Abstract:To intuitively investigate the self-healing performance of asphalt, binder bond strength (BBS) test was conducted to study the influence of temperature, moisture, aging, and SBS modifier on the self-healing performance of asphalt. The healing process of asphalt under different healing conditions was observed by computed tomography (CT) test. BBS test results showed that higher temperature and dry condition were conducive to the asphalt self-healing performance. Moisture was disadvantageous to the short-term and long-term healing of asphalt, but could improve the healing ratio during the mid-term of the healing process. The healing behavior and the stabilized healing strength of 4.5% (mass fraction) SBS-modified asphalt after multiple failure-healing cycles were lower than those of base asphalt, and the healing performance of SBS-modified asphalt was less sensitive to aging. CT scanning results showed that the healing process of asphalt mainly included three steps: gathering, rounding, and moving. CT images confirmed the favorable effect of higher temperature and dry condition on asphalt self-healing and the adverse effect on SBS modifier.

Abstract:To investigate the moving stability of trucks and cars under crosswind, a driving simulator study based on the eight degree-of-freedom motion system was carried out. The simulation platform was established for the bridge-tunnel section of expressway in mountain area under crosswind. Thirty participants were recruited, and six scenarios were designed with each run in a different crosswind condition. By comparing and analysing the lateral displacement, yaw rate, and lateral acceleration of cargo truck and car under crosswind, the sideslip stability, yaw stability, and roll stability of the vehicles were discussed. Based on the static stability factor, the concept of overturning ratio was proposed to compare the anti-overturning ability of different vehicles. Results show that the car was more prone to sideslip when driving in the bridge-tunnel section under crosswind, and the cargo truck had more serious side drift under continuous crosswind. The car, with a shorter crosswind perception time, could respond to the crosswind environment faster than the cargo truck. The risk of truck rollover was higher under crosswind, and there was still a certain risk of rollover after the cargo truck drove out of the crosswind area.

Abstract:To investigate the scour effect on the changes in the vehicle-induced dynamic behaviors of bridges, and to promote the research on the scour diagnosis based on vibrations, a vehicle-bridge coupled vibration analysis model was established which can simulate the excitations of vehicles with various driving conditions (i.e., uniform speed, different speeds, and braking). The influence law of the vehicle-induced dynamic behaviors of a prestressed concrete continuous bridge in time domain and frequency domain was analyzed through numerical simulation. Results show that the local vibration mode frequency of the scoured piers decreased significantly with the increase in the scour depth, and the mode frequency of the non-scouring piers showed no evident change. As the scour depth increased from 1 m to 4 m, the longitudinal displacement and acceleration response peaks of the pier tops increased obviously under the excitations of vehicles in various driving states, and the response peak was the largest at braking under the same scouring condition. Compared with the non-scouring condition, the longitudinal displacement and acceleration response peak grew linearly with an increase of 72.72% and 56.26% respectively. Moreover, the spectral amplitude of the scouring sensitive response increased significantly, and the center frequency shifted to low frequency. The results can provide theoretical basis and index reference for the foundation scour diagnosis of bridges based on vehicle-induced dynamic responses.

Abstract:In order to investigate the effect of low air pressure on the air entrainment of cement-based materials, simulation study of negative pressure was conducted. The foaming performance and bubble stability of air-entraining agent (AEA) solution, pore structure of air-entrained cement paste, and air-void structure of air-entrained cement mortar were tested in different air pressures. The cement paste and cement mortar specimens were stirred and formed in designed air pressures until hardening. Results show that the foams formed by two new types of AEAs (FC-1 and FC-2) were more stable than those formed by triterpenoid saponin type AEA (SJ-2) in corresponding aqueous solutions in negative pressure. However, this finding could not be extended broader to cover cement-based materials. The total porosity and the volume of pores with radii of 100-1 000 nm in hardened cement paste were found higher in negative pressure, while there was little adverse effect of low air pressure on either air content or air-void structure of hardened cement mortar when a suitable AEA was applied. SJ-2, FC-1, and FC-2 AEAs could satisfy the requirements of air entrainment for frost resistant concrete in high-altitude regions with low air pressure, among which FC-1 and FC-2 presented comparative advantages in air-void structure optimization and air content control.

