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This paper is to study the influence of sulphate corrosion on the mechanical behaviors of concrete, especially concrete corrosion under actual corrosion environment. This study investigated different immersion methods for concrete in a corrosion environment, and different strengths of concrete specimens with contents of sodium sulphate and magnesium chloride of 10.36% and 12.00%, respectively. Strength tests were performed on the specimens for the different immersion methods of full immersion, half‐immersion, and dry‐wet cycles, and with different strengths for the concrete of C20, C35, and C50. To study the internal mechanisms of the corrosion of concrete with different immersion methods and different strengths, a microscopic pore structure test was conducted. The field tests were performed after 12 years to evaluate the long term performance of the concrete under the actual environmental conditions. The test results show that full immersion mainly led to chemical corrosion, half immersion led to double damage from salt crystallization and chemical corrosion, and dry‐wet cycles accelerated the corrosions of the salt crystallization and chemical corrosion. The higher the strength of concrete, the better its resistance to the sulphate corrosion. In comparison with the strength of the concrete before corrosion, field test results show that the strength of uncorroded concrete after 12 years is increased by 9.5% and the strength of corroded concrete after 12 years is reduced by 56.2%. The results of the strength tests and pore structure tests show that sulphate has a significant impact on concrete strength after 12 years of field tests. This study will contribute to improve long‐term performance of the concrete, provide guidance for concrete structure design in corroded areas, and suggested that increasing attention should be paid to the effects of sulphate on concrete performance. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In order to comprehensively understand the degradation of cementitious materials when subjected to the coupled action of calcium leaching and freezing/thawing, changes of the pore structure and the Vickers hardness of cement paste are investigated. In this study, the coupled action are designed as Case I (first leaching then freezing/thawing) and Case II (first freezing/thawing then leaching). The effects of Case I on the mechanical degradation are greater than Case II. This is further clarified by changes of the pore structure. For freezing/thawing, the level of deterioration qualified by hardness decrease is sensitive to the proportion of big pores (i.e., >100 nm). For calcium leaching, it is determined by the amount of CH and C-S-H. In addition, the hardness-porosity relationship is discussed. A quantitative prediction model from theoretical deviations considering influences of pore size is established. [ABSTRACT FROM AUTHOR]

Abstract: In order to study the ionic diffusivity of cement-based materials, a numerical model is proposed based on cement hydration and a microstructure model HYMOSTRUC3D, coupled with random walk algorithm (RWA). By computer modeling of random movements of virtual ions in the pore structure of cement paste from the HYMOSTRUC3D model, the ionic diffusivity of cement paste can be obtained according to statistical data analysis. Einstein–Smoluchowski equation which provides the intrinsic relation between the macroscopic diffusion process and the ions Brownian motion under microscopic scale is utilized in the prediction of the ionic diffusivity of cement paste. The proposed model is first validated by experimental results of chloride diffusion coefficient from literature. Simulations show good agreement with the experiments from steady-state diffusion cell test. Next, the proposed model is used to predict the ionic diffusivities of cement pastes with different water-to-cement ratios (w/c) at various hydration times. Then, considering the pore structure of cement paste is an essential part for analyzing the ionic diffusivity of cement paste, the reliability of the pore structures from the HYMOSTRUC3D model is discussed by comparing the pore size distributions by simulation with those by experimental techniques. Finally, the simulated ionic diffusivities of cement pastes in this study are compared with other simulations from literature. [Copyright &y& Elsevier]