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.

This study investigates the influence of freeze–thaw (F–T) cycles on the seismic fragility of double-column bridge piers. Mechanical tests were conducted on standard concrete specimens subjected to 0, 25, 50, 75, and 100 F–T cycles using an HC-HDK9/F rapid freeze–thaw testing machine. The experimental results were used to calibrate and validate the applicability of the selected concrete constitutive model. A nonlinear finite element model of a double-column bridge pier was developed in the OpenSees platform, incorporating material degradation parameters corresponding to varying F–T cycles. Incremental dynamic analysis (IDA) was performed to derive seismic demand curves and quantify fragility corresponding to multiple damage states. The results indicate that the failure probability of the piers increases significantly with the number of F–T cycles, particularly for slight and moderate damage levels. In the low to moderate peak ground acceleration (PGA) range, the exceedance probabilities for slight and moderate damage states show a sharp rise, highlighting the sensitivity of early-stage damage to F–T degradation. It is worth noting that under the extreme condition of PGA = 1.0 g and 100 freeze–thaw cycles, the piers still exhibit a certain degree of redundancy against severe and complete damage, which to some extent reflects the certain rationality of the current seismic design in freeze–thaw environments. These findings underscore the robustness of current seismic design provisions in cold regions and provide theoretical and data-driven support for performance assessment, service life prediction, and maintenance planning of bridges exposed to freeze–thaw environments.

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.

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.