Durability Design of Post Tensioned Bridge Substructures

Bridge decks and superstructures have received considerable research and design effort to improve durability. While this effort has been successful in increasing the service life of the superstructure, bridge substructure condition may now limit service life.

Post-tensioning has been widely used in bridge superstructures, but has seen only limited application in bridge substructures. Post-tensioning can provide structural and economical benefits in substructure designs, and can possibly be used to improve durability.

The overall objective of this dissertation is to develop durability design guidelines for post-tensioned bridge substructures. The design guidelines are based on an extensive literature review and three experimental programs.

The literature review revealed a wealth of information on identifying durability concerns, concrete durability and corrosion protection measures for post-tensioned concrete structures.

A long term corrosion testing using large scale beams was developed to examine the effects of post-tensioning on corrosion protection through crack control. The beams are subjected to aggressive exposure and structural loading. Preliminary results indicate corrosion activity is decreased as the level of prestress increases, and that corrosion activity is largely confined to crack locations. This testing program is ongoing.

A long term exposure testing program using large scale column elements was developed to examine corrosion protection in vertical elements. Post-tensioned designs were compared to standard reinforced concrete designs. Corrosion activity during the reporting period was limited. Chloride samples showed substantially reduced chloride penetration for fly ash concrete. This testing program is ongoing.

A testing program with standard macrocell specimens was used to investigate corrosion protection for internal tendons in precast segmental construction. Findings indicated that match-cast epoxy joints are a necessity for corrosion protection of internal tendons. Severe corrosion damage was found on galvanized steel ducts, suggesting plastic ducts should be used in aggressive exposures. Gaskets used on the joint face around duct openings allowed moisture and chlorides to penetrate the joint. This testing program is ongoing.

Preliminary durability design guidelines were developed to identify durability concerns, to improve substructure durability using post-tensioning and to protect the post-tensioning system from corrosion. Because the experimental programs are on-going, the design guidelines are subject to change.