China Railway Track System Research Articles

Stiffness identification of fixing device in CRTS II slab track based on long-term monitoring data

The fixing device is an important component in the China Railway Track System (CRTS) II slab track, through which the longitudinal force of the track can be transferred to bridges and piers. The stiffness of the fixing device significantly affects the mechanical properties of the track. However, it is difficult to evaluate stiffness using conventional detection methods, because the fixing device is located under the base plate. In this study, the fixing-device stiffness of simply supported bridges was identified using long-term monitoring data and finite element (FE) analysis. First, a monitoring scheme was proposed based on the principle of track–bridge interaction. The strain in the base plate on both sides of the fixing device, as well as the temperature of the base plate and bridge, were obtained from 2015 to 2018. Then, the stiffness of the fixing device was proved to be closely related to the strain difference of the base plate on both sides of the fixing device, and the corresponding nonlinear mapping relationship was established through FE analysis. Finally, the fixing-device stiffness over 4 years was identified using long-term monitoring data and the established mapping function, and the influence of the stiffness degradation on the mechanical property of the track was discussed. This study can provide a reference for the design and maintenance of CRTS II slab tracks on multi-span simply supported bridges.

Study on Optimal Bracketed Duration Scheme of Seismic Time History Analysis for High-Speed Railway Track-Bridge System

ABSTRACT Seismic duration significantly impacts the structural seismic response. In order to obtain a correct time history analysis result of structural seismic design, determining a reasonable seismic duration scheme is imperative. In this paper, a five-span simply supported beam bridge and China Railway Track System (CRTS) II slab ballastless track system is taken as the research focus. A finite-element model for a high-speed railway track-bridge system was developed. The influence mechanism of starting and end threshold on seismic response was analyzed. Different bracketed duration schemes were put forward, and their applicability was assessed. The optimal bracketed duration scheme of time history analysis for a high-speed railway track-bridge system was determined. The results indicate a response curve deviation between the full duration and partial duration in the starting part due to the inconsistent structural movement trends and a deviation in the end part because of the structure entering a free vibration state in advance. When calculating the seismic peak response, the starting threshold A and end threshold B at both ends should be taken as5% of peak acceleration. When calculating seismic residual response, the starting threshold C at the left end should be taken as 25% of peak acceleration.

Experimental study on fatigue damage of self-compacting concrete of CRTS III slab track

To study fatigue performances and establish the fatigue damage model of self-compacting concrete (SCC) of China Railway Track System (CRTS) III slab track under different stress levels, static and fatigue tests were conducted. In the static test, the flexural strength of SCC was obtained. In the fatigue test, fifteen specimens were divided into five groups. The maximum stress levels were 0.65, 0.70, 0.75, 0.80, and 0.85 for each group, and the minimum stress level was 0.10 for every group. The S-N curve of SCC is obtained and shows that the fatigue life of SCC decreases with the increase of the maximum stress level. The strain evolution and elastic modulus degradation can be divided into 'Fast-Slow-Fast' three stages. With the maximum stress level increases, the proportion of elastic modulus degradation increases and decreases in the first and third stages, respectively. The fatigue damage model derived from the improved elastic modulus method and the Chaboche fatigue damage model can reflect the damage evolution of SCC under different stress levels. Moreover, the proposed model can provide theoretical guidance for the further numerical simulation of SCC for ballastless tracks under the fatigue load.

Evaluation of slab track quality indices based on entropy weight-fuzzy analytic hierarchy process

This paper aims to guide the maintenance and repair of ballastless track quality by determining a set of comprehensive evaluation indices. We take the China Railway Track System (CRTS) III slab track as an example and mainly look at two typical types of damage, namely, cracking and interlayer debonding. Based on the entropy weight method-fuzzy analytic hierarchy process (EWM-FAHP), cracking interspace volume (CIV) and debonding interspace volume (DIV) and subsequent weights were proposed to measure the damage degree. In addition, a model containing cracking and debonding was established using the ABAQUS finite element software, and these weights were modified based on the models' calculated results. Then, the quality condition index (QCI) was determined to assess the quality state of the slab track in a comprehensive manner. This index was further used in a case study to evaluate the quality of the slab tracks in real-scenario cases and thereby bring forward appropriate O&M suggestions.

