THE CONCRETE BRIDGE MAGAZINE

Spring 2019

ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.

Issue link: http://www.aspiremagazinebyengineers.com/i/1093138

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C O N C R E T E B R I D G E T E C H N O L O G Y 28 | ASPIRE Spring 2019 by Bruce Osborn, Structural Technologies LLC/VSL Observations from 30 Years of Inspecting Post-Tensioned Structures During the 1970s, the construction of post-tensioned (PT) concrete structures rose in popularity as the technology allowed for the efficient construction o f l o n g e r s p a n s w i t h s m a l l e r concrete sections and less mild-steel reinforcement. While this technological advancement has provided contractors and owners with a durable and cost- effec tive solu tion, it has not been without its challenges. Since their inception, post-tensioning technology and specifications for PT bridges have evolved to ensure that long- lasting structures are being built with higher-quality installation procedures. Early failures of PT systems such as the Niles Channel Bridge (1999), Sunshine Skyway Bridge (2002), Varina-Enon Bridge (2007), and Mid-Bay Bridge (2011) pushed owners to implement closer inspection of PT systems. The following article shares the author's firsthand accounts of inspections of over 70 bridges and approximately 18,500 tendons. The author has inspected PT structures in some of the most benign and harshest environments from Alaska to Puerto Rico, and in many locations in between. The condition of each PT bridge was determined through analysis of data gathered from destructive and nondestructive investigations performed on external and internal post-tensioning systems. In the former type of system, tendons run within the void in box girders or cable-stayed bridges; in the latter type, ducts for tendons are cast within the structural concrete section. The findings illustrate the evolution of PT structures since the 1970s, common types of deficiencies, and how shifts in quality-control procedures have improved installation and extended the life of PT structures. Unforeseen Challenges In the early years of post-tensioning, stressing was regarded to be the most impor tant aspec t of the operation, and other details were believed to not be as s truc turally significant. Even though grouting served as another layer of protection against corrosion of the PT strands, it was often treated as an afterthought and performed with limited supervision or inspection from an outside par ty. Because the construction and inspection standards of past decades do not meet today's requirements, inspection of older PT structures is important. Prior to 2000, grout used for tendons was a simple mixture of Type I or II cement and water. In some cases, additives were used to try to improve the protective characteristics of the grout. In later years, it was proven that this combination of materials had segregation issues that caused excessive bleeding in the tendons. The presence of bleed water increased the risk of voids during the grouting process, thereby making the strands vulnerable to corrosion. Voids discovered in this time period were typically credited to excessive bleed water, improper venting, poor grouting practices, or inadequate grout materials. Notably, when inspections identified voids in the grout within tendons, those voids were not always associated with significant corrosion. In most cases, voided areas could be regrouted to extend the service life of the structure and the PT system within it. Figure 1 shows a borescope picture taken from the inside of a voided tendon in a structure whose exterior condition was well maintained. The void is located at the top of the duct, and the steel within the tendon has not undergone any major corrosion. This type of void is most Figure 1. Bare strand in a tendon at a grout void. All Figures: Bruce Osborn.

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