Deterioration Mechanism And Life Prediction Of Damaged Concrete In Sulfate Environment

With the implementation of the national strategy of “One Belt and One Road” and “Western Development”,a large number of concrete structures are used in the harsh salt lake environment in the west.Meanwhile,as a multi-phase and multi-component composite materials,concrete often produces initial defects and damage during the hydration hardening stage and service period,which cause concrete vulnerable to sulfate attack,volume increase,concrete cracking.Furthermore,with the continuous accumulation of damage,the speed of transmission-erosion damage-cracking process is intensified until the concrete failure.However,the current design of concrete durability is mostly based on the research results of intact concrete,which overestimates the service life of concrete structures.Therefore,the deterioration law of concrete with different damage degree in the harsh environment of the west is studied,and the influence of different factors on the sulfate transport behavior of concrete is discussed.The model of concrete life prediction under the sulfate environment is established,which has great theoretical significance and practical value for the durability design of concrete.This paper focuses on the deterioration mechanism and life prediction of concrete with initial damage in sulfate environment.Meanwhile,reveals the deterioration law of concrete under different influencing factors and analyzes the time-varying regularity of erosion products in concrete during sulfate attack.At the same time,the evolution rules of space cracks and pore structures in cement paste during the process of erosion were quantitatively characterized.The temporal and spatial distribution of sulfate ions in concrete was tested,and the life prediction model of concrete with damage degree in sulfate environment was established.Furthermore,the rationality and reliability of the model were verified by combining experimental data and engineering cases.The main research conclusion and innovation results are as follows:(1)The cyclic loading method was used to realize the different damage degree of concrete,and a method for quantitatively characterizing cracks in concrete with damage was proposed.Furthermore,the determination method of sulfate ion content in concrete was explored.The results show that the crack volume fraction of concrete with damage degree of 20% is 2.55 times that of concrete with damage degree of 10%;The sulfate ion content in concrete can be accurately measured by gravimetric method.(2)The damage and deterioration process of concrete subjected to sulfate attack under drying-wetting cycling was studied.The deterioration evolution law of damage degree,waterto-cement ratio and mineral admixture on concrete was analyzed.Meanwhile,the pore structures and erosion products of concrete were studied by various modern micro-test techniques XRD,MIP,CT,SEM and EDS.At the same time,the relative dynamic elastic modulus is used as the evaluation index of cumulative damage model of concrete,and the cumulative damage model of concrete with erosion time is established,which is verified by experiments.(3)The damage and deterioration process of concrete subjected to sulfate attack under full immersion was studied.The deterioration evolution law of damage degree,water-tocement ratio,mineral admixture and sulfate solution(concentration and type)on concrete under full immersion environment was analyzed.Furthermore,microscopic testing techniques XRD,MIP,CT,SEM and EDS were used to analyze the evolution of erosion products and microstructure of concrete under sulfate attack.The degradation mechanism of concrete under sulfate attack in long-team immersion environment is revealed from marco and micro aspects.The results show that the increase of initial damage degree,water-to-cement ratio and sulfate solution concentration can significantly aggravate the concrete deterioration.In addition,the mineral admixture can effectively improve the sulfate resistance of concrete.The presence of magnesium ions accelerates the damage and deterioration of concrete.When chloride ions coexist with sulfate ions,the formation rate and content of ettringite are reduced,which significantly delays the erosion of sulfate attack to concrete.(4)The damage process of cement paste exposed to sulfate solution was in situ quantitatively investigated by X-CT.The evolution laws of spatial crack distribution,statistical gray value,two-dimensional section structure,three-dimensional pore structure and erosion depth were obtained.The relationship between the crack volume fraction and erosion time is established;meanwhile,the relationship between erosion depth and erosion time is established,which provides a scientific basis for durability design,life prediction,maintenance of concrete structures.(5)The effects of damage degree,water-to-cement ratio,mineral admixture,concentration and type of sulfate solution on the transport characteristics of sulfate ion in concrete were studied by gravimetric method.Based on the experimental results and Fick's second law,the diffusion coefficient and sulfate solution concentration,strength grade,initial damage degree and erosion time were established.Based on the constitutive relation of concrete erodedby sulfate,and according to the fracture criterion of concrete,the life prediction model of concrete in sulfate environment is established.Furthermore,the model is verified by the experimental results and the case of harsh environment.The results show that the model can predict the service life of concrete structure in harsh environment.