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Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements (2006)

Chapter: Chapter 5 - Extension of Service Life with Repair and CorrosionMitigation Options

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Suggested Citation:"Chapter 5 - Extension of Service Life with Repair and CorrosionMitigation Options." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
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Suggested Citation:"Chapter 5 - Extension of Service Life with Repair and CorrosionMitigation Options." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 5 - Extension of Service Life with Repair and CorrosionMitigation Options." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 5 - Extension of Service Life with Repair and CorrosionMitigation Options." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 5 - Extension of Service Life with Repair and CorrosionMitigation Options." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 5 - Extension of Service Life with Repair and CorrosionMitigation Options." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 5 - Extension of Service Life with Repair and CorrosionMitigation Options." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 5 - Extension of Service Life with Repair and CorrosionMitigation Options." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 5 - Extension of Service Life with Repair and CorrosionMitigation Options." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 5 - Extension of Service Life with Repair and CorrosionMitigation Options." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
×
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Suggested Citation:"Chapter 5 - Extension of Service Life with Repair and CorrosionMitigation Options." National Academies of Sciences, Engineering, and Medicine. 2006. Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements. Washington, DC: The National Academies Press. doi: 10.17226/13934.
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26 This chapter briefly discusses various corrosion control strategies that might be considered for rehabilitating a rein- forced concrete structure experiencing deterioration due to chloride-induced corrosion. This chapter provides an overview and should be used as a starting point for obtaining more information on the actual systems being considered because new products, and new forms of old products, are introduced regularly. In each broad category are many prod- ucts and systems offered by numerous vendors; all products and systems in a category are not equal, and each product and system has its own set of applications and limitations. Some, but not all, products and systems currently available have been evaluated in a controlled setting. The best source of information on many of the products and systems are the results of the research studies commissioned by various gov- ernment and semi-government agencies. When considering performance history, it is necessary to consider the type and level of monitoring performed to report the performance. The fact that a product or system was installed on numerous structures does not vouch for its ability to perform. The various categories of corrosion control systems are discussed under the appropriate subdivision of local or global systems. Corrosion inhibitors can be used locally and globally; therefore, they are discussed first in the local cat- egory, and their global application is discussed in the global category. Local Corrosion Control Systems Patching Materials Patching materials are used to replace localized areas of dete- riorated (i.e., spalled and delaminated) concrete. Because of the expansive forces involved in the corrosion process, deteriora- tion of reinforced concrete structures caused by corrosion ulti- mately results in exposure of the reinforcing bars. Patches usually have a short service life because they do not address the cause of the problem (i.e., corrosion of the reinforcing bars). When concrete contaminated with chlorides in concentrations greater than the threshold level is left in place in the area sur- rounding the patches, inadvertent acceleration of the rate of the corrosion process occurs. The patched area acts as a large non- corroding site (i.e., cathodic area) adjacent to corroding sites (i.e., anodic areas), and thus corrosion cells are created. Patching on bridge decks may be full-depth or partial-depth repairs. Deterioration of bridge decks due to corrosion of the reinforcing steel involves either the top layer of reinforcing steel only or both the top and bottom layers of reinforcement. If only the top layer of reinforcing steel is corroding, a partial- depth repair would be appropriate. For partial-depth deck repairs, deteriorated concrete is removed to the depth required to provide a minimum of 1.0 inch clearance below the top layer of reinforcing steel. The maximum depth of removal for a partial-depth repair should not exceed half the deck thickness. Corrosion of both the top and bottom layers of reinforcing bars often requires full-depth repairs. For full-depth repairs, deteri- orated concrete is removed through the entire thickness of the concrete element. Partial-depth deck patching materials include portland cement concrete (PCC) and mortar, quick-set hydraulic mor- tar and concrete, polymer mortar and concrete, and HMA. Because of their short service life (1 year or less), HMA patches should only be considered as temporary patches. PCC is typically used for full-depth deck patches and large partial- depth repairs. The service life of deck patches somewhat depends on the type of patch (full- or partial-depth) and the patch material. The service life of deck patches ranges from 4 years to 10 years [36], although an FHWA TechBrief indicates that the service life of a patch ranges from 4 years to 7 years [37]. The service life of the patch depends largely on the corrosivity of the sur- rounding concrete and the development of the halo effect. Patch rehabilitation involves removing and patching all damaged (i.e., spalled and delaminated), sound but corroding, C H A P T E R 5 Extension of Service Life with Repair and Corrosion Mitigation Options

