corrosion mitigation systems for concrete structures6 concrete repair bulletin july/august 2003...

6 CONCRETE REPAIR BULLETIN JULY/AUGUST 2003

Corrosion Mitigation Systemsfor Concrete StructuresBy J. Christopher Ball and David W. Whitmore

AAAAA ccording to a recently completed study by theU.S. Federal Highway Administration, the

annual direct cost of corrosion to the U.S. economyis estimated at $276 billion, or 3.1% of the U.S.Gross Domestic Product (GDP). If indirect costssuch as loss of productivity are included, the annualcost is conservatively estimated at $552 billion, orover 6% of the GDP.

While these statistics are related to the overallcost of corrosion, all anecdotal information indicatesthat the annual cost for corrosion-related repair orreplacement of concrete structures is considerable.Over the long term, owners of both public and privatestructures are faced with rising costs to maintaintheir structures. Forward-looking owners recognizethe significant incentive to protect their investmentsand consider systems that can economically extendthe life of today’s concrete structures.

Corrosion BasicsPrior to the consideration of corrosion mitigation

systems, it is useful to gain a general understandingof the basics of corrosion of reinforcing steel inconcrete. A brief overview is presented as follows:

General CorrosionThe corrosion of reinforcing steel in concrete is

an electrochemical reaction that is influenced byvarious factors, including chloride-ion content, pHlevels, concrete permeability, and availability ofmoisture to conduct electricity. Five elements arerequired to complete the corrosion cell: an anode,a cathode, an ionic path, a metallic connectionbetween the anode and cathode, and the availabilityof oxygen. In practical terms, the anode is the siteof rust formation on the reinforcing steel, the cathodebeing another area of the rebar protected by thecorrosion of the anode, the metallic connection isprovided by the network of reinforcing steel, andthe ionic path is through the concrete matrix withsufficient moisture for conductivity.

When mild steel is used as reinforcement inconcrete, a protective oxide layer is initially formedon the surface of the rebar due to the alkalinity ofthe concrete. As long as this film is maintained, the

reinforcement will indefinitely remain in a verypassive state.

For corrosion to occur in concrete, the passiveoxide film on the reinforcing steel must be destroyed.In most cases, this is due to the presence of suffi-cient chloride-ions at the level of the steel (generallythought to be 1.0 to 1.4 lb of water-soluble Cl– peryd2 [0.6 to 0.8 kg/m2]). Chloride-induced corrosioncan be commonly found in structures exposed todeicing salts or a marine environment. Chloridescan also be introduced to the concrete during theoriginal construction by the use of contaminatedaggregates or chloride-containing admixtures.

The passive oxide film can also be destroyedby the loss of alkalinity in the concrete matrixsurrounding the reinforcing steel. The reduction inalkalinity is generally caused by carbonation—areaction of atmospheric carbon dioxide with calciumhydroxide (in the cement paste) in the presence ofwater. The result is a reversion of the calciumhydroxide to calcium carbonate (limestone), whichhas insufficient alkalinity to support the passiveoxide layer. The amount of time for the carbonatedzone to reach the level of the reinforcement is afunction of the amount of concrete cover, concreteporosity, humidity levels, and the level of exposureto carbon dioxide gas. Once the carbonation reachesthis steel, the passive layer will be destroyed, andthe corrosion process may commence.

Over time, concrete delaminations result fromthe expansive pressures of the corrosion by-products.If corrosion activity is not arrested, section loss ofthe reinforcment can occur and significant structurerepair or replacement may eventually be required.

Patch-Accelerated CorrosionWhen concrete delaminations are repaired using

typical “chip and patch” procedures, abrupt changesin the concrete chemistry surrounding the reinforcingsteel are created. Typical repair procedures call forremoval of the contaminated concrete around thefull circumference of the reinforcing steel within therepair area, cleaning of bond-inhibiting corrosion by-products from the steel, and refilling the cavity withnew chloride-free, high-pH concrete (ICRI Technical

JULY/AUGUST 2003 CONCRETE REPAIR BULLETIN 7

Guideline No. 03730). In the case of chloride-inducedcorrosion, this procedure creates adjacent chloride-contaminated and chloride-free zones with signifi-cantly different corrosion potentials. This differ-ence in corrosion potential (voltage) is the driv-ing force for new corrosion sites to form in thesurrounding chloride-contaminated concrete.

The new corrosion activity is commonly referredto as “ring anode corrosion,” “halo effect,” or “incip-ient anode formation.” Once again, from a practicalstandpoint, the evidence of this activity can be newconcrete spalls adjacent to the previously completedpatches. The time between repair programs will beinfluenced mainly by the amount of chloride presentand the conductivity of the concrete. It is notuncommon, however, to find additional repairsrequired in 2 to 5 years.

Despite the corrosion concerns, the repair ofdelaminated concrete is necessary for structureserviceability, but having an awareness of thesecorrosion issues is important. Developing a corrosionmanagement strategy will depend on many factors,including the amount of existing damage, level ofcontamination, environmental exposure, and theowner’s requirements and budget. In many cases,longer-term corrosion solutions may be considered.

