Study On The Transport-Degradation Mechanism Of SO₄^2- In Concrete Under Different Conditions

Sulfate corrosion leads to the reduction of bearing capacity of concrete structures,and the phenomenon of peeling,cracking and bare reinforcement can be seen everywhere.It seriously affects the safety,applicability and durability of concrete structures.Accordingly,in this paper,the transport behavior of sulfate ion in concrete is studied by means of experiment,theoretical derivation and numerical simulation for the concrete eroded by sulfate under different mechanisms,and the sulfate ion transport-degradation model in concrete is established.In the study of sulfate ion transport-degradation mechanism in saturated concrete,firstly,the chemical reaction mechanism of concrete by sulfate attack was studied in depth,and the two-stage chemical reaction equation of concrete by sulfate attack was established.Secondly,based on the simplified mesoscopic model of concrete,the volume fraction calculation formula of the interface transition zone is derived by applying the Fuller distribution and the Torquato distribution function theory.The particle size and interface are considered by chemical dynamics theory and ion diffusion theory.Mathematical model of sulfate ion transport in concrete in transition zone and chemical reaction rate is proposed.Finally,on the basis of the transmission model,from the chemical point of view and according to the corrosion mechanism of concrete,a new method for calculating the thickness of corrosive layer of concrete eroded by sulfate is put forward,and the rationality and correctness of the method are verified by comparing the experimental data.At the same time,through the expansion theory analysis,it is concluded that the method is safer for engineering application.In the study of sulfate ion transport-degradation mechanism in unsaturated concrete,a semi-theoretical semi-empirical calculation model of capillary absorption coefficient of water transport is obtained by moisture transport test and theoretical derivation,and based on the theory of water transport in porous media.A two-stage model for moisture transport in unsaturated concrete is proposed.By studying the influence mechanism of water transport on sulfate ion transport,a sulfate ion transport model in unsaturated concrete was established by theoretical deduction.The numerical solution of the derived theoretical model is compared with the measured data by finite element method,which ensures the validity and correctness of the theoretical model.Based on the transmission model,a calculation method of corrosion layer thickness of unsaturated concrete eroded by sulfate is proposed.In the study of sulfate ion transport-degradation mechanism in concrete under the coupling action of compressive stress and sulfate,the sulfate ion experiment in concrete under the coupling action of compressive stress and sulfate was carried out by refitting the external loading test device.The experimental results show that with the increase of compressive stress,the whitening and crystallization of concrete surface decrease,the deterioration degree decreases,and the sulfate ion transport rate is inhibited.In theoretical research,according to the theory of elasticity,a general theory for calculating the coefficient of variation of concrete porosity under compressive stress is deduced.By introducing the variation coefficient of concrete porosity under compressive stress,a sulfate ion transport model in concrete under coupling action is established,and the validity and correctness of the model are verified by comparing with the measured data.According to the transmission model and the deterioration mechanism of concrete,the calculation method of concrete corrosion layer thickness under coupling is proposed,and the damage analysis of concrete is obtained by compressive stress.This model is suitable for compressive stress ratios of not more than 0.4.