Durability Research And Service Life Prediction Of Aeolian Sand Concrete Under Complex Environment

Numerous desert and sandy land as well as abundant aeolian sand(also known as desert sand)resources distributed in the Northwest China and North China,which had complex climate conditions,belonged to the cold,arid and semi-arid areas and contained widely distributed chloride,sulfate,carbonate etc.Issues such as shortage of raw materials,difficulties in transportation,high cost and the urgent need for alternative products of concrete raw materials confronted many concrete projects in the hinterland of the desert and areas near it.The extremely complex service environment of concrete structure resulted in the premature damage and deterioration of concrete,the premature failure of some components and the decrease in durability,which seriously threatened the structural safety of buildings during their service period.Based on detailed investigation and the environmental characteristics of Northwest China and North China,a test system for durability of concrete under the action of complex factors was established in this paper to carry out accelerated simulation of various factors leading to the damage and failure of concrete;the design of the aeolian sand concrete(ASC)ratio under the influence of harsh environment was optimized based on the fact that the canal lining and slope protection were widely used in irrigation areas in Hetao of Inner Mongolia near the desert area;the damage evolution equation of different ASC under the different environmental conditions was established through the research on the rule of damage and failure of the ordinary concrete(OPC)and ASC with the different dosages(20%,40%,60%,80% and 100%)under the action of the single,double and multiple damage factors of the chloride salt and sulfate;besides,the binding law and binding ability of ASC in different chlorine salt environment were summarized through the research on the diffusion law of chloride ion in ASC in chlorine salt environment;the carbonization law of ASC in atmospheric environment was studies as well.An in-depth analysis of the damage mechanism of ASC in the complex environment was carried out through qualitative analysis of the corrosion products in different corrosion solutions and corrosion time by means of SEM,XRD,NMR and other microscale measurement means,through which it established a new life prediction model of ASC structure based on carbonization in atmospheric environment and the theory of chlorine ion diffusion in chlorate environment based on Fick's law.(1)The application status,research background and significance of concrete in the hinterland of desert and areas near the desert and the latest research progress of the prediction method for the durability and service life were reviewed.Problems to be solved in current research were pointed out.The main research direction,research contents and technical route of the paper were clarified.(2)A detailed analysis of the engineering properties of aeolian sand was carried out where the aeolian sand in the middle of Kubuqi Desert was taken as the research object;the design of the ASC mix ratio was optimized in combination with the fact that the canal lining was widely used irrigation areas in Hetao followed by the analysis of the physical and mechanical properties,phase composition,microstructure and pore structure of ASC.The results showed that the 28 d compressive strength of the high-fluidity ASC with the water gel ratio of 0.55 and the strength grade of C25 was only 100% ASC,which was slightly insufficient,yet the 28 d split tensile strength of which was all qualified.The main hydration products of the ASC thereof included plate-like C-H crystals,granular CSH gel,some hexagonal flake AFm and a bit of AFt.It was showed by the analysis of bubble spacing that the pore structure of the ASC thereof mainly included large size transition pores and pores.The independent macropores of(0.1~5)?m were observed after the 100%ASC had been amplified by 100 times via SEM.(3)A continuous record of the loss of appearance and quality and changes of relative dynamic elastic modulus of ASC under the action of single,double and multiple factors was made where two kinds of systems,i.e.,freeze thawing and drying and impregnation were adopted and the sulfate and chloride solution were taken as the environmental solution in this paper.The research on the law of damage and failure of ASC under the action of single,double and multiple factors was carried out and the analysis of changes of pore structure,crystal structure and hydration products was conducted via the microcosmic microscale test.The results showed that under the action of freeze-thaw,the large the dosage of the aeolian sand,the better the ASC durability,among which the anti-dry and wet ability of 80% ASC was the most ideal.The ASC damage mechanisms varied with the different action mechanisms because of the positive and negative effects of frost heave pressure,steam expansion pressure and salt crystallization pressure on ASC damage and the differences of ASC pore structures with different aeolian sand content.(4)The law of chloride ion diffusion of ASC was studied through the tests of the free chlorine ion concentration and total chloride ion concentration of ASC under different working conditions via the chemical method.The results showed that the chloride binding of ASC under different working conditions took on the Langmuir nonlinear adsorption.The chloride binding capacity of ASC was summarized and the conclusion that the factors such as aeolian sand content and environment had great influence on chloride ion diffusion coefficient and surface chloride ion concentration was made.(5)A more complete ASC life prediction system was formed by the establishment of a new life prediction model of ASC based on carbonation after the influence factors such as the aeolian sand content and the thickness of the protective layer of the concrete had been taken into consideration on the basis of experiment,theoretical analysis and formula deduction and a new life prediction model based on chloride ion diffusion theory of ASC after the time dependent boundary conditions of chloride ion binding capacity,chloride ion diffusion coefficient and surface chloride ion concentration had been taken into consideration and the construction of the w,the new damage degree of ASC which was capable of more accurate,objective and comprehensive evaluation of ASC injury in the freeze-thaw environment.The results showed that the prediction model could carry out more accurate prediction of the service life of the newly built or established ASC project and provide theoretical guidance for the durability design of ASC.