Assessment of the Use of Polypropylene Fiber Reinforced Foam Concrete as a Subgrade Material for the China Railway Track System (CRTS) III Slab Ballastless Tracks

This paper presents the application and the development of foam concrete (FC) to replace the traditionally used subgraded materials (typically compacted soil or granular earth rock) in high-speed railway by using cement to sand ratio of 2:1 with a target density of 500 to 700 kg/m3 and by adding polypropylene (PP) fibers with 0.25% and 0.40% by volume to improve the properties of FC. The mechanical properties were evaluated and tested in the laboratory in accordance with ASTM standard method, including compressive strength, tensile strength, modulus of elasticity and Poisson’s ratio, and dynamic strength. In addition, the numerical model investigates the performance and analyzes the effect of applying the full cross-section PP fiber reinforced FC in slab ballastless track, under different thicknesses, on the dynamic response of the whole structure, and then compares them with that of the traditional structure. The results indicate that the strength of FC with a density of 500 to 700 kg/m3 can meet the requirements of static and dynamic loading conditions of railway earth structure. But the density of 600 to 700 kg/m3 shows good strength stability under different curing states. In addition, adding 0.25% PP fiber is an optimal volume that can significantly improve mechanical properties. The prediction of the model shows that FC at a density of 600 kg/m3 by adding PP fibers of 0.25% at a thickness of 1.5 m can reduce the stress and vibration of the track structure better than that of the traditional structure; this indicates a long-term maintainance stability of the subgrade bed.

Method for Tunnel Displacements Calculation Based on Mobile Tunnel Monitoring System

Efficient, high-precision, and automatic measurement of tunnel structural changes is the key to ensuring the safe operation of subways. Conventional manual, static, and discrete measurements cannot meet the requirements of rapid and full-section detection in subway construction and operation. Mobile laser scanning technology is the primary method for tunnel detection. Herein, we propose a method to calculate shield tunnel displacements of a full cross-section tunnel. The point cloud data, obtained via a mobile tunnel deformation detection system, were fitted, projected, and interpolated to generate an orthophoto image. Combined with the cumulative characteristics of the tunnel gray gradient, the longitudinal ring seam of the tunnel was identified, while the Canny algorithm and Hough line detection algorithm identified the transverse seam. The symmetrical vertical foot method and cross-section superposition analysis were used to calculate the circumferential and radial displacements, respectively. The proposed displacement calculation method achieves automatic recognition of a ring seam, reduces human–computer interaction, and is fast, intelligent, and accurate. Furthermore, the description of the tunnel deformation location and deformation amount is more quantitative and specific. These results confirm the significance of shield tunnel displacement monitoring based on mobile monitoring systems in tunnel disease monitoring.

Evaluation of Internal Crack Growth of PPS Thrust Bearings under Rolling Contact Fatigue in Water

In this study, in order to evaluate the progress of internal cracks in PPS thrust bearings under rolling contact fatigue in water, cracks were observed by a full-cross-section observation method using a lathe machining. “Main subsurface crack” initiated at the surface toward the inside, then grew in a direction parallel to the surface. They connected with many “Semi-circular cracks” initiating at the surface from the opposite side to the inside, to from a semi-ellipsoidal flaking damage. It was found that the “Semi-circular cracks” and the “Main subsurface crack” dominated the flaking destruction.

Extraction of Least-Dispersive Ultrasonic Guided Wave Mode in Rail Track Based on Floquet-Bloch Theory

