of a bridge over the Montreal River in Latchford, Ontario. This trial was especially important since the structure contained alkali reactive aggregate and was suffering from a combination of ASR and corrosion-induced damage. The laboratory studies found that unless lithium was in the electrolyte, ASR damage would be aggravated by the chloride removal process. Consequently, a lithium borate buffered solution was used. An anode/blanket composite was installed on each abutment corner, and electrolytes were continuously circulated through the system. The system operated for 23 days at an average current density of 0.15 A/ft2 (1.6 A/m2), accumulating 78 and 83 A-h/ft2 (840 and 890 A-h/m2) of charge in two zones. Post-treatment analyses showed that 0.74 and 0.49 oz (21 and 14 g) of chloride per square foot had been removed from the two zones at current efficiencies of 19 and 12 percent, respectively. Petrographic analysis of the concrete showed that the treatment did not aggravate the alkali-silica reaction occurring in the structure. The main problem was electrolyte dilutions due to excessive rainwater runoff.

The SHRP study identified the following required and desired criteria for ECE:

Required criteria:

Desired criteria:

A candidate structure for ECE should undergo active corrosion caused by the presence of chloride ions at the level of the steel reinforcement. Although it is possible to conduct chloride extraction before corrosion begins, the benefit of such treatment is questionable.

Subsequent to SHRP effort, research and field work on ECE has continued in North America and Europe to address some of the unresolved issues as well as to develop and refine criteria for commercial application of the ECE process. (See references 9–16.)

Commercial Application of ECE

The Norcure System, which includes treatment processes for ECE and realkalization, was acquired by Fosroc International 1994 and is the only commercial process available in Europe and North America. It is operated through licensed applicators. American Concrete Technologies in Boston, MA, also owns licensing rights for the Norcure System but is in the process of being relinquished.

Since the late 1980s and early 1990s, hundreds of structures worldwide have been treated with ECE and realkalization. Table 9 provides a list of the ECE projects completed to date in the United States and Canada, and table 10 summarizes the worldwide ECE and realkalization projects from 1987 to 1999. Table 10 does not contain a complete listing of all projects completed to date; however, the information is accurate up to 1996. It is estimated that the total concrete surface treated by ECE and realkalization at the present time exceeds 4 million ft2 (372,000 m2).

Table 9. ECE Projects completed in the United States and Canada.
No. Location Structure Component Year Area Treated
m2 ft2
Total 12,314 132,505
1 Burlington, Ontario, Canada Burlington Bay Skyway Pillar 1989 Demo Demo
2 Winnipeg, Manitoba, Canada Lakeview parking garage Slab 1990 Demo Demo
3 Toronto, Ontario, Canada Credit view overpass Columns 1990 Demo Demo
4 Winnipeg, Manitoba, Canada Bridge underpass Wall 1990 30 323
5 Winnipeg, Manitoba, Canada Rt-90 underpass Wall 1991 Demo Demo
6 Lucas County, OH Neapolis/Waterville Road Deck 1991 61 660
7 Albany, NY Hawkins Street over NY-85 Columns 1992 57 616
8 New York, NY Watch tower Columns 1993 300 3,228
9 Ottawa, Ontario, Canada Bell Canada parking garage Slab 1993 Demo Demo
10 Saskatoon, Saskatchewan, Canada Hwy-16/Hwy-11 overpass Columns 1994 150 1,614
11 Regina, Saskatchewan, Canada Hwy-1/Hwy-6 overpass Columns 1995 370 3,981
12 Morinville, Alberta, Canada Grandin Avenue overpass Columns 1995 55 592
13 Regina, Saskatchewan, Canada Hwy-6/Hwy-11 overpass Columns 1995 180 1,937
14 Arlington, VA 34th Street over 1-395 bridge Deck 1995 733 7,887
15 Charlottesville, VA Rt-631 over I-64 bridge Columns 1995 488 5,251
16 Sioux City, SD 1-29 overpass Columns 1995 150 1,614
17 Dover, DE Bridge deck Deck 1997 1,550 16,678
18 Burlington, Ontario, Canada Burlington Bay Skyway Columns 1997 268 2,884
19 Toronto, Ontario, Canada Bridge substructure Chambers 1997 180 1,937
20 Syracuse, NY Parking garage Slab 1997 100 1,076
21 Minneapolis, MN Bridge substructure Piers 1997 225 2,421
22 Winnipeg, Manitoba, Canada Bridge deck Deck 1997 270 2,905
23 Peoria, IL Bridge substructure Piers 1998 462 4,971
24 Omaha, NE Bridge substructure Piers 1998 1,525 16,409
25 Council Bluffs, LA Bridge substructure Piers 1998 463 4,982
26 Winnipeg, Manitoba, Canada Bridge deck Piers 1998 220 2,367
27 St. Adolphe, Manitoba, Canada Bridge deck Deck 1998 1,115 11,997
28 Burlington, Ontario, Canada Burlington Bay Skyway Piers 1999 1,533 16,495
29 Omaha, NE Bridge substructure Piers 1999 1,400 15,064
30 Jackson, MI Bridge substructure Piers 1999 109 1,173
31 Minot, ND Bridge substructure Piers 1999 100 1,076
32 Washington, DC Bridge substructure Piers 1999 220 2,367