A novel fatigue cohesive model for interface between ballastless track slab and self-compacting concrete

The interface between track slab and concrete is the weakest zone in a ballastless track. Under the long-term and repeated coupling effect of the environment and train loads, a large area of damage can occur at this interface, leading to the initiation and propagation of interlaminar crack, and the failure of the interlayer bonding area. To reveal the fatigue behaviors and degradation laws of the interface between the prefabricated track plate (C60) and self-compacting concrete (SCC) in China Railway Track System (CRTS) III slab track, a fatigue cohesive zone model is proposed for the analysis of the fatigue bonding performances of the interface C60-SCC, and the fatigue damage degradation criterion is established. Next, static and fatigue tests for C60-SCC composite specimens are conducted, and full-field strain distribution images of these specimens are obtained by the digital image correlation technique. The results show that: (1) the evolution of interfacial crack of C60-SCC specimens under monotonic loading was captured and static cohesive parameters were obtained; (2) the fatigue life curve and interfacial separating displacement curves were acquired, for predicting the interface fatigue life and calculating the degree of interface damage and residual fatigue life of CRTS III slab track in service; (3) the fatigue cohesive response of the interface C60-SCC under cyclic loading were determined for describing the fatigue behaviors of the interface in CRTS III slab track.

Experimental and numerical study on interfacial thermal behaviour of CRTS II slab track under continuous high temperatures

The interfacial thermal behaviour of vertical multi-layer materials under high-temperature has a significant role in structural thermal behaviour of China Railway Track System (CRTS) II slab track in high-speed railway. In order to further clarify the interfacialthermal behaviour under continuous high diurnal temperature, a 1/4 large-scaled model of five CRTS II track slabs in a temperature simulation chamber was first tested to obtain their temperature distribution and stress responses. After the validation of experimental results, three-dimensional heat transfer models of the full-scale CRTS II track-bridge structure were then established to investigate their interfacial thermal behaviours under two continuous high diurnal temperature conditions, respectively. Results show that the transient heat transfer model could reflect thermal behaviours of the CRTS II slab track, and the relationship between the stresses and internal temperature in the track slab is a quadratic polynomial equation. There is no interface damage under a continuous high temperature from 35 °C to 26 °C with the largest vertical displacement of about 0.38 mm at the 108th hour. Under a higher continuous temperature from 45 °C to 36 °C, the largest vertical displacement increases to 1.24 mm. More importantly, the interface damage zones grow from both two sides of the mid-span track slab to the central zone with the upward arching shape.

Optimum maintenance strategy for CRTS II slab track based on time-dependent system reliability

CRTS (China Railway Track System) II slab track is a vertical multi-layer and longitudinal anisotropic strip system, which has been widely constructed in China’s high-speed railway. In this study, an innovative optimum maintenance strategy is proposed for CRTS II slab track, based on the concept of time-dependent system reliability and life-cycle cost analysis. System limit state function of the track structure is constructed with consideration of important serviceability failure modes. The time-dependent system reliability of the track structure is investigated and utilized as the indicator of track performance, which is estimated by an innovative outcrossing method based on the system limit state function considering time-dependent correlation among failure modes. At each maintenance time, all critical failure modes and components are identified and repaired to decrease the system failure probability under the acceptable level. Application of the proposed maintenance strategy is demonstrated by a worked example of a track structural system on bridge. It is found that: (1) appropriate maintenance strategy is important to ensure service reliability of the track system and extend the service life; and (2) the proposed maintenance strategy can be utilized to predict when, where, and what failure mode for maintenance and it can help to minimize the expected life-cycle cost without compromising the service reliability of the track structure.

