China Railway Track System Research Articles

Experimental investigation and fluid-structure interaction modelling of the casting of self-compacting concrete for CRTS III slab track in the curved segments

Ensuring the casting quality of self-compacting concrete (SCC) is crucial for the operational safety and durability of China railway track system (CRTS) III slab track. However, the mechanisms of key factors influencing flow behaviors of SCC and structural responses remain insufficiently understood, nor is the relationship between flow behaviors and structural responses during casting. In this study, fluid-structure interaction (FSI) models of SCC casting were developed and verified by full-scale physical tests, comprising slump flow, L-box flow and casting of SCC in an actual track at the field of Laixi-Rongcheng high-speed railway. Through in-depth analysis on the casting process of SCC in curved segments with a super elevation of 80 mm, the relationship between flow behaviors and structural responses was established. Furthermore, the influence mechanisms of key factors on flow behaviors and structural responses were investigated systematically. Results show that during gravity-fed SCC casting, its flow velocity diminishes, thus increasing the static pressure and the forces exerted on the slab by SCC. These vertical and lateral forces increase to 147.16 and 7.83 kN through three stages of development, acceleration and stabilization, resulting in commensurate structural responses. At the end of casting process, the central region of the slab exhibits maximum vertical deformation of 0.95 mm and tensile stress of 1.07 MPa. The tension rods at the side with lower elevation bear greater vertical and lateral forces than the ones at the side with higher elevation. The rise of funnel height from 1.05 to 1.35 m accelerates the casting process but increases the forces exerted by SCC by up to 26 % and augments the associated response of entire structures, whereas the funnel diameter shows little effect. Increasing yield stress of SCC from 9.3 to 49.3 Pa produces a more uniform flow morphology and thus higher static pressure and force by SCC in the middle region of the slab, impacting the structural responses by up to 15 %. The growth of plastic viscosity from 59 to 99 Pa⋅s significantly affects the volume rate and casting time by up to 66 % but has little effect on the flow morphology, fluid pressure and structural responses. In order to ensure high quality casting of SCC, five clamping beams are suggested with two beams positioned at either side of the slab and the other three evenly placed along the middle region of the slab.

Mechanical characteristics of track slab and clamps during the pouring of self-compacting concrete in the China Railway Track System III slab track

The pouring of self-compacting concrete (SCC) is crucial for ensuring the construction quality and comfort of slab tracks. In this study, a fluid–structure coupling pouring model of SCC was established based on the coupled Eulerian–Lagrangian (CEL) approach. It was applied to trace the forces and displacements of track slab and clamps during SCC pouring. The influences of some key parameters, such as the pouring equipment, technology, and rheological properties, were investigated, including the height and diameter of the funnel, pouring velocity, and plastic viscosity of the SCC. The results indicated that during pouring, the floating force and maximum vertical displacement of the track slab, as well as the vertical forces of the clamps, initially remained stable before undergoing rapid changes, eventually reaching stable values. The flow velocity of SCC in the pouring hole and its rebound primarily influenced these mechanical indicators. The funnel height positively affected the floating force of the track slab, negatively affected the vertical forces of the clamps, and slightly affected the peak values of the maximum vertical displacement of the track slab. By contrast, the funnel diameter had little influence on these mechanical behaviour indicators. As the pouring velocity increased, the floating force of the track slab and vertical forces of the clamps significantly increased. Moreover, the peak values of the maximum vertical displacement of the track slab increased to 0.47 mm at a pouring velocity of 0.03 m/s. In the case of low plastic viscosity, an increase in plastic viscosity led to a visible decline in the floating and clamp forces. However, when the plastic viscosity exceeded 80 Pa•s, further changes had little effect on the mechanical indicators owing to the combined effects of excessively low flow velocity and rebounding.

