Experimental Study On Sulfate And Chloride Erosion Resistance Of Concrete On Yumo Railway

Due to the erosion of chloride and sulfate,the concrete structures in coastal area,Salt Lake area and saline land area will suffer durability damage before the design service life.Among them,the durability of railway concrete structure needs special attention because of its characteristics such as long strip distribution,serving in open air environment,bearing fatigue load and high safety requirements.In the actual construction environment,the corrosion of chloride and sulfate does not begin to appear after the curing of concrete,so it is necessary to explore the durability of early-age concrete in the environment of chloride and sulfate erosion.In this paper,C35 and C50 concrete in actual railway engineering are taken as the research objects,and the main research contents and conclusions are as follows:（1）Based on the diffusion reaction equation and the constitutive relation of concrete under saturated state,the chemical-mechanical damage deterioration analysis model of sulfate attack concrete is established and implemented on COMSOL.Based on the diffusion equation of chloride intrusion into concrete in saturated state and the simplified model of steel corrosion,the analysis process of chemical mechanical damage deterioration process of concrete corroded by chloride ion is established.This analysis method can quantitatively analyze the stress change,strain change and damage evolution in the process of damage deterioration of concrete corroded by sulfate and chloride,and provide the basis for the durability evaluation of concrete.（2）Using the dry-wet cycle method,C35 and C50 concrete specimens after standard curing for 3 d and 7 d were immersed in the Na₂SO₄ solution with a concentration of 8%.The compressive strength,compressive corrosion resistance coefficient and relative mass were taken as the evaluation indexes.The results show that the compressive strength,compressive corrosion resistance coefficient and relative mass of concrete in the process of sulphate dry-wet cyclic erosion first increase and then decrease.At the initial stage of erosion,the compressive strength and relative mass increment of concrete cured for 3 days are larger than those cured for 7 days;at the later stage of erosion,the compressive strength and relative mass of concrete cured for 3 days are lower than those cured for 7 days.（3）Electric flux test was used to investigate the change rule of chloride ion erosion resistance of C35 and C50 concrete with different fly ash content and admixtures at 3 d,7 d,28 d and 56 d.The results show that at the early age of 3 d and 7 d,the electric flux value increases with the increase of fly ash content,and after 28 d,the electric flux value of concrete decreases with the increase of fly ash content.Using low water binder ratio,fly ash as cementing material and water reducing agent can improve the compactness of concrete and help resist the penetration of chloride ions,but still cannot guarantee the impermeability of 3d and 7 d early-age concrete.Therefore,the anti-chloride ion penetration ability of concrete at3 d and 7 d is still weak,and it is easily corroded.（4）The corrosion solutions with concentrations of 3%NaCl+5%Na₂SO₄,3%NaCl,5%Na₂SO₄ were used to explore the durability change of concrete and the interaction of two kinds of corrosion ions in the compound corrosion of chloride and sulfate.The results show that during the 90-day test age,the electric flux value of the concrete in the composite erosion solution is lower than the concrete in the single chloride erosion solution under the corresponding conditions,and the compressive strength is higher than the concrete in the single sulfate erosion solution.That is,the presence of sulfate ions in the compound erosion improves the permeability of concrete against chloride ions,and the presence of chloride ions delays the erosion damage of sulfate ions on concrete.（5）SEM was used to analyze the process of ettringite growing from short and small crystals to crisscross long and dense crystals in a single sulfate corrosion environment,and the role of ettringite in concrete changed from filling to expansion cracking;XRD was used to analyze the change law of diffraction peak intensity of chemical substances such as ettringite,gypsum,Friedel’s salt,calcium hydroxide and other chemical substances in early-age concrete under compound corrosive environment.The decrease of macroscopic mechanical properties of concrete is the apparent phenomenon that concrete is damaged by chloride and sulfate erosion,while the damage of microstructure is the fundamental damage of chloride and sulfate erosion of concrete.