Erosion Resistance And Mechanism Of Recycled Aggregate Concrete Under Multi Coupling Factors

Recycled aggregate concrete(RAC)is a new type of concrete adding recycled coarse aggregate produced by crushing and screening of waste concrete partial or total instead of natural aggregate into the mixture.Nowadays,the study of RAC is a research focus in civil engineering field,especially the study of the RAC durability,which has important theoretical and practical significance for the application of RAC in concrete structures.The microstructure alteration of concrete including multiple interfacial transition zones caused by the recycled coarse aggregate has great influence on the evolution of microstructure damage,the transport process of erosive ions,initiation and extension process of cracks under loading,and wetting-drying process of RAC.Thus,the damage evolution and performance degradation of RAC under multi coupling factors is the core problem of the study of the RAC durability.This study will provide important theoretical and practical significance for service life design of the RAC structure.The main study contents and results in this dissertation are listed as follows:(1)Parameters of the RAC durability experimentThe erosion regimes selected in this paper are consist of long-term immersion in the salt solutions,wetting-drying cycles coupled with salt solutions,and wetting-drying cycles combined with loading and salt solutions.The salt solutions are 5%sodium sulfate solution and 3.5%sodium chloride solution respectively.The proper equipment are used for applying flexural loading,and the stress ratio are 0.3 and 0.5.Finally,the mechanisms of performance degradation and erosive ions transport of RAC are studied under single-factor,dual-factor or multi-factor.(2)Mechanism of performance degradation of RAC under multi-factorThe damage process of RAC and sulfate ions transport in RAC under sodium sulfate solution coupled with wetting-drying cycles,or under sodium sulfate solution combined with wetting-drying cycles and loading are investigated in this paper.The results indicate that the contents of water-soluble(free)sulfate ions first decrease and then increase with the increase of the replacement of recycled coarse aggregate.However,the reaction coefficients of sulfate ions always increase.The relationship between the contents of reaction sulfate ions and the contents of acid-soluble(total)sulfate ions is linear.The replacement level of recycled coarse aggregate would not change the damage process of RAC deterioration under sulfate attack,which is still consist of the initial damage stage,the linear increasing stage and the decreasing stage.But the replacement level of recycled coarse aggregate shows obvious influence on the initial and accelerated damage velocity.The extension of micro cracks in RAC due to the applied loading accelerates the process of sulfate erosion increasing with the increase of stress ratio.The decrease of water to cement ratio and incorporation can improve the resistance of RAC against the sulfate attack.(3)Mechanism of chloride ions transport in RAC under multi-factorThe contents of chloride ions,the chloride diffusion coefficient and the chloride binding capacity of RAC under sodium chloride solution coupled with wetting-drying cycles,or under sodium chloride solution combined with wetting-drying cycles and loading are investigated in this paper.The results indicate that the contents of chloride ions and the chloride diffusion coefficient of RAC first decrease and then increase with the increase of the replacement of recycled coarse aggregate.The relationship between the chloride diffusion coefficient and replacement is quadratic polynomial.The chloride binding capacity is increased with the increase of the replacement of recycled coarse aggregate.The contents of chloride ions and the chloride diffusion coefficient significantly increase as the loading applied,and the relationship between the chloride diffusion coefficient and stress ratio can be described by quadratic function.However,the influence of stress ratio on the chloride binding capacity is no regularity.(4)Transport model of chloride ions and damage model under sulfate attack of RAC under multi-factorThe estimation of chloride ions transport path in molded RAC by X-ray computed tomography(X-CT)shows that the transport path depends on relative relationship between the compactness of the new mortar and the old attached mortar.Also,the transport model of chloride ions in RAC under multi-factor is established according to Fick's second law.Moreover,the replacement of recycled coarse aggregate,water to cement ratio,stress ratio,chloride binding capacity and time dependence are taken into consideration in this model.The mathematical model can be used to describe the damage process of RAC under sulfate attack with coupling factors.According to the basic principle of damage mechanics,the relative dynamic elastic modulus is selected as the damage variable and the two stage damage model is established.The first stage is linear dependence and the second stage is parabolic relation.The physical meanings of regression parameter in this model are specified.