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Design Guide for Bridges for Service Life (2013)

Chapter: 5 Corrosion of Steel in Reinforced Concrete Bridges

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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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Suggested Citation:"5 Corrosion of Steel in Reinforced Concrete Bridges." National Academies of Sciences, Engineering, and Medicine. 2013. Design Guide for Bridges for Service Life. Washington, DC: The National Academies Press. doi: 10.17226/22617.
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235 5.1 introduction This chapter of the Guide provides essential information for addressing corrosion of reinforcing steel in conventionally reinforced concrete structures. The focus is on con- trolling and mitigating corrosion for extended durability and service life. Corrosion in prestressed or posttensioned concrete structures is not discussed. The description of corrosion in Section 5.2 covers the diffusion process that enables the penetration of chlorides through concrete and the creation of corrosion cells once the chlorides infiltrate. It also addresses the patch-accelerated corrosion commonly referred to as ring anode corrosion in repairs. Section 5.3 describes factors influencing corrosion, including chloride contamination and carbonation. Section 5.4 summarizes strategies for addressing corrosion in new and existing structures; different levels of corrosion protection are considered, such as corrosion prevention, corrosion control, corrosion passivation, and electrochemical treatments. Section 5.5 summarizes case studies that address corrosion in existing structures. Faced with rising maintenance costs, many engineers and owners recognize the need to protect existing structures from future corrosion damage. As a result, accord- ing to Ball and Whitmore (2005), the use of corrosion mitigation systems to delay the need for future concrete rehabilitation is increasing. Selecting the appropriate corro- sion mitigation approach is based on many factors, including the amount and depth of contamination (chloride ingress or carbonation), amount of concrete cracking and concrete damage, severity and location of corrosion activity (localized or widespread), expected environmental exposure, use and service life of the structure, and the cost and design life of the corrosion-protection system. 5 CORROSiON OF STEEL iN REiNFORCED CONCRETE BRiDGES

236 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Deicing salts applied during winter months generally contain chlorides. Chloride solutions penetrate existing cracks and diffuse through the concrete cover to the rein- forcing steel, initiating corrosion. Corrosion products exert stresses that can crack the concrete and cause delaminations and spalling. One approach to mitigating the problem is to prevent or minimize chloride pen- etration of chlorides by minimizing cracking using low-permeability concretes and providing adequate concrete cover over the steel, membranes, sealers, or overlays. Another approach is to prevent the steel from corroding or to minimize the rate of cor- rosion by using corrosion-resistant reinforcement or cathodic protection. Depending on the specifics of a project, one or a combination of both of these approaches may be desirable. 5.2 deScriPtion oF corroSion 5.2.1 Corrosion Process The source for this section is Ball and Whitmore (2005). The corrosion of steel in concrete is an electrochemical reaction similar to that in a battery. The corrosion rate is influenced by various factors including chloride ion content, pH level, concrete per- meability, and availability of moisture to conduct ions within the concrete. For cor- rosion to initiate in reinforced concrete, four elements are required to complete the corrosion cell: an anode, a cathode, ionic continuity between the anode and cathode through an electrolyte, and a metallic (electrical) connection between the anode and cathode. The anodic site becomes the site of visible oxidation (corrosion); the cathode is the location of the reduction reaction, which is driven by the activity at the anode. In reinforced concrete, the metallic path can be provided by the mild steel reinforcing or embedded prestressing strands. The ionic path is provided by the concrete matrix with sufficient moisture due to the permeability of concrete. At the anode, iron is oxidized to ferrous ions: Fe → Fe2+ + 2e– At the cathode, a reduction reaction takes place. In an alkaline environment, the reduction reaction is typically 2H2O + O2 + 4e – → 4OH– For the corrosion process to be initiated, the passive oxide film on the reinforc- ing steel must be broken. In most cases breaking the oxide film occurs as a result of the presence of sufficient quantities of chloride ions in the concrete matrix at the level of the steel. Chloride-induced corrosion is most commonly observed in structures exposed to roadway deicing salts or in marine environments with direct exposure to salt water or wind-borne sea spray. Chlorides can also be introduced into the con- crete during the original construction by the use of contaminated aggregates, water, or chloride-containing admixtures.

