Resumen de Design and applicability of an integrated monitoring system for railway sleepers monitoring
Prestressed concrete sleepers (PCS) are one of the main components of classical ballasted railway tracks. Their role is crucial for ensuring track integrity and safe railway operations. Railway sleepers are usually partially embedded in the ballast bed, which makes it almost impossible to detect premature structural damages with the naked eye. As a result, the monitoring of PCS is especially relevant as part of the in-track-maintenance programs, as this enables the detection of sleeper deterioration, which is sign towards structural failure.
To support the preventive maintenance of ballasted tracks, a new monitoring system has been designed for detection of crack-initiation processes on PCS in-service. This dissertation presents the results of the design and development of the new system and its implementation in track, including a proposal for an alternative experimental methodology to assess the structural integrity and wear of sleepers in-service. The proposed method has been developed on the basis of the results obtained from monitoring campaigns of sleepers by the developed system and the mechanical properties of prestressed concrete subjected to cyclic loads.
This research began with a thorough review of the state-of-the-art of PCS. This comprehensive analysis evinced the uneven development of PCS in different parts of the world and indicated that, today, the main causes of sleeper failure at the international level are tamping damage, shoulder wear, cracking due to dynamic loads, center binding, and environmental degradation. In this regard, the current international standards for design and approval of PCS include safety factors in their calculation and test procedures to consider all effects that may have an impact on the integrity and structural performance of railway sleepers in-service. In addition, the state-of-the-art review revealed a large number of monitoring systems and technologies currently used for monitoring railway tracks to provide insights into the performance of the railway track components and the changes in their properties over time. Each monitoring system has advantages and drawbacks; therefore, it was concluded that the selection of a certain monitoring technology should be made on the basis of the applicant’s experience, the scope of the analysis and the available technical and economic resources. The knowledge gained from the state-of-the-art review was applied to establish the design principles of a new monitoring system focused on PCS and capable of compensating for the disadvantages of the current technologies and offering an innovative approach.
The developed system is named the “Integrated Monitoring System” (IMS). It combines two traditional sensing techniques – strain gauges and laser-optic measurement. The strain gauges are housed in the rail pad to build a “sensing pad” that enables the detection and measurement of the wheel loads passing over the sleeper during train operation. The laser optic is encapsulated and embedded in the sleeper body to detect and measure the sleeper deflections caused by the wheel loads and the track condition, regardless of the other elastic components conforming the track structure. These small deflections – which are usually hidden by the greater deformations of the rail, the rail pad, and the ballast – have referred to in this dissertation as “relative sleeper deflections.” The implementation of the new system required the re-design of standard PCS. These re-designed sleepers have been termed “IMS-sleepers.” The materials, methods, and processes involved in the design, development, calibration, and testing of the developed system and the IMS-sleeper are in accordance with the current applicable standards and codes.
The first tests of IMS-sleepers with the integrated system were conducted in a laboratory in Germany, and they allowed reliable verification of the correct system functionality. The experience gained through this testing contributed to the design of the first field experiment conducted on a test track in the United States. IMS-sleepers were installed in track and equipped and monitored by the developed system during train operation in a heavy-haul track. The experiment began with the re-calibration of the system components in the track. The results demonstrated the proper function of the monitoring technology and allowed straightforward quantification of the structural behavior of PCS in-service.
In combination with the IMS, an alternative experimental methodology for determination of the crack-initiation process on PCS in track is proposed. The method is assisted by the monitoring of sleepers through IMS and statistical analysis of the obtained results by probabilistic models. Since the crack initiation process depends on several material properties and track parameters, which in turn are time-dependent, an analytical analysis of the factors affecting the process was conducted. The results and conclusions obtained from this analysis were then analyzed using a finite elements model (FEM) created and calibrated from the results obtained in the field experiment. This numeral analysis allowed the quantification of the influence of the considered sleeper and track factors on the bending moments and stresses that lead to the initiation and development of cracks. The results of the analytical and numerical analysis revealed that the factors that have a crucial impact on the crack initiation process are the concrete flexural tensile strength, the rate of losses of prestressing force, and the trackbed irregularities. These factors were considered variables for the development of probabilistic models able to describe the crack-initiation process. Based on a sample data generated by the FEM, two models were developed and introduced. The first is based on probability distributions and aims to provide insights into the structural condition of the sleepers on a monitored track segment. The second is a multiple regression model based on quantitative and qualitative variables, which is oriented to the prediction of the development of the sleeper condition over time. These experimental and statistical methodologies, together with the innovative and highly practical features of the IMS, expand the monitoring potential from structural analysis to detection and prediction of changes in the sleeper condition in track. This is intended to support and enhance the preventive maintenance of railway tracks. Therefore, some general recommendations are given at the end of this dissertation for implementation of the IMS and its methodology in the preventive maintenance programs of the ballasted tracks.
