Technical Papers
Electrochemical Treatment of Concrete: A New Approach to Extend the Service Life of Chloride Contaminated,
Carbonated, or Alkali Silica Reactive Concrete Structures
D.W. Whitmore, P.Eng. Vector Construction Ltd. 474 Dovercourt Drive Winnipeg, Manitoba Canada R3Y 1G4
Abstract
Much of the deterioration of concrete structures can be attributed to chloride contamination, carbonation, alkali silica reactivity
(ASR), or a combination of these factors. Electrochemical methods have been used to address each of these modes of
deterioration.
Electrochemical techniques remove or address the cause of the deterioration and leave the concrete in a passive and stable
condition. In the case of chloride contaminated concrete, Electrochemical Chloride Extraction may be used to remove
chloride ions. Electrochemical Realkalization can be used to realkalize carbonated concrete. Both chloride extraction and
realkalization will stop rebar corrosion and return the internal reinforcing steel to its normal passive state. Where ASR is the
problem, Electrochemical lithium impregnation may be used to arrest the expansion of certain expansive aggregates. In all
cases, Electrochemical Treatment Methods may be used as a cost effective alternative to other available restoration methods.
This paper describes the electrochemical treatment methods and presents results from some the field installations completed to
date.
Keywords: Electrochemical Treatment of Concrete, Electrochemical Chloride Extraction, Electrochemical Realkalization,
Electrochemical Lithium Impregnation, Reinforcing Steel Corrosion, Concrete Restoration, Alkali Silica Reactivity.
Introduction
Electrochemical treatment of concrete is becoming more popular as a repair and restoration option to stop corrosion and
extend the service life of existing reinforced concrete structures. This paper will discuss the theory behind Electrochemical
Chloride Extraction (ECE) as well as a number of important services and components required to successfully complete an
ECE project.
Electrochemical Chloride Extraction Process
Electrochemical Chloride Extraction (ECE) is essentially a simple process whereby chloride ions are removed from chloride
contaminated concrete through ion migration. A schematic diagram of a typical ECE installation is shown in Figure #1. An
anode embedded in an electrolyte media is applied to the surface of the concrete. The electrolyte media is saturated using an
appropriate electrolyte. The anode and the reinforcing steel in the concrete are connected to the two terminals of a direct
current (DC) power supply such that the anode is positively charged and the rebar is negative.
Chloride ions are removed from the concrete by ion migration. Chloride ions being negative ions migrate toward the positive
electrode, the anode. Since this is external to the concrete, the chloride ions will leave the concrete and concentrate around
the anode. Thus the chloride content of the concrete is reduced, particularly on and around the negatively charged reinforcing
steel where the concrete for all practical purposes becomes free of chlorides. Simultaneously, the electrolytic production of
hydroxyl ions at the reinforcing steel surface results in a high pH being generated around the steel. Therefore, when the
process is terminated and the installation is removed, the reinforcing steel will be situated in chloride free, highly alkaline
concrete. The result is a strong re-passivation of the embedded reinforcing steel. Corrosion of the reinforcing steel is halted.
Site and Construction Considerations
In order to successfully achieve the full potential of a Electrochemical Chloride Removal project it is important that
consideration and planning goes into the design and selection of system components to meet the needs of each specific
project. Three main areas require consideration and design prior to installation. These are: 1. Site Utilities and Services, 2.
Anode, Electrolyte, and Electrolyte Media, and 3. System Operation.
Site Utilities and Services
Electrochemical Chloride Extraction requires a number of site utilities and services. The most notable of these are; a power
supply, a water (electrolyte) supply, and physical access to the site.
ECE requires a temporary electrical power supply for the six to eight week treatment period. Most AC/DC rectifiers designed
for ECE require 240V AC input. Generally this power is readily available near the site. If a power supply is not available
locally, or the cost to access the local system is prohibitive, generators may be required.
An electrolyte solution is also required. Water is generally required on site as the main constituent of electrolyte solution. A
connection to the local water supply is usually the most cost effective source of water. If this solution is not available, water
storage tankage and pumping equipment may be required.
Finally, site access such as scaffolding or portable lifts may be required on substructure projects to install and remove the
system. Access may also be required during system operation to monitor and maintain the installation.
Anode, Electrolyte, and Electrolyte Media
The selection of the anode, electrolyte, and the electrolyte media is probably the most important consideration in assuring a
successful installation. There are numerous materials and combinations available. Each one has its own advantages and
disadvantages.
Anode. The two most common anodes currently being used are catalized titanium mesh and steel mesh. Catalized titanium
mesh has the advantage of being an inert anode which under suitable conditions does not corrode. The catalized coating on
the mesh is consumed over time and after one or two uses this coating is generally significantly deteriorated. Titanium mesh
may require the use of a buffered electrolyte or regular electrolyte replacement since chlorides will concentrate in the
electrolyte resulting in the acidification of the electrolyte over time. The cost of titanium mesh is approximately six times the
cost of steel mesh.