Effect of girder deformation on the dynamic performance of high-speed train-track-bridge coupling system

Aiming at the high-speed railway (HSR)-China Railway Track System III (CRTS III) slab ballastless track (SBT)-simply-supported bridge system, the mapping relationship between girder deformation and rail deformation had been deduced theoretically, and then, the additional irregularity of the rail was obtained. A dynamic model of the high-speed train-track-bridge (TTB) coupling system was established according to existing theory on the high-speed TTB dynamic interactions. Combined with the mapping relationship, taking the superposition of the additional irregularity and the random irregularity as excitation, the dynamic response of two train models (CRH2 and ICE3 trains) was analysed by using the high-speed TTB coupling system model. The recommended thresholds of girder deformation were given based on running safety and ride comfort indicators. The results show that the sensitivities of the two train models exposed to girder deformation were different, and the CRH2 train was more sensitive to the lateral girder deformation. The non-uniform dislocation deformation of multiple girders exerted a slightly more significant effect on the dynamic characteristics of two train models than the dislocation deformation of single girder. Under the same amplitude of girder deformation, the influences of vertical girder dislocation and vertical girder rotation on the running safety and ride comfort of trains were significantly greater than that of lateral girder dislocation and lateral girder rotation. Acceleration and wheel-rail force increase rapidly in the area of girder deformation. To guarantee running safety and ride comfort of trains, this paper proposes suggested values for girder deformation thresholds.

Experimental and Numerical Investigations of Structural Performance of Slab Track under Hygrothermal Environment

The coupled heat and moisture actions could easily cause deformation, cracking and corrosion problems in the slab ballastless track under operational conditions via China Railway Track System (CRTS) Ⅱ. In this paper, two one-quarter-scaled models of CRTS Ⅱ slab ballastless track under hygro-thermal environment were tested in a testing chamber, respectively. To study the effect of constrained conditions on the structural performance of track structure, temperature distribution, stress and displacement of two scaled models with constrained and free boundary conditions were also compared. Then, the temperature and stress field of the track structure was investigated by establishing a finite-element model. The results showed that the temperature transfer between layers has a temperature-lag effect. The positive and negative temperature gradients between the lower part of the track slab and the cement asphalt (CA) mortar layer of the constrained specimen are smaller than those of the free specimen. The longitudinal displacement of the track slab for the free specimen is significantly affected under the hygro-thermal environment and the longitudinal displacement of the concrete base and the track slab forms a displacement difference. Numerical-analysis results showed that the bottom of the concrete base is prone to produce a high-temperature core and that the temperature inside the track structure forms an agglomeration effect. KEYWORDS: Slab track, Hygro-thermal environment, Structural performance, Scale model, Finiteelement model.

Effects of joint stiffness degradation on vertical stability of CRTSII slab ballastless track

Joint damage of the China Railway Track System (CRTS)II slab track leads to typically non-uniformity of the longitudinal stiffness of the track slab, which tends to produce different degrees of arching at the joint. The objective of the present study was to investigate the arching mechanism of the non-uniform track slab. The buckling eigenvalues and arch curves of the non-uniform track slab were obtained using the Newton iterative eigenvalue algorithm. An analytical expression for the equilibrium path of the slab with an initial arch was obtained using the energy method. Then, the arching law and stability of the non-uniform track slab were analysed. The following results were obtained. 1) The reduction of the joint bending stiffness significantly affects the arching deformation and buckling mode of track slab, thus, it is necessary to consider the non-uniform characteristics of the track slab when analysing its stability. 2) The most unfavorable wavelength of track slabs is between 6 m, and 7.5 m considering the different joint bending stiffnesses and elastic moduli, and the reduction of the bending stiffness of the joint is the main factor for reducing the most unfavorable wavelength. 3) The reduction of the joint bending stiffness is the most important factor for causing the instability of the track slab, the maximum allowable temperature rise decreased from 83.97 to 49.12 ℃ when the bending stiffness of the joint was reduced by a factor of 100. 4) For improving the stability of the track slab, releasing part of the temperature force in the track slab by reducing the elastic modulus of the joints is a feasible method, the maximum temperature rise of track slabs can be rapidly increased when the elastic modulus of the joints is < 5000 MPa.