sound but critically chloride-contaminated, and/or carbonated areas. In addition, the bridge element must be protected against future ingress of chloride ions by applying a barrier system on the surface or using an electrochemical system to control cor- rosion in the sound areas. With periodic application of an effective barrier system, the estimated service life of patch reha- bilitation methods can be in excess of 50 years [36]. This esti- mate is based on the assumption that all sources of corrosion in the concrete element have been minimized. However, the bond between the patch and the concrete may deteriorate with time, resulting in a patch life significantly less than 50 years. Reinforcing Bar Coatings Corrosion of reinforcing bars in concrete (excluding cor- rosion attributable to carbonation) is initiated and sustained when sufficient amounts of chloride ions, oxygen, and mois- ture are present at the reinforcing bar surface. Therefore, min- imizing or eliminating access of one or some of these elements is required to prevent, stop, or retard the corrosion process in concrete. One of the most commonly used mechanical means to protect the reinforcing bar against cor- rosion is to coat it with a material that acts as a physical bar- rier to harmful reactants. Coatings on reinforcing bars are either applied to reinforc- ing bars prior to installation in a new concrete structure (i.e., fusion-bonded, epoxy-coated, and galvanized steel) or applied to reinforcing bars for the purpose of corrosion control in damaged concrete structures (polymer- and epoxy-modified coatings, epoxy zinc coating, etc.). The second type of appli- cation will be discussed further in this section. The ideal requirements for a repair coating are that it (a) serve as a barrier to chemical species that take part in a corrosion reac- tion, (b) have no adverse effect on adjacent bars or surrounding patch material, and (c) can be applied easily in the field. Surface preparation of the reinforcing bar prior to applica- tion of repair coatings is of prime importance. Proper surface preparation is essential to achieve maximum adhesion, which is the primary factor governing the performance of any pro- tective coating [38]. Abrasive blast cleaning is typically used to clean reinforcing bars. Specifications clearly state that bars must be cleaned to a “near white”condition; this represents the approximate removal of 95% of rust, mill scale, and other contaminants from the bar surface [39-41].Abrasive blast cleaning is used not only to clean the surface, but also to roughen the surface to facilitate bonding of the coating.The selection of specific abrasives determines the surface roughness,or anchor pattern (a function of the peak-to- valley depth); the number of peaks; and the shape of the peaks. For example, a grit blast media produces an angular shape that provides a better tooth for coating adhesion and more exposed steel surface area than the round profile of shot blast. In general, all grit, or a mixture of grit and shot, yields an optimum anchor pattern for epoxy coatings [42]. To ensure proper adhesion, any residue on the steel surface following the blast cleaning opera- tion should be removed prior to application of the coating. Remaining residue can result in “backside contamination”(con- tamination at the coating-bar interface),which adversely affects adhesion. There is a fundamental difference in the application of coatings on new versus corroded reinforcement bars. The for- mer is performed in a factory where surface preparation, ambient conditions, and the coating process are under strict quality control. In the latter case, the coating is applied on- site, where it is much more difficult to control influential con- ditions. The first step in performing this work is to remove the defective concrete by a suitable method and expose the rein- forcing bar both around its full circumference and for a short length beyond the area of corrosion before any surface prepa- ration can be carried out. Unlike factory-applied coatings, which are performed using machinery, the field-applied repair coatings have to be applied manually. Damaged concrete can be repaired by patching with plain cement mortar without the application of any reinforcing bar coating. Although some level of protection is generally achieved in the repaired area, corrosion of the reinforcement in surrounding concrete can actually be accelerated [43]. The effect is thought to be because the highly alkaline nature of the mortar patch makes the reinforcement in the repaired area more cathodic relative to the reinforcement in the sur- rounding concrete, which becomes anodic (i.e., active). Repair coatings are not stand-alone items. Consideration must be given to the compatibility of the patching mortar material and the use of an appropriate bonding agent with a given coating system. Each type of coating will have compat- ible ancillary products. Specific guidelines are typically pro- vided by each manufacturer. There are several types of commercially available repair coatings, including polymer-modified cement slurry; nonpas- sivating epoxy coatings; passivating epoxy coatings; zinc-rich epoxy coatings (one or two part); and zinc-rich, water-based coatings. Polymer-modified cement slurries, nonpassivating epox- ies, and epoxies filled with passivating fillers (e.g., zinc phos- phate and cement clinker) have been found to be prone to undercutting [44]. As each of these coating types isolates the reinforcement from the highly alkaline repair mortar, it keeps the reinforcement from being passivated. When passivating fillers are used, they appear to be fully bound in the poly- mer/epoxy film and offer little benefit. It has been reported that polymer-modified cement slurries containing rust inhibitors, silica fume, and sand can give better results and are not subject to undercutting. However, field experience with these particular repair coatings is limited. 27

Zinc-rich epoxy (one part) repair coatings have been reported to offer excellent protection to reinforcement in both the repaired area and the surrounding concrete [44]. Using this repair coating, the zinc provides electrical contact between the reinforcement and active zinc. The zinc then acts as an anode, thus protecting the reinforcement. How- ever, it has been found that reinforcement bars with zinc- rich coating can suffer from accelerated corrosion damage. This can occur, with resulting delamination of the patch repair, when the chloride content in the concrete adjacent to the patch is very high and the surrounding reinforcing bar mat has a high corrosion rate. It is suggested that the zinc component of the repair coating undergoes anodic dissolu- tion due to galvanic action (as intended) and that over a period of time the zinc is depleted from the coating. The problem is that the zinc protects both the reinforcement bars on which it is applied and nearby uncoated reinforce- ment that is electrically continuous to the coated reinforce- ment. The resulting dissolution rate of the zinc is much higher than desired. In addition, the underlying reinforce- ment tends to maintain its high anodic potential, and accel- erated corrosion proceeds on the reinforcement because of a relatively large adjoining cathodic area. Water-based, zinc-rich coatings are relatively new. No field data are available to substantiate laboratory data that show this type of coating to be very effective [45]. This type of coating is reported to have very good bonding characteristics. According to the manufacturers, the coat- ing not only acts as a protective barrier to aggressive components, but also affords sacrificial protection and passivation action. Repair of Epoxy-Coated Reinforcing Steel When epoxy-coated rebars are present in a concrete ele- ment, consideration must be given to the repair of the epoxy coating in the damaged areas. The field-applied coating must be compatible with the original coating on the reinforcing steel. Sohanghpurwala et al. evaluated various repair strategies for epoxy-coated rebars and determined that a combination of a compatible epoxy coating and a high-resistance silica fume patch material was most effective in combating corrosion on epoxy-coated rebar concrete