237 Chapter 5. CORROSiON OF STEEL iN REiNFORCED CONCRETE BRiDGES Over time, the corroding area (anode) will become more acidic as hydroxyl ions (OH–) are consumed from the concrete in contact with the corroding area, and the cathode will become more alkaline by the generation of hydroxyl ions (OH–). 5.2.2 Diffusion Process The main source for this section and the next (5.2.3) is Whitmore (2002). To the casual observer, uncracked concrete is a solid and impenetrable material. Viewed under a micro scope, however, concrete is a labyrinth of fine capillaries, pores, and voids be- tween the individual cement and aggregate particles. The degree of porosity depends on the quality and density of the concrete mix. As a result of this porosity, liquids can soak into the exposed surfaces of concrete and carry contaminants such as chloride ions with them. Over time, the concentration of chloride ions within the concrete will tend to equalize as governed by Fick’s law. In a one-dimensional case, Fick’s law can be expressed as shown in Equation 5.1: C C x D t 1 erf 2x t c , 0= −      ( ) (5.1) where C(x,t) = chloride concentration at depth x and time t, C0 = surface chloride concentration (kg/m 3 or lb/yd3), Dc = chloride diffusion constant (cm 2/year or in.2/year), and erf = error function (from standard mathematical tables). This expression indicates that over time the chloride concentration within the con- crete will tend to equalize with the chloride concentration exposed to the surface. As expected, the chloride concentration within the concrete is greater near the exposed surface and increases with time at any point within the concrete. Concrete with a lower chloride diffusion constant (Dc) will provide longer-term protection to reinforc- ing steel located at depth x from the surface of the concrete. The diffusion constant for a particular point in a concrete structure may be deter- mined if chloride data are available for one location at two points in time or if a com- plete chloride profile is available some time after the structure has been constructed. With current chloride data and an estimate of the diffusion coefficient, future chloride profiles can be predicted using the formula in Equation 5.1. Figure 5.1 displays the chloride concentration within concrete over time. Figure 5.2 shows the chloride contents with depth. Based on the chloride profile, the calculated (best-fit) diffusion coefficient Dc is 4.38 × 10 –13 cm2/s. If the concrete element is cracked, chloride penetration at crack locations may greatly exceed chloride levels in the surrounding concrete. This level of chloride pen- etration can lead to corrosion initiation at crack locations long before general corro- sion may otherwise occur.

238 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 5.2.3 Corrosion Cells Once the chloride concentration at the depth of the reinforcing steel exceeds threshold levels, the passive oxide film will begin to degrade, and corrosion may be initiated. Chlorides act similarly to a catalyst in the corrosion process: the chlorides are involved in the corrosion reaction, but they are generally not consumed by the corrosion reac- tion itself, such that a single chloride ion can be responsible for the corrosion of many atoms of iron. Figure 5.1. Chloride concentration within concrete over time. Figure 5.2. Chloride contents with depth. Test Values Fit Using Equation 5.1; C0 fixed

239 Chapter 5. CORROSiON OF STEEL iN REiNFORCED CONCRETE BRiDGES On a localized basis, corrosion cells can be formed as a result of differences in chloride concentration at various locations along a single bar. These variations can result in localized pitting-type corrosion. Similarly, if entire sections of a reinforced concrete structure become contaminated relative to other adjacent areas, an overall corrosion cell or “macrocell” can be created, as illustrated in Figure 5.3. Macrocell corrosion can be very aggressive and is responsible for much of the severe structural damage seen on bridges and other structures. Both pitting-type corrosion and general corrosion result from corrosion cells (Whitmore 2002). The corrosion products (rust) occurring as a result of macrocell corrosion occupy a large volume and cause cracking, concrete delamination, and spalls. 5.2.4 Patch-Accelerated Corrosion Commonly referred to as ring anode corrosion or halo effect, patch-accelerated cor- rosion is a phenomenon specific to concrete restoration projects (Figure 5.4). When repairs are completed on corrosion-damaged structures, abrupt changes in the con- crete surrounding the reinforcing steel are created. Typical concrete repair procedures call for the removal of the concrete around the full circumference of the reinforcing steel within the repair area, cleaning corrosion byproducts from the steel, and refill- ing the cavity with new chloride-free, high-pH concrete. These procedures leave the Fe Fe2+ + 2Fe(O FeCl 2 2O 1 / 2 O 2 Fe 2+ + 2e - 2Cl - FeCl 2 H) 2 + 1 /2 O2 + 2OH - Fe(O H - + H 2 O + 2e - 2O C Fe2 O3 + 2H 2 O H) 2 + 2Cl - H - hloride-Contami Concrete 2e - nated Figure 5.3. Corrosion macrocell in a concrete deck. Figure 5.4. Patch-accelerated corrosion.

240 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE reinforcing steel embedded in adjacent environments with abruptly different corrosion potentials. This difference in corrosion potential (voltage) is the driving force behind new corrosion sites forming in the surrounding contaminated concrete. The evidence of this activity is the presence of new concrete spalling adjacent to previously com- pleted patch repairs. 5.3 FActorS inFLuencing corroSion One of the leading causes of concrete rehabilitation is corrosion-induced concrete damage and spalling in reinforced concrete structures (Figure 5.5). In steel-reinforced concrete, the concrete matrix must be sufficiently strong to resist applied forces from a structural standpoint and to serve as a corrosion-protection mechanism for the em- bedded reinforcing steel. The ability of concrete structures to resist corrosion attack is not related to the mechanical strength of the concrete alone. Instead, two important factors limit the ability of concrete structures to resist corrosion: the presence of cracks and the porosity of the concrete. The presence of cracks and the ability of chlorides to permeate the concrete allow chlorides to get to the reinforcing steel, thus compromising the corrosion resistance provided by concrete’s naturally high alkalinity. As discussed in Ball and Whitmore (2005), numerous factors may influence the durability of concrete, including the water-cement ratio, permeability, curing, shrink- age and cracking, ingredients including admixtures, and the severity of environmental exposure. Due to the high alkalinity of the concrete pore water solution, a thin passive oxide layer is formed and maintained on the surface of the embedded steel that pro- tects it from corrosion activity. Until this passive film is destroyed by the intrusion of aggressive elements or a reduction in the alkalinity of the concrete, the reinforcement will remain in a passive, noncorroding state. The causes of corrosion are summarized in Figure 5.6. Chloride contamination and carbonation are explained in the sections that follow. Figure 5.5. Corrosion-induced delamination on a bridge pier.