Abstract:In view of the problems of current scouring tests on cement stabilized macadam base (CSMB) materials such as difficult to operate and low efficiency, a rotary scouring test equipment for CSMB materials was developed. By using FLUENT software, the scouring force on specimens was analyzed in numerical simulation, and the scouring force on the surface of the specimens was about 8 to 50 kPa under different scouring processes. Based on the influence of different immersion heights, scouring frequencies, and scouring time on the specimens, the optimal rotary scouring test parameters were proposed. The influence of unconfined compressive strength, voidage and compaction degree on the scouring damage of specimens was analyzed. Results show that the scour mass loss rate was approximately linearly decreasing with the increase in the unconfined compressive strength. In terms of anti-scouring, the compressive strength of CSMB should not be less than 4.0 MPa, and the cement content should not be less than 4.5%. The scour mass loss rate changed in the form of “U” with the increase in voidage, and the optimal anti-scouring voidage was between 2.5% and 5.0%. With compaction degree enhanced, the scour resistance of the specimens increased, and the lowest compaction degree should be greater than 98%. According to the influence of top surface void of base layer on the fatigue life of pavement structure in finite element simulation, based on the principle that the fatigue life of asphalt layer reduced by 40% to 60%, the scour resistance technical standard of CSMB was put forward.

Abstract:To further understand the aerodynamic effects and the characteristics of train wind, the RNG κ-ε model was used to simulate the overall process of a high-speed train passing through a tunnel asymmetrically. The movement of the train was simulated using the sliding mesh technique. The aerodynamic effects and the train wind characteristics were investigated, and the accuracy of the numerical method was verified by comparing the numerical results with the results of a full-scale experiment. Results show that there was a large difference of pressure transients between the entrance and inside of the tunnel. The maximum positive pressures and the peak-peak pressure transients amplitude differed by 13.1% and 7.3% at the symmetrical monitoring points on both sides of the train. The longitudinal velocity component and the maximum resultant velocity on the near-tunnel side were 2.1 and 1.9 times those of the far-tunnel side respectively. The train passing asymmetrically had no obvious effect on the aerodynamic pressure, but the impact on the train wind was significant. The boundary layer of the train had a significant effect on the longitudinal component of the train wind, while the effect on the lateral velocity component and the vertical velocity component was small. Complex wake vortex structures alternately appeared behind the tail, which was very different in the open air. The attenuation of the train wind velocity in the tunnel was slower, and the duration was longer.

Abstract:To study the distribution rule of modified time-to-collision (MTTC) in vehicle-to-vehicle (V2V) communications, a mixed distribution model that can describe the MTTC distribution of different driving risk levels was proposed, and the model was verified by field test data. Experiment on V2V communications was carried out based on long-term evolution-vehicle (LTE-V) technology. Real vehicle driving test data was obtained, and k-means clustering method was used to divide the driving risk levels into four levels. Then, three probability distribution models including Weibull, Gamma, and lognormal were utilized, and the goodness-of-fit analysis was carried out for MTTC under four risk levels. Test results show that MTTC under different risk levels could be fitted well by lognormal distribution model. On this basis, an MTTC lognormal mixed distribution model for four risk levels was established, and the model parameters were calibrated using expectation-maximization (EM) algorithm. Three classic distribution models (Weibull, Gamma, and lognormal) were selected for comparison. The effectiveness of the proposed mixed distribution model was verified by goodness-of-fit analysis, and the mixed distribution model was applied to MTTC test data in conventional environment. Results indicate that the proposed lognormal mixed distribution model can fit the MTTC distribution better in V2V communication environment and has good adaptability to the conventional environment, which can provide theoretical support for related research on vehicle operation safety.

Abstract:In order to explore the adaptability of the existing merging area in highway to autonomous vehicles, the influence of the acceleration lane length and the visibility triangle angle of existing highway merging area on the automatic traffic flow was analyzed, and results were compared with those of traditional traffic flow. Based on the characteristics of autonomous vehicles in terms of perception, car-following, and lane-changing behaviors, as well as the collaboration with surrounding vehicles, the Krauss car-following model and LC2013 lane-changing model were improved to adapt to the characteristics of autonomous vehicles. According to the lane-changing characteristics of the vehicles, a formula was established for calculating the acceptable safety gap when changing lanes, and the car-following model parameters were adjusted while fulfilling the requirement of lane-changing safety. Results show that under the existing plane design parameters of merging area in highway, the safety, efficiency, and stability of automatic traffic flow were better than those of traditional traffic flow. Compared with traditional vehicles, the number of collisions of automatic traffic flow reduced by nearly 100%, average delay reduced by 60% to 71%, average vehicle speed increased by nearly 20%, and the speed was more stable. Under different plane design parameters of merging area, the number of collisions of automatic traffic flow was 0, the average delay was maintained at around 0.65 s, and the average speed was stable at 33-34 m/s. The existing plane design parameters of merging area in highway well adapted to the autonomous vehicles in terms of safety, efficiency, and stability, and the parameter values of the merging area had little effect on the autonomous vehicles.

