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Evaluating Concrete Resistivity: Reducing Stray Current From Dc Streetcars
Tinnea, Jack (Tinnea & Associates, LLC) | Tinnea, Ryan (Tinnea & Associates, LLC) | Burke, Dennis (N. D. Burke Associates, Inc.) | Nelson, Les (Seattle Public Utilities) | Cochran, Shaunie (Seattle Public Utilities) | Anderson, Elizabeth (Seattle Public Utilities) | Pham, Ly (Seattle Public Utilities)
ABSTRACT A direct current (DC) streetcar project with rails embedded in the street is under development south of Lake Union in Seattle, Washington. DC trains can produce stray currents which can accelerate the corrosion of buried utilities. As a first line of defense this project employs an insulated formed rubber rail boot encased in the concrete track slab. A rubber boot encasing the rails provides a high resistance to earth and is an effective barrier providing a reduction of stray current discharges. Traditionally, Seattle Public Utilities (SPU) would have replaced any watermains within 10 feet parallel to the rail and crossings. The pipe was evaluated and found to have at least 100 years of useful life left if the pipe environment were to remain the same. If the pipe were replaced, SPU would coat the replacement pipe as a secondary barrier against stray current. If the pipe was to remain in the ground, a secondary protection measure would need to be evaluated. In an effort to augment traditional stray current control methods and significantly increase rail to earth resistance values at boot defects, it was decided to investigate the benefits of increased resistivity of the concrete track slab. Multiple track slab concrete mixes were tested to evaluate the impacts on resistivity of water reducers, a waterproofing agent and mineral admixtures including slag, fly ash, and condensed silica. To provide a comparative baseline, samples from routine City of Seattle concretes were also tested. Surface resistivity measurements of the several concretes were obtained over time using the State of Florida Test Method FM 5-578. Resistivity was also obtained using the Wenner four-pin test method. In addition, correlative testing was conducted using ASTM C 1212 Standard Test Method for the Electrical Indication of Concrete's Ability to Resist Chloride Penetration - what is also known as the "Coulomb Test". After initial screening, final mix designs were again evaluated for electrical resistance and also for resistance to drying and autogenous shrinkage. This paper discusses the several test methods employed in addition to the effects of trial mix composition. BACKGROUND Since the 1890s stray currents generated by DC operated streetcars have caused corrosion of metallic utilities buried in the vicinity of the rails and power substations. Although the causes for stray current were known, effective construction techniques to prevent them were not available. Utilities coped with the problem by grounding facilities directly to transit rails or power station negative busses. Often these measures resulted in an increasing zone of stray current influence. By the 1950s, streetcars had become relics of the past or were maintained as historic curiosities until the need for increased mass transit prompted increased construction of DC powered light rail transit (LRT) systems. With the increase in LRT construction, ways and means to prevent stray current from leaving the transit rails were attempted by designers. Many transit agencies recognized that stray currents were as detrimental to transit facilities as to utility facilities. Products and methods improve each year based on the results on the previous project.
- North America > United States > Florida (0.48)
- North America > United States > Washington > King County > Seattle (0.34)
- North America > United States > District of Columbia > Washington (0.24)
ABSTRACT In August of 2003, water was discovered leaking into post-tensioning (PT) ducts for loop tendons of the Woodrow Wilson Bridge. This structure is being built to cross the Potomac River in Washington D.C. At the bridge location, the Potomac is slightly brackish. To prevent corrosion, the water was pumped out of the ducts, and the ducts were sealed using an acrylic polymer gel. After sealing the ducts, the PT tendons were run, loaded and the duct annulus filled with a cementitious grout. To evaluate the acrylic gel seal corrective strategy, test specimens were prepared that generally followed the configuration given in the Post- Tensioning Institute's (PTI) Accelerated Corrosion Test (ACT). These specimens were modified to include fabricated cracks of differing widths that were sealed with the acrylic gel. The specimens were then subjected to the 1000-hour ACT. This paper details the test method modifications developed, the results obtained and a discussion of the findings. INTRODUCTION Water was discovered leaking into post-tensioning (PT) ducts located at the loop tendons of Woodrow Wilson Bridge in Washington D.C. Careful consideration determined that splices in the PT ducts near the pedestal-pile cap interface were the sources of the leaks (see Figure 1). After a thorough review, a quick setting acrylic polymer (acrylic gel) was selected to seal the duct splices. After removal of any residual water the ducts were to be sealed with the acrylic gel. After sealing, the ducts would then be pressure-washed to remove excess acrylic gel and pressure tested to confirm the absence of leaks. (available in full paper) Tests were performed to confirm that the selected acrylic gel would be resistant to chloride ion diffusion from the surrounding brackish Potomac River. To test the acrylic gel's resistance to chloride ion permeability the Post-Tensioning Institute (PTI) Accelerated Corrosion Test (ACT) was modified to investigate the performance of the acrylic gel crack sealer. This report provides the findings of the modified ACT including evaluation of the use of the acrylic gel as a sealant that is resistant to chloride ions present in the adjacent and slightly brackish Potomac River. Also included is a discussion of the test methods employed including several photographs. TESTING PROTOCOL Materials To reduce shipping costs, branded construction materials are often prepared from locally available raw materials. Given the construction site was on the east coast of the United States and testing was being conducted in Seattle, materials used in the testing regime were obtained from the job site. These materials included 7-wire prestressing strands, the grout used to fill the post tensioning (PT) ducts, and the acrylic gel used to seal the leaking PT duct splices. The grout was Sika PT-300 while the acrylic gel was a mix of Avanti AV100 Chemical Grout and AV103 Catalyst. Specimen Preparation The test specimens each consisted of a 305mm (12") long length of 7-wire prestressing strand centered axially in a clear 25mm (1") PVC mould casing as described in Appendix B of the PTI's Specification for Grouting of Post-Tensioned Structures and shown in Figure 2.
- Government > Regional Government > North America Government > United States Government (0.69)
- Materials > Chemicals > Specialty Chemicals (0.54)