Corrosion Mitigation StrategiesRemoval and Replacement

Widespread removal of chloride-contaminatedor carbonated concrete is a long-term solution

Figure 1: Corrosion cell in concrete

Figure 2: Patch-accelerated corrosion in concrete

Figure 3: Removal of chloride-contaminated concrete


because this method removes the sourceof the corrosion problem. This approachlends itself to structures with severecorrosion problems as evidenced by alarge percentage of the surface areabeing delaminated. This option, however,is generally cost-prohibitive or imprac-tical in many situations. Removal andreplacement can also create structuralconcerns during the repair execution thatcan result in shoring requirements andincreased costs.

Barrier SystemsThe use of sealers, coatings, and

overlays are often considered formitigating future corrosion activity.

Barrier systems operate by preventing the ingressof chloride ions, moisture, and/or carbon dioxideinto the structure. A significant increase in lifeexpectancy can be achieved when the proper systemis applied to a new structure or to a structure with-out significant contamination at the level of thereinforcing steel. If the structure is already contam-inated and showing signs of corrosion distress,the use of barrier systems will generally have alimited impact on the service life of the structure.

Electrochemical TreatmentsAn alternative approach to achieve long-term

corrosion protection is to address the underlyingcause of corrosion without the widespread removalof contaminated but sound concrete. For manyprojects, electrochemical treatments are a cost-effective strategy for providing corrosion protectionover large areas.

Electrochemical chloride extraction (ECE) is ashort-term process where an electric field is appliedbetween the reinforcement in the concrete and anexternally mounted mesh. The mesh is embeddedin a conductive media, generally a sprayed-onmixture of potable water and cellulose fiber forvertical and overhead surfaces. During treatment,negatively charged chloride ions are transportedaway from the rebar and toward the positivelycharged external electrode mesh by means of ionmigration, where they are trapped and removed inthe fibrous electrolyte mixture.

For structures subject to carbonation-inducedcorrosion, a variation of the ECE process can beutilized to re-alkalize (increase the pH of) theconcrete. The primary differences between ECE andre-alkalization are the choice of electrolyte, theduration of treatment, and the procedures to verifya successful application.

Impressed Current Cathodic ProtectionAnother option to consider is the installation

of an impressed current cathodic protection (ICCP)

Figure 4: Electrochemical chloride extraction process

Figure 5: Impressed current cathodic protection

Figure 6: Embedded galvanic anode installed in patch repair forprotection against patch-accelerated corrosion initiation


system. ICCP systems start with theinstallation of permanent anode(s) intothe structure. An external DC powersource is applied with the anode beingconnected to the positive (+) terminaland the reinforcing steel connectedto the negative (-) terminal. Properlydesigned, installed, and maintained,ICCP systems can provide long-termprotection to the reinforcing steel.According to industry standards, anICCP system is considered to be 100%effective when the system polarizes thereinforcing steel sufficiently to result ina 100mV depolarization after the systemis turned off.

Additionally, ICCP systems can becost-effective when used to protect largeareas, and the initial costs are spreadover a long period of time. Periodicinspection and maintenance of the systemis required. If an ongoing monitoring andmaintenance program is unlikely, othercorrosion mitigation strategies shouldbe considered.

Galvanic ProtectionGalvanic protection is achieved when two

dissimilar metals are connected. The metal with thehigher potential for corrosion (generally a zinc-basedsystem in concrete applications) will corrode inpreference to the more noble metal. As the sacrificialmetal corrodes, it generates electrical current toprotect the reinforcing steel.

Potential applications for galvanic systemsinclude balconies, walkways, bridge and parkingdecks, and precast/prestressed concrete. Twotypes of galvanic protection systems are used forthese applications: distributed systems for globalcorrosion protection and discrete anodes forlocalized protection.

Distributed systems consist of galvanicanode(s) that are placed onto the surface ofthe concrete or embedded in a concrete overlay.Discreet systems utilize embedded galvanicanodes (EGAs) tied to the steel which is exposedin the area to be repaired. EGAs can also be usedto protect the remaining chloride-contaminatedor carbonated concrete by installing into drilledholes on a grid pattern.

To address patch-accelerated corrosion, palm-sized EGAs are used in conjunction with traditionalconcrete repair by tying the anodes onto thereinforcing steel at the perimeter of concretepatches. Once the concrete is placed, the zincanodes provide localized galvanic protectionto prevent corrosion initiation of the adjacentreinforcing steel. EGAs can be used to provide

corrosion prevention at the interface of newconcrete and existing contaminated concretesuch as partial- or full-depth concrete repairs,expansion jo in t replacements , and br idgewidening projects.

Unlike impressed current cathodic protectionsystems, the galvanic system voltage is fixed(similar to a battery) and the amount of currentgenerated is a function of the surrounding envi-ronment. Galvanic anodes will generate highercurrent output when in a more corrosive orconductive environment. For example, current outputwill likely exhibit a daily and seasonal variationbased on moisture and temperature changes.