241 Chapter 5. CORROSiON OF STEEL iN REiNFORCED CONCRETE BRiDGES 5.3.1 Chloride Contamination Destruction of the protective oxide film on reinforcing steel is most often caused by the presence of elevated levels of chloride ions. The chloride threshold that initiates corro- sion is generally considered to be around 1.0 to 1.4 lb of water-soluble Cl– per cubic yard of concrete (at the level of the steel). This chloride threshold varies depending on the pH of the concrete. For example, concrete that has experienced a loss of alkalinity requires less chloride to initiate corrosion. Chloride-induced corrosion, illustrated in Figure 5.7, is common in structures exposed to deicing salts, marine environments, or certain industrial processes. In some cases, sufficient amounts of chlorides capable of causing corrosion have been introduced during construction by the use of chloride- containing admixtures or contaminated aggregates. Causes of Corrosion of Steel in Concrete Chlorides Cast- In Chlorides Surface-Applied Chlorides Carbonation Dissimilar Metals Figure 5.6. Causes of corrosion of steel in concrete. Figure 5.7. Chloride-induced corrosion of reinforcing steel.

242 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Non-chloride-bearing salts, including calcium magnesium acetate, magnesium acetate, and calcium acetate, lower the freezing temperature of water and can be used for ice control. However, magnesium-bearing solutions cause severe paste deteriora- tion by forming brucite and noncementitious magnesium silicate hydrate (Lee et al. 2000). The detrimental effects caused by calcium acetate are much less severe than those caused by magnesium, but the use of these non-chloride-bearing salts has not gained wide acceptance due to cost and the distress they cause. 5.3.2 Carbonation The main source for this section is Ball and Whitmore (2005). The passive condition of the reinforcing can also be disrupted by the loss of alkalinity in the concrete matrix surrounding the reinforcing steel. It is generally accepted that a pH greater than 10 is sufficient to provide corrosion protection in chloride-free concrete. The reduction in alkalinity is generally caused by carbonation, a reaction between atmospheric carbon dioxide and calcium hydroxide in the cement paste in the presence of water. The result is a reversion of the calcium hydroxide to calcium carbonate (approximate pH 8.5), which has insufficient alkalinity to maintain the passive oxide layer. The zone of carbonation begins on the surface of atmospherically exposed con- crete. The amount of time for the carbonation zone to reach the level of the reinforcing is a function of the thickness of concrete cover, presence and extent of cracks, concrete porosity, humidity levels, and the level of exposure to carbon dioxide. Carbonation- induced corrosion is more likely to be observed in structures situated in industrial envi- ronments, where airborne pollutants are commonplace; in old or historic structures with a high degree of concrete porosity; or in structures with low concrete cover over the reinforcement. In bridge structures, there is generally good-quality concrete cover of sufficient thickness (about 2.5 in. in decks) to resist carbonation for up to 100 years. 5.4 StrAtegieS For AddreSSing corroSion Because of the magnitude of the corrosion problem, both the public and private sectors have ongoing activities aimed at reducing or eliminating corrosion damage to concrete structures. Although many technologies and materials have been developed for the prevention and repair of corrosion-induced damage, the challenge is to select durable, cost-effective technologies and materials from the numerous choices available. Durability and desired service life must be considered during design. In the case of new structures, it is desirable to avoid, prevent, or delay the initiation of corrosion through the use of low-permeability concretes, proper precautions against cracking (see Chapter 3), and other corrosion-prevention techniques. For existing structures, the condition of the structure should be evaluated to determine whether it is corroding. If the structure is corroding and the chloride content is high in the concrete, the con- taminated concrete is removed; if rust is forming on the surface of the reinforcement, it is cleaned or removed and overlaid with a low-permeability concrete overlay. If the