 

Table 10. Summary of worldwide ECE and realkalization projects.
Year ECE Projects Realkalization Projects Combined ECE/Realkalization Projects
Number of Projects Total Area Treated Number of Projects Total Area Treated Number of Projects Total Area Treated
m2 ft2 m2 ft2 m2 ft2
Total 132 46,733 502,201 247 204,223 204,223 379 249,414 2,683,049
1987 0 0 0 1 400 4,304 1 400 4,304
1988 4 90 968 8 1,560 16,786 12 1,650 17,754
1989 18 1,165 11,890 9 1,161 12,492 27 4,826 51,282
1990 7 2,550 27,438 7 20,542 221,032 14 23,092 248,470
1991 2 300 3,228 23 19,647 211,402 25 19,947 214,630
1992 7 1,040 11,190 20 23,719 255,216 27 24,759 266,407
1993 11 2,076 22,338 37 30,239 325,372 48 32,315 347,709
1994 17 8,740 94,042 41 26,642 286,668 58 34,182 367,789
1995 26 9,938 106,933 46 36,452 392,224 72 46,390 499,156
1996 24 11,094 119,371 53 43,463 467,662 77 51,715 556,453
1997 6 2,593 27,901 0 0 0 6 2,593 27,901
1998 5 3,785 40,727 1 110 1,184 6 3,895 41,910
1999 5 3,362 36,175 1 288 3,099 6 3,650 39,274

ECE Specifications

Currently, there are no specifications for ECE either in the United States or Europe. The projects that have been completed in North America have followed guidelines developed by SHRP with additional criteria introduced by the client organization. However, the ECE ETG set up by FHWA has been discussing the issue of producing a specification since 1995 and is on the verge of producing a state-of-the-art report which will eventually be converted to specifications. The National Association of Corrosion Engineers has also produced a draft state-of-the-art report on ECE.(17) Similarly, the European Corrosion Society is working on ECE specifications as part of their activities for producing standards for electrochemical treatments for reinforced concrete structures.(18)