Due to the limitation in driving voltage, in somesituations, galvanic systems may not achievethe accepted 100 mV depolarization criteria forcomplete cathodic protection. Previous studiesindicate, however, that current density in theorder of 0.4 mA/m2 of steel surface area hasprevented the initiation of steel corrosion inconcrete with chloride levels as high as 2% byweight of cement (10 times the chloride thresholdto initiate corrosion activity).

This range of current density has been shownto provide “cathodic prevention.” When combinedwith the low system maintenance, the lack ofexternal power supply required, and the generalcompatibility with prestressed and post-tensionedsteel, EGAs provide an attractive choice forcorrosion protection for many applications.

Figure 7: Florida condominium with distributed galvanic protection system


Embedded Galvanic AnodesCase Study

The Florida coast provides one of the mostcorrosive environments in the world and is anexcellent test area for corrosion mitigation systems.Elevated chloride levels from sea spray combinedwith high temperatures and humid conditionsprovide a very corrosive environment. In this typeof situation, typical “chip and patch” repair programscan show signs of patch-accelerated corrosion inas little as 2 to 3 years.

In June of 2000, a condominium on the eastcoast of Florida exhibiting concrete spalling frompatch-accelerated corrosion was selected for theapplication of an EGA system. The purpose of theinstallation was to reduce the level of corrosionactivity and to cost-effectively increase theservice life of the chloride-contaminated balconiesand walkways.

Because the removal of spalled concrete was tobe performed in accordance with the ICRI guide-lines, which state that concrete removal be continueduntil clean steel is encountered, the activelycorroding areas were being addressed by the concreterepairs. The primary concern of the owners wasto delay corrosion initiation within the sound butchloride-contaminated areas that remained.

Figure 9: Galvanic anodes installed prior to deck coatingFigure 8: Illustration of galvanicanode spacing

To evaluate the performance of the EGA system,47 anodes were installed into drilled holes on twoseparate balconies. The anodes were installed in agrid pattern at three different spacings—18, 28, and42 in. (45, 71, and 107 cm) on center.

A remote monitoring system was installed tomeasure current output of the anodes and to conductdepolarization testing of the system. Results indicatethat all areas achieved sufficient overall currentoutput for cathodic prevention. As mentionedpreviously, cathodic prevention occurs when suffi-cient current is generated (greater than 0.4 mA/m2

of steel surface area) to provide corrosion mitigation.Assuming that the conditions do not change, theamount of current required to provide cathodicprevention decreases over time as hydroxyl ions aregenerated at the steel (thus increasing pH), chlorideions migrate away from the steel, and passivitydevelops over time.

Readings taken 5 months after anode instal-lation indicated that the polarization of thesteel was less than 100 mV. Although this isconsidered to be less-than-complete cathodicprotection, a significant level of corrosion protectionis nevertheless provided to the steel. Subsequentreadings have indicated increasing polarizationover time.


J. Christopher Ball serves VectorCorrosion Technologies as Director,Sales and Marketing. He has over11 years of construction industryexperience with a specialty inconcrete rehabilitation and corrosionprotection systems. Ball is an

active member of ACI International and ICRI.

As Vice President of VectorCorrosion Technologies, David W.Whitmore is actively involved withprofessional organizations such asICRI, the National Association ofCorrosion Engineers (NACE), andACI International, where he is

involved in repair and corrosion committees thatfocus on the development of specifications andstandards for technologies to rehabilitate andprotect concrete structures.

Table 1: Test results from Florida Condominium Test Balcony A (5 months after installation)

Table 2: Test results from Florida Condominium Test Balcony B (5 months after installation)

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After successfully demonstrating the systemperformance, EGAs were used to reduce the corrosionactivity and delay the initiation of corrosion innew areas on the remaining 15,000 ft2 of balconiesand walkways.

Understanding TechnologiesAs detailed previously, there is a wide range of

corrosion mitigation systems available for concretestructures, each with its advantages and limitations.Developing a basic understanding of the mechanismof corrosion and available corrosion mitigationtechnologies provides a strong foundation for devel-

oping long-term corrosion management strategiesthat increase the service life of concrete structureswhile meeting the owner’s requirements and budget.

ReferencesBertolini et al., 1998, “Cathodic Protection and Cathodic

Prevention in Concrete: Principles and Applications,” Journalof Applied Electrochemistry, V. 28, pp. 1321-1331.

International Concrete Repair Institute, 1995, TechnicalGuideline No. 03730, “Guide for Surface Preparation for theRepair of Deteriorated Concrete Resulting from ReinforcingSteel Corrosion.”

Pedeferri, P., 1996, “Cathodic Protection and CathodicPrevention,” Construction and Building Materials, V. 10, No. 5,pp. 391-402.

corrosion mitigation systems for concrete structures6 concrete repair bulletin july/august 2003...

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