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CHAPTER 1 INTRODUCTION AND RESEARCH APPROACH I.! Introduction The application of grouted post-tensioning utilizing internal tendons has gained acceptance world-wide as an economically viable and popular form of bridge superstructure construction. Serious problems can develop in grouted post-tensioned structures if voids are present within the grout that is used to fill the tendon ducts and protect the prestressing steel. Inadequate grouting may allow water to penetrate into the ducts, initiate corrosion of the tendons and potentially lead to failure of the structure. The sudden collapse of the Bickton Meadows footbridge in 1967 and of the Ynys-y-Gwas Bridge in 1985 due to corrosion of tendons in addition to other documented problems with post- tensioned bridges led to a ban in 1 992 on the construction of new bonded post-tensioned bridges by the U.K. Ministry of Transportationi 2. In 1 996, the moratorium on grouted post-tensioned cast-in- place construction was lifted but remains in force for segmental construction apparently because of concerns about the corrosion protection of tendons as they pass through the bridge segment joints. It is generally perceived that the experience in the United States (U.S.) with durability of segmental construction utilizing internal tendons has been good. Thus, this general perception appears to be contradictory to the experience in the United Kingdom (U.K.) as evidenced by the continued ban there on segmental construction. There is a pressing need for U.S. bridge engineers to gain an understanding of durability issues associated with segmental construction and to be able to judge on a technical and rational basis the veracity of the on-going moratorium in the U.K. pertaining to segmental construction. I.2 Research Objective The principal objectives of the study reported herein were to analyze and summarize the information on durability of precast segmental concrete bridges which utilize grouted internal tendons 3
and to provide definitive recommendations to the AASHTO Highway Subcommittee on Bridges and Structures relative to durability of segmental construction. The focus of the study was principally a comparative assessment of the durability, with the primary emphasis on the corrosion performance, of precast segmental bridges in the U.S. and the U.K. The study includes a summary of selected information available on materials, design, construction, performance data, inspection, structural evaluation, repair and rehabilitation, and corrosion protection of internal tendons across segment joints. 1.3 Research Approach In order to meet the research objectives, the study included a literature search, a survey, and personal contacts. Initially, a review of the literature was conducted regarding the durability of precast segmental bridges with internal tendons. This search was conducted using a variety of technical data bases available through engineering societies, the Internet, and in-house publications and personal contacts. The research team collectively possessed a wealth of personal contacts, both nationally and internationally, that were surveyed regarding the corrosion protection issues of internal tendons across segmental joints. One member of the research team, Mr. Jeffrey West, a doctoral student of Dr. John E. green's at the University of Texas at Austin, met with Dr. George Somerville, who is Chairman of the Working Party in the U.K. charged with reviewing durability of grouted post-tensioned construction in the U.K. Mr. West met with Dr. Somerville in July 1997 in conjunction with his attendance at the Structural Faults Symposium in Edinburgh. This was an invaluable and unique opportunity to gain an understanding of the U.K. perspective. Additional contacts were made with Dr. Sommerv~lle via electronic mail regarding clarification and elaboration on several issues. Key State and Federal governmental agencies, members of the AASHTO Subcommittee on Bridges and Structures, the American Segmental Bridge Institute (ASBl), and selected design consultants were surveyed regarding experience with durability of segmental construction. The survey was in the form of a questionnaire. A key research project was in progress at the time of the publication of this report that was particularly relevant to the study. Interim results of the on-going research study at the Ferguson 4
Structural Engineering Laboratory at the University of Texas at Austin sponsored by the Texas Department of Transportation of accelerated segmental exposure specimens were reviewed. In addition, evaluation of projects recently completed by the principal investigator involving corrosion susceptibility and non-destructive testing of existing segmental bridges provided relevant information to the study issues. I.4 Report Contents The findings from NCHRP Project 20-7, Task 92, "Durability of Precast Segmental Bridges" are reported herein. Chapter 2 summarizes the findings from the study. Chapter 3 presents the implications from the studyin the context ofthe relevance of the U.K. moratorium with respect to the performance experience in the U.S. Chapter 4 presents the major conclusions from the study and suggested research needs. . 5