elements. The barrier provided by the field-applied epoxy is not as good as the factory-applied one; however, the high resistance of the silica fume patch material was able to minimize the corrosion current flow between the patch and the adjacent original concrete [35]. Corrosion Inhibitors Corrosion inhibitors are chemical compounds or formula- tions that are used to control corrosion of metals in aggressive environments. They have been used extensively for many decades. Numerous different commercially available corro- sion inhibitors have been developed for specific applications, in particular water treatment, petroleum refining, and other chemical process industries. The use of inhibitors as a means of controlling corrosion of reinforcement in concrete is now receiving more attention. A number of inhibitors such as sodium nitrite, potassium dichromate, sodium benzoate, and stannous chloride were investigated in the laboratory in the late 1960s and early 1970s for potential application in concrete structures; however, they met with limited success [46–48]. In the late 1970s, calcium nitrite was identified as an effective inhibitor for reinforced concrete [49, 50]. In the early 1980s, organic amine-based inhibitors were introduced [51]. In the late 1980s, a Strategic Highway Research Program (SHRP) project evaluated 17 corrosion-inhibiting systems for reinforced concrete bridge components [52]. Based on the initial evaluation, four inhibitors were selected for additional testing [53]. A second evaluation showed that three of the four inhibitors were effective and that two methods could be used to deliver the inhibitor to the corroding reinforcement. One of the three inhibitors was calcium nitrite, and the other two were based on organic compounds. The inorganic compounds (based on calcium nitrite) are marketed as a liquid admixture containing 30% calcium nitrite and a 15% solution of calcium nitrite. The organic products, which are proprietary formulations, include a water-based, organic amine and an oxygenated hydrocarbon. The amine- based inhibitors are currently marketed as a liquid admixture and a surface-applied inhibitor. Both of these organic amine products are also known as migratory corrosion inhibitors. The method of operation for organic and inorganic cor- rosion inhibitors is vastly different. The calcium nitrite (inorganic)–based inhibitors provide corrosion protection by chemically oxidizing the surface of the reinforcement according to the following reaction [54]: In this process, ferrous, hydroxyl, and nitrite ions react to form nitric oxide, ferric oxide, and water. Generally, chloride ions react with ferrous ions to form a soluble complex that, upon reaction with hydroxyl ions, leads to the formation of ferrous hydroxide, Fe(OH)2. The chloride ions are then released back into solution for further reaction with addi- tional ferrous ions. A typical reaction between ferrous ions and a chloride ion is given as follows [55]: FeCl OH Fe(OH Cl6 4 22 6 − − −+ ↔ +) Fe Cl FeCl++ − −+ ↔6 6 4 2 2 2 2Fe OH NO NO + Fe O + H O++ 2 2 3 2+ + → − 28

The chloride and nitrite ions compete for the ferrous ions produced by the steel, and the relative amounts of the chlo- ride and nitrite ions determine which of the above two chem- ical reactions occur. The effectiveness of the nitrite inhibitor therefore depends on the ratio of the nitrite to chloride ions in the concrete. Recommended dosage rates of the nitrite inhibitor are provided by the manufacturer. However, one investigation pointed out that the recommended dosage lev- els may be insufficient (by as much as 33 to 67%) to offer ade- quate protection against the quantities of chlorides indicated by the manufacturer [56]. The method of operation of the amine-based (i.e., organic) inhibitors is two-fold [57, 58]. First, the inhibitors block the permeation of chloride ions through the concrete matrix. The ingress of chloride (as well as other ions such as sulfates) is greatly reduced as a result of the hydrophobic properties imparted to the concrete by the organic inhibitors. Second, and more importantly, the inhibitors form a thin protective film or barrier on the surface of the reinforcement. This bar- rier prevents chlorides from coming in contact with the rein- forcement. The chloride threshold to initiate corrosion is thus increased. It is claimed that these inhibitors migrate through the concrete for a considerable distance and seek out ferrous members; hence, they are called migratory inhibitors. Unlike nitrite inhibitors, the effectiveness of the amine-based inhibitors does not depend on the ratio of inhibitor to chlo- ride in the concrete. The effectiveness of nitrite as a corrosion inhibitor for rein- forcement embedded in concrete when used as an admixture in new construction has been established through a number of independent studies and through field experience. How- ever, there are advantages and disadvantages, as listed below. Advantages to using nitrite as a corrosion inhibitor include the following: • When used as an admixture in new construction, effective corrosion protection is given to both reinforcement bar mats, thereby providing extensive system longevity com- pared with unprotected reinforced concrete in chloride- laden environments. • Nitrite is fully compatible with portland cement and can be used during batch mixing or at the job site. • Nitrite requires no maintenance after installation. • Nitrite can be used in reinforced and prestressed concrete elements. • Nitrite can act as an accelerator and inhibitor. • Nitrite compares favorably with other corrosion protection systems in protection and cost-effectiveness, versatility, and east of application. • In solution form, nitrite can be used in chloride-contami- nated concrete without extensive surface preparation or exposure of corroded reinforcement bars. Disadvantages to using nitrite as a corrosion inhibitor include the following: • Accelerating properties of calcium nitrite can create diffi- culties in placing, finishing, and curing concrete under cer- tain conditions (such as high ambient temperatures and use of cements with a low C3A content). • Nitrite may cause an increase in slump. • Although an increase in strength may be seen, nitrite also has the detrimental effect of retempering due to stiffening of the treated concrete. • Because the amount of nitrite required depends on the amount of chlorides in the concrete, careful estimates of these quantities are required (nitrite may lead to an over- dosage when used as an admixture). • When nitrite is used in solution form, the concrete cover should not be more than 0.5 inch. A recent study found that the nitrite inhibitor, when used in conjunction with patch repair on field structures, did not provide any benefit [59]. The inhibitors in this study were used as an admixture in the patch concrete and were applied either to the surface of the exposed reinforcing steel or to both the surface of the reinforcing steel and the surface of the patch repair. The test structures were exposed to a very corrosive environment, and the adjacent sound concrete was heavily contaminated with chloride ions. As with nitrites, there are several advantages and disadvan- tages for the amine-based corrosion inhibitors, as listed below. Advantages to using organic inhibitors include the following: • Organic inhibitors can be used as either admixtures or penetrating coatings. • Organic inhibitors are cost-effective and easy to apply. • When organic inhibitors are used as an admixture, they do not have any detrimental effects on setting time, slump, and stiffness. • When organic inhibitors are used as an admixture, they greatly reduce the ingress of chloride, sulfate, and other aggressive ions. • When organic inhibitors are used as a coating, no concrete removal is required (however, the surface must be cleaned). • When organic inhibitors are used as a coating, the product can be spray or brush applied. • Organic inhibitors are water based, easy to handle and use, and are environmentally safe. Disadvantages to using organic inhibitors include the following: • Because of a more compact pore structure, organic inhibitors reportedly are not as effective when used with concretes having a low water-to-cement ratio. 29