243 Chapter 5. CORROSiON OF STEEL iN REiNFORCED CONCRETE BRiDGES existing structure is not corroding, some of the techniques used on new structures, such as applying sealers and membranes, may also be used on the existing structure. An overview of the available options is provided in Figures 5.8 through 5.11. Many strategies have been used successfully to improve the corrosion resistance and durability of new structures. These strategies include • The use of low-permeability concrete; • The use of increased concrete cover; • The use of improved construction methods such as curing to minimize cracking; • The use of corrosion-resistant reinforcement; • The use of corrosion inhibitors to increase the corrosion initiation threshold; • The use of membranes, coatings, and sealers; and • The use of improved design details to keep elements dry and to prevent exposure to chlorides. In the United States, epoxy-coated reinforcement has been widely used as a corrosion-protection system for concrete bridges. However, because recent work and observations in the field have shown that the longevity desired (75 years and beyond) may not be achievable, other corrosion-resistant reinforcements are being considered (see Chapter 3 on materials). Each of these methods can be effective if it significantly extends the time for cor- rosion to initiate. In many cases it is preferable to employ more than one technique, which will generally reduce the overall risk of corrosion. Additional information on these topics is provided in Chapter 3 on materials. Figure 5.8. Reduced service life of reinforced concrete. Reduced Service Life of Reinforced Concrete Structures New Structures (Not Corroding) Avoid Corrosion Prevent/Delay Corrosion Initiation Electro chemical Passivation (Immunity) Existing Structures Currently Not Corroding Currently Corroding Figure 5.8. Reduced service life of reinforced concrete.

244 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE Figure 5.10. Options for preventing or delaying corrosion initiation. Prevent/Delay Corrosion Initiation External— Eliminate Exposure Internal— No Contamination. No Cast-in Cl– Delay Corrosion Initiation Use Protective Barrier Use Low- P ermeability Concrete Reduce/Eliminate Cracks Increase Concrete Cover Thickness Increase Corrosion Threshold (i.e., Inhibitor) Electrochemical Passivation (Immunity) Galvanic Impressed Current Treatment Figure 5.10. Options for preventing or delaying corrosion initiation. Figure 5.11. Electrochemical passivation. Figure 5.9. Options for avoiding corrosion. Avoid Corrosion Use Corrosion- r esistant Reinforcing Materials Stainless Steel Corrosion- resistant Steel Fiber-reinforced Polymer (FRP) Reinforcing Eliminate Cause of Corrosion External Exposure (CI– or Carbonation) Internal— No Contamination. No Cast-in CI .– Figure 5.9. Options for avoiding corrosion.

245 Chapter 5. CORROSiON OF STEEL iN REiNFORCED CONCRETE BRiDGES 5.4.1 Existing Structures Options for protecting structures from corrosion and extending their service life are much more limited when dealing with existing structures, as many of the physical parameters are already defined and cannot be changed or easily altered. In particular, the concrete and reinforcing steel of existing structures are already in place, and the characteristics of these materials, including type, quality, cover thickness, permeability, resistance to corrosion initiation, and presence of cracks, are already fixed. Because of these limitations, many of the options that are viable for new (noncorroding) construc- tion, as shown in Figure 5.12, are not possible or may not be economically practical for use with existing (corroding) structures. Figure 5.13 shows the options for corrod- ing structures. Despite the reduced number of options available for existing structures, it is still beneficial to be as proactive as possible. Preventing or delaying corrosion is generally preferable to managing corrosion after it has initiated. Additional options (and often more economical options) exist to prevent or delay corrosion activity if the structure is not chloride contaminated or has not already started to corrode. Figure 5.12. Noncorroding structures. Structures not Currently Corroding Cl– Contaminated Carbonated to Steel Treatment to Delay Corrosion Initiation Remove Source Do Nothing Not Cl– Contaminated Corrosion Prevention Treatment to Slow Deterioration Galvanic Impressed Current Electrochemical Treatment Figure 5.12. Noncorroding structures.

246 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE 5.4.2 Levels of Corrosion Protection for Existing Structures 5.4.2.1 Selecting an Active Corrosion-Protection Strategy for Reinforced Concrete Structures The selection of the appropriate level of corrosion protection is based on many fac- tors, such as the level of chloride contamination and carbonation, amount of concrete damage, location of corrosion activity (localized or widespread), the cost and design life of the corrosion-protection system, and the expected service life of the structure (Ball and Whitmore 2005). The levels of corrosion protection described in this section are summarized in Table 5.1. Figure 5.13. Options for corroding structures. Figure 5.13. Options for corroding structures. Options for Currently Corroding Structures Do Nothing Treatment to Reduce Rate of Deterioration Partial Removal and Replacement Cathodic Protection Electrochemical Treatment Impressed Current Galvanic tAbLe 5.1. SummAry oF LeveLS oF corroSion Protection For eLectrochemicAL corroSion mitigAtion SyStemS Level of Protection Description Cathodic protection Stops active corrosion by applying ongoing electrical current Corrosion prevention Prevents new corrosion activity from initiating Corrosion control Significantly reduces active corrosion Corrosion passivation– electrochemical treatment Stops active corrosion by changing the chemistry of the concrete around the steel