Ultrasonic guided wave (UGW) has shown great potential in the field of structural health monitoring of rail tracks due to its long-range capability and full cross section coverage. However, the practical application of UGW has been hindered by the complicated signal interpretation because of the natures of multiple modes and dispersion. Therefore, it is desirable that the effective UGW modes with high excitability and least dispersion can be identified and extracted for practical applications. In this paper, a numerical study on the guided wave propagation was carried out on a standard rail with 56E1 profile. Firstly, Floquet-Bloch theory was applied to obtain the dispersion curves of guided wave in a rail. Then, a 3D FE model was built to investigate the UGW propagation along the rail within the frequency range of 0–120 kHz. Wavenumber-frequency analysis method was applied to decompose and identify the propagating UGW modes. With a carefully designed 2D bandpass filter, a specific mode W0 was extracted in the wavenumber-frequency domain. Finally, a frequency band sweep technique was also proposed to get the optimal frequency band to achieve a pure and least-dispersive UGW mode along the rail web. The proposed method provides an effective way to extract efficient UGW modes to assess the integrity of the rail track, as well as other waveguides with complex geometry.

Impact Response of Preplaced Aggregate Fibrous Concrete Hammerhead Pier Beam Designed with Topology Optimization

This research aimed to study the impact response of topology optimized hammerhead pier beam (HPB) based on the density approach. The HPB is prepared with the concept of preplaced aggregate fibrous concrete (PAFC) comprising two primary approaches; first, the coarse aggregate and fiber are prepacked into the designed formwork. Second, the gaps between the aggregate and fiber are filled with cement grout. In this work, an attempt has been made to study an impact response of HPB made with PAFC. Five HPBs were prepared and strengthened with steel fibers with two different schemes, Firstly, the HPB was reinforced with a full cross-section at 2 and 4% of steel fiber, while another set of beams were only reinforced in the tension zone with the same amount of fibers. The study parameters included compressive strength, impact strength, impact ductility index, number of main and secondary cracks, and failure pattern. It was observed that the PAFC had an increase in compressive strength up to 56.9%, compared with nonfibred concrete. A fully fibered concrete beam with 4% fiber addition was the best at taking impact, and the initial crack and failures were observed at 2725.1 J and 3009.8 J, respectively, compared with non-fibered and tension zone fibered concrete beams. Compressive local damage and transverse flexural cracks were observed, which had caused initial cracks and final failure. The HPB with a full reinforced scheme at 4% dosage exhibited higher impact strength than the normal concrete and beam reinforced only in the tension zone.

Research on the Mechanical Characteristics of H-section Steel Frame under the Action of Thermo Mechanical Coupling

The research on the mechanical characteristics of concrete-filled steel tubular composite frame under high temperature fire environment is one of the research hotspots. In this paper, the finite element simulation software is used to analyze the concrete-filled steel tubular composite frame structure. The failure mode of the flexural deformation of the composite frame structure under high temperature fire environment is introduced. The simulation results of the deformation and displacement of the single-layer single span and two-layer two-span composite frame structure are deeply studied, including the different temperature field, structural field, structural field of each beam and column The results show that: with the temperature rising, the horizontal plastic strain, vertical displacement and local plastic region of beam and column are redistributed and changed in high temperature fire environment, and the flexural effect of two-story two-span concrete-filled steel tubular composite frame under different fire positions is analyzed. The results show that: with the temperature rising, the horizontal plastic strain at the concentrated load is not the results show that the deflection and deformation redistribution are obvious, and the deflection and deformation redistribution are obvious at the joint points of beams and columns. Finally, a mechanism is formed and destroyed. The flexure effect of mode 1 is larger than that of condition 2, which indicates that the flexural effect of two-story two span CFST composite frame under full cross-section fire is larger than that of condition 2 It should be better. The research results can provide reference value for the reinforcement and repair of CFST composite frame under high temperature fire.

Analysis of the effect of the secondary moment on curved beams of full cross section

abstract This paper analysis the effect of the secondary moment on curved beams using the equivalent nodal loads method. A case study was carried out applying the equivalent nodal loads method to a two-span prestressed curved beam, using a curved finite element as the structural model, analyzing the effect of the secondary moment for each case and comparing it with its equivalent in straight beam. It was found that the stiffness parameters E I and G J influence the secondary moment. The results demonstrates that the beam opening angle reduces the effect of secondary moment, and that the greater the angle, the greater is the reduction that occurs in its secondary moment compared to its equivalent straight beam.