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Corrosion Resistant Alloys for Reinforced Concrete

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1.0 INTRODUCTION

1.1 BACKGROUND

The United States has a major investment in its highway system, whose operational performance, in conjunction with that of other transportation modes, is critical to economic strength and societal well-being. While deterioration of structures with time is a normal and expected occurrence, the rate at which this deterioration has occurred for highway bridges since the advent in the 1960s of a clear roads policy and application during wintertime of deicing salts in northern locations has been abnormally advanced and posed significant maintenance challenges. Also important is similar advanced deterioration of reinforced concrete bridges in coastal locations, both northern and southern, as a consequence of sea water and spray exposure. In either case (deicing salt or marine exposure), the deterioration is a consequence of the aggressive nature of the chloride ion in combination with moisture and oxygen.⁽¹⁾ Over half of the total bridge inventory in the United States is of the reinforced concrete type, and these structures have been particularly affected. A recent study indicated that the annual direct cost of corrosion to bridges is $5.9 billion to $9.7 billion.⁽²⁾ If indirect factors are also included, this cost can be as much as 10 times higher.⁽³⁾

As this problem has manifested itself during the past 40+ years, technical efforts have been directed towards understanding the deterioration mechanism and developing prevention and intervention strategies. With regard to the former, steel and concrete are in most aspects mutually compatible, as exemplified by the fact that in the absence of chlorides, the relatively high pH of concrete pore solution (pH ≈ 13.0-13.8) promotes formation of a protective oxide (passive) film such that corrosion rate is negligible, and decades of relatively low maintenance result. However, in the presence of chlorides, even at concentrations at the steel depth as low as 0.6 kg/m³ (1.0 pcy) on a concrete weight basis, the passive film may become locally disrupted, and active corrosion will commence.⁽⁴⁾ Once this occurs, solid corrosion products form progressively near the steel-concrete interface and lead ultimately to concrete cracking and spalling. Figure 1 shows a photograph illustrating such damage for the case of a coastal bridge piling. Because corrosion-induced deterioration is progressive, inspections for damage assessment must be routinely performed. Present Federal guidelines require a visual inspection every 2 years.⁽⁵⁾ If indicators of deterioration are not addressed, public safety is at risk. For example, corrosion-induced concrete spalls occur as potholes in a bridge deck and contribute to unsafe driving conditions. In the extreme, structural failure and collapse result.

Figure 1. Photo. A cracked and spalled marine bridge piling.

1.2 Modeling of Reinforced Concrete Structure Deterioration and Service Life Projection

Corrosion-induced deterioration of reinforced concrete can be modeled in terms of three sequential component steps or periods, which include the following:

1. Time for corrosion initiation (T_i).
2. Time, subsequent to corrosion initiation, for appearance of cracking on the external concrete surface (crack propagation) (T_c).
3. Time for surface cracks to evolve into spalls, which progress to the point where maintenance beyond what is routine is required (T_s).

The sum T_c + T_s is termed the corrosion propagation period, T_p. Maintenance-free service life, T_mf, is then T_i + T_p. As defined, maintenance-free service life is not intended to include occasional minor or routine repairs, as are likely to be required for any structure of significant size prior to T_mfor even T_i being reached. Figure 2 illustrates these parameters schematically in conjunction with a plot of cumulative damage versus time (adapted from Tutti).⁽⁶⁾

Figure 2. Graph. Schematic illustration of the various steps in deterioration of reinforced concrete due to chloride-induced corrosion.

Thus, the critical challenge for T_mf determinations is to develop data from laboratory and test yard experiments, service experience, or both that facilitate projection of T_i and T_p. Of course, T_i for actual structures cannot be determined directly from laboratory experimentation, since T_i for laboratory specimens is necessarily more brief than for structures. However, it is generally recognized that passive film breakdown and initiation of active corrosion for reinforcing steel in concrete commence once a critical chloride concentration, C_T, is achieved at the reinforcement depth.⁽⁷⁾ Consequently, if C_T is known from test yard exposures and the same value applies to actual structures, then T_i for the latter can be calculated using the solution to Fick's second law of diffusion, assuming that diffusion is the predominant Cl^- transport mechanism in both cases (test yard specimens and structure). Fick's second law for one-dimensional diffusion is as follows:

      (1)

where indicates the partial derivative, C(x,T) is chloride concentration, [Cl^-], after time T at distance x into the concrete in the direction of diffusion measured from the exposed surface, and D_e is the effective diffusion coefficient. As equation 1 is written, if D_e is assumed to be independent of x and T, then its solution is as follows:

      (2)

where, C_s is [Cl^-] at the concrete surface, also assumed constant with time, and ERF is the Gaussian error function. Further, at corrosion initiation C(x,T) is C_T, x is the rebar cover, and T = T_i. Thus,

      (3)

This solution assumes that C_T, C_s, x, and D_e are spatially and chronologically constant, whereas they are, in fact, distributed parameters with the range for C_T varying by more than an order of magnitude.^{(8, 9)} In addition, C_s and D_e may vary with exposure time and concrete age. Equation 3, as written, considers that initial [Cl^-] in the concrete is 0. Also, implicit in this expression is that the diffusion media (concrete) is homogeneous and without cracks. Nonetheless, analyses based on equation 3 are generally accepted as a viable engineering tool for projection of T_i.

Less focus has been placed upon T_c; however, some authors have developed sophisticated models that consider the tendency for solid corrosion products and corroding reinforcement to develop tensile hoop stresses and ultimately concrete cracking.⁽¹⁰⁾ Influential variables that influence T_c include corrosion rate, specific volume of solid corrosion products and rate at which these form, concrete microstructure and strength, and ratio of concrete cover to rebar diameter. Alternatively, T_c can be assumed as a specific time, such as 5 years, for surface cracks to appear in the case of black bar (BB).⁽⁹⁾ Less attention has focused upon T_s and any subsequent period that might lapse before maintenance intervention commences.

Based upon the corrosion deterioration model represented in figure 2, methods of life-cycle cost analysis (LCCA) are now commonly employed to evaluate and compare different material selection and design alternatives. This approach considers both initial cost and the projected life history of maintenance, repair, and rehabilitation expenses that are required to achieve the design life (synonymous with T_mf). These are evaluated in terms of the time value of money from which present worth is determined. Comparisons between different options can then be made on a cost normalized basis. Thus, materials selection choices define C_T which, in combination with design parameters, allows calculation of T_i. With estimation of T_c and T_s, T_mf can be projected. Iterations may be required depending upon cost and design life considerations. Figure 3 schematically illustrates this progression.

Figure 3. Chart. Representation of the sequential steps involved in the design process.

1.3 Epoxy-Coated Reinforcing (ECR) Steel

In the early 1970s, research studies were performed that qualified epoxy-coated reinforcing (ECR) steel as an alternative to BB for reinforced concrete bridge construction.^{(11, 12)} For the past 30 years, ECR has been specified by most State departments of transportation (DOTs) for bridges, decks, and substructures exposed to chlorides. At the same time, concrete mix designs were improved by specification of low water-to-cement ratio (w/c), possibly admixed with pozzolans or corrosion inhibitors (or both), and covered over reinforcement of 65 mm or more.⁽¹³⁾ However, premature corrosion-induced cracking of marine bridge substructures in Florida indicate that ECR is of little benefit for this type of exposure. (See references 14, 15, 16, and 17.) While performance of ECR in northern bridge decks has been generally good to date (30+ years), the degree of corrosion resistance afforded in the long term for major structures with design lives of 75 years to 100 years is still uncertain.

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