U.S. Department of Transportation
Federal Highway Administration
1200 New Jersey Avenue, SE
Washington, DC 20590
Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
|This report is an archived publication and may contain dated technical, contact, and link information|
Publication Number: FHWA-HRT-4-093
The United States has a major investment in its highway system; the system's performance, in conjunction with that of other transportation modes, is critical to the Nation's economic health and societal functioning. Although deterioration of structures over time is normal and expected, the rate at which this has occurred for highway bridges since the 1960s, when officials began applying deicing salts in northern locations in the winter, has been abnormally advanced and has posed significant challenges, both economically and technically. Also important is the fact that similar advanced deterioration has occurred for bridges in coastal locations, both northern and southern, because of seawater or spray exposure. In both cases (deicing salts and marine exposure), the deterioration is a consequence of the aggressive nature of the chloride ion. More than half of the total bridge inventory in the United States is reinforced concrete, and these structures have proved to be particularly susceptible to deterioration. A recent study has indicated that the annual direct cost of corrosion to bridges is 5.9-9.7B$. If indirect factors are included also, this cost can be as mush as 10 times higher than that estimate.
As this problem has manifested itself during the past 40 years, technical efforts have been directed toward first, understanding the deterioration mechanism and second, developing prevention and intervention strategies. The objective of the present review is to evaluate efforts in both categories from the perspective of high-performance (corrosion-resistant) reinforcing steel for concrete bridge deck and substructure service.
Although concrete has evolved to become the most widely used structural material in the world, the fact that its capacity for plastic deformation (so its ability to absorb mechanically imparted energy is essentially nil) imposes major practical service limitations. This shortcoming most commonly is overcome by incorporating steel reinforcement into specific locations in the concrete where tensile stresses are anticipated. Consequently, concerns regarding performance must not only focus upon properties of the concrete but also of the embedded steel and, in addition, the manner in which these two components interact. In this regard, steel and concrete are, in most aspects, mutually compatible, as exemplified by the fact that the coefficient of thermal expansion for each is approximately the same. Also, while boldly exposed steel corrodes actively in most natural environments at a rate that requires instituting extrinsic corrosion control measures (for example, protective coatings for atmospheric exposures and cathodic protection in submerged and buried situations), the relatively high pH of concrete pore water (pH 13.0-13.8) helps form a protective oxide (passive) film about 10 nanometers thick. This film effectively insulates the metal and electrolytes so that the corrosion rate is negligible, allowing decades of relatively low maintenance.
Disrupting the passive film upon embedded reinforcement and onset of active corrosion can arise in conjunction with either of two causes: carbonation or chloride intrusion (or a combination of the two). In the case of carbonation, atmospheric carbon dioxide (CO2) reacts with pore water alkali according to the generalized reaction,
which consumes reserve alkalinity and reduces pore water pH to the 8-9 range, where steel is no longer passive. For dense, high-quality concrete (for example, high cement factor, low water-cement ratio, and pozzolanic admixture), carbonation rates are typically on the order of 1 mm per decade or less; loss of passivity from this cause within a normal design life is not generally a concern. Carbonation must be anticipated at concrete cracks, however, where air essentially has direct access to the reinforcement, irrespective of concrete cover and quality. Older structures are also at issue because of their age, because earlier generation concretes were typically more permeable when compared to more recent concretes, and because of relatively low concrete cover.
Chlorides, on the other hand, arise in conjunction with deicing activities upon northern roadways or from coastal exposure, as noted above. While this species (Cl-) has only a small influence on pore water pH, concentrations as low as 0.6 kilograms per cubic meter (kg/m3) (concrete weight basis) have been projected to compromise steel passivity. In actuality, it probably is not the concentration of chlorides that governs loss of passivity but rather the ratio of chlorides-to-hydroxides ([Cl-]/[OH-]), because the latter species (OH-) acts as an inhibitor. This has been demonstrated by aqueous solution experiments from which it is apparent that the Cl- threshold for loss-of-steel passivity increased with increasing pH.[4-5] However, in cementitious materials, this interrelationship is more complex due to Cl- binding and the dependence of such binding upon pH. Thus, Cl- binding evidently decreases with increasing OH- above pH 12.6, such that a decrease in pH can result in decreasing [Cl-]/[OH-]. Considerable research efforts have focused on identifying a chloride threshold; however, a unique value for this parameter has remained elusive, presumably because of the numerous influential variables, including type of cement, cement alkalinity, concrete mix design, environmental factors, potential, and reinforcement composition and microstructure. Because Cl-, not carbonation-induced loss of passivity, is of primary concern for bridge structures, subsequent focus is placed upon this cause of corrosion alone.