Steel mesh is not inert and is consumed during the operation of the system. By the completion of the treatment a large
percentage of the steel mesh will be reduced to rust. This rust will stain the surface of the concrete during treatment.
Depending on the application this may be a concern. Generally however, any staining can be removed by a light sandblast. In
many cases this does not increase the cost of the project since sandblasting may be specified as surface preparation for a
protective coating or sealer which is applied to prevent re-contamination by chlorides in the future.
Electrolyte. Numerous types of electrolyte solutions have been used for some of the various projects which have been
completed. The most common of these are; water, calcium hydroxide (lime) solution, and "lithium borate" solution. As is the
case with the anodes, each of these electrolytes have unique benefits and disadvantages. Some of the major benefits and
disadvantages are outlined below.
Water. Water is the most efficient electrolyte available. It is also inexpensive and readily available at many sites. No
environmental protection or containment is required when using potable water. The greatest disadvantage of using water as an
electrolyte is that it has no buffering ability. If water is used as an electrolyte with an inert anode in a closed system, electrolyte
acidification will occur if the water is not regularly replaced.
Calcium Hydroxide Solution. Calcium hydroxide solutions can be used to provide a limited buffering capability where it is
desired. Calcium hydroxide has a very low solubility in water, but if a reservoir of solid calcium hydroxide is maintained this
material will slowly dissolve over time to replace the spent solution. Calcium hydroxide solutions are somewhat more
expensive than straight water and require some time to prepare and maintain. The efficiency of this type of electrolyte is
somewhat less than potable water.
"Lithium Borate". Lithium borate does not actually exist, but this term is commonly used to refer to an electrolyte containing a
mixture of lithium hydroxide and boric acid. This electrolyte provides a very highly buffered solution which is suitable for some
closed system applications. Lithium based electrolytes have been specified for work where concrete suffering from
Alkali-Silica Reactivity (ASR) is involved since experiments have shown lithium to be effective in mitigating ASR.
Disadvantages of lithium electrolyte solutions include their relatively high cost and the need for recirculating systems which are
often installed to minimize the quantity of electrolyte required.
Electrolyte Media. Electrolyte media are simply materials used to suspend, hold, or contain the electrolyte solution on the
surface of the concrete. Electrolyte media are also used to provide separation between the anode and the concrete surface.
There are three main types of electrolyte media currently in use at the present time. These are; 1. Sprayed cellulose fibre
which is primarily used for vertical surfaces, 2. Synthetic felt mats which are primarily used in horizontal "ponding" type
applications, and 3. Surface mounted tanks which are used to create a ponded environment on a vertical surface.
Sprayed cellulose fibre has the advantage of being able to conform to any concrete shape and is self adherent. The cost of the
cellulose fibre is not expensive. Disadvantages of using cellulose fibre include the need to keep it wet during the treatment
process, and the clean-up and disposal of the used material at the end of the project. Depending on the type used, cellulose
fibre can provide some buffering and generally provides a greater spacing between the anode and the concrete surface.
Synthetic felt mats also have the advantage of being inexpensive, but from a practical point of view, they are limited to
horizontal surfaces. Depending on the application, the felt mats may be rolled up and re-used on future projects. The felt mats
must also be maintained wet during the treatment process.
Surface mounted tanks can be built to fit a given structure or built in panels to cover a larger area. The initial cost is high, but if
they can be re-used many times, the cost per usage can be reduced. Tanks do not require frequent "topping up" as long as
evaporation and leakage are minimized.
System Operation
Electrochemical Chloride Extraction treatment duration varies from 2 weeks in an easy case, to more than 3 months in difficult
cases. Treatment times are typically 6 to 8 weeks on average. During this time frame the system must be maintained and
operated. Different system configurations will require varying degrees of maintenance and periodic inspection.
The installation must remain wet during operation. For cellulose and synthetic felt installations this will require either daily
wetting by a person on site or the installation of an automatic wetting system. Tank systems or other ponded systems will
require electrolyte circulation, occasional topping up to replace electrolyte lost to evaporation or leakage, and electrolyte
replacement at intervals to reduce the chloride content and prevent the acidification of the electrolyte. Generally a buffered
electrolyte is required for these systems.
The electrical operation of the system must be discussed and agreed to prior to operation. Operating limits and modes,
operating durations, and provision for maintenance and testing must be designed into an effective system.
Conclusions
Electrochemical Chloride Extraction can be used to remove chloride ions from chloride contaminated concrete. A number of
anodes, electrolytes, and electrolyte mediums are available for this purpose. Each material or system option has its own
advantages and disadvantages. The most appropriate choice of materials and configuration will depend on site conditions and
project requirements. Electrochemical Chloride Extraction will occur with any selection of anode, electrolyte, and electrolyte
media, but the efficiency of the chloride removal process, the time required to complete the work, the cost, and the ease of
operation will depend on the combination selected and the specific site conditions.
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