Study On Deterioration Of Concrete Under Drying-wetting Cycles And Sulfate Attack Based On Interface Characteristics

Marine engineering and saline soil concrete structures works in nature are often severely damaged by the double action of drying-wetting cycles and sulfate attack.The deterioration of concrete is closely related to the damage of the interface transition zone.The composition of the interface transition zone of concrete interface is significantly different from that of cement paste,and it is the weakest area in concrete.To study the deterioration characteristics of concrete,the microdamage in the interface area must be considered first.Therefore,it is of great significance to study the degradation of concrete in the study of concrete degradation based on the interfacial characteristics of the degradation of concrete under the drying-wetting cycles and sulfate attack.In this paper,sodium sulfate,magnesium sulfate and clear water were used as erosion solution.Concretes with different designed strengths(C30,C45,C60)were subjected to macroscopic and microscopic tests on deterioration of concrete under the effect of drying-wetting cycles and sulfate.Analysis of effects of sulfate species and concrete strength grade on sulfate ion migration rate.And through a layered slice method,a concrete interface transition zone sample was prepared.Atomic force microscopy(AFM)and scanning electron microscopy(SEM)were used to study the microstructure and characteristics of the interfacial transition zone of the concrete under the coupling of drying-wetting cycles and sulfate attack.Through research,it has been found that the deterioration of concrete by a single drying-wetting cycles can be divided into two stages: Sharply falling segments and slowly deteriorating segments.The concrete with higher strength has better anti-permeability,and the damage caused by drying-wetting cycles is small.Comparing with C30 and C45 concrete,C60 concrete enters the stage of slow deterioration in advance.The role of sulfate causes the formation of gypsum and ettringite inside the concrete,it compensates the destructive effect of the drying-wetting cycles on the concrete in the early stage,and the degree of compensation for the C30 concrete is higher than that of the C60 concrete.In the early stage of sulfate attack,Magnesium ion and concrete hydration produce a dense protective layer of magnesium hydroxide,which provides more compensation for the strength of larger porosityconcrete(C30 concrete).But no viscous dolomite is produced in the later stage,ultimately lead to the greatest deterioration of concrete in magnesium sulfate.The presence of magnesium ions will hinder the inward transmission of sulfate ions.Single drying-wetting cycles increases porosity in the interface area,the intrusion of sulfate causes the grain structure of the interface transition zone of the concrete to grow up,fills the transitional holes in the interface zone and sparse structures.The outer layer of the concrete is denser than the inner layer,and the sodium sulfate solution is denser than the magnesium sulfate solution.Sulfate reduces porosity,but on the other it extends the initial cracks in the interface zone.The compaction effect of sulfate on the pores can't compensate the damaging effect,and the cracking of concrete in the interface area causes the destruction of the overall structure.