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43 CHAPTER 8: FUTURE RESEARCH The research team suggests further research to investigate the use of plastic composite ties to insulate the rails from the ground. Plastic ties have the potential to prevent leaks of stray electric currents traveling from the rails to ground, thereby reducing rail base corrosion. The research team has been evaluating the field performance of plastic ties from several manufacturers since 1997. The first group of ties that were installed in track has been subjected to more than 1 million MGT (million gross tons) of heavy axle load traffic at the Facility for Accelerated Service Testing. The majority of the plastic ties tested has the same dimensions and weigh about the same as typical hardwood ties. The in-track tests conducted at TTCI indicate their performance is generally comparable with wood ties although cracking and some fractures have been documented (40, 41). More recently, during the summer of 2006, a laboratory test was conducted by TTCI to characterize the electrical resistance of plastic ties from three manufacturers. Figure 25 shows one of the plastic ties tested. The minimum allowable electrical impedance (resistance) of 20,000 ohms for concrete crossties, as specified in the AREMA Manual for Railway Engineering (18), was used as a baseline. Since the plastic ties tested are almost impermeable on their exterior, the test was conducted on cross sectional slices to expose any internal porosity. The purpose of exposing the porosity was to simulate a worse case condition, where water entered the interior of the tie potentially increasing electrical conductivity. Figure 25 shows the internal porosity of one of the test samples and the seven locations where the electrical resistance was measured under dry and water-soaked conditions. Figure 26 shows that some of the measurements were taken using typical screw spikes and some were taken directly on the surface of the sample. The electrical resistance of the tie samples was measured under dry conditions soon after they were cut from the three test ties and again after 3 weeks of total submersion in water. The results indicate that measurement location 1 (see Figure 26), screw spike to screw spike, provided the least resistance in all three tie types. The dry samples of the three tie types provided more resistance than the water soaked samples. The electrical resistance of tie type 1 at measurement location 1 was significantly lower than that of tie types 2 and 3 after being submerged in water for 3 weeks. However, at 65 k ohms, it was 3.25 times higher than the 20 k ohm minimum specified by AREMA. The remaining measurements ranged from 500 k ohms (25 times higher than the AREMA minimum) to over load indications on the ohmmeter. The laboratory test results indicate that although the 3 tie types provided different levels of electrical resistance, those levels were well above the AREMA specification. The in-track performance of plastic composite ties under heavy axle loads has been documented. That and the result of the recent electrical resistance lab tests suggest that test plastic ties be tested on a transit system where rail base corrosion has been a problem.
44 Figure 25. One of the plastic composite ties used for the electrical resistance laboratory test at TTCI. Figure 26. The electrical resistance measurements were taken at seven locations under dry and water-soaked conditions using a typical ohmmeter. 1 2 3 4 5 6 7
45 AUTHOR ACKNOWLEDGEMENTS TTCI thanks those in the transit industry who responded to the survey on âpractices and problems associated with rail base corrosion.â The authors express their gratitude to the personnel who assisted during site visits to the various transit locations and to those who provided corroded rails to the study. A special acknowledgement goes to Dr. Dingqing Li for his professional guidance and excellent support during the program. Special recognition is also given to Dr. David Jaramillo Vigureras for his contribution and support of the program through the CIITEC-IPN, Mexico. A special thanks is also given to Mr. Rafael Jimenez for his expert advice on plastic ties and for providing the appropriate literature for this research.
