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51315-6133-Accelerated Corrosion Study of Direct-Fixation Fasteners

Picture for Accelerated Corrosion Study of Direct-Fixation Fasteners
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Product Number: 51315-6133-SG
ISBN: 6133 2015 CP
Author: Jonah Kurth
Publication Date: 2015
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Two of theChicago Transit Authority's heavy-rail transit linesrun beneath the downtown core of Chicagothrough reinforced concrete subway tunnels constructed in the 1950s.Traction power is delivered to the train cars through a +600VDC third rail at grade but elevated slightly above the track level. The rail cars pick up the current through shoesand then return current through the wheels to the running rails (i.e. negative return rails).The running rail track in these subway tunnels was renovated over the last decade with work that included replacing formerly wooden half-ties with cast-in-place or precast concrete ties with direct-fixation fasteners. Corrosion of these fasteners and components was observed at various locations in the tunnels within a few years.This paper presents the findings of a recent accelerated corrosion study commissioned by the CTA. The study included inspection of track sections and characterization of the exposure conditions that might be conducive to corrosion. Study of the stray currents had to be performed while the system was in operation due to the 24-hour 7-day a week schedule of the train lines. The influence of stray current was evaluated with the Lateral Gradient Technique using half-cells supplemented with concrete surface resistivity and AC resistance measurements between track components. In addition debris and water in the tunnels was sampled and analyzed.The study found that stray currents as measured by the Lateral Gradient Technique were associated with severely corroded areas. In addition these corroded areas had lower rail-to-concrete resistance and lower concrete surface resistivity. The study concluded that the electrical isolation of the direct-fixation fasteners had been short-circuited by either debris build-up or water run-off resulting in accelerated corrosion of the fasteners.
Two of theChicago Transit Authority's heavy-rail transit linesrun beneath the downtown core of Chicagothrough reinforced concrete subway tunnels constructed in the 1950s.Traction power is delivered to the train cars through a +600VDC third rail at grade but elevated slightly above the track level. The rail cars pick up the current through shoesand then return current through the wheels to the running rails (i.e. negative return rails).The running rail track in these subway tunnels was renovated over the last decade with work that included replacing formerly wooden half-ties with cast-in-place or precast concrete ties with direct-fixation fasteners. Corrosion of these fasteners and components was observed at various locations in the tunnels within a few years.This paper presents the findings of a recent accelerated corrosion study commissioned by the CTA. The study included inspection of track sections and characterization of the exposure conditions that might be conducive to corrosion. Study of the stray currents had to be performed while the system was in operation due to the 24-hour 7-day a week schedule of the train lines. The influence of stray current was evaluated with the Lateral Gradient Technique using half-cells supplemented with concrete surface resistivity and AC resistance measurements between track components. In addition debris and water in the tunnels was sampled and analyzed.The study found that stray currents as measured by the Lateral Gradient Technique were associated with severely corroded areas. In addition these corroded areas had lower rail-to-concrete resistance and lower concrete surface resistivity. The study concluded that the electrical isolation of the direct-fixation fasteners had been short-circuited by either debris build-up or water run-off resulting in accelerated corrosion of the fasteners.
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