1. Introduction
Because of its manifold advantages, including the reduction in structure height, the lower maintenance requirements and the increased service life, the ballastless slab track has been widely used in the construction of high-speed railway (HSR) in many countries, such as Japan [
1], Germany [
2] and now in China [
3]. In the structure of the ballastless slab track, the cement asphalt mortar (hereinafter abbreviated CA mortar) injected between the concrete roadbed and the track plate is one of the most important parts, which functions as a cushion layer to play important roles such as supporting, adjusting, load transferring, vibration absorption and shock insulation, etc., and improve the durability of the track structure as well as the ride safety and comfort level of the high-speed trains [
1,
2,
3].
The CA mortar is an organic-inorganic composite with mechanical properties that are highly sensitive to time and temperature, consisting of cement, asphalt and/or polymer emulsion, fine aggregate and chemical admixtures, such as superplastisizer, expansive, air-entrained, and de-foaming agent, etc. Two kinds of CA mortar are often used according to the structure of the ballastless track, one with high elastic modulus (7000–10,000 MPa) and high compressive strength (more than 15.0 MPa at the curing age of 28 days is required) used in Bögl slab track firstly developed in Germany [
2], in which anionic asphalt emulsion is utilized, and another one with low elastic modulus (100–300 MPa) and low compressive strength (only require a compressive strength of over 1.8 MPa at 28 days) previously employed in the shinkansen slab track [
1], which uses cationic asphalt emulsion as the main raw material. Both CA mortars are now introduced into China and employed in the construction of high-speed railway, i.e., China Rail Track System, CRTS I and CRTS II. The CRTS I ballastless slab track adopts CA mortar with low elastic modulus, whereas CRTS II ballastless slab track employs CA mortar with high elastic modulus [
3].
In recent ten years, a large HSR network has been built in China and more than 6000 km of HSR lines implemented with CRTS I or II ballastless slab tracks. Numerous studies, especially in China, have been or are being carried out and reported on fabrication [
4] and improved properties [
5], the rheological behavior of the fresh CA mortar [
6,
7,
8,
9], the mechanical properties [
10,
11,
12,
13,
14,
15], the dynamic or impact characteristics [
16,
17,
18,
19], the creep [
20], and the durability of the hardened CA mortar. For example, the mixing kinetics of CA mortar was studied by using a power consumption method and the theory of air entrainment was used to interpret the effects of mixing speed, fluidity and mixing time on the air content of the mortar [
4]. In order to improve properties of the hardened CA mortar, a novel mixing method was developed and the colloidal silica sol was incorporated to improve the expanding stability and to avoid bleeding of the fresh CA mortar during hardening [
5]. About the rheological behavior of the fresh cement asphalt paste and mortar, influence of many factors, such as type of asphalt emulsion, superplasticizer dosage, water to cement ratio, viscosity-modifying agent, solid volume fraction (Vs), mass ratio of asphalt to cement (A/C), have been investigated and discussed in details [
6,
7,
8]. A predictive model of CA composite particles was also proposed to determine the maximum particle packing density of CA pastes by the maximum particle packing density of cement paste, the maximum particle packing density of asphalt emulsion, and the volume fraction of asphalt in asphalt-cement system [
9].
After hardening, it has been revealed that the framework formed by the hardened cement paste was the primary skeleton and the asphalt phase a filling phase for the CA with high elastic modulus, but for the CA mortar with low elastic modulus, the demulsified asphalt exhibited as a dominant phase, thus two weak structure skeletons, formed by the asphalt membrane and the hardened cement paste, respectively, were responsible for the strength [
10]. Because of the existence of the asphalt phase, the compressive and dynamic behavior of the CA mortar were both found to be highly dependent on the temperature and the loading rate [
11,
12,
13,
14,
15,
16,
17,
18,
19]. Besides, the creep of the CA mortar on different load levels were also studied by using a developed creep testing apparatus and for the CA mortar, two creep stages, i.e., the attenuation creep stage in which the deformation rate decreases gradually and the steady creep stage in which the creep rate is relatively stable were found. A creep model based on the thermodynamic theory was then established to help improve the structural design theory of ballastless slab track [
20].
