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CHAPTER 3 STUDY IMPLICATIONS 3.! Comparative United Kingdom/United States Experience The study has indicated that the experience with precast segmental construction has been significantly different between the U.K. and the U.S. The U.K. has experienced two corrosion-induced failures of precast segmental bridges to date. In addition, there have been a number of durability problems with bonded post-tensioned structures. The problems with segmental bridges in the U.K. have been specifically with thin mortar joints and poor design and construction practices. The problems with bonded post-tensioned structures have also been related to mistakes made during the design and/or construction of the bridge. Therefore, the U.K. experience with problems with bonded post-tensioned structures and specifically with precast segmental bridges has been related to poor design and/or construction practices and not due to significant intrinsic durability problems with precast segmental construction utilizing bonded post- tensioning. Thus, the moratorium currently in effect in the U.K. on precast segmental construction could be categorized as a very conservative response to past design and construction errors. In contrast to the U.K. experience, there have been no known durability problems with precast segmental bridges, related to the post-tensioning, in the U. S. to date. Although there have been some problems with grouting in the U.S., there has been very little reported corrosion-induced damage to bonded post-tensioning. It is important, though, to keep in perspective that there has not been exhaustive field investigation of the corrosion performance of precast segmental bridges with internal tendons. Most of the reported information is based on simple visual inspection. Although the U.S. experience with durability of precast segmental construction has been much better than in the U.K.: lessons can be learned from the U.K. experience. Certainly high quality design details and proper construction practices must be continued, especially when using internal tendons. It is important to adopt more stringent grouting requirements and inspection procedures, similar to what has been done in the U.K., to insure that the tendons are completely encapsulated in a high quality grout. 25
While lessons can, and should, be learned from the problems in the U.K. with precast segmental construction, the current U.S. experience and laboratory research indicates that precast segmental bridges, when properly designed and constructed, have good long-term durability. 3.2 Design and Construction Practice The results of this study provided valuable information about the state of design and construction practice in the U.S. Overall, durability design and construction has been good for precast segmental bridges. However, some things can be done to improve the durability design and construction and ensure good fixture perfo~ance. Although the study survey indicated that designers are using, and owners are adopting, the AASHTO Segmental Guide Specifications, AASHTO should formally adopt the guide specifications as part of the Standard Specifications for Highway Bridges to insure that durable segmental design and construction practices continue to be implemented. In addition, the AASHTO Specification should specifically preclude the use ofthin mortar segment joints. Moreover, epoxy joints should be specified for all corrosive environments, not just where de-icing salts are used. Grouting provides the main line of defense against corrosion. Thus, proper grouting is essential. Currently, there is a need for more comprehensive grouting guidance than what is provided by AASHTO and PTI. The review of the current draft of the new PTI Specifications on grouting indicates that it will provide more comprehensive guidance and should be used by designers and contractors upon its publication. In the interim, the recent references (18-27) on grouts should be reviewed for guidance. Although current research indicates that even dry segment joints may provide adequate corrosion protection when the tendon is well-grouted, epoxy joints performed significantly better; thus, match-cast epoxy joints or cast-in-place concrete joints should be used exclusively with internal tendons. Additionally, adequate end cover for the mild reinforcement at the segment joints should be maintained. It is recommended that the amount of end cover be consistent with the clear cover specified by AASHTO for the top deck slab. Metal tendon ducts seem to be performing well, but they are susceptible to corrosion and will most likely not provide a long-term barrier to the ingress of chlorides to the grouted tendons. 26
Corrosion resistant materials, such as polyethylene or polypropylene, should be used if a more corrosion resistant detail is required. A continuous duct across segment joints is probably not needed if proper construction and design procedures are followed. These include proper epoxying of the segments, adequate Recompression across the joints' complete grouting with a high quality grout' and high quality concrete in the precast segments. 3.3 Inspection and Evaluation Techniques Currently, there is a need for better non-destructive evaluation and inspection techniques for the condition of post-tensioning tendons insitu. At this time, some design or construction choices will limit the available inspection options; thus, future inspection and maintenance should be considered during the design of precast segmental bridges. The use of plastic ducts, while affording better corrosion protection, will limit the non- destructive testing ART) options for in-service inspection. Namely, traditional corrosion evaluation electrical techniques, such as half-cell corrosion potentials and linear polarization corrosion rate measurements (3LP), will be precluded. A1SO7 impact-echo testing will not be able to detect voids in the grout nor will impulse radar be able to be used to locate the duct (although it may be able to locate the tendon). Utilizing metal ducts, while not an effective long-term corrosion barrier, will allow the use of half-cell corrosion potentials and 3LP measurements to detect corrosion although the tests will likely detect both duct and prestressing steel corrosion. Additionally, impulse radar will be able to locate the duct for invasive probing; and impact-echo can be used to locate grouting voids. The use of radiography, while the only technique that has been successful in evaluating the condition of the prestressing steel, is limited by its high cost, safety issues, and the limited access to the tendons in many structures. 3.4 Repair Although the number of repairs related to durability of precast segmental bridges has been few to date, those repairs that have been done have been highly successful. 27
The reinfection of cementitious grout where voids in the grouted tendons were discovered has worked very well. Additionally, the installation of additional post-tensioning has been successful in strengthening precast segmental bridges when required. Thus, designers should be encouraged to provide additional ducts and good access to their structures so as to allow the installation of future additional post-tensioning, if required. 28