• Organic inhibitors produce a slight decrease in concrete strength. • When organic inhibitors are used as a coating, they cannot be applied below 36°F, nor should they be applied if the temperature is expected to fall below 36°F within 12 hours of application. • The concrete surface should be as dry as possible before applying the organic inhibitor product. The organic corrosion inhibitors evaluated along with the nitrite corrosion inhibitors in the SHRP study also did not exhibit any benefit when used as admixtures in patch concrete and applied either to the surface of the exposed reinforcement or to both the surface of the exposed reinforcement and the surface of the patch repair [59]. Corrosion Inhibitor Patching (Superstructure Elements) There are two types of superstructure corrosion inhibitor rehabilitation patching methods. Type I is a standard patch- ing method using a corrosion inhibitor-modified concrete patching material. Type II uses the same patching materials; however, it also includes four applications of a spray-on inhibitor on the exposed reinforcing bars and patch cavity prior to patching. For both methods, all damaged concrete, sound but actively corroding concrete, and critically chloride- contaminated concrete is removed. For the Type I method, concrete removal areas are marked out and scored 1.0 inch deep along the patch perimeter with a dry concrete saw. The concrete is removed at least 1.0 inch below any reinforcing bars, the patch area and exposed rein- forcement are properly cleaned, and the patch area is backfilled with a corrosion inhibitor modified concrete. A penetrating sealer is then applied to the entire structural element. For the Type II method, concrete is removed to the depth of reinforce- ment, the patch area surfaces and reinforcement receive four applications of spray-on corrosion inhibitor, and then the patch area is backfilled. Because of potential bond problems between the patch material and the original concrete due to spray application of some inhibitors, the patch area has to be sandblasted to remove any surface residue. Global Corrosion Control Systems Overlays Overlays are used to restore the deck riding surface to orig- inal construction quality and to increase the effective cover over the reinforcing bars. Examples of overlays include latex- modified concrete (LMC), low-slump dense concrete (LSDC), and HMA with a preformed membrane (HMAM). All of these overlays increase the dead load on a structure and thus reduce the live load capacity. In addition, LMC and LSDC are not suitable for use on bridge decks where existing concrete may be susceptible to alkali-aggregate reactions (sil- ica or carbonate) unless low-alkali cement is used or other preventative measures have been taken [60]. These overlays are considered repair methods because sound concrete (that may be chloride contaminated or car- bonated) is left in place. The type of overlay has some influ- ence on the service life of the repair; however, the amount and degree of contaminated concrete left in place remains the most important factor. Since the primary factor that influences the decision to overlay a bridge deck is the extent of surface damage, the amount of contaminated concrete left in place becomes a critical factor in determining the service life of the overlay system. The HMAM repair overlays tend to increase the moisture content of the concrete because they use an elastomeric membrane between the original deck sur- face and the overlay. Moisture enters through the damage in the elastomeric membrane and is trapped between the mem- brane and the concrete surface. This increase in moisture content can increase the average annual corrosion rate. Rehabilitation of an existing corrosion-damaged deck con- sists of removing and patching all damaged (i.e., spalled and delaminated), sound but corroding, and sound but critically chloride-contaminated areas and overlaying the patched deck with LSDC, LMC, or HMAM. The economics of concrete removal must be considered when determining the extent of material to be removed. At some break-even point of removed surface area, the price for removing the entire con- crete surface to a depth of at least 1.0 inch below the top layer of reinforcing bars will be less than the price for large areas of spot removal. The chloride content and carbonation of the concrete below the removal depth must also be determined. The chloride content of the concrete left in place must be low enough that existing chlorides do not diffuse into, and initi- ate corrosion of, the lower layer of reinforcing bars. In addi- tion, carbonated concrete left in contact with the lower layer of reinforcing bars can initiate corrosion. The service life of rehabilitation overlays is limited by the rate of diffusion of chloride ion through the LSDC and LMC and the leakage of chloride ions through the membrane of an HMAM. Thus, the overlays are significantly influenced by envi- ronmental exposure conditions (i.e., chloride concentration and temperature).An LSDC overlay applied on a contaminated concrete bridge deck was observed to fail by disbondment and delaminations in the original concrete in 20 years [61]. A study by the Virginia Transportation Research Council found that the service life of LMC overlays applied on several bridge struc- tures in the state of Virginia was about 20 years [62]. The estimated cost for constructing a rehabilitation overlay on a prepared surface is approximately the same as the cost for 30

