Microstructure Evolution Of Concrete Under Multi-actions And Chloride Transport Modelling

Concrete is the most widely used construction materials in the world(second only to water in total volumes consumed annually by society),especially the marine concrete,as well as a kind of high energy consumption and high pollution materials,is the main killer of global environmental pollution.Considering the environmental cost,emphasis should be placed on enhancement way of concrete durability,to save global resources and minimize environmental burden.With Concrete being used in a wide variety of structures and applications in marine environment for many decades,the durability of concrete in marine environment face a big challenge due to the aggressive marine environment and the long service life that is expected for marine infrastructures.Aiming at the exploring durability of concrete in marine,the environmental factors concrete suffering in marine were simulated,concrete under multi?actions of soluble salts,dry-wet cycles and loading was systematically investigated from the three aspects including microstructure evolution,deterioration process and chloride transport,and eventually,chloride transport models under the multi-actions were established in this study.The main contents and achievements are as follows.In the present thesis,the damage process of concrete under the Multi-actions of soluble salts,dry-wet cycles and loading was evaluated by the relative dynamic elastic modulus change mass.The results indicated that the temperature during drying process is the key factor that cause deterioration for concrete;in the case of drying temperature is certain,the dry/wet ratio is the key factor causing deterioration for concrete,the higher dry/wet ratio,the more severely deterioration;the higher content of sulfate in composite salts solution could relieve the damage for concrete,when the concrete suffer from dry-wet cycles,as well as dry-wet cycles combined loading.Additionally,the addition of mineral admixtures in concrete could enhance the resistance for concrete to multi-action of soluble salts,dry-wet cycles and loading.The change of micro-structure and corrosion productions in cement pastes exposed to soluble solution were detected by X-CT,SEM,XRD and TG/DSC.The results showed that the presence of sulfate could reduce the formation of Friedel's salts,and the presence of chloride could reduce the formation of expanded substance,such as ettringite,when concrete exposed to composite salts solution under dry-wet cycles.The addition of mineral admixtures could enhance the chloride combined capability and promote the productions of Friedel's salts.From the chloride profiles for concrete under multi-actions,combined with grey relation analysis,it is concluded that the dry-wet cycles is the dominated factor that influnce the chloride penetrate compared to loading,due to the higher temperature during drying stage.Moreover,chloride transport in concrete was restricted by sulfate when concrete exposed to composite salts solution(the presence of chloride and sulfate).The theoretical derivation of moisture transport meodel for concrete under dry-wet cycles was presented,as well as the definition to parameters in the model.The theoretical moisture transport model is:(1)Moisture distribution after drying:,integration solution to the equation,Moisture loss during drying could be expressed:;(2)Moisture distribution after wetting:c(x,t)= Cmax-c(x,td),integration solution to the equation,moisture gain could be expressed:.Additionally,the water diffusion coefficient Dd and water absorption k could be obtained by drying test for concrete and water absorption test for concrete.And the moisture contend could be determined through sorption isotherm curve.The chloride transport model in concrete when it subjected to the action of single soluble salts solution and dry-wet cycles was established based on the moisture transport model for concrete under dry-wet cycles.Furthermore,the environmental deterioration factors(such as composite salts,loading)were integrated to the chloride transport model step by step,finally,the chloride transport in concrete was simulated when it subjected the multi-action of soluble salts,dry-wet cycles and loading.It should be emphasized that the transition from single salts to composite salts was turned to water activity and the loading factor problem was solved by the function of diffusion coefficient depending on loading rate.