Fatigue life prediction for CA mortar in CRTS II railway slab track subjected to combined thermal action and vehicle load by mesoscale numerical modelling

The fatigue life of the cement asphalt (CA) mortar in the slab track of the China Railway Track System (CRTS) II is crucial to the smooth and safe operation of high-speed railways. To investigate the fatigue life of the CA mortar under the combined thermal action and vehicle loads, this paper develops a new coupled thermal-mechanical finite element (FE) model for the concrete slab track at the mesoscale. First, the meteorology and heat transfer principle are adopted to simulate the temperature distribution of the concrete slab track. The heterogeneous characteristics of the concrete of the precast slab are modelled by the three-phase composite material at the mesoscale using the new developed random aggregate algorithm. Then, the energy-based exponential cohesive zone model is adopted to simulate the interface between the CA mortar and the precast concrete slab. The proposed mesoscale numerical model can provide reliable predictions for the temperature distribution and the deformation of the precast concrete slab under thermal action, which are confirmed by the relevant field measurements. Finally, the fatigue life of the CA mortar is estimated by the nonlinear damage cumulative method associated with the stress time history under the combined thermal action and vehicle load. From the obtained results for the numerical example, the thermal action can decrease severely the fatigue life of the CA mortar, and the fatigue life varies significantly at different positions, with the most vulnerable zone located near but not directly below the rails.

Quasi-distributed optical fiber sensing for the coupled vibration analysis of high-speed train-bridge coupled system under earthquakes

Due to the extensive construction of high-speed rail lines passing through seismically active areas, the possibility of a sudden earthquake occurring during train operation is very high. Additionally, the earthquake will intensify the dynamic response of trains and bridges, causing serious bridge damage. In this work, the dynamic response behavior of the bridge structures in the coupled train-bridge system under the earthquake is investigated based on the 4-table shaking table system. Shaking table experiments are also conducted on a high-speed railroad model of simple-supported girder bridge at a scale of 1/10 with the simulated layers of China Railway Track System type II (CRTS II) ballastless track slabs. The strain responses of various components, including rails, track plates, base plates, and beams, were measured on the bridge using quasi-distributed fiber optic gratings, both with and without seismic actions. Additionally, the seismic isolation characteristics of the fasteners and the CA mortar layer were investigated. The dynamic performance of the coupled train-bridge system under earthquakes was studied in both the time and frequency domains.

On the deformation of CRTS-Ⅲ ballastless track during the casting process of self-compacting concrete: Numerical simulations and random forest-based prediction models

CRTS (China railway track system)-Ⅲ slab track is a main structural type of ballastless track in Chinse high-speed railway (HSR), which requires precise positioning of prefabricated concrete track slabs. However, the cast-in-place process of the self-compacting concrete (SCC) filling layer induces a large deformation as the flowing SCC generates an up-lift force on the bottom of the track slab. Currently, the deformation mechanism and influencing factors of the track slab during the SCC casting process have not been thoroughly studied. In this work, a fluid-solid interaction finite element model (FEM) is created to calculate the upward deformation of the CRTS-Ⅲ track slab caused by the flowing SCC. The main influencing factors, including the SCC’s casting height and speed, SCC’s viscosity, constraint stiffness of the limiting beams, and vertical temperature gradient, have been systematically analyzed and discussed. The relationship between the influencing factors and the track slab’s deformation is obtained. Furthermore, machine learning prediction models using random forest algorithms are developed to evaluate the deformation and up-lift force. A dataset comprising 603 calculation cases obtained from FEM simulations is used to train the prediction models. The random forest-based models developed in this study give an accurate estimation of the deformation and up-lift force based on the SCC properties and construction parameters. In addition, the feature importance of each influencing factor is obtained and studied. The findings of this study can provide guidance for the control of the deformation in CRTS-Ⅲ track slab during the SCC construction process.

Experimental study on interface performance of CRTS Ⅱ slab ballastless track under temperature loading