constructing a repair overlay on a prepared surface plus the cost for additional concrete removal and replacement required for rehabilitation. A more complete discussion on the eco- nomics of concrete removal is presented by Vorster et al. [38]. The construction procedures, quality assurance/construction inspection measures, and material performance specifications are identical for repair and rehabilitation overlays. Hydrodemolition—a process that involves a high-pressure water jet to break apart concrete by demolishing the concrete matrix—has been used to remove concrete to a specific depth in a cost-effective manner so that an overlay can be placed. This process is completed by several simultaneous mecha- nisms: cavitation, pressurization, and direct impact [63]. Cav- itation, formed by rapidly changing pressure of flowing water, produces shock waves of such magnitude that the cement matrix is broken apart. Pressurization acts to break the con- crete in tension along previously existing cracks and voids. Direct impact from the water spray nozzle removes loosened particles. Vorster et al. state that high production rates, because of significant automation, can be attained (once setup is complete) and selective areas of concrete can be removed to any desired depth [38]. Hydrodemolition has several advantages over typically used concrete removal techniques (e.g., hammering and rotary milling). The primary advantage is the ability to remove concrete between the reinforcement bars, which is usually inaccessible by rotary milling. Hammering may be used to remove selective areas of concrete between reinforce- ment bars; however, this technique is labor-intensive and may produce additional microcracking in previously sound areas. Disadvantages of hydrodemolition include the necessity to clean up removed particles and flying debris. After the hydrodemolition equipment has passed over an area of concrete, aggregate, small pieces of concrete, and slurry are left behind. If this slurry is allowed to harden before removal, it may rebond to the newly scarified surface and present problems of adhesion when the new concrete is applied. In addition, a significant amount of runoff water (up to 31.7 gallons/minute/nozzle [38]) is created that must be collected and disposed of in an appropriate manner. The equipment used with hydrodemolition is large and complex. It requires significant mobilization and cleanup times. In addition, this method of concrete removal is most productive, in terms of cost, when large areas of concrete are to be removed. Once the concrete is removed and the debris is cleaned from the deck, one of the aforementioned patch materials or overlay techniques can be applied. Membranes Membranes are elastic materials applied to bridge decks and are normally used in conjunction with HMA overlays. The main reason for employing a membrane is to provide a waterproof barrier to prevent the intrusion of chloride ions into the concrete deck. Proper application of an approved membrane and HMA wearing surface on bridge decks can greatly reduce the intrusion of chlorides into the concrete, thus serving as an effective bridge deck protection system. Currently, membranes and concrete overlays (latex-modified and low-slump dense concrete) are the most widely used cor- rosion protection systems for bridge decks. Twenty-two states in the United States use membranes as a standard bridge deck protective system; of the remaining states, 19 have used mem- branes in the past. For existing, and in particular older, reinforced concrete structures, the performance of membranes depends on the initial chloride content in the concrete. If the chloride content is high near the surface, the chlorides will diffuse into the con- crete after the membrane is applied and may critically con- taminate the concrete at the reinforcement level over time. As protective systems, membranes have several advantages and disadvantages. The advantages include the following: • Membranes can be applied relatively rapidly, including application of the HMA wearing surface. • Membranes can bridge and prevent reflection of most moving concrete cracks because of their elastic nature. • The HMA wearing surface can provide a good riding surface. • Membranes can be applicable to almost any deck geometry. The disadvantages of membranes as protective systems include the following: • The service life of the membrane may be limited by the wearing surface when exposed to heavy traffic. • The HMA overlay is a nonstructural component of the deck slab, adding to the dead load without increasing struc- tural capacity. • The system is not suitable for grades in excess of 4% because the bond capacity is limited and shoving and debonding can occur under traffic. Membranes are either preformed sheet systems or liquid, applied-in-place materials. The liquid-applied membranes can be applied as hot liquid and cold liquid. Preformed membranes are supplied in rolls of continuous sheets that are bonded to the bridge deck with an adhesive primer. They are placed in an overlapping configuration to provide a waterproofing layer. Hot-liquid membranes are placed on the bridge deck using brushes, spray, or rollers. The liquid must be preheated and applied above a minimum temperature (specified by the man- ufacturer) to ensure a waterproof layer. Cold-liquid mem- branes are placed similarly to the hot-liquid membranes, but without the need to preheat the material prior to placement. 31

The following performance criteria may be used to identify suitable membrane products for concrete bridge decks: • Water permeability: Membranes should demonstrate effective waterproofing capabilities yet allow for vapor transmission to ensure long-term adhesion to the deck. If vapor transmission is not permitted, the membrane system may increase the moisture content of the bridge deck, thereby promoting corrosion. • Chloride ion permeability: Protection of concrete from chloride ion intrusion is a major requirement for mem- branes. It is suggested that concrete that is waterproofed with a membrane be tested for permeability in accordance with the modified version of AASHTO T-277, “Rapid Chloride Permeability Test,” and the charge passed should not exceed 100 coulombs. • Low-temperature flexibility: Membranes should possess adequate flexibility to withstand the stresses caused by deck movements at low temperatures. No visible damage should occur when wrapping a sample of membrane around a 1-inch mandrel at 9°F. • Crack bridging: Cracks already in existence on the bridge deck will grow with temperature and load changes; the membrane must have elastic properties to be able to accommodate changes in width. It is suggested that mem- branes be able to bridge a crack width of 0.06 inch at 32°F. • Bond strength: A strong adhesive bond between the mem- brane and wearing surface reduces deformation of the HMA wearing surface layer by heavy wheel loading. The adequacy of the bond should be evaluated in both tension and shear, with minimum allowable values of 690 kPa and 172 kPa, respectively. • Resistance to indentation: Because of the thermoplastic nature of some membranes, indentation and puncture by aggregates may occur during application and rolling of the HMA wearing surface. Testing for resistance to indentation shall result in no penetration at the expected maximum placement temperature. Membrane deterioration may be expected because of repeated loading from traffic and age embrittlement. A SHRP project developed a nondestructive procedure for determin- ing the effectiveness of a membrane [64]. It also concluded that the service life of bridge deck can be extended by 25 years by properly installing a membrane. Another factor that determines the service life of the mem- brane is the service life of the overlying wearing surface. Membranes cannot resist damage during removal and replacement of a deteriorated HMA wearing surface. There- fore, the membranes must also be removed and replaced. Depending