After steel in concrete becomes active, either in conjunction with chlorides achieving the threshold concentration or pore solution pH reduction from carbonation at the embedded steel depth, then the classical anodic iron reaction,
and cathodic oxygen reduction reaction,
occur at an accelerated rate. Ferrous ions subsequently react to form sequential oxides according to
where the latter ferric product (-FeOOH) is more protective than the ferrous. Because the ferrous-to-ferric conversion occurs over time and is never complete, passive film disruptions invariably are present. In addition, neither product is protective in the presence of Cl- or at pH below about 11.5. Despite the normally high alkalinity of concrete, acidification may occur in the vicinity of anodic sites because of oxygen depletion and hydrolysis of ferrous ions.Thus,
The product H+ may be reduced and, along with O2 reduction at more remote cathodic sites, further accelerate the anodic process. Further oxidation can occur as
Interestingly, corrosion seldom causes failure in reinforced concrete components and structures. Failure occurs because the oxide products (ferrous and ferric) have specific volumes that are multiples of that of the reactant steel; their accumulation in the concrete pore space adjacent to anodic sites leads to development of tensile hoop stresses around steel which, in combination with the relatively low tensile strength of concrete (typically 1-2 megapascals (MPa)), ultimately cause cracking and spalling. Figure 1 shows a photograph of corrosion-induced concrete spalling on a bridge piling in Florida.
Because corrosion-induced deterioration is progressive, inspections for damage assessment must be performed routinely; present Federal guidelines require a visual inspection every 2 years. Because the corrosion is progressive and the resultant damage distributed in severity, repairs are needed continually. If such visual indicators are not addressed, then public safety is at risk. As an example, corrosion-induced concrete spalls occur as potholes in a bridge deck and contribute to unsafe driving conditions. As an extreme, structural failure and collapse may occur.
The situation is more critical and challenging in the case of post-tensioned structures, where advanced corrosion often is not revealed by visual inspection and loss of tendons is more critical to integrity than in the case of conventional reinforcement. Here, nondestructive testing methods such as magnetic flux leakage and natural frequency measurements are helpful, but these do not address all aspects of the problem and are expensive to apply.
Representation of Corrosion-Induced Concrete Deterioration
Corrosion-induced deterioration of reinforced concrete can be modeled in terms of three component steps: (1) time for corrosion initiation,Ti; (2) time, subsequent to corrosion initiation, for appearance of a crack on the external concrete surface (crack propagation), Tp; and (3) time for surface cracks to progress into further damage and develop into spalls, Td, to the point where the functional service life, Tf, is reached. Figure 2 illustrates these schematically as a plot of cumulative damage versus time. Of the life component terms, Ti occupies the longest period in most cases, so corrosion control measures generally focus on this parameter. In the case of epoxy-coated reinforcement, corrosion is thought to initiate at coating defects and holidays such that Ti is the same as for black steel; however, propagation rate is low (relatively large Tp) because of both high resistance between anode and cathode and small cathode surface area (assuming the bottom mat steel, as well as the top, is coated).
|Figure 2. Schematic illustration of the various steps in deterioration of reinforced concrete due to chloride-induced corrosion.|
During the past several decades, the approach taken by the Federal Highway Administration (FHWA) and most State Departments of Transportation (DOT) has been to specify epoxy-coated reinforcing steel (ECR) for bridge decks augmented by low water-to-cement ratio (w/c) concrete, possibly with pozzolans or corrosion inhibitors (or both), and concrete covers of 65 mm or more. In Florida coastal waters, ECR has proven ineffective because of higher average temperatures, chlorides, and moisture.[13,14,15,16] Here, practitioners are relying on pozzolans, corrosion inhibitors, and relatively large cover, and are projecting a 75-year life for bridge members in and near the splash zone. Likewise, the methods of life cycle cost analysis (LCCA) are employed to evaluate and compare different materials selection and design alternatives. This approach considers both initial cost and the projected life history of maintenance, repair, and rehabilitation that are required until the design life is reached. These are then evaluated in terms of the time value of money, from which present worth is determined. Comparisons between different options then can be made on a cost-normalized basis.
The above process becomes accelerated if concrete cracks wider than 0.3 mm are present because, in this case, detrimental species, chlorides in particular, and water may have direct access to the reinforcement, irrespective of the depth of cover.[17,18] Here, corrosion resistance relies upon inherent properties of the reinforcement in an electrolyte of pH lower than that of normal pore water.
The mechanism of Cl- intrusion into concrete invariably involves both capillary suction and diffusion; however, for situations in which the depth of the capillary suction is relatively shallow compared to the reinforcement cover, diffusion alone normally is assumed. Analysis of diffusion is accomplished in terms of Fick's second law,
where c(x,t) is the Cl- concentration at depth x beneath the exposed surface after exposure time t, and D is the diffusion coefficient. As equation 9 is expressed, D is assumed to be independent of concentration. The solution in the one-dimensional case is
co is the initial or background Cl- concentration in the concrete, and
cs is the Cl- concentration at the exposed surface.
