To study the influence of different water–binder ratios on the corrosion, permeability, and freezing properties of concrete, we produced different strengths of concrete with respective water–binder ratios of 0.32, 0.38, 0.50, and 0.66. The corrosion resistance of the concrete was studied via three corrosion methods: full immersion, half immersion, and dry and wet cycles. The impermeability and frost resistance of concrete with different water–binder ratios were tested and analyzed. The test results show that the corrosion modes in order from strong to weak were dry and wet cycles, half soaking, and full soaking. The relative dynamic elasticity modulus and freeze–thaw index were used to evaluate the frost resistance of concrete based on the analysis of three indices of frost resistance. To study the internal mechanism of corrosion of concrete with different water–binder ratios, microscopic pore structure testing of the concrete was conducted using a Micromeritics AutoPore IV 9500 Series instrument. The porosimeter studies show that the smaller the water–binder ratio, the more small pores and the denser the concrete. The smaller the water–binder ratio, the higher the strength and the better the corrosion, permeability, and frost resistance.

In order to study the influence of different casting temperatures and curing conditions on the hydration reaction, strength, and micropore structure of concrete cured in a low negative temperature environment, a hydration heat test of high-speed railway durability concrete in a permafrost area was carried out. The external temperature was $-5°\mathrm{C}$ , while the casting temperature was either 5°C, 10°C, 15°C, or 20°C. A concrete compressive strength test was carried out, in which the dosage of air-entraining agent used was 0.00%, 0.05%, 0.10%, 0.15%, or 0.20%. Based on a mercury injection test, the mechanisms of concrete pore structure formation under the variation of casting temperature and air-entraining agent content were analyzed. The age of equal strength under different casting temperatures and different dosages of air-entraining agent was obtained, based on the theory of equal strength. The results of our research show that compared with standard curing, the hydration reaction of cement was significantly inhibited and the hydration rate and degree were decreased under the condition of constant $-5°\mathrm{C}$ curing. Under negative temperature curing, the micropore structure of concrete underwent obvious deterioration. The air introduced by the air-entraining agent formed dispersed and nearly spherical bubbles in the concrete, changing the bubble spacing and improving the antifreeze performance of the concrete at low temperatures. The concrete showed an obvious age lag phenomenon under negative temperature curing. Low negative temperature cured samples with 20°C, 15°C, 10°C, and 5°C casting temperatures met the equal strength requirements in 42, 49, 56, and 63 days, respectively. These research results provide guidance for future concrete design efforts and high-speed engineering construction activities in frozen soil areas.

To study different lateral restraints, different constituents of expansion agents, the influence of different steel ratios, and concrete creep properties, we carried out experiments with lateral restraint and without lateral restraint conditions separately on 12 specimens with the expansion agent content accounting for 4%, 8%, and 12% respectively. In addition, the creep tests were performed on specimens with different steel ratios of 0.0%, 3.8%, 6.6%, and 9.2%. The test results show that the lateral restraint improves the strength of the system (concrete-filled steel tubes) which resists further load after the concrete ultimate strength is surpassed and reduces the creep. The creep degree of the concrete-filled steel tube with lateral restraint is about 0.09–0.30 times smaller than that of the tube without lateral restraints. The creep degree of the concrete-filled steel tube increases as the steel ratio decreases. Creep tests with different amounts of expansion agent indicate that the creep degree of the concrete structure increases as expansion agent content decreases. To study the internal mechanism of the creep of concrete-filled steel tubes with different lateral restraints and different expansion agent concentrations, a microscopic pore structure test on the steel core concrete was conducted using the RapidAir457 pore structure instrument. Microscopic studies show that the air content and the length of the bubble chord of the laterally restrained core concrete are lower than those without lateral restraint core concrete. The amount of air content and the length of the bubble chord of core concrete specimens increase as the expansion agent content in the core concrete specimens decreases from 12% to 4%. Under the same external loading conditions, as steel ratio increases, the lateral restraint causes a further reduction of creep. The results of this study suggest that the creep of concrete can be reduced by selecting appropriate lateral restraint conditions and an optimal amount of expansion agent in the mix design of concrete for concrete-filled steel tubes.

To reasonably predict the creep law of concrete-filled steel tubular (CFST) members under different working conditions, four groups of creep tests were carried out, and the creep curves of CFST specimens with defects were obtained. Based on the creep prediction formula given in the code for the design of CFST highway arch bridges issued in 2015, the creep prediction models of CFST under different core concrete conditions are studied. The results show that the concrete microcracks around the cavity defects are continuously generated and expanded during the compression process, resulting in the dislocation and sliding of the concrete. It increases the additional deformation of CFST; debonding defects hinder the redistribution of stress between the steel tube and the core concrete, weaken the interaction effect between the steel tube and the concrete, and increase the creep deformation of the core concrete. According to Different core concrete states, appropriate reference factors are selected to modify the creep prediction formula of CFST, which greatly improves the creep prediction accuracy of CFST members and defective members.