on the severity of the traffic and environment, some HMA overlays have required removal and replacement in 10 years or less. Because HMA does not prevent the intru- sion of water, water accumulates above the membrane and weakens the bottom portion of the HMA layer. This leads to debonding and stripping of the HMA overlay. Other dis- tresses associated with HMA are excessive wear of the surface in the wheel paths (especially in areas where use of studded tires is prevalent), lateral movement of the HMA layer, and rutting. Sealers and Surface Coatings Concrete sealers and surface coatings are used to prevent chloride ions from diffusing into the concrete. Measures that prevent water from entering the concrete will also minimize chloride intrusion. However, chloride ions that already exist in the concrete, especially near the surface, will diffuse into the concrete after the sealer or coating is applied and may crit- ically contaminate the concrete at the reinforcement level over time. A sealer is a solvent-based liquid applied to a prepared con- crete surface. There are generally two categories of sealers: penetrating sealers and surface sealers. Penetrating sealers, such as silanes and siloxanes, react with the pore structure within hardened concrete to create a nonwettable (i.e., hydrophobic) surface. Surface sealers, such as linseed oil and epoxy (with a solids content of less than 50%), block the pores of concrete. For bridge deck applications, penetrating sealers are preferred because surface sealers can have an inadequate depth of penetration and may quickly wear away when exposed to abrasion from traffic. However, surface sealers would be adequate for application on beams and girders. Penetrating sealers prevent liquid water from entering the concrete; however, they are very permeable to water vapor. Vapor permeability of sealers is desirable because it prevents moisture from being trapped inside the concrete element when the concrete is sealed. As the concrete dries out, the elec- trical resistance increases significantly, further inhibiting the corrosion process. Several environmental exposure conditions may influence the service life of sealers applied to bridge decks. These include ultraviolet light, moisture, and surface wear due to traffic. The service life of penetrating sealers applied to bridge decks ranges from 5 to 7 years [36]. Typically, penetrating sealers should be reapplied every 6 years. A coating is a one- or two-component organic liquid (such as epoxies, acrylics, methacrylate, and urethanes) that is applied in one or more coats to a prepared concrete surface. The primary purpose of the coating is to prevent the ingress of water into the concrete and, hence, the diffusion of chloride ions. However, unlike sealers, the vapor permeabil- ity of a coating is very low. Coating materials have high solids 32

33 content, usually 100%, and the typical thickness of coatings after drying is in the range of 0.001 to 0.003 inch. The selection of a coating material depends on individual site conditions. Epoxies are abrasion resistant and have a high adhesive strength; however, they are susceptible to degrada- tion by UV light. Acrylics are brittle and normally have low impact strength. Urethanes have high impact strength and good weathering characteristics, but low abrasion resistance. The following performance criteria should be used to iden- tify suitable products for sealing concrete bridge decks: • Chloride screening: Sealers should be able to reduce chlo- ride ingress into concrete by at least 90% after 30 weeks of ponding with saltwater. • Penetration depth: The initial depth of penetration should be 0.125 inch, and ideally 0.25 inch, to provide for protec- tion from wear and UV light degradation. • Moisture vapor permeability: The minimum vapor trans- mission should be 80% after sealing of the concrete. The percentage of vapor transmission is determined by com- paring the vapor loss of sealed concrete to that of an unsealed concrete over a 14-day period. The concrete used in the test should be in a saturated, surface-dry condition. • Surface friction: The surface should exhibit acceptable frictional characteristics after it is sealed. Penetrating sealers, surface sealers, or coatings may be applied to protect superstructure elements. Environmental exposure conditions that influence the service life of sealers and coatings applied to superstructure components include UV light, moisture, and abrasive wave and ice action. Sur- face sealers not exposed to abrasive wave or ice action have a service life of 1 to 3 years. In the presence of abrasive wave or ice action, the service life of surface sealers may be less than 1 year. The service life of coatings depends on the type of coating material applied and the field exposure condi- tions. Coated bridge components subjected to sea spray may have a shorter life than those exposed to deicer salt runoff water [36]. Polymer surface treatments are primarily applied to bridge deck surfaces to reduce the infiltration of chloride ions and water. They are essentially a two-component, abrasive- resistant, organic coating applied in one or more coats to the concrete. The exception is that each layer is impregnated with aggregate to improve friction. Polymer binders that are com- monly used include epoxy, acrylic, urethane, methacrylate, and high-molecular-weight methacrylate. Polymer surface treatments cure quickly. As a result, traffic can be permitted on the bridge deck a few hours after treat- ment. Hence, polymer surface treatment is considered a rapid bridge deck treatment. Because polymer surface treatments are thin (0.25 inch to 0.50 inch thick) and tend to follow the contours of the deck, they do not provide the substantial improvement of ride quality or drainage of the deck obtained by overlays, but they result in much less increase in dead load. Waterproofing with a polymer surface treatment or with a sealer/coating results in similar performance of an existing concrete. Chloride ions already present in the concrete, espe- cially near the surface, will diffuse into the concrete after treatment and may eventually critically contaminate the con- crete at the reinforcement level. Corrosion Inhibitor Overlays (Bridge Decks) The corrosion inhibitor overlay method was developed to limit the amount of sound chloride-contaminated concrete that had to be removed. Using this method, all sound but chloride-contaminated concrete surrounding the top layer of reinforcing bars is to remain in place. The procedure for this method is as follows: 1. Dry mill the concrete cover; 2. Remove all damaged concrete, clean any exposed rein- forcing bars, replace damaged reinforcing bars (if neces- sary), patch with a corrosion-inhibitor-modified concrete, and shotblast the entire deck surface; 3. Apply three spray applications of the corrosion inhibitor; and 4. Overlay with a corrosion-inhibitor-modified microsilica concrete (MSC), LSDC, or LMC. The spray-on corrosion inhibitor may be either organic or inorganic. If the spray-on corrosion inhibitor is inorganic, then the patch and overlay material should be inorganic- inhibitor-modified concrete. Similarly, if the spray-on corro- sion inhibitor is organic, the patch and overlay material should be organic-inhibitor-modified concrete. In many cases, the thickness of the 2-inch overlay will be greater than the milling depth, thus adding an additional dead load to the entire structure. In these cases, the reduction in live load capacity must be checked against present and future requirements. In addition, the spray-on organic corro- sion inhibitors leave a surface residue that significantly reduces the bond between existing concrete and the overly concrete. Hence, the entire deck surface must be lightly sand- blasted or shotblasted to remove the bond-reducing surface. During the inhibitor treatment process, all damaged (pre- viously patched, spalled, and delaminated) and highly cor- roded areas are patched with a corrosion-inhibiting concrete. Also, highly chloride-contaminated concrete is removed by dry milling, and these areas are treated with a spray-on corro- sion inhibitor. The spray-on inhibitor is intended to diffuse to the reinforcing steel and retard corrosion. The corrosion inhibitor in the overlay is intended to work the same way by