Interlayer interface damage poses a prevalent challenge in China Railway Track System (CRTS) Ⅱ slab ballastless tracks, with temperature playing a pivotal role in its occurrence. This study aims to scrutinize the mechanism of interlayer interface damage of track structure under temperature loading, vertical full-scale composite specimens were constructed for push-out and normal-tension model tests, and digital image correlation (DIC) technology was adopted to obtain the interlayer strain field distribution during loading and to get the mechanical property degradation, damage evolution law and damage mechanism of cement asphalt (CA) mortar layer interface under different temperatures. Based on the experimental study and fracture mechanics, the temperature cohesion damage model of the track slab-CA mortar interlayer interface was established. The results show that the bonding surface between the track slab and CA mortar layer is particularly susceptible to damage, and the interface damage exhibits "cold brittleness" and " thermal ductility" at low and high temperatures, respectively. Cohesion zone models (CZM) were constructed for both the normal and tangential interfaces at temperatures of 0 °C, 15 °C, 30 °C, 45 °C, and 60 °C. Furthermore, by defining field variables in the ABAQUS software, a temperature variation function describing key parameters of interface damage fracture behavior was integrated into the CZM, using defined field variables. The simulation results exhibited excellent agreement with the experimental findings.

Study on physical and mechanical properties of cement asphalt emulsified mortar under track slab

PurposeDuring the construction process of the China Railway Track System (CRTS) I type filling layer, the nonwoven fabric bags have been used as grouting templates for cement asphalt (CA) emulsified mortar. The porous structure of nonwoven fabrics endowed the templates with breathability and water permeability. The standard requires that the volume expansion rate of CA mortar must be controlled within 1%–3%, which can generate expansion pressure to ensure that the cavities under track slabs are filled fully. However, the expansion pressure caused some of the water to seep out from the periphery of the filling bag, and it would affect the actual mix proportion of CA mortar. The differences in physical and mechanical properties between the CA mortar under track slabs and the CA mortar formed in the laboratory were studied in this paper. The relevant results could provide important methods for the research of filling layer materials for CRTS I type and other types of ballastless tracks in China.Design/methodology/approachDuring the inspection of filling layer, the samples of CA mortar from different working conditions and raw materials were taken by uncovering the track slabs and drilling cores. The physical and mechanical properties of CA mortar under the filling layer of the slab were systematically analyzed by testing the electrical flux, compressive strength and density of mortar in different parts of the filling layer.FindingsIn this paper, the electric flux, the physical properties and mechanical properties of different parts of CA mortar under the track slab were investigated. The results showed that the density, electric flux and compressive strength of CA mortar were affected by the composition of raw materials for dry powders and different parts of the filling layer. In addition, the electrical flux of CA mortar gradually decreased within 90 days’ age. The electrical flux of samples with the thickness of 54 mm was lower than 500 C. Therefore, the impermeability and durability of CA mortar could be improved by increasing the thickness of filling layer. Besides, the results showed that the compressive strength of CA mortar increased, while the density and electric flux decreased gradually, with the prolongation of hardening time.Originality/valueDuring 90 days' age, the electrical flux of the CA mortar gradually decreased with the increase of specimen thickness and the electrical flux of the specimens with the thickness of 54 mm was lower than 500 C. The impermeability and durability of the CA mortar could be improved by increasing the thickness of filling layer. The proposed method can provide reference for the further development and improvement of CRTS I and CRTS II type ballastless track in China.

Investigating the fatigue performance of conventional reinforced concrete CRTS III ballastless track structures using a fatigue damage constitutive model

Compared to prestressed track structures, the mechanisms underlying fatigue degradation in conventional reinforced concrete track structures under high-cycle train loads remain elusive. This paper introduces a finite element model for CRTS (China Railway Track System) III slab ballastless track. In this model, fatigue damage constitutive relationships for both concrete and reinforcement are incorporated as material subroutines. The study focuses on investigating the fatigue characteristics of composite plate structures, which consist of track slabs and self-compacting concrete (SCC). An accelerated calculation method for high-cycle fatigue issues is utilized. Several key findings emerge from this study. First, the onset of fatigue cracks is observed on the lower surface of the SCC, directly beneath the applied load. These cracks tend to propagate laterally from the most critical location toward the slab edge. Second, a significant interrelationship exists between material fatigue degradation and stress redistribution. Traditional decoupled fatigue analysis methods are likely to overestimate the rate of fatigue evolution at critical points. Third, increasing the stiffness of geotextiles can partially optimize stress distribution within the track structures, thereby extending the time to initial cracking.