247 Chapter 5. CORROSiON OF STEEL iN REiNFORCED CONCRETE BRiDGES 5.4.2.2 Cathodic Protection Cathodic protection provides proven corrosion protection and is intended to effec- tively stop ongoing corrosion activity. It should be selected when the highest level of protection is necessary and the cost is economically justified. Cathodic protection sys- tems are grouped into two general categories: impressed current and galvanic. Impressed current systems may use titanium- or zinc-based anodes and an outside power source. For long-term performance, these systems should be monitored and maintained. Discrete anodes are ideal to protect heavily reinforced concrete, thick structural sections such as columns or beams, or steel-framed masonry buildings; tita- nium ribbon or mesh anodes are placed in slots cut into the concrete surface or cast into a concrete overlay. Galvanic systems may be designed to provide corrosion control or cathodic protec- tion. These systems are self-powered and typically require less monitoring and main- tenance than impressed current systems. Galvanic jackets are used to protect marine pilings and other structures. Galvanic anodes may be arc-sprayed zinc or otherwise applied to the concrete surface, or they may be cast into a concrete overlay, jacket, or encasement to provide galvanic cathodic protection over a desired area. If galvanic anode systems are cast into concrete or are not directly exposed to a marine envi- ronment, they should be activated to ensure that sufficient current is supplied to the reinforcing steel to provide long-lasting corrosion protection. Cathodic protection systems are generally designed to meet National Associa- tion of Corrosion Engineers (NACE) cathodic protection standards and typically use 100-mV depolarization as the acceptance criteria (NACE 2000). The current density required to achieve cathodic protection is higher than the current required for corro- sion prevention or corrosion control applications. Typical cathodic protection systems operate in the range of 2 to 20 mA/m2 of steel surface area. At these current densities and polarization levels, cathodic protection has demonstrated a very high level of cor- rosion protection (Ball and Whitmore 2005). Galvanic cathodic protection systems were evaluated in the research phase of SHRP 2 Project R19A, the report for which is forthcoming. In this study, four gal- vanic anodes were evaluated for the purpose of minimizing corrosion in reinforced concrete members: a commonly used (ordinary) anode, an anode with a zinc surface area four times larger than ordinary (OA4) anodes, and two high-voltage anodes with different degrees of output voltage (a higher-level high-voltage anode and a lower-level high-voltage anode). Concrete test slabs were cast in two layers; the concrete in the upper layer was contaminated with salt to accelerate the corrosion activity, but the lower layer was not modified. (Salt was not added to the concrete in the lower layer.) The test results indicated that there was no corrosion in any of the specimens in the given time period. The testing further indicated that specimens with the higher-level high-voltage anode provided increased corrosion protection by having higher current and generating more negative potential values than specimens with the lower-level high-voltage anode. OA4 anodes provided additional current and corrosion protec- tion compared with the ordinary anodes. Lower-level high-voltage anodes exhibited

248 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE current and potential values similar to the OA4 anodes. The two high-voltage anodes and the OA4 anodes provided higher current and generated more negative potential values, indicating better corrosion protection, than ordinary anodes. Because of time constraints, the tests were terminated without observing corrosion in the specimens. Further research with an extended time frame is recommended. 5.4.2.3 Corrosion Prevention Corrosion prevention is used to keep corrosion activity from initiating in contaminated concrete. In concrete repair projects, the removal and replacement of damaged con- crete, if completed in accordance with industry guidelines (ICRI 2008), will address the areas with the highest levels of corrosion. However, new corrosion sites are likely to form in the surrounding contaminated concrete that was passive before the repairs. To mitigate new corrosion activity from occurring around concrete repairs or at other interfaces between new and old concrete, such as bridge widening, joint repairs, and slab replacements, a localized corrosion prevention strategy may be employed using embedded galvanic anodes to extend the life of the concrete repairs. Size and spacing of embedded galvanic anodes should be adjusted to suit site conditions, including quantity and existing condition of reinforcing steel, level of chloride contamination, and environmental conditions. Ball and Whitmore (2005) point out that there has been a significant amount of research in corrosion prevention, some of which has indicated that applied current densities as low as 0.5 to 2.0 mA/m2 of steel surface area are effective at preventing the initiation of corrosion for concrete with chloride concentrations up to at least 10 times the chloride threshold (Pedeferri 1996). Other research has shown beneficial effects of applied currents between 0.25 and 1.0 mA/m2 (Bertolini et al. 1996). 5.4.2.4 Corrosion Control Corrosion control systems are used when corrosion has initiated but has not yet pro- gressed to the point of causing concrete damage. The use of corrosion control systems will either stop ongoing corrosion activity or provide a significant reduction in the corrosion rate and thus increased service life for the rehabilitated structure. In many cases, this level of protection can be provided with low incremental cost, as the protec- tion can be targeted at specific areas of contamination or corrosion activity. Galvanic anodes embedded in drilled holes or installed on a grid pattern in a concrete repair or overlay can be used to provide targeted galvanic corrosion control to columns, beams, decks, posttensioned anchorages, and other areas where ongoing corrosion activity threatens the service life or serviceability of the structure. The applied current necessary to control active corrosion is significantly higher than the current required for corrosion prevention (Ball and Whitmore 2005). Davison et al. (2003) achieved corrosion control of specimens with very high initial corrosion rates. Their research indicated that the typical current density to control active corro- sion is in the range of 1 to 7 mA/m2 (Davison et al. 2003). Some polarization of the reinforcing steel will typically occur at these current densities, although the level of

