ABSTRACT

As structural tendons are routed through deviation blocks throughout post-tensioned bridges, the outer ducting material changes from an insulating polymeric duct to a conductive galvanized steel duct. During casting of the bonding material, workable grout is pumped into the duct where the increased initial conductivity can promote a galvanic couple to from between the galvanized steel duct and the steel strands. When this couple is formed, the less noble zinc in the galvanized steel duct becomes the anode and the steel strands become the cathodically polarized. This polarization can drive the hydrogen evolution cathodic reaction which can lead to hydrogen embrittlement. The objective of this paper is to quantify the level of galvanic coupling achievable as a function of grout conditions and to assess whether such conditions may promote hydrogen evolution and absorption. Experimental tendon assemblies were used to quantify galvanic coupling considering a single wire and various grout conditions. The results were related to more realistic geometric configurations by simulations considering the appropriate kinetic boundary conditions. Understanding the significance of this galvanic couple can allow for better structural tendon life prediction and may lead to possible design changes that reduce the usage of galvanized steel ducts for use in structural tendons.

INTRODUCTION

External post-tensioned tendons are used in segmental precast box girder bridge construction to hold segments together and prevent service cracking. The tendons consist of multiple 7-wire prestressing strands contained within a high-density Polyethylene (HDPE) duct located within the inner opening of the box girders. They run continuously through deviator blocks, which helps form the profile of the tendon. Within the deviator blocks, the ducts are made from a more rigid material such as galvanized steel. The ends of the tendon are anchored down at bulkheads and stressed, after which the duct is filled with cementitious grout.1 The alkaline grout is intended to provide protection against corrosion, but due to voids in the grout and/or areas of improper mixing, cases of severe corrosion have occurred.

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