Research On Fatigue Performance Of Concrete Under Large Temperature Difference Environment

In areas with large temperature difference,such as Inner Mongolia Autonomous region and Xinjiang Uygur Autonomous region,the concrete used in roads and bridges will not only be subjected to the repeated cyclic load of vehicles,but also bear the large temperature difference caused by seasonal change and day-night replacement.The range of temperature difference and climatic environment in large temperature difference areas are different from the test conditions of rapid freezing method in the Standard for Test methods for long-term performance and durability of ordinary concrete(GB/T 50082-2009).The negative temperature is lower(-50?)and the positive temperature is higher(70?)than that in the standard,and the climate is dry.The damage of concrete in these areas is more serious and the damage mechanism is more complex.In this paper,the experimental study on the fatigue performance of concrete,the analysis of fatigue damage mechanism and the establishment of prediction model are carried out according to the actual temperature difference range in large temperature difference areas.It has important theoretical significance and engineering application value for the mechanical performance evaluation,life prediction,maintenance and reinforcement of concrete structures in these areas.The main research work includes the following aspects:1.The bending strength tests of concrete(water-binder ratio 0.36,0.39,0.42)are carried out after 0,15 and 30 large temperature difference cycles(temperature difference range-50??70?)with two kinds of cooling rates(3 hours,6 hours).Fatigue tests at stress levels of0.80,0.85 and 0.90 are carried out on concrete(water-binder ratio 0.36,0.39,0.42)which experienced 0,15 and 30 temperature difference cycles(temperature difference range-50??70?).It is found that the bending strength of concrete decreases with the increase of water-binder ratio,temperature difference cycles and the cooling rate.At the same water-binder ratio,the fatigue life of concrete decreases gradually with the increase of temperature difference cycles.However,the fatigue life of concrete with different water-binder ratio is different.After 15 and 30 temperature cycles,the lower the water-binder ratio is,the smaller the fatigue life reduction is.2.The maximum tensile strain,fatigue strain and the average strain of the whole field of concrete specimens are obtained by using Digital Image Correlation(DIC)technology,and the bending and fatigue deformation rules of concrete are analyzed.Based on damage mechanics,the damage variables in the process of bending and fatigue deformation of concrete are defined.It is found that the more the temperature difference cycles and the higher the cooling rate,the more serious the damage to concrete.The defined damage variable reflects the damage degree of concrete specimens more comprehensively and intuitively.3.According to the effect of temperature difference cycles on the fatigue performance of concrete,the fatigue damage evolution model of concrete based on damage mechanics is modified.Using this model and the damage variable based on the definition of fatigue strain,the relationship between residual fatigue life and residual strain is established.Through this relation expression,the corresponding residual fatigue life of concrete can be obtained from fatigue life,fatigue strain,stress level and the number of temperature difference cycles,which provides a reference for the safety assessment of concrete structures in large temperature difference areas.4.The effect of temperature difference on pore structure is studied by Nuclear Magnetic Resonance(NMR)pore structure testing technique.It is found that the temperature difference has the greatest influence on the pore with the size of 0.1?27.98nm.The bending strength and fatigue life of concrete decrease gradually with the increase of the pore structure parameters,and the influence of temperature difference on the fatigue life of concrete cannot be ignored.The coarsening rate of pore structure is increased by the combined action of temperature difference and load,and the load has the greatest influence on the pore with the size of 524.26nm?10000nm.5.The characteristic parameters of three-dimensional pore structure are proposed,including the comprehensive pore size change rate S_Z,which comprehensively reflects the pore size change rate of concrete,the pore structure complexity coefficient C_Z,which comprehensively reflects the complexity of concrete,and the total porosity P_total.The characteristic parameters of three-dimensional pore structure characterize the pore structure of concrete more comprehensively from three different aspects.It is found that the effect of temperature difference on S_Z is the most significant,and the effect of load on C_Z is the most significant.The bending strength and fatigue life decrease with the increase of the characteristic parameters of three-dimensional pore structure.The correlation coefficient R²between the characteristic parameters of three-dimensional pore structure and the bending strength and fatigue life of concrete is very high,which provides a basis for predicting the bending strength and fatigue life of concrete.6.Considering the limited amount of concrete test data,the grey system theory is used to establish the NMGM(1,N)prediction model of bending strength and total porosity,the NMGM(1,N)prediction model of bending strength and pore porosity,and the NMGM(1,N)prediction model of bending strength and three-dimensional pore structure parameters.Co MPared with the models established by other scholars using a large amount of data,it is found that the NMGM(1,N)prediction model of bending strength and three-dimensional pore structure parameters has the highest accuracy.The NMGM(1,N)model of fatigue life and total porosity and the NMGM(1,N)model of fatigue life and three-dimensional pore structure parameters are established respectively,and the accuracy of the model is co MPared.The NMGM(1,N)model using three-dimensional pore structure parameters can predict the fatigue life of concrete more accurately.The application of grey system theory in the prediction of flexural strength and fatigue life of concrete provides a new basis and method for the research of concrete in the case of small amount of data.