The influence of uneven frost heave and thermal conditions on the deformation and damage of slab track in seasonally frozen regions

To ensure the secure operation of high-speed trains in seasonally frozen regions, this paper investigates the deformation and failure patterns of slab tracks subjected to uneven frost heave and environmental issues. Utilizing our team’s previous research findings, finite element analysis software was employed to construct a mechanical transfer calculation model for the subgrade and slab track. This model was used to explore the relationship between uneven frost heave and slab track deformation at different time periods, along with the track structure’s damage behavior. In addition, comprehensive temperature field tests were conducted on the China railway track system (CRTS) III slab track, probing temperature variations, warping deformations, and stress distributions within the track structure. Finally, the interaction of uneven frost heave and thermal loads unveiled the interlayer debonding and stress characteristics of slab track structures across different months in seasonally frozen regions. The results revealed that the greater amplitude of frost heave and the smaller wavelength led to a higher change rate of rail irregularity. The uneven lateral deformation of the subgrade surface due to the sunny-shady slopes effect resulted in significant rail level irregularity of slab tracks. The combined impact of short-wavelength frost heave and temperature gradient loads emerged as the primary cause of macroscopic damage and stiffness degradation of concrete materials within slab track structures. These findings will help to understand the mechanical behavior of slab tracks in cold regions, thereby improving the quality of high-speed railway operations.

Experimental study of friction performance degradation in sliding layer of CRTS II slab track considering dust incursion

The track-bridge interaction force in the China Railway Track System II slab track can be significantly reduced due to the effect of the sliding layer, thus its friction performance is crucial for the operational safety of the track. However, because of the micro delamination between the base plate and bridge deck under certain conditions, the dust inevitably infiltrates the sliding layer. This study performed abrasion experiments to investigate the influence of dust incursion on the friction performance of the sliding layer. The experimental apparatus was designed to simulate repeated sliding and normal pressure between the base plate and the bridge under actual working conditions. Different sizes of dust particles were employed, and the friction performance, characterized by the friction coefficient and surface topography of the geomembrane, was obtained and analyzed. The experimental results show that the growth rate of the friction coefficient was 1.46–2.88 times greater than that without dust incursion. The results of this study can serve as a reference for the actual friction performance degradation of the sliding layer and shed light on maintenance decision-making.

Influences of cement asphalt emulsified mortar construction on track slab geometry status

PurposeThe construction of cement asphalt (CA) emulsified mortar can obviously disturb the slab status after the fine adjustment. To decrease or eliminate the influence of CA mortar grouting on track slab geometry status, the effects of grouting funnel, slab pressing method, mortar expansion ratio, seepage ratio and grouting area on China Railway Track System Type (CRTS I) track slab geometry status were discussed in this paper.Design/methodology/approachCombined with engineering practice, this paper studied the expansion law of filling layer mortar, the liquid level height of the filling funnel, the pressure plate device and the amount of exudation water and systematically analyzed the influence of filling layer mortar construction on the state of track slab. Relevant precautions and countermeasures were put forward.FindingsThe results showed that the track slab floating values of four corners were different with the CA mortar grouting and the track slab corner near CA mortar grouting hole had the maximum floating values. The anti-floating effect of “7” shaped slab pressing device was more efficient than fixed-joint angle iron, and the slab floating value could be further decreased by increasing the amount of “7” shaped slab pressing devices. After CA mortar grouting, the track slab floating pattern had a close correlation with the expansion rate and water seepage rate of CA mortar over time and the expansion and water seepage rate of the mortar were faster when the temperature was high. Furthermore, the use of strip CA mortar filling under the rail bearing platform on both sides could effectively reduce the float under the track slab, and it could also save mortar consumption and reduce costs.Originality/valueThis study plays an important role in controlling the floating values, CA mortar dosage and the building cost of projects by grouting CA mortar at two flanks of filling space. The research results have guiding significance for the design and construction of China's CRTS I, CRTS II and CRTS III track slab.