Abstract:In order to investigate the corrosion damage of bridge pile foundation under the action of freeze-thaw cycles in alpine salt swamp areas, laboratory model test was conducted to simulate the effect of pile foundation concrete mixture ratio and compound salt solution concentration on the corrosion resistance of pile foundation concrete under the conditions of curing freeze-thaw cycles. The corrosion damage mechanism of pile foundation was analyzed by measuring the appearance, quality, compressive strength, dynamic elastic modulus, and SEM of the concrete. Results show that concrete mixture ratio had significant impact on the corrosion resistance of pile foundation. When fly ash, silica fume, and expansive agent (proportion III) were added to concrete, the relative dynamic elastic modulus was reduced by less than 10%. The anti-erosion coefficient was more than 0.9, the mass loss was less than 5%, and there was only a slight shedding on the concrete surface, which could improve the anti-erosion performance of the pile foundation. The compound salt solutions with different concentrations were prepared. The pile foundation concrete showed different degrees of corrosion under the conditions of freeze-thaw cycles, the higher the concentration was, the more obvious the mutual inhibition of compound salt ions was, and the corrosion of concrete was weakened. When the concentration reached 15.4%, the mass loss showed a negative growth. According to the SEM test results, ettringite, wollastonite gypsum, and Friedel′s salt were the main products leading to the corrosion damage of pile foundation concrete structure. It is suggested that the reduction factor of the design life of pile foundation is 0.6, which can prevent the adverse effect of freeze-thaw cycle on the design life of pile foundation under the corrosion of reference concentration solution.

Abstract:For the dynamic reliability analysis of complex nonlinear bridge structures under stochastic ground motions, an efficient seismic reliability analysis method was developed based on the principle of maximum entropy. First, the relationship between the seismic reliability and extreme value distribution (EVD) of bridges was clarified. The seismic reliability of bridge structures was transformed into the corresponding EVD estimation. Then, the maximum entropy method (MEM) for estimating the EVD of nonlinear seismic responses of complex bridge structures was established. In view of the fact that the iterative solution of the existing MEM is greatly affected by the initial value and is not easy to converge, a method based on likelihood function was proposed to solve the EVD and seismic reliability of bridge structures more efficiently. Finally, a typical high-pier and long-span continuous rigid frame bridge was taken as an example. The accuracy and efficiency of the proposed method were verified via Monte Carlo simulation, and the results were compared with the results of kernel density estimation (KDE) and lognormal distribution fitting. Results show that the proposed MEM based on likelihood function could obtain the global optimal solution of EVD, and the solution process was not affected by the initial value and had good numerical stability. The method could accurately estimate the EVD and dynamic reliability of complex bridge structures under stochastic earthquakes. Both KDE and lognormal distribution failed to estimate the EVD of the structural seismic responses at small failure probability levels. It is thereby recommended that MEM is adopted for the seismic reliability analysis of bridge structures.

Abstract:The seismic vulnerability analysis method has been widely used to evaluate the seismic performance of bridge structures. In order to make up for the shortcomings of common analysis methods of bridge seismic vulnerability and improve the efficiency of vulnerability analysis, the Bayesian estimation method was introduced to the analysis of seismic vulnerability for bridge structures, and a bridge seismic vulnerability method based on Bayesian estimation(referred to as BEM) was proposed. Taking a typical four-span railway simply supported beam bridge in strong earthquake area as the engineering background, the vulnerability analysis of the bridge with different pier heights was carried out. The correctness of the proposed method was verified by taking the analysis results of common methods as reference. Results indicate that BEM can overcome the shortcomings of common bridge vulnerability analysis methods, improve the analysis efficiency of bridge seismic vulnerability analysis method, and greatly reduce the computational burden on the premise of ensuring accuracy. The method has good stability and a great prospect in engineering application.

