Study On Durability Of Subway Concrete Structures Under High Salty Soil In High Altitude Region Of Northwest China

The high altitude salinized soil environment in northwestern China,represented by Xining City in Qinghai Province,is a coexistence environment of high concentration chloride,sulfate and other strong corrosive media.It is a strong alkali environment with large alkali content.The durability of concrete structures in metro projects is diverse and complex in this region.Based on the Xining traffic track project,6 types of high performance concrete(HPC)are prepared with the concrete structure of metro project under high altitude salinized soil in Northwest China as the research object.The research on the performance of HPC has been carried out,including corrosion resistance,chloride ion diffusion,alkali-aggregate reaction(AAR)and its inhibition,as well as prediction of service life of concrete structures in metro project.The results show that the six HPCs which have long-life characteristics have the ability to resist the composite destruction caused by chloride diffusion,sulphate corrosion,and AAR.The research results can be used as an important reference for the durability design and safety assessment of the concrete structures of metro projects in high altitude salinized soils in Northwest China.The main research work and achievements are as follows:In the first chapter,it summarizes the significance and progress of research on the concrete structure of subway engineering in the harsh environment of saline soil in the northwest high altitude region.Further,according to the negative influence factors of the durability of concrete structures in the environment of high-altitude saline soils in Northwest China,the current problems are pointed out,and on this basis,the content of this paper is proposed.In the second chapter,according to the service-life requirements of concrete structure in the saline soil of northwestern China,the proportioning design and preparation of HPC were completed by several kinds of optimization designs,and the proportioning design of mortar-bars specimens based on HPC proportioning was completed.In addition,the related test methods and test calculation methods in this paper were introduced.In the third chapter,the performance of HPC in corrosion medium brine was determined by the HPC natural diffusion test.Moreover,the AAR activity of aggregate was determined using accelerated mortar-bars test(AMBT)and petrographic analysis.The analysis of ions concentration of HPC at different depths by plasma spectrometer showed that,the key constraints to determine the durability of concrete structures in 100 years of service life in high altitude saline soils in Northwest China are as follows:alkali-silica reaction(ASR),sulfate corrosion damage,as well as steel corrosion caused by chloride ion diffusion.Sulfate ions,alkali metal ions,etc.are highly contained in concrete,and their diffusion behavior in HPC is similar to that of chloride diffusion.In the fourth chapter,based on the high concentration of chloride ion in the saline soil of Northwest China,natural diffusion method was used to study the chloride ion diffusion in HPC.The results showed that the adsorption of chloride ion in HPC belongs to the linear adsorption relationship.The contents of supplementary cementitious materials(SCM),water-cement ratio and rust-inhibiting admixture all affected the chloride ion binding ability of HPC.In addition,the development of Cs,the free chlorine ion concentration on the surface of HPC,conforms to the boundary conditions of Cs=kt 1-m/2+C0,which will be used as the basis for predicting the chloride ion diffusion life.In the fifth chapter,according to the characteristics of strong alkali environment of saline soil environment in northwestern China,test method to extend the age of AMBT was used to investigate the effect of SCM,Li2CO3 and a new low-cost nitrate additive called MN used as inhibition of ASR.Finally,the applicability of AMBT was evaluated by the long-term test results.The results of SCM inhibition showed that,fly ash(FA)and ground granulated blasted furnace slag(GGBFS)with low content of CaO could inhibit ASR continuously under high temperature alkaline solution;and low levels(less than 5%)of silica fume(SF)may have a negative effect on ASR inhibition,and made recommendations for follow-up studies of SF.In addition,Ca/Si(molar ratio)in the cementitious system can effectively measure the inhibitory effect of SCM on ASR,the lower the Ca/Si,the lower the expansions of the mortar bars,and it is also confirmed in the long-term test results.The Li2CO3 inhibition results showed that,the alkaline infiltration duo to the environment of strong alkali,seriously affecting the inhibition of Li2CO3.Single doped with[Li]/[Na]=0:80 of Li2CO3,can not effectively inhibit the ASR.However,the complex doping of Li2CO3+SCM,then showed good inhibitory effect.The results of MN inhibition of new admixture showed that,the inhibition effect of MN with single admixture was not good.As dosage of MN was 1.0%,MN+SCM complex incorporation showed excellent inhibition,which fully exceeded the SCM inhibitory effect.According to the research,the optimal dosage range of MN is from 0.75%to 1.25%.Finally,the applicability of AMBT is evaluated.The results showed that,the results of AMBT and the results of prolonged age were different;the prolonged age test method can truly reflect the pros and cons of each inhibition scheme.A criterion(14d expansion rate of not more than 0.03%)in GB/T 50733-2011 is too strict,not as a refusal to suppressing program on ASR.In contrast,the criteria in TB10424-2010 and ASTM C1567 were appropriate.Furthermore,AMBT can evaluate the effect of ASR inhibition in strong alkaline environments and in common alkaline environments.The scope of application should include SCM and chemical admixtures.In addition,the microcosmic morphology analysis of SCM mortar bars shows that SCM can effectively inhibit ASR in a longer age,but can not permanently inhibit ASR in a strong alkali environment.The use of air-entraining agents can effectively relieve the rate of ASR expansion failure,but it is not the main control method to suppress ASR.In the sixth chapter,based on the diffusion model and the reliability theory,the service life of the concrete structure under the 3 kinds of destructive effects of chloride ion diffusion,AAR destruction and sulfate corrosion was predicted.Moreover,the theoretical model of damage-reaction velocity is put forward to predict the service life of AAR,and the service life of sulfate corrosion was predicted by using 2 empirical models(Atkinson and Hearne empirical model and Kurtis empirical model).Combined with the life prediction results of various theoretical models,the service life of concrete structures is comprehensively evaluated.The results are shown that,AAR has the strongest destructive effect on the durability of concrete structures and it accelerates the chloride ion erosion damage and sulfate corrosion damage of concrete structures to shorten the life of concrete structures further.Finally,the key factors to ensure 100a service life are proposed,including increasing the thickness of protective layer of concrete reinforcement,strictly controlling the initial alkali content of concrete,effectively controlling the alkali infiltration in the external environment,selecting the cement with low-content C3A(less than 3%),low water-cement ratio(recommended w/c take 0.37 or less),and additional waterproof coating on the surface of concrete components,further,the pile foundations,tunnels and other structural parts need the backfill soil improvement or other measures.In the seventh Chapter,it summarized the research work of the full text,and put forward some suggestions for the future research.