From Weeks to Days: Advancements in Electrochemical Realkalization for Historic Concrete

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Reinforced concrete defined the architectural ambitions of the 20th century. Bridges, monuments, civic halls, and cultural landmarks were built with the confidence that concrete would endure. However, the high alkalinity that makes concrete’s cement matrix protective to the embedded steel, passivating it and preventing corrosion, is not permanent. Carbonation, the gradual reaction of atmospheric carbon dioxide with calcium hydroxide in the concrete pore structure, progressively lowers pH. Once the pH drops below the threshold needed to sustain the passive oxide layer on reinforcing steel, corrosion can initiate in the presence of moisture. For historic structures, where original fabric must be preserved, this presents a formidable conservation challenge (Fig. 1).

Conventional repair, which involves removing and replacing carbonated concrete, is often incompatible with historic preservation goals. It alters original surface appearance, risks structural damage during removal, and does nothing to address the underlying electrochemical condition of the remaining concrete. Electrochemical realkalization (ER) offers a fundamentally different approach: it restores concrete alkalinity without removing original material, leaving historic surfaces essentially unchanged. Over more than three decades, the technology has evolved significantly. Power supply systems, monitoring protocols, anode materials, and electrolyte formulations have all advanced—enabling treatments that are faster, more precisely controlled, and dramatically less expensive to deliver than the systems available when the technique was first commercialized in the late 1980s.

The ideal condition for carbonation to occur is when concrete’s relative humidity is in the range of 50% to 70%. Carbonation induced corrosion has a slower reaction rate and is less aggressive than chloride-induced corrosion. So long as the carbonated concrete remains dry, carbonation itself is not an issue. However, in today’s ever-changing climate, in construction with low concrete cover (as seen in many historic buildings), and where the concrete may be subject to variations in use, such as a historic warehouse converted to lofts, moisture migration into the carbonated concrete will lead to corrosion (Figs. 2-3).

“Realkalization is a process where an electrical field is established which controls the movement of ions where the charged ions are attracted to the opposite electrical field. For this to occur, an alkaline electrolyte solution within a containment is temporarily applied to the surface of the concrete. An electrical current is set up between the surface anode in solution and the embedded reinforcing steel. The electrical field is applied from a direct current (DC) power supply, where the positive circuit (anode) is connected to the material in the containment and the negative circuit (cathode) is connected to the reinforcing steel. The electrical current draws the alkaline solution into the concrete toward the embedded reinforcing steel and restores the alkaline environment that allows repassivation.

Historically, the circuit was achieved on reinforced concrete primarily by two methods: 1) applying steel wire mesh to the concrete surface and spraying a cellulose fiber over the mesh, saturating it with a highly alkaline solution; or 2) using Perspex cassette shutters which were secured to the concrete and filled with the alkaline solution.

Two complementary mechanisms can raise the pH in the concrete cover zone: electrochemical reactions at the cathode generate hydroxyl ions (OH-), while electromigration drives alkali cations (positively charge ions) from the electrolyte into the concrete pore structure. Together, they raise the pH from values as low as 8.5–9.5 to levels exceeding 12, restoring the protective alkaline environment around the reinforcement. Success is verified through phenolphthalein indicator testing of post-treatment cores. The process is governed by the Association for Materials Protection and Performance (formerly the National Association of Corrosion Engineers) SP0107, Electrochemical Realkalization and Chloride Extraction for Reinforced Concrete, and the European Committee for Standardization’s (the Comité Européen de Normalisation) CEN/TS 14038-1, which specify current densities, total charge passed (typically 100–200 A·h/m², ampere hours per square meter), electrolyte composition, and verification protocols.

Unlike many chemical interventions, realkalization is considered reversible which is a requisite under the United States Secretary of the Interior’s Standards for the Treatment of Historic Properties (Guidelines). Realkalization requires no cutting or chasing of concrete surfaces, and architecturally significant finishes are left intact, properties that make it uniquely compatible with listed and landmark structures where minimal intervention is a regulatory requirement.

Electrochemical realkalization was developed and patented by John Miller in Norway in the late 1980s (European Patent EP 264,421, filed 1987), commercialized under the NORCURE trademark from 1988, and subsequently acquired by Fosroc International, which is an international manufacturer of concrete and cement-based construction chemicals, in 1994. The technique gained early regulatory approval in the United Kingdom for use on listed historic structures and was applied to significant protected buildings and infrastructure across Europe and a select number of North America buildings through the 1990s and 2000s, establishing a track record on some of the most architecturally sensitive reinforced concrete structures of the twentieth century. By the time North American practitioners began adopting the technique in earnest, European case studies had already demonstrated its compatibility with the demands of statutory heritage consent—no alteration of historic surfaces, no structural intervention, and full compatibility with continued use of treated structures—and its ability to arrest carbonation-induced corrosion in settings where conventional concrete removal was either impractical or expressly prohibited. The slower adoption of this technology in the U.S., despite its widespread use in Europe and the United Kingdom decades earlier, was in part due to the delayed recognition among engineers in North America that carbonation-induced corrosion posed a significant risk to concrete structures.