Abstract:To scientifically screen out drivers with different accident proneness and make road traffic safety education more targeted, this paper designs a questionnaire and scales suitable for non-professional drivers of motor vehicles, analyzes and identifies factors influencing driver′s accident proneness, and calculates corresponding weight values. The fuzzy mathematics model was used to construct membership function so as to complete grading evaluation of driver′s accident proneness, and the rationality of the grading evaluation method was verified via collected road traffic accident data. Results show that six influencing factors including driver′s age, driving frequency, illegal operation score, stable driving personality, smart driving personality, as well as daily work and driving environment were screened out. Driver′s accident proneness scales and membership function could effectively identify drivers with different levels of accident proneness. When the score was within (0,3.5], the driver′s accident proneness was high; when the score was within (73.5,4], the driver′s accident proneness was relatively high; when the score was within (4,0), the driver′s accident proneness was ordinary. It was verified by examples that the proposed method can effectively screen out drivers with different accident proneness and provide reference for driver training and safety education.

Abstract:To improve the operational service capacity of flex-route transit under uncertain travel demand, a slack arrival strategy and a meeting point strategy were proposed. First, the operating mechanism of flex-route transit service under the proposed strategies was described in detail, and the process was divided into a two-stage optimization problem. Then, mixed integer programming (MIP) was employed to formulate the problem with a twofold objective: to maximize the number of accepted requests in the first stage and to minimize the total trip time of the accepted passengers in the second stage. A memetic algorithm was proposed to solve the model in a reasonable amount of time. Simulation experiments based on a real-life flex-route transit service were conducted to evaluate the proposed strategies. Results show that the two strategies, which relaxed the departure time constraints of checkpoints and the space constraints of reserved pick-up and drop-off locations, could improve the routing flexibility of flex-route transit service and significantly reduce the passenger rejection rate without any additional operating cost. Besides, when the two strategies were applied at the same time, the rejection rate could be reduced by up to 22%.

Abstract:In order to improve the calculation accuracy of moisture index of subgrade in highway climatic zone Ⅱ₁, an optimal calculation method of subgrade moisture index was established by employing the Food and Agriculture Organization Penman-Monteith (FAO-PM) method instead of the Thornthwaite method to estimate the average monthly potential evapotranspiration (PE) of soil subgrade, which considers the evapotranspiration of soil subgrade as zero under negative temperature conditions. After analyzing the influence of meteorological parameters on the monthly PE of soil subgrade, the applicability of the proposed method was verified and the calculation process of subgrade moisture balance was simplified. Based on the seasonal change characteristics of subgrade moisture, a staged moisture index calculation method was proposed to refine the range of subgrade moisture index in climate zone Ⅱ₁. Results show that when the temperature was below zero, the average monthly PE of the subgrade was positively correlated with average sunshine hours, average temperature, and average wind speed, but negatively correlated with average relative humidity. Moreover, the total PE accounted for 9.8%-15.7% of the total evapotranspiration throughout the year. The subgrade moisture equilibrium states were classified into three types: runoff, no runoff with or without drought, for which the most unfavorable seasons could be distinguished by the four-stage statistical analysis of the moisture index. For the seven meteorological stations studied, the minimum and maximum moisture indexes of different soil groups were calculated as -16.5 and 33.2 respectively, which are less than the current standard recommendation (-8.1-35.1), indicating that the subgrade moisture in these regions are more arid than expected and gradually decreases from southwest to northeast. The method can not only be used to effectively calculate the evapotranspiration of soil under negative temperature, but also ensure the accuracy of subgrade moisture index estimation in climatic zone Ⅱ₁.

Abstract:Track stiffness irregularity, indistinguishable on rail surface, causes dynamic impact or track deformation when trains running through, which greatly affects the operation safety and stability of the system. In view of this practical issue, a determination method of track stiffness irregularity was analytically deduced. Then, a high-speed train-track-bridge dynamic interaction model was established based on the train-track-bridge dynamic interaction theory. On this basis, the influence of normal track stiffness irregularity on the dynamic behaviors of the system was investigated in time and frequency domains. Finally, taking the fastener failure as an example, the influence of abnormal track stiffness irregularity on the dynamic performance of the system was studied. Results show that track stiffness had obvious effect on the vibrations of the system. Wheel-rail force, wheelset acceleration, and vertical acceleration of frame were obviously influenced by fastener spacing and length of slab. Pitch acceleration of frame was most sensitive to track stiffness irregularity. Considering structural elasticity, track vibrations at the slab-edge were much larger than those at the mid-span. The amplitude ratio of rail acceleration at different locations was 1.17, while that of slab acceleration reached 2.2. Normal track stiffness irregularity caused periodical vibrations, which might lead to the resonance of the system and speed up the structure damage, while abnormal track stiffness irregularity caused wheel-rail impact and affected the running safety of the system in serious situations. When the train ran at 250-350 km/h, the allowable number of failure fastener was one.