Interface behaviour analysis of China railway track system Ⅱ slab ballastless track under temperature action and initial gap damage

The interface damage is considered to be one of the main diseases of China Railway Track System (CRTS) Ⅱ slab ballastless track, which will affect the long-term performance of the track structure and safety operation of high-speed trains. This study aims to reveal the interface damage mechanism between the track slab and the cement asphalt (CA) mortar layer of CRTS Ⅱ slab ballastless track, under different combinations of temperature actions and initial gap damage. A three-dimensional finite element model of CRTS Ⅱ slab ballastless track was established, in which a cohesive constitutive model was incorporated to simulate the interaction behavior of the interface. The interface damage evolution under different temperature actions and initial gap damage was analyzed. The analysis results show that: (1) Overall temperature has a more obvious effect on interface damage compared with temperature gradient, and the greater the overall temperature drops, the lower the decrease of interface damage will be; (2) When initial gap damage occur at the slab end, the growth rate of interface damage under temperature gradient is greater than that under overall temperature; The interface will begin to delaminate when the overall temperature drop reaches −50°C and the gap length becomes greater than one fastener spacing; and (3) When initial gap damage occur at the slab edge, the influence of overall temperature on interface damage is greater than that of temperature gradient; The interface gap damage reaches level II (according to TG/GW 115-2012) at the slab edge under the combination of −50°C and −50°C/m, while the slab center interface is unlikely to be damaged under negative temperature.

Assessment of the Simultaneous Use of Asphalt Concrete and Foam Concrete as a Full Cross-Section Layer in the Substructure of the China Railway Track System (CRTS) III Slab Ballastless Tracks

This paper proposes the simultaneous application of asphalt concrete (AC) and foam concrete (FC) as a full cross-section layer in the substructure of the high-speed railway ballastless track. AC is designed as a waterproof layer and is placed as part of the top surface layer of the subgrade bed where the mixture is made from three types of performance grade (PG) binders (normal, high, and modified grade binders) named PG64-22, PG70-22, and PG76-22, respectively. FC is designed as a subgrade material to replace the traditional material in the bottom layer of the subgrade bed. It has a target density of 500–700 kg/m3 and polypropylene (PP) fiber is added at a ratio of 0.25% and 0.40% by volume. The mechanical properties of both materials are reviewed from the authors’ previous research, in which they were assessed through laboratory testing in accordance with ASTM standards. In addition, the numerical model analyzed the dynamic response of the whole structure when AC and FC were applied as full cross-section layers under different thicknesses of FC and then compared with traditional track structures. Laboratory test results indicate that PG76-22, or modified asphalt binder and FC at a density of 600 kg/m3 with the addition of 0.25% PP fiber, is suitable for use in slab ballastless tracks. In addition, the prediction of the model shows that AC at a thickness of 0.12 m and FC at a thickness of 1.00 m can reduce the stress and vibration of the track structure better than the traditional structure, resulting in stabilization and long-term service life.

Adhesion performance tests and analysis of interface damage for CRTS III prefabricated slab tracks

Debonding between the track slab and self-compacting concrete (SCC) is a common form of damage in China railway track system (CRTS) III prefabricated slab tracks. To investigate the influence of interlayer cohesive parameters on the interface connection, composite specimens of track slabs and SCC were made to be used for splitting tensile and shear tests. The variation characteristics between the interface’s tension and displacement were obtained using digital image correlation (DIC) techniques, and the key parameters of a cohesive zone model (CZM) were determined and used to simulate the cracking behavior. Also analyzed was the interlayer damage behavior of the composite specimen using ABAQUS, the results of which were compared with those of the tests. Furthermore, a finite element simulation model of the CRTS III prefabricated slab track with CZM was used to analyze the impact of different cohesive parameters on interlayer damage under train loads and varying temperature gradient loads. The results show that: I). The DIC technique accurately describes the entire process from the damage initiation and propagation, to complete failure of the interface-bonded components; II). The finite element simulation results of the concrete composite specimens agree with the experimental results, indicating the CZM accurately reflects the interlayer cracking behaviors; III). Reducing the interface stiffness and increasing the critical fracture energy can slightly delay the onset of interface damage and debonding, respectively; IV). An increase in the cohesive strength can effectively inhibit the development of interface damage, for example, increasing the normal and tangential cohesive strength can improve the starting temperature gradient for interlayer damage initiation by 150 % and 45 %, respectively; and V). Under large positive temperature gradients (>70 °C/m), the interlayer debonding rate corresponding to most cohesive parameters reaches 100 % (complete failure of adhesion), making extreme temperature areas a necessary focus on the interlayer debonding of slab tracks. The research results of this paper can be used to analyze the damage behavior between the track slab and SCC layer, and provide a theoretical basis for the design and maintenance of CRTS III prefabricated slab track.

