ABSTRACT

An indirect impedance technique is assessed as a possible method to detect corrosion within external post-tensioned bridge tendons. The indirect impedance aims to extract the impedance of the steel-grout interface from the impedance measured at the surface of the grout. However, the electrode configuration required for the indirect impedance measurement yields geometry-induced frequency dispersion that confounds interpretation. A proof of concept was established for using indirect impedance to detect corrosion in post-tensioned tendons. Bench-top experiments were performed to show that the indirect impedance was sensitive to the properties of the steel and grout interface. Finite-element models were used to simulate the indirect impedance and determine how the geometry of the unusual electrode configuration influences the impedance. The indirect impedance was able to distinguish between locations of corroding steel and locations of passive steel, even if the actual damage to the steel was minimal. However, the complex nature of the grout impedance limits the indirect impedance measurement to providing only a qualitative assessment of the corrosion of bridge tendons.

INTRODUCTION

External post-tensioned tendons are incorporated in segmental bridge construction to force bridge segments into compression. This allows for longer spans between piers and provides greater concrete durability. In segmental bridge construction, precast-concrete box-girders are linked outward from bulkheads to form a bridge span between piers. The tendons are located within the inner opening of the box girders and are run continuously through deviator blocks which help form the profile of the tendon. They consist of multiple 7-wire pre-stressing strands contained within a High-Density-Polyethylene (HDPE) duct. The ends of the tendons are anchored and stressed after which the duct is filled with cementitious grout.1 The alkaline grout is designed to provide protection against corrosion but, due to possible voids in the grout and areas of improper mixing, incidents of severe corrosion have occurred. In Florida, the first reported post-tensioned corrosion issue was at the 18-year old Niles Channel Bridge.2,3 Similar issues were reported at the 7-year old Mid Bay Bridge in the Western Panhandle1,4 and the 15-year old Sunshine Skyway Bridge in Tampa.5 A number of studies were commissioned by the Florida Department of Transportation (FDOT) to address these issues. An important conclusion from the study related to the Mid Bay Bridge was that a non-destructive technique for testing corrosion and corrosion-risk in these post-tensioned members was needed.

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