U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
|This report is an archived publication and may contain dated technical, contact, and link information|
|Publication Number: FHWA-HRT-10-069 Date: September 2011|
Publication Number: FHWA-HRT-10-069
Date: September 2011
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)
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
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:
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.)
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).
|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|
|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|
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)