"How do we extend the service life of our existing concrete bridges and structures?" That question, posed by Claude Napier of FHWA's Virginia Division, set the tone for the pilot electrochemical chloride extraction (ECE) showcase workshop held in Arlington, Virginia, in July. The workshop, sponsored by FHWA's Office of Technology Applications and the Virginia Department of Transportation, attracted almost 50 attendees from State departments of transportation, private industry, and FHWA.
Following Napier's introduction, FHWA's Louis Triandafilo updated participants on the status of Strategic Highway Research Program (SHRP) implementation. Donald Jackson, FHWA's project manager for the ECE showcase, provided a historical background and future directions of ECE; and David Whitmore, of Vector Construction Ltd., gave a contractor's perspective of the ECE process. Gerry Clemena, principal research scientist at the Virginia Transportation Research Council, presented findings from Virginia's pilot projects using the ECE process on concrete bridge decks and piers.
Much of the deterioration in concrete bridges and structures is caused by chloride ions that are present in de-icing chemicals. The ions permeate the concrete and eventually reach the reinforcing steel, where they create conditions that encourage corrosion of the steel, which in turn leads to spalling of the concrete. To be effective, the repair strategy must include halting the corrosion and removing the chlorides. Electrochemical repair techniques fit the bill: they remove the chloride ions and halt the corrosion.
ECE is similar to cathodic protection of concrete, but ECE requires 100 to 500 times the current density necessary for cathodic protection. Cathodic protection systems are permanent systems, whereas the ECE process is temporary, requiring 6 to 10 weeks.
"There is no doubt that ECE is effective in reducing corrosion," said Jackson. "These ECE projects and workshops are one way of getting the new technology out in the field, and sharing the results with others in the highway industry."
With the ECE process, an anode and electrolyte are applied to the concrete surface. The anode and the concrete surface are kept wet and charged with electricity. The negatively charged chloride ions are drawn away from the reinforcing steel toward the positively charged anode. The treatment also repassivates the steel. After the treatment, the concrete should be sealed, if possible, to prevent additional chloride ions from reentering the concrete.
Workshop participants spent the morning in the classroom, learning how the ECE process works. They listened to colleagues talk about their experiences with ECE projects and watched videotapes of the ECE treatment being applied. Later, they visited one of Virginia's pilot projects, a bridge deck in Arlington currently undergoing the ECE process. Half of the bridge deck had already been treated, and participants got a firsthand look at the treatment in progress on the second half.
Participants walked over the treated area and inspected each section. "I wanted to find out more about how the ECE process works, since we have a similar bridge project coming up in Seaford," said Muhammad Chaudhri, of the Delaware Department of Transportation. "I came to find out what the pitfalls are, and what our expectations should be. There's nothing like walking on the bridge and seeing the treatment in progress. You can read as much as you like and attend lectures, but actually seeing it firsthand is extremely useful."
For the Arlington project, two of the bridge's five spans were treated, and the other three spans were kept as controls. The treatment area totaled 733 m2 (7,890 ft2), and the price for treatment was about $102/m2 (9.50/ft2), totaling $75,000.
Eight weeks after the treatment process, results from the first half of the bridge look promising. Changes in chloride concentration were most dramatic toward the surface of the concrete, suggesting that chlorides might be easier to draw out at shallow depths, and that the amount of chloride ions might be higher near the surface, said Clemena. Chloride concentration dropped by 50 percent in some places and as much as 90 percent in others. The average reduction in chloride concentration for the treated half of the bridge was 75 percent.
One side effect from the treatment is softening of the concrete. Hardness decreased 6 percent to 10 percent in the two spans that were treated. Clemena attributes this to difficulty in maintaining the pH level of the electrolyte and says this is one aspect of the treatment that must be adjusted. He noted, however, that he is not too concerned about the decrease in hardness because the top layer of the bridge will be scarified prior to placing a new overlay.
Preliminary results of another ECE pilot project completed near Charlottesville, Virginia, are not as conclusive as the Arlington project. This project involved applying the ECE process to columns and piers of the Route 631 bridge connecting Interstate 64 in Albemarle County (see the July 1995 Focus for details).
Preliminary data suggest that the ECE process may not be as effective on bridge piers and columns. While chloride concentration was lower for the piers and columns, the results from the Arlington project are much more dramatic. Clemena attributes this to a couple of factors. The Arlington bridge deck had a much higher concentration of chloride to begin with, so it was not surprising that results were more dramatic. In addition, because the bridge deck was a flat surface, it was easier to keep the surface wet (by ponding).
According to Jackson, more research needs to be conducted in several areas of ECE. One of those areas is the impact of the ECE process on aggregates prone to alkali silica reactivity (ASR).
"If there is an existing or potential ASR problem, the ECE process could exacerbate it," said Philip Boon, business manager of the Lithium Division of FMC Corporation, which participated in the expert task group for the Arlington project. FMC helped set the parameters for testing a lithium electrolyte in the ECE process, as a means of potentially stopping or preventing the formation of ASR.
"The workshop is a great way to demonstrate the technology in the marketplace," said Boon. "It's too soon to tell whether or not the process was successful--it will take a number of years to test and evaluate. But ECE has shown itself to be used practically in the field." Other areas that need further investigation, according to Jackson, include the following:
The ECE process just may, however, answer the question, "How do we extend the service life of our existing concrete bridges and structures?" For more information, contact Donald Jackson at FHWA (telephone: 202-366-6770; fax: 202-366-7909; email: djackson@intergate.dot.gov.
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Updated by Martin Beaudette