Abstract:In order to solve the problem of excessive impact on the system due to the movement state changes in robots, CNC machine tools, lifting machinery, and other equipment, two hyperbolic tangent functions were used to construct the S-type velocity curve with the junction point at uniform motion stage. Combined the S-type velocity curve with tracking differentiator, a high-order continuous point-to-point motion trajectory (HCPPMT) was obtained. By adjusting the relevant parameters of HCPPMT, the speed at acceleration and deceleration stages, as well as the maximum speed and operation time at constant speed stage could be adjusted, so that the smoothness and impact strength during the acceleration, constant speed, and deceleration processes could be controlled, and the states of each stage could be adjusted in the process of point-to-point motion. Therefore, the impact of motion state switching on the mechanical equipment could be reduced and the stability of the motion could be enhanced. The convergence of HCPPMT was proved by Lyapunov method, and the physical properties of HCPPMT were mathematically proved. The superiority of HCPPMT was illustrated by comparing with two other trajectories and the application of trajectory planning for end-effector of three-degree-of-freedom manipulator.

Abstract:To broaden the working range of on-board charger (OBC) for bidirectional charging and discharging and improve the charging efficiency, the power factor correction (PFC) cascaded high-efficiency CLLC resonant topology was used to realize the bidirectional conversion of energy between grid voltage and battery voltage. Meanwhile, in order to solve the problem of difficult to design parameters when CLLC resonant topology works in a wide range, a parameter design and optimization method was proposed. Fundamental harmonic analysis (FHA) was used for the modeling of resonant network. The normalized equations of charging and discharging gains were derived, and the key parameters affecting the gain and efficiency of the resonant converter were extracted. The influences of the changes in key parameters on the topology performance were compared and analyzed, and the parameter design and optimization principles were summarized. Experimental results show that the most important parameters that affected the bidirectional operation of the resonant converter were transformer turns ratio and resonant capacitance. The transformer turns ratio mainly affected the charging and discharging efficiency of the CLLC converter, and the value of the resonant capacitor affected the best working mode of the wide range output between the charging and discharging mode. The best value range of transformer turns ratio and resonant capacitor was given when the bidirectional transmission power was at the same level. By applying CLLC topology, the bidirectional on-board charger could achieve ultra-wide range operation. The charging efficiency of the whole machine reached 94.5%, and the efficiency of CLLC reached 97.5%.

Abstract:To address the occlusion problem of pedestrian detection algorithm when applied in traffic scenarios, this paper presents an occlusion-aware algorithm combined with dual attention mechanism for pedestrian detection. Based on the RetinaNet framework, the spatial-wise attention mechanism and channel-wise attention mechanism were utilized in regression and classification branches respectively, guiding the detector to pay more attention to the visible parts of pedestrians. Moreover, visible bounding box information of pedestrians was introduced to optimize the traditional regression loss function, so that it can adaptively adjust the weights of predicted boxes according to the degree of occlusion. Experiments on Caltech and CityPerson datasets show that the proposed algorithm had better robustness and higher accuracy than other eight advanced algorithms such as RetinaNet. Especially in the case of heavy occlusion, the log-average miss rate of the proposed algorithm was only 45.69%, which was 12% lower than those of other algorithms. Furthermore, the proposed algorithm could detect pedestrians in quasi-real-time. It processed 11.8 frames per second on Caltech dataset and 10.0 frames per second on CityPerson dataset. The detection methods of dual attention mechanism and occlusion-aware regression loss function proposed in this paper are feasible and effective, and have significant advantages for the processing of occluded pedestrians.

Abstract:In view of the environment recognition problems for bionic robot fish working near the target, a near wall flow recognition method based on artificial lateral line (ALL) is proposed in this paper. Firstly, the feasibility of near wall flow field environment recognition by means of ALL was analyzed theoretically. Then, the ALL virtual pressure sensor array was established. The surface pressure data of bionic robot fish were calculated and extracted under different parameters (inflow velocity v, near wall distance d, and swimming frequency f) by using computational fluid dynamics (CFD) method. Finally, the regression model of inflow velocity and near wall distance based on multilayer feed forward neural network was established, and the model structure and data characteristics were optimized. Results show that the wall effect caused asymmetric distribution flow around the fish. The lateral sensors at the head and tail of the fish had high identification for flow field parameters. Eliminating the weak correlation features had little effect on the prediction indexes of inflow velocity and near wall distance, and was helpful to reduce the complexity of the prediction model. The results can provide theoretical and methodological basis for information extraction and processing of underwater robot environment recognition.