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Job Site Evaluation of Corrosion Resistant Alloys

BACKGROUND AND INTRODUCTION

For a Nation to be productive, its transportation system must be efficient and reliable. While deterioration of highway structures over time is a normal and expected occurrence, the rate at which this has occurred for bridges in the United States, since the advent in the 1960s of a clear-roads policy and the use of roadway deicing salts in northern locations, has been severe and posed significant challenges, both economically and technically. Also important is the accelerated deterioration of bridges that has occurred in coastal locations, both northern and southern, as a consequence of exposure to sea water (chlorides) and sea spray. In both cases (deicing salts and marine exposure), the deterioration is a consequence of the aggressive nature of chlorides in combination with moisture and oxygen.⁽¹⁾ More than half of the total bridge inventory in the United States is of the reinforced concrete type, and these structures have proved to be particularly susceptible. A recent study has indicated that the annual direct cost of corrosion to bridges is $5.9 to $9.7 billion. If indirect factors are included also, this cost can be as much as 10 times higher.^(2,3)

In response to this problem, research studies that focused upon the utility of epoxy-coated reinforcing steel (ECR) were initiated. In the early 1970s, ECR was qualified as an alternative to black bar.^(4,5) Consequently, for the past 30 years, ECR has been specified by State Departments of Transportation (DOTs) for major decks and substructures exposed to chlorides. At the same time, ECR was augmented by use of low water-to-cement ratio (w/c) concrete, possibly with pozzolans or corrosion inhibitors (or both), and covers of 65 mm or more.⁽⁶⁾

However, in Florida coastal waters, ECR has proven ineffective ^(7–10) because of the combined effects of higher average temperature and more prolonged moist exposure. Several comprehensive research studies, including evaluations on actual bridges, were conducted that further investigated, first, the suitability of epoxy coatings for reinforcement corrosion control and, second, in-service ECR performance.^(11–13) These studies generally found that time-to-corrosion initiation for ECR and black bar are approximately the same but that the propagation period for ECR to cause concrete surface cracking can range from about the same as for black bar, as noted for Florida bridge substructures, to decades in northern bridge decks. Thus, while ECR performance in the latter type application has been generally good to date and results from long-term testing programs indicate that two mats of ECR in bridge decks should provide a 75–100-year service life with minimal maintenance as presently specified for major bridge structures,⁽¹³⁾ still this is not known with certainty. In response to this, interest has focused during the past decade upon alternatives that afford more corrosion resistance than ECR—stainless steels in particular. Such corrosion-resistant steels become particularly competitive on a life-cycle cost basis, since the higher initial expense of the steel may be recovered over the life of the structure via reduced repairs and rehabilitations.

The Innovative Bridge Research and Construction Program (IBRC)¹ was authorized by Congress in the Transportation Equity Act for the 21st Century (TEA-21) legislation initially as a 6-year effort (fiscal year (FY) 1998–2003) but was subsequently extended through May 2005. The program objective was to provide resources whereby States could demonstrate the utility of innovative materials and technology in construction of bridge and highway structures. The majority of the funding ($142 million) was for actual repair, rehabilitation, and replacement of existing structures and for new construction with a lesser amount ($4 million) for research, both based upon innovative materials. Corrosion-resistant reinforcements constitute one component of the program.

¹The description for this bridge utilizes English and not metric units since the project documents and specifications were so based.