Interlayer shearing and bending performances of ballastless track plates based on high-order shear deformation theory (HSDT) for laminated structures

Based on higher-order shear deformation theory (HSDT), the bending of China Railway Track System type II (CRTS-II) track laminated plates on elastic foundations and interlayer shearing performances are analyzed by meshfree radial point interpolation method (RPIM), which has the properties of the Kronecker function and discretizes the governing equations derived from HSDT. To verify the applicability of the model, the geometric configuration and key parameters of the CRTS-II slab ballastless track structure are analyzed, and the interlaminar slip model of the track structure is derived as a benchmark solution when calculating the track structure using HSDT-RPIM. The accuracy of the derivation is verified by comparing the HSDT-RPIM model with a double-layer slab model considering bidirectional interlayer slip. Then, the effect of resistant slip coefficient on the bending deformation of the track structure was analyzed using the bidirectional interlaminar slip model. The effect of elastic foundation coefficient on the bending of the track structure and the interlayer slip of the track structure (track slab – base slab) was analyzed. Finally, the effects of boundary conditions and elastic foundation coefficients on the interlaminar stresses of the track structure are investigated using HSDT-RPIM, and some interlaminar damages of the track structure are explained.

Experimental study on interfacial damage characteristics of CRTS II slab track and CA mortar with AE and DIC techniques

Since the interface is often regarded as the weakest part of the ballastless track structure, there is a great need to investigate the interlayer bonding performance to ensure the safety of train operations for high-speed railways. In this study, the damage characteristics of the interface between China Railway Track System II (CRTS II) slab track and cement emulsified asphalt (CA) mortar (T-CA interface) under shear loading were thoroughly investigated. The interfacial deformation and damage development process during the direct shear experiments were recorded by a combination of acoustic emission (AE) and digital image correlation (DIC) techniques. Consequently, a bilinear cohesive zone model (CZM) of the T-CA interface was proposed. The results show that the AE activities are rare in the initial stage of loading, while they increase sharply when the shear loading approaches the interface bonding strength. The AE parameters can well characterize the entire process of interfacial failure. Through the DIC technique, the macroscopic deformation process was well captured and the shear stress versus relative displacement curves were derived. Subsequently, the relationship between the shear stress and relative displacement of the interface was well fitted by a bilinear CZM, of which parameters are calibrated through the experimental data. The effectiveness of the proposed bilinear CZM is validated by using a three-dimensional finite element model. This work provides a better understanding for the damage mechanism of the bond performance between CRTS II track slab and CA mortar.

Study of the Target Earthquake-Induced Track Irregularity Spectrum under Transverse Random Earthquakes

Railway transportation, as an important lifeline during earthquake relief and post-disaster reconstruction, has an extremely significant role. The study of track irregularity caused by earthquakes is the basis for ensuring traffic safety after their occurrence. In this paper, a finite element model of a five-span simply supported high-speed railway beam bridge with the China Railway Track System (CRTS) II was established and an experimental verification was performed. Eighty arbitrarily selected seismic waves were extracted from the Pacific Earthquake Engineering Research Center (PEER) strong ground motion database and a nonlinear time-history analysis was performed on the finite element model. The frequency–domain distribution law of earthquake-induced track irregularities was studied. A stable target earthquake-induced track irregularity spectrum model was constructed, and its expression was fitted. According to the results, in the case of transverse earthquakes, the rails experienced noticeable alignment irregularity and cross-level irregularity, while the amplitude of the gauge and vertical irregularity were relatively small. The target irregularity spectrum has a higher amplitude in the low-frequency components. When peak ground acceleration (PGA) was low, earthquake-induced track irregularity was not obvious, but the deteriorating effects of earthquakes on track irregularities increased significantly with increasing PGA.

