Study On Mechanism Of Cracking And Ion Propagation In Sulfate Attacked Concrete

Sulfate is widely distributed in marine environment and saline-alkali environment and sulfate attack is one of the main problems of concrete structure durability.Microcracks are easy to develop during the service of concrete structures.The presence of micro-cracks promotes the propagation of ions inside the concrete,which exacerbates the deterioration process of the concrete.This paper calculated the cracking concentration of concrete,which is based on the chemical reaction of ettringite formation,and studied the cracking mechanism of concrete under different external sulfate concentration and the development of concrete crack with attack cycle.The numerical simulation method is used to continuate the concrete surrounding crack and the systematic study of sulfate attack was carried out systematically.According to the test and literature investigation,this paper discussed and analyzed the main products of sulfate-eroded concrete,and determined that the main product of concrete corrosion by sulfate is ettringite.According to the various component fractions of the concrete formula table,the influence of the specificity of the ettringite crystal on the expansion was considered through the reaction of forming ettringite in the concrete pores.Then the expansion stress of the ettringite produced by each chemical reaction was determined.When the expansion stress is greater than or equal to the tensile strength of the concrete,crack occur on concrete.At this time,the concentration of the sulfate was defined as the cracking concentration of the concrete,that is,the sulfate concentration reached when cracks are generated inside the concrete.The concrete was immersed and eroded by two sulfate solutions with mass fraction of 7.5% and 10%.The sulfate ion concentration curve under various attack cycles was obtained by chemical titration test.The results of SEM observation cracks were compared to confirm the cracking,which correct the concentration calculation.Based on the comprehensive analysis of the existing sulfate attack and cracking mechanism,two different concrete cracking mechanisms was proposed,which are surface expansion cracking mechanism and delamination damage mechanism.This paper discussed the different causes of concrete failure caused by the two mechanisms,and clarified the method to distinguish the two concrete cracking mechanisms.The experiment investigated the tendency of different cracking mechanisms of concrete under different ages and different external sulfate concentrations,and selected four kinds of environmental attack ion concentrations with mass fractions of 0%,2.5%,5%,7.5%,and 10%.Sampling studies was carried out with two attack depths of 3 mm and 6 mm.The damage cracking of concrete with different concentrations and different ages was observed by scanning electron microscopy.It was found that in the specimens with the external sulfate concentration of 0%~5%,the concrete is delaminated damage;in the specimen with the external sulfate concentration of 7.5%~10%,the concrete is surface expansion cracking damage.As the age increases,the cracking mechanism of the concrete does not change.The existence of cracks in cracked concrete affects the propagation of sulfate ions near the cracks.Based on the influence of chemical reactions and simplified crack models,this paper established the governing equations for the propagation of sulfate ions in concrete near cracks in cracked concrete.The finite difference method was used to solve the solution,and the simulated concentration curve of sulfate ion were obtained.The calculation results was compare with the sulfate ion test data curves under the same conditions,and the reliability of the propagation model was verified.By introducing the crack influence coefficient and the crack affected zone,the influence of crack on the propagation of sulfate in concrete was quantified.