To arrive at this solution, assume cs and D are constant with time, and the diffusion is "Fickian," that is, there are no Cl- sources or sinks in the concrete. In actuality, cs increases with exposure time, although Bamforth reported this to reach steady-state after about 6 months for marine exposures. Based on a literature study of marine exposures, values in the range 0.2 to 1.0 percent (concrete weight basis) were reported; however, Howell and Tinnea measured cs as high as 2.1 percent for an Alaskan viaduct. Factors that affect cs have been projected to include (1) type of exposure, (2) mix design (cement content, in particular), and (3) curing conditions. Also, the diffusion coefficient that is calculated from equation 10 is termed an effective value, Deff, because it is weighted over the relevant exposure period as a consequence of cs varying with time, progressive cement hydration, and the possibility of chloride binding, which renders the migration non-Fickian.
By the approach represented by equation 10, c(x,t), co, and cs are measured experimentally (normally by wet chemistry analysis), and Deff is calculated based upon knowledge of reinforcement cover and exposure time. Experimental scatter and error may be minimized by measuring c(x,t) at multiple depths and employing a curve-fitting algorithm to calculate Deff. Also, if Deff is known from one sampling set, then cth, the Cl- threshold for passive film breakdown and onset of active corrosion, can be determined by measuring c(x,t) at the reinforcement depth (crd) at the time of corrosion initiation and solving equation 10, recognizing that for this situation, crd cth. In any case, the parameters that affect Cl- intrusion rate are cs and Deff, where the former is exposure dependent and the latter is a material property (actually, cs is also sensitive to material composition and microstructure, and Deff is affected by exposure conditions (relative humidity and time of wetness, for example).
Corrosion control options for reinforced concrete structures can be represented in terms of two general categories according to whether they apply to existing Cl- contaminated structures or to new structures. This topic is addressed below in terms of each of these subdivisions.
The inventory of corrosion-damaged bridge decks in the United States and other countries is such that damage intervention normally transpires only after major repair or rehabilitation (or both) is needed. Rehabilitation includes (1) installation of physical barrier systems such as coatings, sealers, membranes, and overlays to forestall subsequent Cl- ingress, and (2) applying electrochemical methods such as electrochemical chloride extraction (ECE) that revert the concrete to a lesser Cl- contaminated state with enhanced alkalinity in proximity of the reinforcement and cathodic protection for corrosion protection. However, the physical barrier-type repair and rehabilitation methods have no lasting effect if Cl- contaminated or carbonated concrete remains in place. Cathodic protection, in contrast, is the only methodology for which long-term service data are available that has been judged effective for controlling ongoing steel corrosion in Cl- contaminated, atmospheric, or splash zone exposed structures. While the theory and principles of cathodic protection have been known for more than a century, its early application for protecting reinforcing steel in concrete often was not successful because of (1) difficulties in assuring a uniform current density in the high resistivity concrete pore water environment; (2) an absence of adequately performing impressed current anodes; and (3) a lack of understanding and appreciation of cathodic protection technology on the part of transportation maintenance personnel. The first of these difficulties has been overcome largely by proper design and by specifying that anodes be distributed. Concerns regarding the second factor have been reduced by a combination of proper design and the advent of distributed mixed metal oxide-type anodes. Cathodic protection is not necessarily effective in protecting steel exposed at concrete cracks and at spalls in atmospheric applications, but this is probably not an impediment in water or wet soils. This technology cannot, of course, restore structural integrity to cracked or spalled concrete or to embedded steel that has already corroded. Protection criteria normally are based on polarization or depolarization of a prescribed magnitude, typically 100 millivolts (mV). As such, potential need not be reduced to the reversible value for the anodic reaction (as is normally specified for aqueous exposures), and corrosion rate may not be reduced to nil but simply to a relatively low value. Protection may result, not only from the polarization but also from electromigration of Cl- away from the reinforcement and resultant steel repassivation by this and by generating OH- in conjunction with the cathodic reaction.
ECE is a relatively new technology for which long-term service data are limited.[23,24] This method employs a temporary anode that is operated at current density orders of magnitude higher than for cathodic protection, such that anions, including chlorides, electromigrate away from the embedded steel cathode. Repassivation can then occur, similar to what was discussed above in conjunction with cathodic protection, although this occurs in a shorter period of time (1-2 weeks to several months). Not all chlorides are removed, but sufficient amounts are displaced from the steel-concrete interface. This technology's effectiveness probably rests as much, or more, on hydroxide generation at the steel and an associated decrease in [Cl-]/[OH-] as on chloride removal or redistribution.
