The corrosive nature of sodium chloride (salt) is well known. It can bring about rust on automobiles, and wreak havoc on concrete, where corrosive concentrations of chlorides are often found.
The porous nature of concrete allows chloride ions to peculate deep into structures, eventually reaching the reinforcing steel (rebar) and causing these structures to corrode. Concrete surfaces exposed to de-icing salt, such as parking structures, highways, and bridges, are particularly vulnerable to its damaging effects. Even relatively new structures can exhibit rebar corrosion if chlorides are cast into them.
Once chlorides attack a structure, they quickly eat away at the reinforcing steel, causing cracks to appear. If left untreated, these chloride ions can cause concrete spalls, expose steel reinforcement bars and eventually reduce the structural integrity of the structure to dangerous levels.
The conventional repair of concrete by chipping and patching is an expensive operation and in many circumstances provides only a Band-Aid solution rather than a cure. Since the general level of chloride contamination is not addressed using this method, new damage will often occur in areas adjacent to patches since these areas remain contaminated. In order to stop corrosion and to prevent continuing deterioration, the actual cause of the corrosion must be removed -- the salt itself.
This revolutionary technology has been successfully utilized on projects in both North America and Europe. The implementation of the Norcure technology by a Canadian Contracting Firm (The Vector Construction Group) has won Vector the NOVA Award from the Construction Innovation Forum (Hard Hat, April 5), an international body dedicated to improving the quality and reducing the cost of construction.
The Norcure process is an electrochemical method of removing chloride ions and migrating them out of the concrete. Using this method, concrete can be rejuvenated without the inconvenience and disruption of conventional demotion and repair, resulting in significant savings.
Concrete to be treated is first tested to determine the levels and location of chloride contamination and carbonation.
After preparing the surface, an electrode In the form of a conductive mesh is attached to the structure. It is embedded in a non-toxic biodegradable electrolyte which is applied to the concrete.
Next, electrical contacts are established to the attached electrode and to the reinforcing steel inside the concrete.
When an electric field is applied, chloride ions migrate away from the rebar and toward the attached electrode. Eventually the chlorides leave the concrete and end up in the electrolyte. Simultaneously, alkali is produced within the concrete thereby raising the pH back to its original level.
When the process is complete, the chloride concentration has been reduced and the alkalinity of the concrete is re-established above the critical value (greater than pH 9.5) such that the passive oxide layer on the reinforcing steel has been reformed. The rebar is now protected from corrosion.
The electrode and electrolyte are removed leaving a revitalized concrete structure.
Depending on the structure and condition of the concrete, the process takes about four to ten weeks for the treatment to be complete. After treatment, concrete structures are given a virtual new lease on life.
The idea of electrochemical treatment started In the 1970s. In the late 1980s, trial demonstration projects using the Norcure system began. The first trial project in North America was for the Ontario Ministry of Transportation on a portion of the substructure of the Burlington Skyway. This project was completed as part of the Strategic Highway Research Program (SHRP) which is a jointly funded program of the U.S. and Canada. The first full-scale commercial project in North America was completed in 1994 on a bridge substructure for Saskatchewan Highways and Transportation. The project included the treatment of 24 salt contaminated columns.
In 1995, the electrochemical chloride extraction (ECE) technology was used in three trials in the U.S.: Charlottesville, VA, 5th Street over Interstate I-64; Arlington, VA, 34th Street over Interstate I-395: and Sioux City, SD, Overpass on Interstate I-29. Donald Jackson, an engineer with Federal Highway Administration (FHWA) and a corrosion expert managed the ECE project. "The SHRP research left several unanswered questions. These pilot projects are intended to help us find answers to those questions. The Virginia and South Dakota departments of transportation volunteered to be our guinea pigs. We are very pleased at having these two States out in front with this technology"
The SHRP study, conducted by Eltech Research Corporation, investigated how to determine whether a bridge would be a good candidate for ECE treatment. The resulting report, Chloride Removal Implementation Guide (Publication No. SHRP-S-347;$10), is available from the Transportation Research Board's Business Office (telephone (202) 334-3214; fax (202) 334-2519). The report includes criteria for evaluating candidate structures and provides information on installing an ECE system.
Acceptance of the system seems to be growing rapidly now that the long-term results of the early SHRP test projects are available. The Federal Highway Administration is actively trying to promote the use of this technology to state DOTs.
There are desalination projects in Norway, Sweden, England, Germany, USA, Canada, Japan, Switzerland and Italy. Altogether, about 50 chloride removal projects have been installed. World-wide, approximately two million square feet of concrete has been electrochemically treated with new uses and applications being developed every day.
For more Information on the Virginia and South Dakota
projects, contact Donald Jackson at
(202) 366-6770.
For additional technical information on the chloride removal
process, contact:
David W. Whitmore, Vector Construction Ltd.
474
Dovercourt Drive,
Winnipeg, Manitoba,
Canada R3Y 1G4.
(204)
489-6300.
Or in the U.S.:
Vector Construction, Inc.
417 Main
Ave.
Fargo, ND 58103.
(701) 280-9697.
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