Structural Behavior And Mix Design For Asphalt Concrete Substructures In High-Speed Rail

Presently the cement concrete ballastless track is widely applied in China’s high-speed rail lines. However, due to high brittleness and stiffness of the cement concrete materials the railway substructure is easy to crack. In addition, it causes high noise level, poor resistance to subgrade deformation and difficulty in maintenance. Dense asphalt concrete with the characteristics of considerable strength and flexibility as well as easy construction should be one of the most important materials for new types of substructures in high-speed lines. However to meet the high level of traffic speed, ride quality and safety of the high-speed rails the substructures require high stability and durability. This increases the need for research on the structural behavior and asphalt mixture design of railway substructure.This research presents a key and comprehensive work on the classification, visco-elastic modeling, dynamic behavior, noise attenuation and vibration control, mix design and field construction of asphalt railway substructures. The required task was accomplished through literature review, field investigation, laboratory testing and numerical analysis. The research was jointly-supported by the National Natural Science Foundation of China(Project No.50978222), and the China’s Ministry of Railway (Project No.2007G042-N) as well as Southwest Jiaotong University (2010Doctoral Innovation Project). The major outcomes of this research are summarized as follows,Firstly, by the literature review and field investigation at home and abroad, the related substructures were classified Pouring Asphalt Substructure (PAS), Waterproofing Asphalt Substructure (WAS), Isolating Asphalt Substructure (IAS) and Supporting Asphalt Substructure (SAS). The material of WAS was named SAMI (Surface Asphalt Mixture Impermeable), and the related name of both IAS and SAS was RACS (Railway Asphalt Concrete Substructures). By reviewing the structural theories of track-subgrade and flexible pavement as well as the method of asphalt mix design in highway, considered the layered visco-elastic theory and the Marshall Design as the basis of the research on structural behavior and mix design respectively for railway asphalt substructures.Secondly, by analyzing the train loading features and summarizing the layered visco-elastic mechanics, a simple mathematical analysis with layered visco-elastic system was modeling for IAS and SAS, and a simple calculation method of the predicted depth for asphalt layer was forming with quasi-static analysis based on the model. Meanwhile, a parameter K introduced in this model was to evaluate the vertical buffering capacity of the fastener system, and then the maximum pressure Rd of a single fastener was calculated. Based on superposition principle, the vertical load transforming of sleepers and slabs were simply modeling with multiple non-uniformly distributed strip load and single uniformly distributed rectangular load respectively, then expressed both of them using the equivalent circular load as the input for the layered system. By doing this, the simple predicted method for the depth of asphalt layer could perform using the vertical deflectometer on subgrade surface layer and the largest horizontal tension stresses on the bottom of asphalt layer. Meanwhile, the visco-elastic solution of layered model derived from the related elastic solution by the "corresponding principle" between the elastic and visco-elastic.Thirdly, according to the numerical analysis by means of ABAQUS software, the vertical acceleration and deformation on subgrade surface as well as the transversal and longitudinal tension strain on the bottom of asphalt layer were taken as the key mechanical parameters. By comparison using linear elastic constitution, the structures with asphalt layer on the subgrade surface named RAC-T and RAC-S were considered as the optimum substructure patterns for ballasted and ballastless track respectively. From the linear visco-elastic FEM analysis with the Prony series parameters fitted in Maxwell model from the creep test of asphalt mix, the asphalt concrete substructures have the major mechanical behaviors as follows, ·To the different train speeds from160km/h to400km/h, the maximum acceleration amplitude of the subgrade surface in asphalt ballasted and ballastless trackbed was about±25m/s2and-15/+10m/s2respectively, and the attenuation amplitude generally increased over the speed. In addition, the train speed effect was negligible to the vertical deformation on the top of subgrade and to the horizontal strain on the bottom of asphalt layer.·The four indices could be lower if asphalt layer thickness increased. The acceleration and deformation reduce about20%and30%respectively when the asphalt layer increased from5cm to30cm. Meanwhile, the strains obviously showed fluctuated feature with larger amplitude in the thickness less than10cm. However, the difference among each response was not too much if the thickness was over20cm.·If modulus of asphalt layer increased the acceleration could be slightly lower, but the modulus can positively effect on the vertical deformation and the strain response as well, even though the lowering degree was limited when the modulus was great enough. Therefore, the dense-graded asphalt mix with higher modulus and10-20cm thickness is good for RACS.Fourthly, the mechanizations of noise and ground vibration induced by train were concisely reviewing by theoretical analysis. From the noise evaluation, the performance of noise attenuation from IAS and SAS were less about1.9-2.2dB and0.3-0.4dB respectively, and the thickness increased or temperature decreased of the asphalt layer can make the noise lowering about0.1-0.2%. However, the train speed decreased from350km/h to200km/h can make the noise decreased more than2%. As for the vibration, the level of the RAC-T and RAC-S were lower about58%and71%than the related standard structure respectively. The vibration of RAC-S was generally increased over the train speed increased while the rate of vibration change would increase about twice when the speed over300-400km/h more than200-250km/h. The vibration decreased more than10%when the depth of asphalt layer increased from10cm to20cm, but when the depth over20cm to30cm the decreased degree was only about1%. The vibration level would be lower when the asphalt layer with appropriate resilient modulus about8000-15000MPa.Fifthly, taking the climatic zones of highway asphalt mix as a reference, the railway asphalt mix climatic zones was divided as three parts as the Hot (>-9℃), the Warm (≤·9℃,>-21.5℃) and the Cold (≤-21.5℃) using the proposed specification of the minimum temperature in past30years. Based on the zones and the structural behaviors as well as the lab tests results, the optimum gradations for the mix of SAMI and RACS were in recommendation as SAMI-10and RACS-25, respectively. The mix design method was put forward with the key parameters as Permeability Coefficient (K≤10-4cm/s), Air Voids1%≤Vg≤3%), Marshall Stability (MS≥5kN), Tensile Strength Ratio (Hot-zone TSR≥60%, Cold-zone TSR≥70%) and Linear Contraction Coefficient in Low Temperature (C≤30με/℃) for SAMI. The Bending-Tension Strength (RR≥4MPa) as well as Bending Creeping Ratio in Low Temperature (BS≥20με/s/MPa) was added more with the above5terms (except for Hot-zone TSR≥75%, Cold-zone TSR≥85%) for RACS.Finally, according to the experiences of highway pavement, the field construction technologies of SAMI was summarized and the construction proposal of RACS was optimally analyzed, including the construction time arrangement and the related preparation, production and transportation as well as paving, compaction and quality control of asphalt mix. Meanwhile, the processing continuity including temperature control is the key point.The conclusions showed that RACS has perfect functions in waterproofing and strengthening, which is exactly beneficial to the long-term service for the high-speed rails. The findings and recommendations of this research, as the first comprehensive framework on domestic asphalt railway substructure, contribute a theoretical reference for the engineering application of asphalt substructure for China’s high-speed railway network construction.