Mechanical Properties of Slab Ballastless Track Subjected to Combined Effects of Loading and Temperature

Temperature has a significant impact on the structural performance of the China Railway Track System (CRTS) Ⅱ slab ballastless track-bridge and the structure is susceptible to fatigue damage under long-term loading. Therefore, it is crucial to conduct cyclic-loading test on the track-bridge structure to reveal the evolution of its mechanical properties under coupled temperature-load effect. In this study, a 1:4 scaled-down model of a ballastless track-bridge was produced and placed in a large-size environmental chamber for temperature-load coupled cyclic-loading tests. The results showed that after 1×10^6 cycles of loading, no cracks were observed on the surface of the track structure. The structural workability and load capacity of the track-bridge met the required service standards. During the temperature-load coupling test, the load-displacement curves of the structural system exhibited intervals, with a more significant increase in static-deflection values. The dynamic deflection of the structural system under the coupling action experienced a higher growth rate and more abrupt changes compared to single-load conditions. These observations indicated that the ambient temperature amplified the deflection of the structural system. The strain values in the track structure exhibited significant non-linearity, with temperature amplifying this effect. The dynamic-response test results revealed a negative correlation between the inherent frequency of the track structure and the ambient temperature, further emphasizing the influence of temperature on the stability of the track structure. Consequently, it is essential to enhance the monitoring of track structures in high-temperature climates to ensure their safe operation.

Mesoscale modelling of CRTS II slab tracks subjected to thermal action and vehicle loads

Evaluation of the damage and deformation of cast-in-situ concrete joints between the precast concrete track slabs of the China Railway Track System (CRTS) II is crucial to the safe operation of high-speed railways. To investigate the damage and deformation evolution of concrete joints under thermal action (caused by the natural meteorological environment) and vehicle loads, a two-dimensional thermal–mechanical coupled numerical model of concrete joints at the mesoscale was developed. This model analyses the influence of three factors – concrete strength, maximum diameter of the joint concrete aggregate and vehicle speed. First, meteorology and heat transfer theory were used in thermal simulations. Then, the non-linear characteristics of the joint concrete were modelled as a two-phase composite material based on the random aggregate algorithm and strain-based elastic damage theory at the mesoscale. The cohesive zone model was used to simulate the interfaces between precast slabs. The reliability of the proposed model was assessed using field measurements. The results of a numerical example showed that the maximum aggregate diameter of the joint concrete significantly affected damage evolution in the joint concrete and concrete strength has a slight effect on joint uplift.

Seismic Analysis of High-Speed Railway Bridge-CRTS III Slab Ballastless Track System Under Transverse Earthquake

The new generation of China Railway Track System III (CRTS III) ballastless track structure has been found vulnerable under high-level earthquakes. However, the seismic characteristics and the damage mechanism had not been well studied. In this paper, a 4-span high-speed railway (HSR) simply supported bridge-track system model is established to investigate the seismic response and damage mechanism of the bridge-CRTS III slab ballastless track system under transverse earthquakes by nonlinear history analysis. In accordance with the results, the piers have a good seismic performance under high-level earthquake (0.57 g). The non-uniform vibration of unequal-height piers results millimeter-level inconsistent displacements between the girders. The fixed bearing is vulnerable under the earthquakes with PGA of 0.3 g and 0.57 g, indicating that the seismic design of fixed bearing should be optimized to enhance the seismic resistance under high-intensity earthquake. After the damage of fixed bearing, the friction is not enough to pull the girders to vibrate together with piers which leads the sliding of girders. The sliding of girder exacerbates the non-uniform displacements of girders and roadbed. The non-uniform vibration and residual slips of girders lead deformation of the rail, especially forming turning angles at the girder joints which may impact the safety of train operation.