249 Chapter 5. CORROSiON OF STEEL iN REiNFORCED CONCRETE BRiDGES polarization may be significantly less than the NACE 100-mV depolarization criteria for cathodic protection. In many situations corrosion activity is moderate or localized so that corrosion control is an appropriate approach because large-scale cathodic protection may not be economically justifiable (Ball and Whitmore 2005). Examples of such situations include localized areas beneath leaking expansion joints or decks with isolated areas of high corrosion potentials. In these cases, targeting the protection to address the specific contaminated zone, or hot spot, rather than the entire structure may make sense from a cost–benefit point of view. 5.4.2.5 Corrosion Passivation by Electrochemical Treatment Corrosion passivation is provided by electrochemical treatments that are aimed at directly addressing the cause of the corrosion activity. Electrochemical chloride extrac- tion (ECE) is used to address corrosion caused by chlorides in chloride-contaminated structures. Electrochemical realkalization is used to address corrosion resulting from carbonation of the concrete. These systems are installed on the structure, operated for a short duration, and then dismantled and removed, leaving the structure in a pas- sive condition. Electrochemical treatments provide many of the long-term corrosion mitigation benefits of cathodic protection systems, but without the need for long-term system maintenance and monitoring. Additional information about the implementa- tion and evaluation of the two systems follows. ECE is an electrochemical treatment in which an electric field is applied between the reinforcement in the concrete and an externally mounted mesh (Figure 5.14). The mesh is embedded in a conductive media, generally a sprayed-on mixture of lime, water, and cellulose fiber. During treatment, as pointed out by Ball and Whitmore (2005), the concrete is kept saturated, which allows chlorides to go into solution within the pores of the concrete. The negatively charged chloride ions (Cl–) are repelled from the negatively charged rebar and attracted toward the positively charged external electrode mesh as a result of the applied electric field. This process lowers the amount of chloride in the concrete, particularly adjacent to the steel. An ECE treatment generally takes 4 to 8 weeks to complete. In instances of structures with carbonation-induced corrosion, a different electro- chemical treatment process, realkalization, can be used to increase the pH of the con- crete cover. The installation for realkalization is essentially the same as for ECE except the conductive media is saturated with an alkaline potassium carbonate solution. The potassium (K+) ions in the alkaline solution are transported into the concrete by the application of the electric field. A realkalization treatment generally takes 4 to 7 days to complete and will not recarbonate (Ball and Whitmore 2005). Electrochemical treatment was evaluated in the research phase of SHRP 2 Project R19A; detailed results will be presented in the forthcoming final report. In the SHRP 2 study, corrosion-resistant reinforcement and electrochemically treated black bars were evaluated to determine if the chloride threshold levels increased to a level to resist corrosion. Both low and high levels of electrochemical treatment were used. The test

250 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE matrix included black bars, electrochemically treated black bars, stainless steel bars, and titanium bars, each subjected to salt solution. Because of time constraints, testing was terminated after 26 cycles consisting of 4 days of wet cycle and 10 days of dry cycle for a total duration of 1 year. At termination, only one specimen from a set of three with black bars and electrochemically treated black bars showed an increase in current or potential values indicative of uncertain corrosion activity. The remaining specimens indicated no corrosion activity. Thus, initial observations indicate that elec- trochemically treated black bars may not provide the protection expected of stainless steel or titanium; however, whether they provide benefits over the black bars without treatment could not be concluded from this study due to time constraints. Further research with an extended time frame is recommended. 5.5 cASe StudieS AddreSSing corroSion in exiSting StructureS 5.5.1 Project overview: impressed Current Cathodic Protection, Corrosion Control, and Electrochemical Chloride Extraction During summer 1989 and continuing until 1994, the Ontario Ministry of Transporta- tion completed various restoration and protection projects on the reinforced concrete piers of the Burlington Skyway, a major viaduct located between Toronto and Niagara Figure 5.14. Schematic diagram showing ECE treatment process. Figure 5.14. Schematic diagram showing ECE treatment process.

