U.S. Department of Transportation
Federal Highway Administration
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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 that extracts chloride ions 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 the popularity of the abbreviation ECR 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 mitigate ongoing corrosion of the embedded steel.
The technique of removing chloride ions from contaminated reinforced concrete by electrochemical means was first studied in the 1970s by the Kansas Department of Transportation (KDOT) and Battelle Columbus Laboratories.(1,2) Both studies were funded by FHWA, and some adverse effects were observed, including a reduction in rebar-to-concrete bonding and increased concrete permeability. Questions were raised with regard to the impact of the migration of ions other than chloride and the rate of remigration of the residual chloride ions.
In 1987, section 128 of the Surface Transportation and Uniform Relocation Assistance Act initiated SHRP. The structures portion of SHRP reevaluated ECE technology in detail and determined its feasibility through a laboratory study. Additionally, it performed four field validation studies from 1987 to 1992. The laboratory portion of the study affirmed the feasibility of ECE application, and three of the four field validation studies were successful.(3,4)
Laboratory work under the SHRP contract addressed several concerns with regard to the passage of large amounts of current through concrete. The effect of current and charge passed on concrete-steel bond strength was evaluated. A reduction of bond strength compared to controls containing salt was noted at a high current density 4.995 A/ft2 (53.75 A/m2) and/or high amount of total charge (200 A-h/ft2 (2,150 A-h/m2)) passed. Lower current density and/or lower charge were not found to have any adverse effects. Even at the highest current density and charge, concrete-steel bond strength was reduced only to the values equal to those of no-salt control specimens.
Lower current densities did not result in a reduction of concrete compressive strength, but a softening of the cement paste around the reinforcing steel was observed on specimens treated at high currents (2.00 A/ft2 (21.5 A/m2)) for long periods of time (500 A-h/ft2 (5,375 A-h/m2)). It was hypothesized that softening the paste could result in the reduction of concrete-steel bond strength.
Hydrogen embrittlement of conventional reinforcing steel was evaluated, and only a temporary loss of ductility was noted on smooth specimens. To control the generation of chlorine gas, several buffer solutions used to maintain the pH of the electrolyte were studied. A sodium borate buffer was identified to be the most effective and practical.
The acceleration of ASR by the generation of hydroxyl ions during the application of ECE was demonstrated. The use of lithium ion in the electrolyte was found to control ASR and prevent the resulting damage.
The SHRP study identified a safe upper current density limit of 0.50 A/ft2 (5.4 A/m2). Optimal chloride removal efficiencies were obtained with total charge accumulation ranging from 80 to 120 A-h/ft2 (860 to 1,290 A-h/m2). Additional charge accumulations resulted in diminishing chloride removal efficiency.
It was determined that the ECE treatment process could extract approximately 20 to 50 percent of the chloride ions from the concrete. The amount of chloride ions removed depended on several factors including the total amount and the distribution of chloride ions in the concrete, the reinforcing steel design, etc.
Although sufficient levels of chloride ions remained in the structure after the application of ECE, it was concluded that the distribution of chloride ions in concrete (i.e., lower concentrations around the reinforcing steel and higher concentrations away from the reinforcement) and the production of hydroxyl ions at the concrete-steel interface could significantly delay the initiation of corrosion and provide an extension in service life. A 40-month evaluation of ECE-treated laboratory concrete slabs was conducted to evaluate the remigration of chloride ions with time and to study the impact of the higher levels of hydroxyl ions at the concrete-steel interface in delaying the initiation of corrosion, At the end of the evaluation, it was found that ECE was effective in mitigating corrosion. To ascertain the length of time that ECE would be effective, a long-term evaluation was necessary.
Four field validation trials were conducted between fall 1991 and fall 1992. Chloride removal was conducted on structures at four locations: a bridge deck in Ohio, marine bridge pilings in Florida, columns in New York, and an abutment in Ontario. Each field site was selected based on criteria established by the laboratory studies and the SHRP ETG. There was active corrosion on a substantial portion of each selected component of each structure, and chloride contamination was well above the threshold level. To establish the effectiveness of lithium ion in controlling ASR, the abutment of the Ontario structure containing alkali reactive aggregate was selected.
Field worthiness of various ECE application systems was evaluated using a variety of anode-electrolyte configurations, and field application costs were determined. To ascertain the effectiveness of these field applications, long-term monitoring was recommended.
Although the SHRP study clearly established the feasibility of extracting sufficient amounts of chloride ions from concrete bridge elements, it was not designed to ascertain the long-term effectiveness of the technology in mitigating corrosion. The primary goal of this study was to monitor 10 SHRP concrete laboratory specimens and 3 SHRP field validation sites for 5 years to determine the long-term effectiveness of ECE. The secondary objective was to identify the most appropriate laboratory and field test method(s) for evaluating and monitoring ECE performance.
The concrete laboratory specimens were exposed to Northern Virginia climate and monitored once every quarter from 1995 to 1998. Quarterly monitoring of the specimens was terminated in 1998 due to noncommitment of funds to the contract. Data collected every quarter included visual data, delamination, half-cell potential surveys, macrocell current and driving volt data, AC resistance between the two mats, slab temperature, and corrosion rate measurements. At the conclusion of the study in 1999, one core was collected from each slab, and powdered concrete samples were collected from various depths to determine the distribution of chloride ions as a function of depth.
Of the four field validation sites, the marine piles in Florida were not included in this study due to concerns about the applicability of ECE in a marine environment. The Ontario site could not be monitored because it had been replaced with a new structure. FHWA had funded several piloted ECE treatments on bridge structures, two of which were selected for long-term evaluation in this study. One pilot application had been performed on a bridge deck, and the other was performed on columns and cap beams. These structures are located in Arlington, VA, and Charlottesville, VA.
With the exception of the structure in Arlington, VA, three evaluations of treated elements and elements designated as controls on each bridge were performed during the 5-year monitoring period from 1994 to 1998. The Arlington, VA, structure was evaluated twice due to noncommitment of funds to the contract.