While the underlying electrochemical principles of realkalization remain unchanged, the engineering infrastructure that delivers, monitors, and controls the treatment has been transformed. Advances across three interconnected areas now define the difference between a viable and an impractical treatment program for many historic structures: power supply systems, anode and electrolyte technology, and real-time monitoring.

Early realkalization systems relied on conventional DC power supplies adapted from industrial electrochemical applications. These units delivered current at fixed output levels with limited ability to respond dynamically to the changing resistance of a concrete structure as treatment progressed. Lack of monitoring data for the power supply outputs could lead to longer run times. Extended timelines meant prolonged scaffolding and access system costs, greater disruption to occupied or publicly accessible structures, and higher overall project expense. These factors made realkalization difficult to justify on cost alone for many owners, even when it was clearly the most preservation-appropriate solution.

The most transformative advancement in recent years has been the development of purpose-engineered power supply systems specifically designed for electrochemical concrete treatment. The latest generation of these units deliver precise voltage and amperage continuously calibrated to achieve the required charge criteria, expressed in A·h/ft² (ampere hours per square foot) or A·h/m² (ampere hours per square meter), across complex multi-zone configurations. Critically, they incorporate real-time current monitoring at 100 Hz (Hertz)/ 0-1KHz (Kilohertz) resolution, allowing the system to detect and respond instantaneously to fluctuations in concrete resistance, zone geometry, and connection integrity. This dynamic response capability is a fundamental departure from the static output of earlier units and the primary driver of dramatically reduced treatment timelines (Fig. 7).

The practical impact on project economics and preservation outcomes is substantial. On a recent application to a significant historic landmark structure in the United States, a 60-foot reinforced concrete monument treated from its interior—specifically to preserve intact exterior statuary—the newest generation of Electrochemical Treatment (ECT) power supply systems enabled the full multi-zone treatment to be completed in 10 days while assuring that the system optimized the performance of each treatment zone. The original program, based on conventional power supply technology, had projected a 30-day timeline across all zones. The reduction from 30 days to 10 was achieved not by compromising charge delivery but by optimizing it, i. e., high frequency monitoring allowed the system to continuously maximize efficient current delivery within the parameters required to meet the 19 A·h/ft² treatment criterion, rather than running conservatively against a fixed output schedule.

For historic structures, the cost implications of this compression are far-reaching. Scaffolding and temporary access systems are among the largest cost drivers on concrete treatment projects, particularly on monumental or vertically complex structures where custom rigging is required. Reducing treatment duration by two-thirds directly reduces the rental period for these systemss—often representing savings that run to tens of thousands of dollars on a single project. For landmark structures with ongoing visitor access or operational requirements, the period during which a protected building is subject to the presence of construction systems, noise, and restricted access is similarly compressed. On the aforementioned monument project, the interior-only treatment approach preserved the most significant exterior statuary in place throughout the treatment period, and the accelerated timeline meant that the structure’s public presence was restored sooner than a conventional program would have permitted. These are not just marginal efficiency gains; they are the difference between a treatment program that is economically feasible for a preservation budget and one that is not (Figs. 8-10).

Alongside power supply advances, anode and electrolyte technology has evolved significantly. Early systems used steel or iron mesh anodes embedded in cellulose fiber pulp saturated with sodium carbonate electrolyte. While effective, iron-based anodes corrode during treatment, producing iron hydroxide products that can stain historic concrete surfaces. This is an unacceptable outcome on architecturally significant finishes, and the cellulose fiber was difficult to maintain constant saturation. Contemporary systems use mixed metal oxide (MMO)-coated titanium mesh anodes. Dimensionally stable throughout the treatment period, titanium MMO anodes produce no corrosion products and conform reliably to complex geometries. The transition to potassium carbonate electrolyte in many applications has further reduced post-treatment efflorescence, improving aesthetic outcomes on exposed architectural concrete. Plastic wrap methods for the saturated fiber electrolyte ensures the electrolyte does not evaporate for the duration of the period and conforms to the concrete surface without leaks, which were concerns of the shutter tank construction.

Zoning methodology has also become more sophisticated. Systems are now designed with discrete power zones calibrated to the specific steel mass, cover depth, and geometry of each treatment area, ensuring even current distribution across complex 3D structural forms. On structures with irregular geometries such as vaulted interiors, sculptural elements, or projecting members, precise zoning prevents under-treatment, risks associated with excessive current delivery to localized areas, and wasting current due to lack of sophisticated monitoring. This precision is especially critical on listed structures where any surface damage from treatment would be unacceptable under heritage consent. In addition, using industrial grade plastic wrap to contain the anode in solution allows these complex geometries to receive treatment without risk.