References

  1. Morrison, G.L., Virmani, Y.P., Stratton, F.W., and Gilliland, W.J. (1976) Chloride Removal and Monomer Impregnation of Bridge Deck Concrete by Electro-Osmosis, Report No. FHWA-KS-RD 74-1, Kansas Department of Transportation, Topeka, KS.
  2. Slater, J.E., Lankard, D.R., and Moreland, P.J. (1976). “Electrochemical Removal of Chlorides from Concrete Bridge Decks,” Transportation Research Record 604, 6–15, Transportation Research Board, Washington, DC.
  3. Bennett, J.E., Thomas, T.J., Clear, K.C., Lankard, D.L., Hartt, W.H., and Swiat, W.J. (1993). Electrochemical Chloride Removal and Protection of Concrete Bridge Components: Laboratory Studies, Report No. SHRP-S-657, Strategic Highway Research Program, Washington, DC.
  4. Bennett, J.E., Fong, K.F., and Shue, T.J. (1993) Electrochemical Chloride Removal and Protection of Concrete Bridge Components: Field Trials, Report No. SHRP-S-669, Strategic Highway Research Program, Washington, DC.
  5. (1985). Norwegian Patent No. 156729.
  6. Miller, J.B. (1994). Electrochemical Treatment of Reinforced Concrete According to Accumulated Current Flow Per Unit Area of Steel Reinforcement, UK Patent Number GB2277099.
  7. Bennett, J.E. and Shue, T.J. (1993). Chloride Removal Implementation Guide, Report No. SHRP-S-347, Strategic Highway Research Program, Washington, DC.
  8. (1996). U.S. Patent No. 4865702.
  9. Mietz, J. and Isecke, B. (1993). “Rehabilitation of Corrosion Induced Damage of Reinforced Concrete Structures by Electrochemical Protection Methods,” Progress in the Understanding and Prevention of Corrosion, 1, 611–618, Institute of Materials, Barcelona, Spain.
  10. Hope, B., Ihekwaba, N.M., and Hansson, C.M. (1995). “Influence of Multiple Rebar Mats on Electrochemical Removal of Chloride From Concrete,” Materials Science Forum, 1992–1994(2), 883–890, Switzerland.
  11. Ihekwaba, N.M., Hope, B.B., and Hansson, C.M. (1996). “Pull-Out and Bond Degradation of Steel Rebars in ECE Concrete,” Cement and Concrete Research, 26(2), 267–282.
  12. Whitmore, D.W. (1996). Electrochemical Chloride Extraction from Concrete Bridge Elements: Some Case Studies, Paper No. 299, presented at CORROSION/96, Denver, CO.
  13. Elsener, B., Molina, M., and Bohni, H. (1993). “The Electrochemical Removal of Chlorides from Reinforced Concrete,” Corrosion Science, 35(5–8), 1563–1570.
  14. Elsener, B., Zimmermann, L., Burchler, D., and Bohni, H.D. (1997). Repair of Reinforced Concrete Structures by Electrochemical Techniques—Field Experience, Proceedings from EUROCORR '97, Norwegian University of Science and Technology, 1, 517–522, Trondheim, Norway.
  15. Clemena, C.G. and Jackson, D.R. (1996). Pilot Applications of Electrochemical Chloride Extraction on Concrete Bridge Decks in Virginia, Report No. VTRC 96-IR3, Virginia Transportation Research Council, Charlottesville, VA.
  16. Clemena, C.G. and Jackson, D.R. (1996). Pilot Applications of Electrochemical Chloride Extraction on Concrete Bridge Piers in Virginia, Report No. VTRC 96-IR4, Virginia Transportation Research Council, Charlottesville, VA.
  17. National Association of Corrosion Engineers (NACE). (1998). Rapid Electrochemical Treatment of Steel in Concrete—Part I: Electrochemical Chloride Extraction, Group Committee T-11, NACE International, Houston, TX.
  18. Wyatt, B.S. (1996). “European Standards for Electrochemical Treatment of Steel in Concrete—In an International Context,” Corrosion Standards II: National, European and International Standards, 1990–1995, 135–137, Institute of Materials, London, UK.

 

 


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The Federal Highway Administration (FHWA) is a part of the U.S. Department of Transportation and is headquartered in Washington, D.C., with field offices across the United States. is a major agency of the U.S. Department of Transportation (DOT). Provide leadership and technology for the delivery of long life pavements that meet our customers needs and are safe, cost effective, and can be effectively maintained. Federal Highway Administration's (FHWA) R&T Web site portal, which provides access to or information about the Agency’s R&T program, projects, partnerships, publications, and results.
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