34 diffusing into the concrete and providing protection through- out the life of the rehabilitated deck. However, chlorides may still diffuse through the overlay from future winter mainte- nance or exposure to seawater. Thus, the service life of the cor- rosion inhibitor deck rehabilitation method is influenced by the amount of inhibitor on the reinforcing steel surface, by chloride concentration, by temperature, and by type of overlay. Cathodic Protection Systems Cathodic protection technology has been used on ships and pipelines for many decades as a method to control cor- rosion. Stratfull offers one of the first examples of using cathodic protection on a highway structure [65]. As a result of extensive government and private industry research in the development of cathodic protection systems for reinforced concrete structures, the Federal Highway Administration has stated that cathodic protection is the only rehabilitation tech- nique that has proven to stop corrosion in salt-contaminated bridge decks regardless of the chloride content in concrete [66]. Another publication describes the satisfactory operation of more than 350 systems in 37 states and 8 provinces in Canada [67]. A more recent evaluation of various types of cathodic protection systems suggests that cathodic protection is effective if designed, installed, and operated in accordance with recommended practice [68, 69]. Although cathodic pro- tection can be applied during new construction, it is most often found in conjunction with rehabilitation of existing concrete structures. However, cathodic protection is not always needed, nor is it applicable on every structure [70]. Cathodic protection is achieved by supplying an external source of current to counteract the internal corrosion cur- rent. During the corrosion process, current flows from anodic areas to cathodic areas. The actual corrosion (i.e., reduction of cross section) occurs at these anodic areas. During cathodic protection, current flows from an auxiliary anode material through an electrolyte (i.e., concrete) to the surface of the reinforcing bars. Through this process, corrosion is greatly minimized. Cathodic protection can be grouped into two basic types of systems: impressed current and galvanic anode systems. In both cases, the reinforcing bars are forced to func- tion as a cathode (hence, the name cathodic protection). An impressed-current cathodic protection is achieved by driving a low-voltage direct current (generally less than 50 volts) from a relatively inert anode material, through the concrete, to the reinforcing bars. Direct current of sufficient magnitude and direction is applied to oppose the natural flow of current resulting from the electrochemical process. The direct current is supplied by an external power source, often a rectifier that converts alternating current to direct current. Recently, solar power and specially designed batteries have been successfully used as external power sources. The direct current is distributed to the reinforcing bars by an anodic material. The uniformity of the current distribution is criti- cal; therefore, an anode is one of the most important compo- nents of a cathodic protection system. Current distribution is also a major consideration in the design of cathodic protec- tion systems. Galvanic cathodic protection is based on the principles of dissimilar metal corrosion and the relative position of spe- cific metals in the galvanic series. No external power source is needed with this type of system, and much less mainte- nance is required. Because of the limited power provided by these systems, actual installations have primarily been on bridge substructure elements in marine environments where the concrete resistivity is generally much lower. An example of the use of galvanic cathodic protection systems is by the Florida Department of Transportation, where much success has been reported on bridge structures in marine environ- ments [71, 72]. Prior to selecting a cathodic protection system for a given structure, several issues need to be considered: • Long-term rehabilitation: Cathodic protection is most cost-effective when long-term rehabilitation (greater than 15 to 20 years) is desired. • Electrical continuity: A closed electrical circuit (i.e., unbro- ken electrical path) between all reinforcing bars is required for a cathodic protection system to function properly. • Chloride concentration: If chlorides at the reinforcement bar depth are in sufficient concentration to initiate corro- sion, cathodic protection may be the only viable method of rehabilitation. • Alkali-silica reaction: Because the application of cathodic protection system increases alkalinity at the steel-concrete surface, alkali-silica reaction can be accelerated. The design of the specific cathodic protection system depends upon the type of surface to be protected (horizontal, vertical, soffit, etc.), geometry, reinforcement cover depth, envi- ronmental considerations (temperature and moisture), and structural considerations (additional dead load capacity). Sev- eral cathodic protection systems have been in operation for over 17 years [69]. The titanium-anode-based cathodic pro- tection systems can be expected to last over 50 years under cer- tain circumstances. Electrochemical Chloride Extraction Electrochemical chloride extraction is similar in principle to cathodic protection. However, the total amount of charge (i.e., current with time) is approximately 50 to 500 times that used for cathodic protection. Another important difference is that chloride extraction is a short-term treatment, whereas

