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Suggested Citation:"Chapter 1 - Introduction." 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 1 - Introduction." 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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1The National Bridge Inventory Database, maintained by the Federal Highway Administration, as of 2002 contained a total of 587,964 bridges. The average age of bridge structures in this database is 40 years, and 41 percent of the bridges are at least 40 years old. Over the past two decades significant attention has focused on the condition of the nation’s aging highway bridge infrastructure. Several independent evalua- tions of the condition of the nation’s infrastructure based on condition ratings contained in the database have been per- formed [1-3]. These studies ascertained that 14 percent of the bridges were rated structurally deficient and the primary cause of the deficiency was corrosion of reinforcing steel. The cost to maintain the nation’s bridges during the 20-year period from 1999 to 2019 is estimated to be $5.8 billion per year, and the cost to improve and eliminate deficiencies over the same period is $10.6 billion [3]. A cost-of-corrosion study determined that the annual cost of corrosion to all bridges (including steel bridges) is $8.29 billion, and this esti- mate does not include indirect cost incurred by the traveling public due to bridge closures [2]. Considering the magnitude of the problem, a well- designed strategy needs to be developed to identify, maintain, repair, and/or replace the existing bridge infrastructure. The primary goal of this manual is to provide such a strategy. The scope of this manual is limited to managing the impact of corrosion of conventional reinforcing steel in bridge decks and other superstructure elements. To address the ongoing corrosion deterioration, a bridge owner has to make decisions to maintain, repair, or replace the structure based on its present and future expected con- dition and to determine what alternative materials and methodologies should be used. Because of a lack of nation- ally accepted decision-making processes, most owners presently use local experience and expertise to make such complex decisions. Such decision-making processes have often resulted in inefficient, costly, nonstandard, and nonop- timal solutions. There is a strong demand for a protocol capable of determining the optimal course of action (main- tenance, repair, or replacement) and assisting in the selection of materials and methodology. To determine the optimal course of action, information on the present condition of the structure and the expected deteri- oration in the future is required. The present condition of the structure provides information on the quantity of repairs required and the type of repairs necessary. However, the expected future deterioration allows the owner to determine the efficacy of the repairs and assists in selecting a repair and pre- vention strategy that could minimize the life cycle cost of main- tenance for the period of life desired from the structure. For example, if all delaminations and spalls were repaired on a con- crete bridge deck that was uniformly contaminated with chlo- ride ions, then, in the ear future, damage in areas that were not previously repaired can be expected. The repairs do not impact corrosion in areas not presently requiring repairs and may, under certain circumstances, accelerate it. If corrosion-induced damage continues to occur after the first repair is performed, then regular repair cycles will be required that will result in sig- nificant expense. The regular repair cycles can be avoided if the propensity for future corrosion-induced damage was known and appropriate measures were taken to control it. On the con- trary, if it is known that the expected future damage was mini- mal, then that structure can be slated for maintenance at a later date and the present funds focused on structures with a higher level of urgency. The knowledge of future corrosion activity on the structure helps owners to identify cost-effective mainte- nance options. The knowledge of future activity can be obtained in two formats, one as the remaining service life of the structure and the other as a function of concrete deterioration with time. To use remaining service life, the owner has to formulate a criterion that defines the end of service life. Numerous such criteria have been defined by researchers and owners. The service life approach provides the owners with the informa- tion as to when in the future, according to specific criteria, C H A P T E R 1 Introduction

the structure will need to be maintained, repaired, or replaced. The other approach, function of concrete deterio- ration with time, provides the owners with information on the progression of damage in the future that can be used to decide what action is most appropriate and when. The own- ers can also use that information to determine the end of service life based on specific criteria. The difference in the two approaches is simply in the presentation of the output of the modeling and not in the modeling process. The process for predicting remaining service life can also be used to pro- ject future damage as a function of time. Because the second approach is more flexible, it is used in this manual. However, for simplicity and keeping in conformance with the naming convention in the literature, the modeling process is termed “service life modeling.” There are several approaches for estimating the structure’s future damage. One such approach is to assume that the process that generated the present damage will continue to do so at the same rates in the future. In this approach, one would model the deterioration process and validate it against pres- ent damage on the structure and then use the model to pro- ject into the future. This validation can be performed for one or more data points from the past. However, it should be rec- ognized that the rates and the processes that resulted in the present damage may not remain the same. For determining a cost-effective bridge maintenance strategy, the order of the magnitude of repair is more important than the exact amount of damage, and this approach is reasonable. Although several mathematical models have been pro- posed to model the corrosion process based on the extent of deterioration and the presence or absence of deleterious agents, none have been verified in a scientific manner or have been standardized for use by the bridge community. In this study the model developed was statistically validated on three bridge structures located in Kentucky, Ohio, and Maryland, and a report documenting the results is presented in the appendix. For any valid model to be successful, standardized input data must be available. Numerous destructive and nonde- structive technologies are available for ascertaining the exist- ing condition of the concrete element and quantifying the presence of deleterious agents. Several protocols for evaluat- ing reinforced concrete bridge superstructures have been pro- posed.A functional protocol for evaluation must be complete, comprehensive, and specific to the model used for estimating remaining service life. Thus, such a protocol was developed to provide the necessary input for the model validated in this effort. Accuracy of any model is dependent on the sufficiency and quality of the input data. However, considering the lim- ited resources of various state highway agencies, a practical compromise was devised to obtain reasonable accuracy with reduced data collection. In addition to selecting the course of action, materials, and methodologies, an evaluation of the impact of various alterna- tive materials and methodologies on the extension of remain- ing service life must be performed to allow for the selection of an optimal solution. Many materials and methodologies intro- duced to counteract corrosion have been evaluated on field structures, and field performance information is available in literature. This is provided to better ascertain the future per- formance and need for maintenance. Objective and Audience The primary objective of this project was to develop a man- ual, for consideration and adoption by AASHTO, that pro- vides step-by-step procedures for the following: 1. Assessing the condition of reinforced concrete bridge superstructure elements subjected to corrosion-induced deterioration. 2. Predicting the remaining service life of such elements. 3. Quantifying the service life extension for such elements expected from alternative maintenance and repair options. The scope of work for this project was limited to bridge superstructure elements reinforced with black reinforcing steel and, to some extent, epoxy-coated reinforcing steel. It did not include prestressed concrete elements and other mod- ified steels. This manual is targeted toward engineers and maintenance personnel charged with maintaining the bridge structures. This manual can be used by state and other highway agencies and adopted with or without modifications as guidelines for use within the organization. Manual Organization Chapters 1 to 5 provide information and explanation of the various facets of the protocol proposed in this manual, and Chapters 6 to 8 provide detailed step-by-step procedures for implementing the protocol. 2

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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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