For the engineering structures, the durability has been paid much attention in the worldwide. For the CA mortar, the most noteworthy is its anti-fatigue performance under the action of high frequency vibration. Studies have shown that [
21] the high-speed train will induce a strong high-frequency vibration on the track structure caused by the ground waves when the train speed reaches or exceeds a certain critical speed, which will affect the safety and comfort of the high-speed train. In serious cases it may cause the train derailment. The natural frequencies of the CA mortar are significantly different from those of track plate and concrete base, which plays an important role in preventing resonance in high-speed train operation [
22]. Therefore, the fatigue degradation of the CA mortar by high frequency vibration should not be ignored. According to a practical investigation on the test section of Qin-shen high-speed railway [
23], although the conventional performance and anti-freeze performance of the CA mortar meet the requirements, there still existed some durable problems. After 1-2 years of actual operation, the elastic modulus of the CA mortar used increased significantly and the local fragmentation occurred.
According to Xiang Jun et al. [
24], the fatigue deterioration of the CA mortar not only reduces its own function of vibration and noise reduction, but also significantly affects the dynamic performance of the ballastless slab track. Compared with the normal working state of the CA mortar, the deterioration of the CA mortar will cause the track slab to be suspended, which will cause the vertical acceleration of the track slab to increase more than 10 times and the displacement to increase more than 20 times. The pressure of the rail on the slab increases much sharply. As the running speed increases, other dynamic response values of the system also increase rapidly. Therefore, in the maintenance of the ballastless slab track, the deterioration of the CA mortar should be carefully avoided.
However, there is no anti-fatigue performance requirement for the CA mortar used in CRTS I ballastless slab track. For the CA mortar used in CRTS II ballastless slab track, although an anti-fatigue performance is required, its frequency of the dynamic load is only 5 Hz in the testing method. Wang et al. [
25] studied the fatigue characteristics of the CA mortar under various ambient temperatures, stress levels and sine wave loading conditions, but the loading frequency was only 10 Hz. However, with the increase of the running speed of the train, a very strong high-frequency vibration on various structural layers of the slab track may be produced and the frequency can reach as high as up to more than 50 Hz [
26], and that is why a cushion layer such as the CA mortar is used to insulate the high-frequency vibration to improve the degree of comfort while taking the high-speed train. Typically, the CA mortar as a vibration-insulating layer will encounter this high-frequency vibration and the deterioration under fatigue may happen during service of the high-speed railway. In this study, the performance degradation and its mechanism of the CA mortar with and without polymer emulsion incorporated under the coupling fatigue effects of the high-frequency vibration, load and temperature were studied by using an anti-fatigue testing device specially developed for the CA mortar used in the ballastless slab track of the high-speed railway [
27].
4. Conclusions
The CA mortar used in the ballastless slab track may encounter the coupling effect of the high-frequency vibration, load, and high-and-low temperature cycles during service, and the deterioration under fatigue may happen during service of the high-speed railway. In this study, the performance degradation and its mechanism of the CA mortar with and without polymer emulsion incorporated under the coupling fatigue effects of the high-frequency vibration, load and temperature were studied by using an anti-fatigue testing device specially developed for the CA mortar used in the ballastless slab track of the high-speed railway. The conclusions can be obtained as follows:
(1) The deformation capacity of the CA mortar for CRTS I slab ballastless slab track decreased after fatigue test under simulated service environment, presenting a typical brittle characteristic and an obvious reduction of the ductility and toughness. The variation of compressive strength and elastic modulus of the CA mortar after the fatigue test is approximately in a piecewise linear relationship with the increase of vibration frequency. The strength and elastic modulus are reduced under freeze-thaw action only without vibration as compared to that cured at the standard curing condition.
(2) The SEM observation and the MIP analysis showed that the internal pore structure of the material changed after the fatigue test. The volume of the macropore decreased whereas that of the micropore increased. Meanwhile, the asphalt in the hardened CA mortar revealed a softening and migration from the bulk paste to fill the pore and make the structure denser and even ooze out of the CA mortar under the high-frequency vibration and high temperature.
(3) Through incorporating the polymer emulsion, the anti-fatigue property of the CA mortar was obviously improved, which can prevent the CA mortar from losing its elastic adjustment function too early. The performance of CA mortar without polymer emulsion is more similar to that of ordinary mortar after the fatigue test. Though increase of the strength and elastic modulus for the CA mortar after severe service is beneficial to the stability of train running, the comfort level and safety of the train operation may decline due to the gradual reduction of the ductility & toughness and the gradual loss of the elastic damping adjustment function of the CA mortar layer between the base concrete slab and the track slab.