251 Chapter 5. CORROSiON OF STEEL iN REiNFORCED CONCRETE BRiDGES Falls, Ontario. This work was monitored by the Ministry’s Materials and Research Branch. Some of this research was conducted under Project SHRP-C-620 and is docu- mented in Evaluation of NORCURE Process for Electrochemical Chloride Removal from Steel-Reinforced Concrete Bridge Components (Bennet and Schue 1993). An impressed current cathodic protection system was installed on more than 200,000 ft2 of reinforced concrete substructure. The system was designed to operate as an impressed current system with an applied current density of 10 mA/m2 (1 mA/ ft2). After an initial period of operation at the design current density, and because of operational issues, the system was run at an average current density of 1 mA/m2 (0.1 mA/ft2). Thus, instead of being operated at a cathodic protection current density (2 to 20 mA/m2), the system was effectively operated at a corrosion control current density (1 to 7 mA/m2). The low applied current density meant that much of the area did not meet the 100-mV NACE criteria for cathodic protection. Despite operating at approximately 10% of the intended cathodic protection current density, the sys- tem operation fell within typical corrosion control current densities and experienced a significant reduction in concrete deterioration and damage. Over the study period, concrete delamination within the protected area was reduced by 96% compared with the rate of deterioration of unprotected concrete piers. In 1989 the Ontario Ministry of Transportation also completed an ECE trial on a section of the same structure. The treated portion comprised rectangular piers and bents. The piers were contaminated with chlorides from long-term leakage of the deck joints above. ECE was used to reduce the chloride content of the concrete to below the thresh- old level of corrosion at the rebar. This process eliminated high corrosion-potential readings, and corrosion potentials in the treated area were shifted into the passive range, as shown in Table 5.2; the ECE-treated section exhibited a high percentage of area with a low risk of corrosion. Corrosion current as measured by linear polariza- tion was greatly reduced, as shown in Table 5.3; the ECE-treated sections exhibited a high percentage of passive area. Thus, corrosion potentials and corrosion currents were reduced to the passive, noncorroding range as a result of the ECE treatment for a 20-year duration (Tables 5.2 and 5.3). These readings have remained stable and show no significant changes over the period. Additional piers were treated during the fall of 1997 as part of the next phase of work on this project. The long-term results from these more recent tests are expected to be similar to the ECE trial completed in 1989. 5.5.2 Project overview: galvanic Cathodic Protection Using galvanic Encasement of Severely Corroded Elements The source for this section is Sergi (2009). Galvanic anodes have also been developed for more global corrosion control. One such configuration is the system installed to repair and protect severely damaged and corroding bridge abutments in the Midwest (Figure 5.15). The abutments had been contaminated with chlorides causing corrosion of the reinforcing steel and significant concrete damage.

252 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE tABLE 5.2. corroSion PotentiAL meASurementS Time Since Treatment Untreated (Control) ECE Treated Area (%) with Low Risk of Corrosion <200 mV Area (%) with Moderate Risk of Corrosion 200–350 mV Area (%) with High Risk of Corrosion >350 mV Area (%) with Low Risk of Corrosion <200 mV Area (%) with Moderate Risk of Corrosion 200–350 mV Area (%) with High Risk of Corrosion >350 mV Pretreatment 0 85 15 0 96 4 1 Year after 41 59 0 98 2 0 2 Years after 41 59 0 100 0 0 3 Years after 26 74 0 96 4 0 4 Years after 26 70 4 98 2 0 6 Years after 26 59 15 96 4 0 8 Years after 11 78 11 96 4 0 10 Years after 15 78 7 96 4 0 15 Years after 20 70 10 98 2 0 20 Years after 15 70 15 96 4 0 Note: Corrosion potentials measured in –mV versus Cu-CuSO4. Values represent percentage of readings within range. tABLE 5.3. corroSion current meASurementS Time Since Treatment Untreated (Control) ECE Treated Area (%) with Passive 0 <0.22 mA/cm2 Area (%) with Low Corrosion 0.22–1.08 mA/cm2 Area (%) with High Corrosion >1.08 mA/cm2 Area (%) with Passive <0.22 mA/cm2 Area (%) with Low Corrosion 0.22–1.08 mA/cm2 Area (%) with High Corrosion >1.08 mA/cm2 Pretreatment 0 46 54 0 87 13 1 Year after 0 52 48 87 13 0 2 Years after 0 84 16 78 20 2 3 Years after 4 88 8 89 11 0 4 Years after 8 86 6 90 10 0 6 Years after 0 40 60 65 35 0 8 Years after 0 29 71 63 37 0 15 Years after 0 62 38 65 34 1 20 Years after 15 70 15 96 4 0 Note: Values represent percentage of readings within range.

253 Chapter 5. CORROSiON OF STEEL iN REiNFORCED CONCRETE BRiDGES As part of the rehabilitation, which also included enlargement and strengthening of the abutment, the cracked and spalled concrete was removed. Elongated anodes were connected to the existing reinforcing steel and encased in a new layer of concrete to reface the abutment wall. The purpose of the anodes was to protect the existing steel from chloride-induced corrosion. This allowed uncracked chloride-contaminated concrete to remain in place and thereby reduce concrete breakout and the need for structural shoring. The cross-sectional configuration of the repaired abutment wall and adjoining structural elements is shown in Figure 5.16. Figure 5.15. A distributed anode system used for corrosion control bridge abutment in the Midwest. Distr anod ibuted es Figure 5.16. Cross-sectional detail of the abutment rehabilitation system. Distributed anode system

