Durability is a fundamental property of concrete, and research on concrete durability differs markedly among service environments. The deterioration mechanisms of concrete materials encompass a complex interplay of physicochemical processes, including carbonation, freeze-thaw damage, wet-dry cycling, sulfate erosion and chloride ion erosion. Considering the coupled erosion mechanism of chloride ingress and freeze-thaw cycling that persistently affect concrete structures in frigid coastal regions of northern China, the non-steady-state transport characteristics and spatiotemporal distribution patterns of chloride ions within concrete media are critical parameters for durability assessment in civil engineering applications. The development of predictive models for chloride ion transport in concrete matrices has therefore attracted increasing research attention. Conventional chloride ion transport models often ignore the dynamic effects of low-temperature environments on the micro-pore structure of concrete, resulting in insufficient model prediction accuracy. In this study, the microscopic mechanism of pore structure and chloride ion transport in concrete in low-temperature environments was studied, and a chloride ion diffusion model with higher engineering applicability was constructed.

The chloride ion transport model suitable for low-temperature environments was established in three steps. The first step was to determine the evolution mechanism of concrete pore structure. Under the combined effects of cryogenic conditions and saline solution infiltration, crystals tended to form within the concrete matrix. These crystals filled concrete capillary pores, densified the matrix, and helped improve concrete impermeability. Crystalline particles could create stress differences around pores, promoting pore expansion and crack formation, thereby reducing the durability of concrete. The second step was to construct a pore structure change model. The pore structure change model included two parts: a time-varying model of temperature field was constructed based on trigonometric functions. and a dynamic evolution equation of porosity considering the coupling effect of temperature fluctuation on crystal filling and microcrack propagation was established to achieve the quantitative characterization of multiphysics response of pore structure. The third step was to construct a chloride ion transport model considering age attenuation, multi-ion coupling, damage to the critical pore size and most probable pore size. The Bruggeman model is combined with the dynamic porosity model to modify the chloride diffusion coefficient. Based on the mathematical and physical nature of Fick's second diffusion law, a chloride ion transport model with time-varying parameters was constructed.

The microstructure evolution of pores had a significant influence on the transport characteristics and spatial distribution of chloride ions during chloride-ion erosion of structures at low temperature. Compared with the traditional diffusion model, the results showed that the chloride ion penetration depth and concentration predicted by the new model established in this study were more consistent with the exposure test data. The evolution of pore structure of concrete under low temperature could be divided into two characteristic stages: crystallization filling stage and micro-crack propagation stage. In the chloride ion transport mechanism, the enhancement effect of microcrack propagation on the effective chloride ion diffusion coefficient was significantly higher than that of crystal filling at low temperature. This phenomenon revealed the key controlling effect of crack network connectivity on ion migration channels.

This study constructs a modified model that accounts for the influence of concrete pore changes on chloride ion diffusion under time-varying temperature in northern coastal areas. Comparison with exposure test data verifies the applicability of the model, which is then used to study the transport behavior of chloride ions under the influence of temperature, tortuosity and initial porosity of concrete through finite element analysis. The main conclusions are as follows: Temperature has an important influence on chloride ion transport. Regulated by the crystallization-cracking mechanism, the temperature decreases from 273.15 K to 253.15 K, and the chloride ion concentration in concrete decreases first and then increases. This effect is more significant with time, and the effect of crystallization-cracking mechanism on chloride ion transport is most obvious at the depth of 9 mm. As a dynamic driving factor, the time-varying temperature has a stronger accelerating effect on chloride ion transport than the constant temperature condition. Under temperature cycling, chloride ions achieve faster and deeper migration through the crack network generated by freeze-thaw damage, and its peak concentration can reach 2.46 times that under constant temperature conditions. As an initial structural characteristic of concrete, the average particle size of aggregate is closely related to the tortuosity of concrete pore network and chloride ion diffusion capacity. The chloride ion transport process in concrete with different aggregate sizes is simulated under time-varying temperature conditions. A larger aggregate size, corresponds to lower tortuosity of the internal concrete pores, and the chloride ion diffusion coefficient of the concrete with an average aggregate size of 20 mm is 1.38 times that of the concrete with an average aggregate size of 16 mm. Affected by differences in the initial pore structure, under the cumulative effect of long-term chloride ion erosion, the difference of chloride ion concentration in concrete with different aggregate sizes gradually increases as the erosion duration increases. The initial porosity of concrete is closely related to the chloride ion transport behavior. The higher the initial porosity, the higher the chloride ion concentration, and as the erosion time increases, the acceleration effect of large porosity on chloride ion transport becomes more significant; in practical engineering applications, when the initial porosity of concrete increases from 0.10 to 0.15, the corrosion time of steel bars at a depth of 20 mm in the structure is shortened by 12.42 years.