Where earlier projects relied on periodic manual readings of applied voltage and current, contemporary systems capture operating parameters continuously at millisecond resolution, generating complete treatment records that verify compliance with charge-passed criteria across every zone. This data trail supports both quality assurance during treatment and long-term asset management by providing a quantitative baseline for future condition assessments, an increasingly important consideration as owners of significant concrete structures look to document and defend preservation expenditures over the long term. This detailed level of measurement also reduces the requirement for core extraction to test the pH as was done in early systems to demonstrate alkalinity had been restored.

The case for electrochemical realkalization on historic carbonated concrete structures rests on a principle that conventional concrete repair cannot satisfy: preserving the authenticity of the original material. Every cubic inch of original concrete removed in a conventional repair program is original fabric lost permanently. On landmark buildings, where the texture, color, aggregate exposure, and finish of the original concrete are intrinsic to the structure’s historic character, that loss is irreversible and, under most preservation frameworks, unacceptable. Realkalization addresses the root cause of carbonation induced-corrosion, which is the loss of alkalinity in the concrete cover zone, without significant alteration of the surface.

The compatibility of realkalization with preservation ethics has been consistent throughout the literature. The use of the technique in the 1990s for Grade II listed structures, which are structures of special architectural or historic interest in the United Kingdom, under the auspices of English Heritage, stewards of landmark properties in England, established a precedent that preservation authorities in other countries have since followed. The technique is classified under BS EN European Standard EN 1504-9 Publication for Products and Systems for the Protection and Repair of Concrete Structures* as a Principle 7 intervention aimed at restoring passivity to reinforcing steel, and it is consistent with the United States Secretary of the Interior’s Standards for the Treatment of Historic Properties. For practitioners working within the demanding constraints of statutory heritage consent, it remains the most preservation-compatible electrochemical treatment available.

The economic argument for realkalization has also strengthened as the technology has matured. When project costs are evaluated holistically, accounting for scaffolding, access, specialist labor, concrete replacement materials, surface reinstatement, and the long-term cost of re-treatment as carbonation re-advances, realkalization combined with surface treatment, such as a coating, silane or siloxane, consistently outperforms patch repair as a lifecycle investment for structures where large areas of concrete cover are carbonated. The dramatic reduction in treatment duration enabled by next-generation power supply technology has further closed the gap, making realkalization competitive on upfront cost as well as lifecycle value.
*Note BS EN (British Standard English) 1504 standard sets the general rules and core principles for protecting and repairing concrete structures. It provides a guide for choosing the right products, managing defect assessments, and planning safe repair work in a 10-part document. Section 9 defines products and systems for concrete repair, based on material degradation and principles of repair.

Electrochemical realkalization has evolved considerably since its introduction in the late 1980s. It is now a recognized conservation methodology for historically significant reinforced concrete structures, with a proven track record on some of the most architecturally and culturally sensitive buildings in the world. The core electrochemical principles are unchanged, but the systems that implement them have been transformed.

Purpose-engineered power supply systems with millisecond real-time current monitoring now enable treatment timelines that would have been unachievable with earlier technology—compressing 30-day sequential zone programs into 10 days of optimized charge delivery. MMO-coated titanium anodes eliminate staining risks while improving conformance to complex historic geometries. Sophisticated multi-zone designs ensure even current distribution across irregular structures. Data records generated by continuous monitoring provide the quantitative documentation that preservation clients and heritage authorities increasingly require.

For preservation engineers and architects working with significant historic concrete, these advances matter in direct, practical terms: shorter project durations, lower scaffolding and access costs, better treatment documentation, and, above all, superior preservation outcomes than traditional repairs. Realkalization’s fundamental value proposition has not changed since 1987: preserve original material, restore the electrochemical condition of the concrete, and leave the historic surface unchanged. The technology that delivers it is now fully equal to the complexity of the structures it is called upon to save. ■

About the Authors

Gina Crevello, Msc, FAPT is an expert in corrosion diagnostics and electrochemical conservation of historic buildings, including masonry clad steel frame buildings and historic concrete.

Paul Noyce, BEng, is an electrochemist, innovator, and concrete durability expert, applying electrochemical treatments to existing, historic and new structures. He has been instrumental in the earliest uses of ICCP, realkalization and electrochemical chloride extraction on historic concrete structures from the 1990s to today.

Chris Ball is senior vice president, Vector Corrosioni Technologies. He has 25 years of construction industry experience with a specialty in concrete rehabilitation and corrosion protection systems.

References

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