Evaluation of Shelf Life in Post-Tensioning Grouts
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Evaluation of Shelf Life in Post-Tensioning Grouts

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    Final report, Oct. 2014-Aug. 2018
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  • Abstract:
    Post-tensioning (PT) grout is commonly used in post-tensioned bridge girders to create a bond between the prestressing strands in the tendon and the surrounding concrete; it also provides corrosion protection to the prestressing strands. In recent years, however, soft grout has been found in several bridges in Florida and around the world. Soft grout has the potential to cause severe corrosion of the prestressing strands. The purpose of this study was to investigate the shelf life of proprietary prepackaged grout along with limited testing of their respective constituents under various levels of exposure to humidity and heat. Prehydration of the fresh portland cement, when exposed to high humidity, causes an increase in mass and a change in the cement particle size; Modified Inclined Tube Test (MITT) results have shown that prolonged exposure to high temperature and humidity increased the susceptibility of prepackaged grout to the formation of soft grout, thus linking prehydration to soft grout. In this research, MITT was conducted on large-volume samples to evaluate the effect of exposure on the susceptibility to soft grout formation. Mixtures were prepared using a water dosage that was 15% higher than the maximum recommended by the manufacturer. All commercial PT grouts, if given sufficient exposure time, eventually formed soft grout. MITT testing was conducted along with a number of other test methods that evaluated the effects of prehydration on mass and particle size, including mass gain, particle size analysis, Blaine fineness, loss on ignition, thermogravimetric analysis, and microwave moisture content. In addition, behavior of individual PT grout constituents such as portland cement, supplementary cementitious materials (SCM), and admixtures under high humidity and temperature was investigated. It was found that each PT grout constituent was susceptible to either physical and/or chemical changes during exposure to adverse levels of temperature and humidity. These changes in properties of PT grout constituents contribute to the formation of soft grout; prehydration of portland cement appears to be the primary cause. Finally, this report provides a proposed screening protocol for field determination of the condition of a PT grout prior to use. If a PT grout performs adequately in the screening tests, it should perform as expected in the grouting operations. This would provide a more convenient and economical method for evaluating exposure of the bagged material than the MITT method.
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