51316-7757-Embrittlement Susceptibility of Stressed High Strength Ungrouted Post-Tensioning Strands

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Product Number: 51316-7757-SG

ISBN: 7757 2016 CP

Author: Joseph Fernandez

Publication Date: 2016

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“Embrittlement Susceptibility of Stressed High Strength Ungrouted Post-Tensioning Strands”By: Joseph Fernandez* and Alberto Sagüés***Department of Chemical & Biomedical Engineering**Department of Civil and Environmental EngineeringUniversity of South Florida Tampa FLPost-Tensioning strands made with highly cold worked eutectoid steel are protected from corrosion by the presence of high pH cementitious grout injected in the ducts that contain the tendon strands. However grouting does not take place immediately after strand placement and tensioning so there is a period when the steel surface is vulnerable to corrosion. Corrosion during those periods may have consequences in the form of strand failure under tension before or after grouting.Much concern exists about hydrogen embrittlement that might develop in tensioned ungrouted strands but remain undetected in conventional mechanical tests. A field investigation was conducted with near-full-scale concrete piles with tendon ducts containing ungrouted strand with various levels of moisture and chloride ion exposure simulating near-seashore exposure over periods up to 4 weeks. After exposure the strands were examined for surface corrosion products and hydrogen content and evaluated mechanically with standardized tensile tests plus limited reverse bending and slow 3-point bending tests.Chloride presence even in small amounts resulted in surface rust development after periods as short as one week. Mechanical performance per standardized tensile tests was not appreciably degraded but hydrogen content measurements indicated significant hydrogen buildup. The effect of hydrogen on strand ductility was further investigated with cathodic hydrogen charging to identify regimes of vulnerability. Considerable embrittlement resulted in those experiments. Fracture morphology with a prominent shear component along the rolling direction was observed. Conditions of relevance to this application are examined and discussed toward establishing regions of vulnerability and an envelope of acceptable exposure.