254 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE The current output, shown in Figure 5.17, appears to be strongly related to tem- perature. Its magnitude varied considerably with temperature on an annual basis, with the mean current density gradually reducing year by year. After supplying an initial current of over 35 mA/m2 of steel area in the first few days, the output averaged over 8 mA/m2 during the first year, decreasing gradually to around 5 mA/m2 in the fourth year. These levels of current density are within the design limits of 2 to 20 mA/m2 of steel area for cathodic protection as specified in European Standard BS EN 12696:2012 (BSI 2012). Current densities in impressed current cathodic protection systems are also normally reduced with age as the steel becomes easier to polarize. Depolarization levels were measured to be well in excess of 100 mV, as specified in the same standard, suggesting that the galvanic system was able to satisfy the criteria for cathodic protec- tion of steel reinforcement. Depolarization and corrosion-protection status are given in Table 5.4. 5.5.3 Project overview: Corrosion Prevention Using galvanic Anodes The oldest site trial of discrete galvanic anodes is over 10 years old (Figure 5.18). To verify the performance of the anodes, 12 anodes were installed in an otherwise con- ventional patch repair on the soffit of a bridge beam (Figure 5.19). The performance of these anodes was monitored over time. 0 20 40 60 80 100 120 0 5 10 15 20 25 30 35 40 45 50 55 60 May-05 Nov-05 May-06 Nov-06 May-07 Nov-07 May-08 Oct-08 May-09 Oct-09 May-10 Oct-10 May-11 Oct-11 May-12 Te m pe ra tu re , ° F G al va ni c C ur re nt , m A Date Galvanic Current Manual Current Temperature Figure 5.17. Current output of anode system and its relationship to temperature. Figure 5.17. Current output of anode system and its relationship to temperature.

255 Chapter 5. CORROSiON OF STEEL iN REiNFORCED CONCRETE BRiDGES tABLE 5.4. dePoLArizAtion And corroSion-Protection StAtuS Date Temperature (°F) Current Density (mA/ft2) Depolarization (mV) Status May 6, 2005 88 >3.5 na Corrosion protected Aug. 16, 2005 87 1.23 344 Corrosion protected Oct. 26, 2005 54 0.52 368 Corrosion protected Dec. 7, 2005 51 0.28 310 Corrosion protected May 1, 2006 57 0.70 313 Corrosion protected Dec. 20, 2006 40 0.36 459 Corrosion protected May 30, 2007 79 0.72 449 Corrosion protected Sept. 20, 2007 75 0.88 482 Corrosion protected Dec. 19, 2008 40 0.34 450 Corrosion protected July 9, 2009 74 0.27 471 Corrosion protected May 11, 2010 54 0.33 485 Corrosion protected Oct. 16, 2011 72 0.57 488 Corrosion protected Note: na = not applicable. Figure 5.18. North side of bridge with discrete galvanic anodes. Source: Sergi 2009. Figure 5.19. Installation of anodes within the repaired area of a beam; control box and wiring are also shown. Source: Sergi 2009.

256 DESiGN GUiDE FOR BRiDGES FOR SERviCE LiFE The anodes were inserted around the perimeter of the repair area between 600- and 700-mm centers (Figure 5.19). The anodes were installed to enable monitoring by connecting a single wire from each anode to a control box such that the anodes could be monitored via the box. Monitoring of the anodes consisted of measuring the current output for each installed anode and, on occasion, performing a depolarization test over a 4- or 24-hour period after disconnection of the anodes. Monitoring started in April 1999. The main source for the rest of this chapter is Sergi (2009). Ten-year results of the current output of each anode are presented in Figure 5.20. They indicate a variable current depending on the ambient temperature and moisture content in the concrete. For example, the same anode could generate up to 400 to 600 μA of current during hot periods and less than 100 μA during cold spells. Corrosion of the steel is expected to have similarly varying corrosion rates so that the current output of the anodes is thought to be self-regulating, producing higher levels when the steel is corroding most. Systems such as this are designed to prevent the onset of corrosion of the reinforce- ment. The current density required for corrosion prevention (referred to as cathodic prevention in Europe) is 0.2 to 2 mA/m2, as reported by Bertolini et al. (1993) and Pedeferri (1996), and adapted in the European Standard BS EN 12696:2012 (BSI 2012). Based on the steel surface area within the repaired and affected adjacent areas, the mean current density ranged between 0.6 and 3.0 mA/m2 through the duration of Figure 5.20. Protective current from galvanic anodes over a 10-year period. . Figure 5.20. Protective current from galvanic anodes over a 10-year period.

257 Chapter 5. CORROSiON OF STEEL iN REiNFORCED CONCRETE BRiDGES this trial, with a mean current density of around 1.4 mA/m2 over the 10-year period. This current density is within the suggested range of 0.2 to 2.0 mA/m2 for corrosion prevention (cathodic prevention). Monitoring the depolarized potential of the steel in the vicinity of the repair with time may be another way of determining the effectiveness of the system. Figure 5.21, which shows the mean depolarized potential with time both within and outside the repaired area, indicates that the mean potential is moving to a more noble level with time. This change indicates increasing passivation of the steel over time. Figure 5.21. Mean depolarized steel potentials with time (4 or 24 hours after disconnection of the anodes). Source: Sergi 2009. Sourc : Sergi (2009). Figure 5.21. Mean depolarized steel potentials with time (4 or 24 hours after disconnection of the anodes). -400 -350 -300 -250 -200 -150 -100 -50 0 50 100 10 000010001001 Time (Days) 300 mm Outside Repair 50 mm Outside Repair Within Repair St ee l P ot en ti al (m V v s. C u- Cu SO 4)

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TRB’s second Strategic Highway Research Program (SHRP 2) Report S2-R19A-RW-2: Design Guide for Bridges for Service Life provides information and defines procedures to systematically design new and existing bridges for service life and durability.

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