Guide Specification for Service Life Design of Highway Bridges (2020)

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NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 2 C H A P T E R 1 Introduction Background Explicit design of bridges for service life is an idea that has been germinating for some time, and despite significant effort, has yet to find traction in day-to-day design. The reasons for this lack of impact on current practice have little to do with resistance from owners, who generally view such an evolution as necessary and needed, but rather to a lack of sound and rational guidance. This in turn stems from a lack of focused and integrated research into the factors and mechanisms that govern the service life of bridges. Although much work has been done in Europe focusing on the penetration of chlorides into uncracked vertical concrete surfaces, and some recent projects in the United States have considered such mechanisms, to date such efforts have not been effectively translated into useable guidance for the bridge community. Although the underlying challenge of a lack of comprehensive data on the deterioration mechanisms of bridges is currently being addressed by numerous research efforts, the slow deterioration rates of bridges limit the rate of data collection and impede the near-term impact of such efforts. As a result, much of the work on service life conducted thus far has focused on those areas for which tractable analytical methods have been developed; but these may not necessarily be the ones that are most important to the definition of a component’s or a bridge’s service life. In order to provide a Guide Specification for bridge service life, a variety of methods of varying sophistication and development were considered such that the document could be used immediately, as well as developed over time. Therefore, based upon the research team’s collective experience with the AASHTO LRFD Bridge Design Specifications, SHRP2 Projects 19A (Azizinamini et al. 2014a) and 19B (Kulicki et al. 2015) and the FHWA Long-term Bridge Performance (LTBP) Program, the research team envisioned the evolution of service life design through a comprehensive framework. The goals and purpose of the Guide Specification are to: • provide practical guidance to designers of highway bridges on how to implement service life design during the design phase, including how to define a target service life and how to provide adequate implementation methods; • utilize currently available data as well as bridge owner experience to tie various design practices to service life, and • allow for the future incorporation of improved deterioration and service life models as they become available that will eventually evolve into calibrated partial factor semi-probabilistic limit states. Research Objective The two main objectives of this research were to develop: 1. a proposed AASHTO Guide Specification for Service Life Design of Highway Bridges 2. a set of case studies demonstrating the application of the proposed guide specification

NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 3 The vision of the research team was to provide AASHTO with a specification that lays the groundwork for modern service life design as it relates to bridge structures. The Guide Specification was created with the understanding and intent that the document would be revised and improved periodically in accordance with the future state of practice. Research Approach The aforementioned research objectives were accomplished over a period of four phases made up of a total of twelve tasks. The main aspects of the project included • a review and synthesis of literature; • a survey of current industry practice in relation to service life design; • the proposal, development, and execution of a service life design methodology; • the development of the Guide Specification, and • the development of the case studies. The information gained from both the literature review and survey were used to develop a proposed methodology centered around a three-tiered “good-better-best” approach to service life design. This three- tiered approach was tied to three categories of service life for nonreplaceable elements: Normal, Enhanced, and Maximum. The severity of the environment is accounted for in the Guide Specification using exposure zones and exposure classes. From a service life standpoint, the environment surrounding a bridge and its components represents the load on the structure. These environmental loads are typically represented using exposure classes. Within the Guide Specification, definitions of exposure classes are presented for the most prevalent deterioration mechanisms of the most common bridge materials (i.e., concrete, steel). The exposure classes are grouped by deterioration mechanism and vary by the severity of the environment. For each section of the Guide Specification that is element or material specific (e.g., concrete, steel, foundations), service life design provisions are provided as a function of the service life category and exposure class. The majority of the design provisions take the form of deemed-to-satisfy and avoidance of deterioration approaches. For the limit state of chloride-induced corrosion of reinforced concrete, calibrated deemed-to-satisfy provisions based on probabilistic methods are implemented. Each section also contains an article that provides widely accepted detailing practices that are beneficial in preventing deterioration and extending service life. A framework for using the fib Bulletin 34 methodology for chloride-induced corrosion is included in an appendix of the Guide Specification. Background information on the probabilistic method is presented along with guidance on how to implement this method in design. It is anticipated that this appendix, as it currently stands, will be used for atypical bridges requiring a rigorous service life design or when alternate design provisions than those given in the Guide Specification are required for a particular project. A set of case studies was developed to illustrate the implementation of the Guide Specification. Modifications to the original designs are made using the provisions of the Guide Specification. The bridges are then relocated to sites with different environmental conditions to highlight the effect of exposure classes on the design. The resultant different designs are summarized together for comparison.

NCHRP Web-Only Document 269: Guide Specification for Service Life Design of Highway Bridges 4 Contents of the Guide Specification The Guide Specification was organized in a manner similar to the AASHTO LRFD Bridge Design Specifications. The first few sections contain introductory and general guidance information. A section is dedicated to defining the environmental “loads” on a bridge structure, followed by several sections for the main types of materials and common bridge elements, and ending with a section on life cycle cost analysis. An appendix is included that provides a framework for expanding the Guide Specification to include probabilistic service life limit states once they have been sufficiently developed through research. A second appendix includes two case studies demonstrating the application of the Guide Specification. The contents of the Guide Specification are organized as follows: • Section 1—Introduction • Section 2—Classification • Section 3—General Design Guidelines • Section 4—Concrete Structures • Section 5—Steel Structures • Section 6—Foundations and Retaining Walls • Section 7—Renewable Elements • Section 8—Life Cycle Cost Analysis • Appendix A: Probabilistic Service Life Design Framework • Appendix B: Case Studies