An FHWA Special Study: Post-Tensioning Tendon Grout Chloride Thresholds

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This document is a technical summary of the Federal Highway Administration report, An FHWA Special Study:
Post-Tensioning Tendon Grout Chloride Thresholds (FHWA-HRT-14-039).

INTRODUCTION

Since the 1970s, the number of pre-tensioned and post-tensioned (PT) concrete bridge structures utilizing high-strength seven-wire strands has increased steadily. For bonded PT tendons, the grout functions as the last layer of corrosion protection for the highly stressed seven-wire strands by providing a high pH environment to form a protective oxide film on the strand surface and also acting as a physical barrier to water, oxygen, and chloride ions. Corrosion of strands initiates when the protective oxide film is compromised due to chloride attack or carbonation of the surrounding grout upon exposure to water and air. Once corrosion starts, it propagates at a certain rate controlled by many factors such as oxygen availability, moisture content, electrical resistance, grout pH, and chloride concentration. In this TechBrief, a prestressed seven-wire strand will be simply referred to as a “PT strand.”

Corrosion problems have been observed in some PT bridges in Florida, Virginia, and Minnesota mainly due to grout voids, poor quality grout, water infiltration, and duct holes/cracking. These deficiencies can lead to severe corrosion or failure of the PT tendons. A recent discovery of a grout product with elevated levels of chloride used in a PT concrete straddle cap located in Corpus Christi, TX, resulted in a full investigation of a potential corrosion problem. Chloride concentrations were reported to be as high as 5.27 percent by weight of cement.^(1,2) These numbers exceed the current limits set by all domestic and international regulatory committees. For example, the chloride limits for prestressed concrete imposed by American Concrete Institute committees are either 0.06 percent water-soluble chloride by weight of cement or 0.08 percent acid-soluble chloride by weight of cement. These chloride limits are arbitrary concentrations determined by the code authorities and specifications after taking into account a factor of safety. Conversely, chloride threshold is the actual amount of chloride needed to initiate corrosion. There is limited information reported in the literature pertaining to the chloride threshold of the PT tendons. As chloride information is needed to assess corrosion risk of the PT bridges containing the chloride contaminated grout, the Federal Highway Administration sponsored a 6-month accelerated corrosion testing program to determine chloride threshold(s) of PT strands exposed to chloride-contaminated grout. This TechBrief explains how two chloride threshold values of 0.4 and 0.8 percent by weight of cement were determined for corrosion initiation and corrosion propagation, respectively, using supporting test results.