To investigate the mechanical properties of steel fiber-reinforced concrete under freeze-thaw cycles and the accuracy of its finite element simulation, a constitutive model and its functional expressions for steel fiber-reinforced concrete under tension and compression before and after freeze-thaw cycles were developed. This was based on the stress-strain curve characteristics obtained from experiments, combined with the Hognestad model, the Guo Zhenhai model, and the tensile-compressive model. Finite element simulations were conducted using ABAQUS to model the evolution of the mechanical properties of the lining structure during freeze-thaw processes, revealing the damage characteristics and failure modes of the lining mechanical properties induced by freeze-thaw cycles. The results indicated that after experiencing freeze-thaw cycles, the peak strength of the specimens decreased from 43.3 GPa to 35.3 GPa. Validation through scaled model tests confirmed that the established constitutive model and the corresponding finite element method accurately reflect the cumulative process of freeze-thaw damage, with the numerical simulation results showing good agreement with the experimental data. This study verifies the feasibility of accurately simulating the structural performance of steel fiber-reinforced concrete by developing a freeze-thaw constitutive model, thereby providing a theoretical basis and analytical method for the design and durability assessment of tunnel linings in cold regions.

Tunnel lining models were cast at a 1:20 scale using four different materials: plain concrete (PC), steel fiber-reinforced concrete (SFRC), reinforced concrete (RC), and rebar-reinforced steel fiber-reinforced concrete (R/SFRC). Loading tests were performed on these models before and after freeze–thaw cycles to investigate the failure modes, analyze the mechanical behavior, and determine the optimal reinforcement scheme in this study. The results indicated that freeze–thaw cycling reduced the load-bearing capacity of tunnel linings by 12% to 28% compared to non-freeze–thaw linings. Adding steel fibers significantly enhanced the ductility of the lining models. The mechanical performance of linings with an optimal steel fiber content surpassed that of models with either increased rebar alone or steel fibers alone. In this study, an optimal combination of a 0.36% rebar ratio and a 1.5% steel fiber volume fraction effectively improved the tensile performance of the lining while reducing rebar consumption, without compromising the inherent mechanical performance of the tunnel structure.

In high-altitude highway tunnels, the efficiency of jet fans significantly impacts the performance and energy consumption of ventilation systems. To optimize jet fan efficiency under such conditions, this study combines outdoor model experiments with numerical simulations of physical models in longitudinal jet ventilation systems. A model was established using SpaceClaim (ANSYS 2022 R1), and numerical simulations were conducted using Fluent software (ANSYS 2022 R1) to obtain results. The effect of different mounting inclination angles (0° to 10°) on the performance of a jet fan was experimentally investigated, and a correlation formula for the lift pressure of the jet fan under different inclination angles was established. Comparative results demonstrate that the numerical simulations accurately capture the variation trend of fan lift pressure under different tilt angles observed in the experiments. Specifically, the lift pressure of the jet fan initially increases and then decreases with increasing tilt angle. Comparative analysis of pressure rise at installation angles of 0°, 2°, 3°, 4°, 5°, 6°, 8°, and 10° revealed that a peak pressure rise of 19.66 Pa was observed at 4° installation, demonstrating optimal performance at this angle. The velocity distribution indicates that tilt angles between 0° and 4° increase the airflow influence range, beyond which efficiency decreases due to kinetic energy loss at the base. The study determined that under these conditions, a jet fan installed at a 4° inclination angle exhibits optimal performance in high-altitude straight tunnels and is thus identified as the optimal installation angle. At this angle, both pressure-rise efficiency and airflow stability are effectively balanced; this configuration provides a critical design basis for energy-saving optimization in high-altitude tunnel ventilation systems.

To investigate the structural dynamic response of pile-column bridge piers in mountainous regions under rockfall impact, this study takes the No. 4 double-column pier of Changba Bridge in Nanchuan, Chongqing, as a prototype. Based on similarity theory and differential equation analysis, a scaled model test was designed and conducted. By considering different rockfall impact angles (30°, 45°, 60°) and different impact positions on the pier (top, middle, bottom), the strain response characteristics of the pier concrete and reinforcing steel were systematically analyzed. The results indicate that the peak strain at the impacted location increases significantly with the increase in the rockfall impact angle; when the impact angle increases from 30° to 60°, the peak strain increases by approximately 22.8%. The peak strain decreases as the impact position approaches the bottom of the pier, with the most pronounced strain response observed at the middle position. The strain response of the reinforcing steel follows the same pattern as that of the concrete, albeit with a brief delay. Furthermore, the stirrups exhibit predominantly transverse orthogonal strain, while the longitudinal reinforcing bars exhibit predominantly longitudinal orthogonal strain. It is concluded that the impact angle is a key parameter affecting local damage to the pier, and the middle section of the pier should be regarded as a priority protection zone. This study provides a theoretical basis for the design of bridge piers in mountainous regions against rockfall impact.

To investigate the deterioration patterns of concrete performance under different numbers of freeze–thaw cycles (F-T cycles) with varying water-to-binder ratios, steel fiber (SF) contents, and fly ash contents, freeze–thaw tests were conducted on concrete specimens incorporating different volume fractions of steel fibers and fly ash at various water-to-binder ratios and subjected to different numbers of freeze–thaw cycles. The results indicate that: (1) F-T cycles led to the deterioration of the internal structure of steel fiber-reinforced fly ash concrete, significantly reducing its mass and relative dynamic elastic modulus (RDEM), as well as degrading its overall mechanical performance. (2) With increasing steel fiber and fly ash contents, the mass loss rate initially increased and then decreased. (3) As the water-to-binder ratio (w/c) increased, the mass loss first rose and then declined. The compressive strength decreased from 32.95 MPa to 19.75 MPa. The optimal performance was achieved at a water-to-binder ratio (w/c) of 0.32 and a steel fiber volume fraction of 2%; the axial compressive strength of the specimen reached 32.95 MPa and the splitting tensile strength reached 4.26 MPa. (4) A freeze–thaw damage regression model based on the relative dynamic elastic modulus (goodness-of-fit > 0.95) can accurately reflect the regularity of mechanical properties. The study can provide a reference for engineering applications of steel fiber concrete in cold regions.

To determine the optimal mix proportion of steel fiber-reinforced concrete in cold regions, this study adopted a multi-factor orthogonal experimental design method. A series of mix proportion schemes was formulated based on different water-to-binder ratios, steel fiber volume fractions, and combinations of mineral admixtures such as silica fume. Mechanical performance tests and freeze–thaw cycle tests were conducted to obtain the strength, deformation characteristics, and durability degradation patterns of specimens with different mix proportions before and after freeze–thaw exposure. Meanwhile, scanning electron microscopy (SEM) was employed to observe the microscopic surface morphology of specimens, both pre- and post-freeze–thaw cycles, and to analyze the damage evolution in pore structures and the fiber–matrix interfacial transition zone, thereby elucidating the microscopic mechanism of freeze–thaw damage. Ultimately, by comprehensively comparing the macro-mechanical properties, freeze–thaw durability, and microstructural characteristics, the experimental results of different groups were evaluated to identify the optimal mix proportion for steel fiber-reinforced concrete, which exhibits excellent mechanical performance and durability under freeze–thaw conditions. The results indicated that freeze–thaw cycles significantly reduced the mechanical properties of the concrete. The optimal mix proportion was achieved with a water-to-binder ratio of 0.4, a silica fume content of 10%, and a steel fiber volume fraction of 1.5%. This optimal mix proportion can provide a direct reference for the material design and application of steel fiber-reinforced concrete in engineering projects located in cold regions.