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Publication Number: FHWA-HRT-09-044
CHAPTER 1. INTRODUCTION
The United States has a major investment in its highway infrastructure because its operational performance, in conjunction with that of other transportation modes, is critical to the Nation's economic health and societal functionality. While deterioration of structures with time is a normal and expected occurrence, the rate at which this has occurred for reinforced concrete highway bridges is affected by winter application of deicing salts in northern locations. Since the advent of a clear roads policy in the 1960s, deterioration has been abnormally advanced and has posed significant challenges, both economically and technically. Also important is similar advanced deterioration of reinforced concrete bridges in northern and southern coastal locations as a consequence of sea water or spray exposure (or both). In either case, 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 susceptible to corrosion. A recent study indicated that the annual direct cost of corrosion to bridges is $5.9–$9.7 billion.(2) If indirect factors are also included, this cost can be as much as 10 times higher.(3)
Steel for concrete reinforcement
As this problem has manifested itself during approximately the past 40 years, technical efforts have been directed toward understanding the deterioration mechanism, monitoring the rate of deterioration and condition assessment, and developing prevention and intervention strategies. With regard to understanding the deterioration mechanism, steel and concrete are in most aspects mutually compatible. This is 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 the corrosion rate is negligible, resulting in decades of relatively low maintenance result. In the presence of chlorides even at concentrations at the steel depth as low as 1.0 pcy (0.6 kg/m3) (concrete weight basis), the passive film may become locally disrupted, and active corrosion commences.(4) Once this occurs, solid corrosion products form near the steel-concrete interface and cause tensile hoop stresses around the reinforcement. This ultimately leads to concrete cracking and spalling. Because corrosion-induced deterioration is progressive, inspections for damage assessment must be routinely performed, and 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 form as potholes in a bridge deck, and they contribute to unsafe driving conditions. In the extreme, structural failure and collapse may result.
Methods of life-cycle cost analysis (LCCA) are commonly employed to evaluate and compare different materials selection and design alternatives for bridge construction. 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. These methods are evaluated in terms of the time value of money from which present worth is determined. Comparisons between different material selection and design options can then be made on a normalized cost basis.
In the early 1970s, research studies were performed that qualified epoxy-coated reinforcing (ECR) steel as an alternative to black bar for reinforced concrete bridge construction.(6,7) For the past 30 years, ECR has been specified by most State transportation departments for bridges, decks, and substructures exposed to chlorides. At the same time, ECR was augmented by the use of low water-to-cement ratio (w/c) concrete possibly with pozzolans or corrosion inhibitors (or both) and concrete covers of 65 mm or more.(8) However, premature corrosion-induced cracking of marine bridge substructures in Florida indicated that ECR is of little benefit for this type of exposure. (See references 9–12.) While performance of ECR in northern bridge decks has generally been good to-date (30+ years), the degree of corrosion resistance afforded in the long term for major structures with design lives of 75–100 years is still uncertain.
In response to the above concerns regarding ECR, interest has focused on more corrosion-resistant alternatives to ECR—stainless steel (SS) in particular—during the past 15 years. Such alloys may become 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 maintenance costs arising from corrosion-induced damage.
Steel for structures and cables
Chloride and moisture can have major impacts on infrastructure components other than reinforced concrete. Structural steel with damaged paint, weathering steel, and high-strength steel in suspension bridge cables deteriorate because of wet-dry cyclic exposures in the presence of aggressive ions that accelerate corrosion processes. An essential aspect of these processes is the formation of corrosion products. The corrosion products can accelerate corrosion by undercutting paint on painted steel or by retaining aggressive chloride and ionic species in nonprotective oxide layers on any steel. Conversely, corrosion products can form protective oxide layers on weathering-type steels in favorable but not adequately defined environments. A better understanding of the mechanisms resulting in the formation of corrosion products in cyclic wet-dry environments in the presence of certain aggressive ions would enable more effective corrosion control measures. The automotive industry has successfully identified and found suitable solutions to specific issues pertaining to the paint undercutting mechanisms (known as cosmetic corrosion). This industry has developed the methodology of correlating accelerated testing and corrosion product identification with field studies for better understanding corrosion mechanisms and has applied the methodology to automotive perforation corrosion. Following similar methodologies, the steel industry has worked with the Federal Highway Administration (FHWA) and State transportation departments to identify issues related to steel and uncoated weathering steel materials, yielding useful information in design and maintenance guidelines. While peripheral issues have been identified, no jointly funded research has been initiated on these issues, and an adequate understanding of the processes is needed.
Weathering steel and, to a lesser extent, structural steel develops a protective oxide layer when exposed to wet/dry conditions in the absence of aggressive environmental influences. Steel suppliers and the FHWA provide guidelines for determining the suitability of weathering steel for specific bridge applications.(13) Included in these guidelines is the recommendation for assessing the environmental suitability of weathering steel at a specific site. The wet/dry conditions required for the development of protective oxides on weathering steel and the presence of aggressive corrosive agents should be determined. The determination could lead to an assessment of the performance of weathering steel under the existing conditions at the specific site.
Many details addressing the procedures for macro and microenvironment assessment for assuring the performance of weathering steel are described elsewhere.(14) In that work, bimetallic couples including the steel material of interest were used to generate a galvanic current that was proportional to the structure's corrosion rate. For this proposed work, corrosion rate assessment by modified corrosion sensors that correlate better with the field performance will be developed. The correlation to field performance requires several years and considerably impedes both development time and understanding of specific parameters affecting corrosion rates. It has been determined that field performance can be simulated in laboratory cyclic tests by comparing corrosion products observed in the field to those generated in the laboratory.(15) The laboratory conditions must be adjusted to yield similar corrosion products. Modifying chloride and/or sulfate exposure and drying conditions in the simulated tests can result in the development of corrosion products similar to those observed in the field. This correlation can speed validation and development of monitoring and control methods by a factor of about 30. Specifically, corrosion products on field samples, samples in simulated tests, and corrosion sensors must be closely matched so that appropriate conclusions can be drawn from testing and monitoring.
The present report includes two research components of concern for highway bridges exposed to chloride contaminated service environments: (1) corrosion properties of 2304 SS reinforced in concrete and (2) monitoring of steel corrosion in atmospheric exposures. Accomplishments regarding each of these components are also presented and discussed in this report.
Topics: research, infrastructure, structures, steel bridges, reinforced concrete, corrosion, sensors
Keywords: Reinforced concrete, Reinforcing steel, Stainless steel, Bridges, Corrosion resistance, Atmospheric corrosion, Steel, Corrosion sensors