Investigation of the seismic performance of CRTS III slab ballastless track of high-speed railway under low-cycle reciprocating load

In the China Railway Track System (CRTS) III slab ballastless track (SBT) system, the self-compacting concrete filling layer (SCCFL) and the base slab are connected via a rubber pad, geotextile, convex platform, and groove. This is a complex interaction between the SCCFL and base slab. As a buffer member between SCCFL and base slab, the rubber pad significantly impacts the seismic performance of CRTS III SBT. To explore the failure mode and seismic performance of CRTS III SBT, four CRTS III SBT specimens with different rubber pad thicknesses were designed and subjected to low-cycle reciprocating load tests in this study. The characteristics of the load-displacement hysteresis curve and load-displacement skeleton curve of CRTS III SBT specimens were then analysed. The failure mode, energy-dissipation capacity, ductility, stiffness degradation of CRTS III SBT specimens, and the influence of rubber pad thickness on the seismic performance of CRTS III SBT specimens were studied. It was found that when the low-cycle reciprocating loading was applied on the CRTS III SBT specimens, the interlayer bonding between the SCCFL and the base slab was the first to fail, followed by compressive deformation of the rubber pad. The concrete at the convex platform-SCCFL interface cracked, and the concrete in the four corners of the groove cracked until failure. When the rubber pad thickness was increased, the ultimate bearing capacity of the CRTS III SBT specimens under low-cycle reciprocating loads remained constant. However, the corresponding energy-dissipating capacity, ductility, and seismic performance all improved.

Long-term friction performance monitoring of sliding layer in China railway track system Ⅱ slab track on bridge superstructure

The sliding layer is an important component of the China Railway Track System (CRTS) Ⅱ slab track on bridges, and its friction performance has a significant influence on the intensity of track–bridge interaction (TBI). However, the friction performance deterioration of this sliding layer under actual repeated abrasion and harsh environmental conditions is difficult to grasp since structural health monitoring implementation into in-service high-speed railway tracks is extremely hard to be permitted and only limited valuable monitoring datasets are available at present. In this study, the friction performance degradation of the sliding layer was therefore investigated using long-term monitoring data from a multi-span simply supported box girder bridge. First, the friction-induced strain in the base plate was decoupled using the monitored strain at the fixing point and mid-span based on mechanical properties of TBI. Then, structural health monitoring and finite element analysis both indicated an approximately linear relationship between the decoupled friction-induced strain and the temperature of the investigated bridge under certain circumstances. Furthermore, the correspondence between the slope of this linear relationship and the friction coefficient was modelled. Finally, the friction coefficients of the sliding layers on target spans were identified using 4 years of monitoring data and the established correspondence model to analyse the statistical characteristics and degradation performance of the layers. This investigation of the friction performance degradation of the sliding layer in the CRTS II slab track system provides guidance for future maintenance and replacement decision making.

Numerical Investigation of Interlaminar Stress of CRTS II Slab Ballastless Track Induced by Creep and Shrinkage of Concrete.

The secondary development of ABAQUS was carried out to calculate the shrinkage and creep of concrete, and a finite element model of the China Railway Track System (CRTS) II slab ballastless track was established. Then, the interlaminar stress of CRTS II slab ballastless track at different ages of the track slab during laying (AOTSL) caused by concrete shrinkage and creep was studied. The obtained results showed that the stress redistribution occurred in the sliding layer, which resulted in the generation of a gap. Although the gap length was slightly reduced due to the shear cogging, the sliding layer at the slab edge is more prone to produce gaps. Under the effect of shrinkage and creep of the ballastless track, large additional shear stress, up to 0.676 MPa, was induced at the interface between CA mortar and the track slab. Meanwhile, the appearance of additional vertical and lateral forces of the shear cogging was caused by the shrinkage and creep of the ballastless track. Additionally, by further analysis, the recommended AOTSL ranges from 120 days to 180 days.

Research on Dynamic Performance of CRTSⅢ Type Slab Ballastless Track under Long-Term Service.