Factors to be considered in designing corrosion control of new reinforced concrete structures include (1) embedded steel surface protection, (2) concrete mix design, (3) structural design considerations, (4) concrete surface modification, (5) cathodic prevention, and (6) corrosion-resistant reinforcement (CRR). The first of these (embedded steel surface protection) exemplified by epoxy-coated reinforcement. These coatings permit movement of moisture to the steel surface but restrict oxygen penetration such that a necessary reactant at cathodic sites (see equation 3) is excluded. ECR has been employed in bridge decks for almost 30 years with generally good results reported for this application. However, cracking and spalling of bridge substructure concrete components from corrosion of ECR occurred for splash and near splash zone locations of the overseas highway to Key West, FL, only 7 years after construction.[17,18] Different opinions have been provided as explanations for this, with some considering that the semitropical splash zone is particularly harsh with regard to corrosion at coating defects and undercoating corrosion, and that this same type of deterioration may also occur with more modest exposures but simply at a reduced rate. Presently, there is concern that ECR may not provide the target 75-year design life that is now specified for reinforced concrete bridge decks without some maintenance.
Historically, black bar with a corrosion-resistant metal or alloy cladding has been considered for reinforcing concrete undergoing severe exposure. Of the options that have been studied, galvanized reinforcing steel is the most prominent and has been employed in concrete to a limited extent. Here, a relatively thin zinc surface layer is applied by either hot dipping or electro-deposition. This methodology relies on a relatively low corrosion rate for zinc and its potential for being active to the substrate steel, thereby providing galvanic cathodic protection at defects and penetrations. However, the results of research programs have been mixed, probably as a consequence of zinc being amphoteric, such that passivating corrosion products apparently do not form at pH > 13.3.[25,26] Noble metal claddings such as copper, nickel, and stainless steel have historically been investigated but have not been used extensively due to initial cost considerations. Renewed interest has recently focused upon the utility of reinforcements of this type, however, because of their potential for providing greater corrosion resistance than black bar alone but at a reduced cost compared to situations where the entire bar cross section is corrosion resistant. This, coupled with development of unique manufacturing methods, has made available stainless clad bars in the price range $1.08-$1.65/kg.
Concrete mix design modifications involve such factors as (1) reduced w/c, including use of water-reducing admixtures or superplastizers; (2) type of cement; (3) permeability reducing admixtures such as fly ash, silica fume, and blast furnace slag; and (4) corrosion inhibiting admixtures. In effect, options 1 and 3 reduce Deff, whereas options 2 and 4 can function by elevating cth.
Structural design aspects of corrosion control involve factors such as configurational (geometrical) considerations that minimize or, if possible, eliminate exposure to corrosives. For example, Florida DOT now requires, wherever possible, elevation of bridge superstructures to a minimum of 4 meters (m) above mean high tide. Of particular importance is also depth of concrete cover over the reinforcing steel. The significance of this parameter is apparent from equation 10.
Coatings, sealers, and membranes (physical methods) also can be specified for new structures. However, it is generally accepted that such treatments will not provide a 75-year design life unless they are supplemented by other options, such as use of impermeable concrete and large cover.
Cathodic prevention is, in effect, identical to cathodic protection, except that it is applied to new, Cl--free structures for which current demand is less than for Cl- contaminated ones. In addition, the objective here is not to reduce corrosion rate itself (because the reinforcement is passive), but instead to establish a potential gradient that opposes the inward diffusional migration of anions, specifically chlorides. In this regard, the approach functions similarly to ECE, except that, instead of removing chlorides, it retards their entry.
Historically, the added initial cost of CRR, such as stainless steel, has largely precluded it from being competitive for concrete construction. However, with the advent of LCCA and the FHWA requirement in 1995 that bridge projects that cost more than $25 million have a 100-year design life, including reinforcements of this type has become a more viable option. However, although there have been research studies and scientific and engineering literature reviews pertaining to CRR in concrete and simulated concrete environments, the long-term corrosion performance of such materials in bridge applications still cannot be projected confidently.[27,28,29,30] The issue is complicated by the fact that there are a variety of CCR choices that cover a range of manufacturing processes, cost, and corrosion performance. This, combined with the very real need to reduce the social and economic impact of bridge maintenance costs, has resulted in a need for additional research. This critical review of CRR performance in concrete has been prepared as a prelude to further FHWA-sponsored research on this topic.
 Numerous methods exist in the literature for reporting the concentration of chlorides in cementitious materials. These include either weight or percent Cl- in reference to either the concrete or cement. Conversion of the Cl- amount from a concrete to cement basis or vice versa requires that the cement content be known. In the absence of this information, a cement content can be assumed.