cathodic protection is normally intended to remain in oper- ation for the life of the structure. Chloride extraction is par- ticularly suited for structures in which active corrosion is occurring, but only minor concrete damage is present. In addition, the structure must be conventionally reinforced and have an expected remaining service life of 5 to 10 years. Chloride extraction is accomplished by applying an anode and electrolyte to the structure’s surface and passing direct current between the anode and the reinforcing bars, where the reinforcement acts as a cathode. Conduction of direct current through the concrete is accomplished by the move- ment of charged ions. Since anions (i.e., negatively charged ions) migrate toward the positively charge anode, it is possi- ble to cause chloride ions to migrate away from the reinforc- ing bars. The speed at which this process is accomplished largely depends on the magnitude of the applied current. The simple movement of ions through concrete does not appear to have any deleterious effects on the concrete. How- ever, chemical changes occur at both the surface of the anode and the surface of the reinforcing bars. These changes are the result of electrochemical reactions that occur wherever cur- rent enters or exits the concrete. Reduction reactions that result in an increase in alkalinity and possible evolution of hydrogen gas take place at the rein- forcement according to the following reactions: The first of these reactions occurs slowly because the avail- ability of oxygen in concrete is limited. Most of the current entering the reinforcement will therefore result in the pro- duction of hydrogen at the reinforcement surface. The evolu- tion of hydrogen can cause hydrogen embrittlement on prestressed reinforcement. The production of hydroxyl ions in these reactions increases the alkalinity at the surface of the reinforcement. This tends to repassivate the reinforcement and thus helps to prevent the re-initiation of corrosion. The electrochemical reactions that may take place at an inert anode include the following: The importance of these reactions depends on the conditions present within the concrete. If the electrolyte is very acidic (pH  4), a significant amount of chlorine gas will evolve, resulting in safety and environmental concerns. If the electrolyte is alkaline (pH  7), then the evolution of oxygen becomes the favored anodic reaction. The small amount of chlorine that evolves under such conditions is rapidly hydrolyzed to form 2 22Cl Cl e − −→ + 2 42H O O H2 → + + 2 2 22 2H O e H OH+ → + − −( ) O H O e OH2 22 4 4+ + → − −( ) hypochlorous acid and hypochlorite according to the following reactions: Under these conditions, the evolution of oxygen and chlo- rine increases acidity (H+). This outcome raises concern about etching of the concrete surface that is in contact with the electrolyte. These concerns were addressed by Morrison et al. in a project that identified conditions under which the chloride extraction process can be conducted safely and effec- tively [73]. Current density is maintained low enough to avoid damage to the concrete and reduction in bond. The electrolyte pH is controlled to avoid etching of the concrete surface and generation of gaseous chlorine. A typical chloride extraction system uses an anode/blanket composite and a contained borate-buffered electrolyte. The borate buffer maintains the electrolyte at a high pH despite acid generation at the anode. The anode used is an inert cat- alyzed titanium mesh, which resists corrosive anodic reac- tions. The system is installed by fastening the anode/blanket to the concrete surface. A sump tank is provided at the base of the structure to act as a reservoir for the electrolyte. The electrolyte is continuously pumped from the tank to the top of the structure and flows by gravity back into the tank. The recommended current used for chloride extraction is in the range of 100 to 500 mA/ft2 of treated surface. Current levels below 500 mA/ft2 have proven to be harmless to the concrete structure. Current on field structures will usually be limited because of the 50-VDC maximum operating voltage. The National Electrical Code (NEC) requires exposed live electrical parts operating at 50 V or greater to be guarded; this is not possible with most chloride extraction installations. The electrical resistance of the concrete in most structures is such that the process will usually operate at maximum volt- age; in addition, the current will be well below 500 mA/ft2. The duration of the treatment process is usually expressed in terms of total charge (i.e., current with time). Effective chloride extraction is typically accomplished by applying a total charge of 600 to 1,500 A-hr/ft2. Too little charge will not remove sufficient chlorides or allow enough alkalinity to build up at the reinforcement to effectively prevent further corrosion, and too much treatment involves unnecessary expense. The recommended amount of total charge is usually reached in 10 to 50 days. A higher charge and longer treat- ment time is recommended for structures in which the chlo- ride content at the level of reinforcement is high. The treatment process described above will remove approx- imately 20% to 50% of the chloride present in the concrete, depending on the amount of chloride present, the distribution HClO ClO H→ +− + Cl H O HClO Cl H2 2+ → + + − + 35

of chloride within the structure, and the design of the rein- forcement. Typically, after treatment is complete, sufficient chloride will remain in the structure to initiate corrosion. However, the remaining chloride is usually distributed well away from the reinforcement, and much time is required for the chlorides to redistribute in sufficient quantities at the rein- forcement to initiate corrosion. The return to corrosive con- ditions is further delayed by the buildup of alkalinity that occurs at the surface of the reinforcement. This chloride extraction process was successfully demonstrated on a bridge deck in Ohio and substructures in Florida, New York, and Ontario [74]. Clemena and Jackson showed this technique to be effective on two structures in Virginia [75], and Broomfield and Buenfeld demonstrated the effectiveness of chloride extraction on a bridge structure in England [76]. Tritthart stated that the effectiveness of the extraction technique is lim- ited if significant chlorides have moved beyond the depth of reinforcement [77]. Sohanghpurwala verified the effectiveness of electrochemical chloride extraction on bridge superstruc- ture and substructure elements and on slabs in the laboratory [78]. He reported that slabs treated with electrochemical chlo- ride extraction had not exhibited any signs of corrosion 10 years after the treatment and concluded that protection for another 10 years could be expected. 36

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Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements Get This Book
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TRB's National Cooperative Highway Research Program (NCHRP) Report 558: Manual on Service Life of Corrosion-Damaged Reinforced Concrete Bridge Superstructure Elements examines step-by-step procedures for assessing the condition of corrosion-damaged bridge elements. It also explores procedures that can be used to estimate the expected remaining life of reinforced concrete bridge superstructure elements and to determine the effects of maintenance and repair options on their service life. NCHRP Web-Only Document 88 contains the data used in the development and validation of the service life model described in NCHRP Report 558. Also, the computational software (Excel spreadsheet) for the service life estimation process is available.

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