by John G. Everett
The Norcure Electrochemical Removal System is a revolutionary method developed to remove salt (chloride ions) from salt-contaminated concrete structures.
Corrosive concentrations of chlorides are often found within concrete. Concrete's porous nature allows chloride ions to percolate deep into the structure, eventually reaching the reinforcing steel and causing corrosion, which in turn leads to delamination of the concrete. Concrete surfaces exposed to de-icing chemicals and marine environments are especially vulnerable. Even relatively new structures can exhibit rebar corrosion if chlorides are cast into the structure. Calcium chloride was commonly used as an accelerator for many years, and those structures which contain sufficient amounts are now suffering significant corrosion damage. Another problem is carbonation. Highly alkaline fresh concrete creates a passive oxide layer on the reinforcing steel, protecting it from corrosion. Carbonation occurs when carbon dioxide from the atmosphere reacts with lime in the concrete to form carbonates. As carbonation progresses, the pH falls below the critical value of about 9.5 and the protective oxide layer on the rebar is destroyed. Without this natural protection, the steel rebar begins to corrode. The conventional repair of concrete by chipping and patching is a first aid operation rather than a cure. Conventional methods are noisy, dirty, time consuming, costly, and in some cases structurally detrimental. Also, the general level of chloride contamination is not addressed, and new damage may occur if surrounding areas remain contaminated. To stop corrosion and prevent continuing deterioration, the cause of the corrosion must be removed. The Norcure Electrochemical Chloride Removal System is a revolutionary method developed to remove salt (chloride ions) from salt contaminated concrete structures. Removal of chloride ions and reestablishment of the passive layer on the reinforcing steel eliminates or greatly reduces ongoing rebar corrosion activity. The halting of corrosion will extend the service life of existing chloride-contaminated structures.
How it Works
The concrete to be treated is first tested to determine the levels and location of chloride contamination and
carbonation. An electrode in the form of a conductive mesh of titanium or steel is attached to the structure. A wet cellulose
fiber slurry is sprayed onto the mesh, giving it the appearance of paper mache. The structure is then wrapped in plastic to
minimize evaporation. Electrical contacts are established to the mesh, which acts as the anode, and to the reinforcing steel,
which acts as the cathode. When direct current is applied, the negatively charged chloride ions migrate away from the rebar
and toward the mesh and electrolyte, leaving the concrete, as shown in the figure. At the same time, alkali ions move from the
electrolyte into the carbonated concrete, raising the pH level. When the process is complete, the alkalinity of the concrete is
reestablished and the passive oxide layer is restored. The rebar is now protected from corrosion. The electrode and
electrolyte are removed, leaving a revitalized structure. The Norcure process is non-destructive, quiet, clean, and less
expensive in many situations than conventional methods. Norcure has some similarities to cathodic protection which has been
around for about 20 years. Cathodic protection is a permanent process requiring continuous monitoring. Norcure treatment
takes 6 to 10 weeks. (Norcure uses current densities on the order of 100 mA/ft²; cathodic protection typically uses only
about 1 mA/ft².)
History
Some early experimental work was performed in the United States during the 1970s. Very high voltages were applied
for a short period of time. Unfortunately, this heated the concrete and caused other undesirable side effects. This work was
abandoned. In the mid 1980s, the Norwegian company Norwegian Concrete Technologies (NCT), now a division of Fosroc,
and their lead research engineer, John Miller, began further work in this area. They were able to determine the causes of the
side effects and developed processes and equipment to eliminate these concerns. Trial demonstration projects began in the
late 1980s with the first trial project in North America being completed for the Ontario Ministry of Transportation on a portion
of the substructure of the Burlington Skyway. This project was completed as part of the SHRP (Strategic Highway Research
Program) which is jointly funded by the United States and Canada. In 1994, the first full-scale commercial project completed
on a bridge substructure in North America was completed for Saskatchewan Highways and Transportation. This project
included the treatment of 24 salt contaminated columns. Vector Construction Inc. of Fargo. North Dakota and Winnipeg,
Manitoba completed five projects in the fall of 1995: two in Virginia. two in Saskatchewan, and one in Alberta. Additional
projects are scheduled to commence in the spring and summer of 1996. Acceptance of the system is growing rapidly now that
long-term results of early SHRP test projects are available. The Federal Highway Administration is actively trying to promote
the use of this technology to state DOTs. To date, approximately 1.5 million square feet of concrete have been treated on
about 50 projects in the United States, Canada, Norway, Sweden, England, and Germany. Licenses to use the Norcure
system are available from NCT. Research is underway to accelerate the process, but higher energy inputs bring back some of
the original side effects. Also under investigation are the benefits of sealing the surface after the treatment is complete to
prevent re-migration. Different types of electrolytes are being evaluated to reduce alkali-silica reactivity. In some early
projects, there were problems in treating bridge piers and columns because the electrolyte had a tendency to gravitate
downward or leak out. New forms that provide a sealed chamber for the electrolyte have solved this problem. Piers and
columns are now easier to treat because there is no traffic to interfere with the treatment, and disruptions to traffic are
minimized. Application costs are roughly $10 per square foot but are expected to decrease as are more experience and
refinements in the process occur.
John G. Everett is Assistant Professor of Civil and Environmental Engineering at the University of Michigan.
Some of the descriptions of Norcure have been adapted from literature provided by Vector Construction Inc. and Norwegian Concrete Technologies, Olso, Norway.)
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