Study on the interface damage of CRTS II track slab under high- and low-temperature cycles based on acoustic emission

China Railway Track System (CRTS) II slab tracks have been widely used in high-speed railway construction. Interface debonding between the track slab and cement-emulsified asphalt (CA) mortar is one of the typical diseases of the CRTS II slab tracks. Properly evaluating the interfacial strength of the track slab and CA mortar under high- and low-temperature cycles is crucial for optimizing the repair in engineering practices. In this study, the track slab and CA mortar (TSCAM) composite specimens were prepared for high-low temperature cycle (HLTC) tests. Then, the shear and tensile tests were carried out to study the interlayer bonding properties of TSCAM under different HLTCs. In addition, the acoustic emission (AE) technique was employed to monitor the signals generated in the specimen during the tests. Furthermore, the AE parameters were analyzed, including ringing counts, energy, b-value and amplitude-frequency. The results show that the number of HLTCs significantly contributed to the deterioration of the interface bond strength between the track slab and the CA mortar. The interface normal bond strength showed a higher decrease than the tangential bond strength with the increase of HLTCs. In addition, the relationships of the shear and tensile bond strength of the TSCAM composite specimen with HLTC were deduced. The tangential and normal bond strength degradation properties of the composite were well fitted by the function of quadratic polynomial and power exponents, respectively. The AE activities of the composite specimens in the shear and tensile tests were gradually reduced with increased HLTC. The AE signals were the strongest when the specimen failed along the interface. Furthermore, the intensity of AE events during the shear test was higher than that during the tensile test under the same HLTC. The overall b-values of the shear and tensile specimens without HLTC were the largest and did not change significantly with the increase of HLTC. The amplitude range of the peak frequency gradually decreased as the HLTC increased. The amplitude and peak frequency of the tensile test was smaller than that of the shear test under the same HLTC. This indicates that the AE technique can monitor the interface damage evolution pattern of the TSCAM composite specimen throughout the tests.

Experimental study on the static and fatigue splitting failure behaviors of bonding interface between bi-concrete materials

The bi-concrete materials structure, steam-cured concrete (SC) and self-compacting concrete (SCC), has been widely utilized in the China Railway Track System (CRTS) III slab ballastless track structure. In this work, the static and fatigue splitting failure behaviors of the bonding interface between SC and SCC have been investigated. The specimen with a bonding interface between SC and SCC was prepared for the static and cyclic splitting tests, and cyclic splitting tests were conducted on four stress levels, including 0.6 Pt (static ultimate load), 0.65 Pt, 0.75 Pt, and 0.85 Pt. The results show that the debonding process of the interface under static loading contains three stages, linear stage, acceleration stage, and brittle failure stage. The evolution of fatigue strain of bonding interface also has three stages, initial increase stage, steady increase stage, and rapid increase to failure stage. The fatigue splitting life of the bonding interface obeyed the two-parameters Weibull distribution, the S-N-Pf model established in this study could predict splitting fatigue life of the bonding interface between SC and SCC well. The value of the fatigue strength was 0.418 of the static splitting ultimate load. In addition, a calculation of fatigue damage reliability based on Miner model is conducted to describe the fatigue damage process of bonding interface quantitatively.

Thermal response of the bridge supported longitudinal CRTS II slab track subject to diurnal temperature variation

As a kind of multi-layer concrete structure, thermal responses China Railway Track System type II (CRTS II) slab track subject to diurnal temperature variation could significantly affect its long-term structural performance of slab track, and ultimately the safety of the high-speed train. However, the thermal responses of the bridge supported longitudinal CRTS II slab track have not been fully understood due to the complex of the structural system. The purpose of this study is to systematically investigate the thermal responses of the bridge supported CRTS II slab track system by conducting experimental testing on the scaled models with different boundary conditions in conjunction with theoretical modelling. The results show that the boundary conditions of the CRTS II slab track haveasignificantinfluenceon thermal responses of the slab track. For example, the heat transfer from the track slab to the cement asphalt mortar (CAM) layer could be accelerated under fixed boundary condition. In addition, the longitudinal thermal stress within the CRTS II slab track under the fixed boundary condition is higher that under free boundary condition. Importantly, it indicates that the vertical thermal gradient in the CRTS II slab track induced by the diurnal temperature variation could lead to the damage of the interface between the track slab and CAM.