To fill the blank in the research on the dynamic performance of track structure under long-term service, the dynamic response study of China Railway Track System Ⅲ type slab ballastless track (CRTSIII SBT) under the action of fatigue for 30 million times and the parting between track slab and self-compacting concrete (SCC) was carried out. By establishing the finite element model of the CRTSIII SBT structure and taking the stiffness change of isolation layer and fastener under fatigue state and the parting during service as the research objects, combined with the full-scale model test, the dynamic response amplitude and vibration law of track structure was analyzed based on the finite element model of axle falling test method. The results show the following: (1) Under the fatigue load, the acceleration of rail and base increases obviously, the longitudinal tensile stress of SCC surface decreases, the longitudinal tensile stress of base surface increases, and the vertical stress of each layer of track structure increases as well. (2) Under the action of the parting, the dynamic response of each structural layer increases, and the change of acceleration and stress of each layer under the action point of axle falling is the most obvious. (3) The fatigue load will weaken the vibration damping performance of the track, and the parting will continue to develop under the action of the falling axle, resulting in partial or total failure of the SCC layer. Both of them will aggravate the dynamic response of the track structure and affect driving safety, which should be paid attention to during maintenance.

Feasibility analysis of magnesium phosphate cement as a repair material for base slab of China railway track system II ballastless track

This study proposed using magnesium phosphate cement (MPC) to repair the base slab of the Chinese ballastless railway track. Laminated members were subjected to static and fatigue bending tests. Results showed that the damage to the laminated members was caused first by bending and then by horizontal shear at the interface between the old and new materials. Fatigue test results showed that the stress level on the base slab was below the endurance limit and thus should not cause failure. Microscopic tests also revealed that the bond between Portland cement and MPC played a crucial role in the repair process.

Research on Fatigue Damage Evolution of the Base Plate Structure of China Railway Track System III Type Slab Ballastless Track under Heavy Haul Train Load

To meet the transportation requirements of heavy-haul trains, it is necessary to consider the use of prestressed reinforced concrete ballastless tracks. Therefore, the CRTSIII SBT (China Railway Track System III-type slab ballastless track) has been considered for its convenient construction and geometric adjustment. Considering the problems existing in the base plate of CRTSIII SBT in heavy-haul applications, an indoor test and simulation were conducted to explore the cracking phenomenon of the base plate concrete under fatigue load. The reliability of the simulation was verified through the experimental results. A fatigue crack development law of the base plate structure was established. Furthermore, the influence of the load position, the thickness of self-compacting concrete and axle loads on the fatigue damage of the base plate structure were investigated. For the CRTSIII SBT subjected to heavier axle loads, it is appropriate to thicken the self-compacting concrete layer and strengthen the concrete and steel reinforcement at the plate end to reduce fatigue damage and cracks. The research results can serve as a basis for the optimization of the CRTSIII SBT base plate structure in heavy-haul railway applications.

Study on the effects of solar reflective coatings on the interfacial damage of the CRTS II slab track

In this work, the effects of novel solar reflective coatings on interfacial damage of the China Railway Track System (CRTS) II slab track were studied. Firstly, the effects of two different types of solar reflective coatings on the non-linear temperature distribution of the track slabs were investigated. Then, the influence of the solar reflective coatings on the damage of the interface between the concrete slabs and the cement asphalt mortar layer was studied based on a finite element model of the CRTS II slab track incorporating a bi-linear cohesive zone model of the interface. Finally, the effects of interface bonding properties on the interfacial damage of the coated tracks were analyzed by the finite element model. Results show: The interfacial damage can be underrated if linear rather than non-linear temperature distribution of the track slabs is applied. Both types of coatings can mitigate interfacial damage and reduce slab vertical displacement significantly compared with the uncoated tracks, while the organic solar reflective coating shows better performance than the inorganic solar reflective coating. To avoid interface debonding, the interface bond strength of the coated tracks should be larger than 0.02 MPa.