Influence of Early Disturbance on the Concrete and Mechanism Analysis

Author:Pan Hui Min

Supervisor:zhao qing xin


Degree Year:2017





As a heterogeneous mixing material,the hardening of concrete depends on the formation of its early microstructure.The microstructure of concrete has not been fully formed before final setting,and it will be very sensitive to the influence of external disturbance before final coagulation.Disturbing force may cause tensile stress within the concrete,resulting in microcracks that can not be bridged,and affecting the strength and durability of concrete.In view of the influence of early disturbance on concrete performance,scholars at home and abroad have made some related explorations.The current research is mainly focused on the influence of disturbance on the macro mechanical properties of concrete.However,the formation mechanism of the microstructure of disturbed concrete is still unclear,and there are few reports on the damage evolution process in the course of service.Based on this,starting from the meso level of concrete,by studying the early disturbance of concrete,the influence of disturbance on the macro mechanical properties of concrete was studied,and the mechanism of its influence was revealed in this research.The influence of disturbance on sulfate attack process and acoustic emission characteristics of concrete subjected to sulfate attack and its failure process were evaluated by acoustic emission data acquisition.The damage evolution model of disturbed concrete under the action of sulfate attack and pressure load was established.In the test of the macroscopic mechanical properties of the disturbed concrete,the stage of the pre setting and final setting of the concrete was defined as "early stage".According to penetration resistance value,the early stage was divided into 5 stages.The influence of disturbed age and disturbance duration on the mechanical properties of concrete was studied by simulating the disturbance test of concrete.The test results show that compared with the reference concrete,the compressive strength and flexural strength of disturbed concrete at 28 days are reduced.When the disturbance force is in the middle of concrete setting(penetration resistance value 10.7~14.8 MPa),the maximum compressive strength and flexural strength are more than 25%.The influence of the disturbance near the initial setting and the near end condensing on the concrete is relatively small.The disturbed concrete structure will inevitably be in the environment of sulphate erosion.In order to study the influence of early disturbance on the sulfate attack performance of concrete,the performance change rule of disturbed concrete under sulfate attack was systematically described by using quality change rate and relative modulus of kinetic elastic modulus to evaluate the degree of deterioration of the disturbed concrete subjected to sulphate erosion.Through observing the surface of corroded concrete specimen and analyzing the microstructure of the surface and interior of the specimen,the influence mechanism of disturbance on the erosion process was studied.The results show that the quality of concrete and relative modulus of elasticity increase faster when concrete is disturbed by condensation and hardening in the medium term,and the quality of concrete reaches its maximum at 50 days after erosion.After 130 days erosion,the relative modulus of elasticity decreased sharply,and the rate of decline is obviously higher than that of other specimens.Disturbing force accelerates the deterioration of concrete under sulfate attack.To analyze the load damage characteristics of disturbed concrete,the pressure load of concrete in early stage disturbed by sulfate attack was applied.The acoustic emission characteristics and damage degree of corroded concrete under uniaxial compression were investigated by acoustic emission data acquisition.The influence of the disturbed ages on the peak stress,the elastic modulus,AE characteristics and damage development of concrete was analyzed.The experimental results show that the peak stress and modulus of elasticity of concrete are reduced in varying degrees,and the maximum reduction of concrete at the middle stage of hardening is the greatest.In the initial loading stage,the acoustic emission events of the disturbed concrete specimens are less,the energy release rate is small,and the acoustic emission active region shows a trend of backward movement.To study the performance changes and damage deterioration of early disturbed concrete during service process,based on the basic theory of concrete setting and hardening,the mechanism of the influence of disturbance on concrete performance was further analyzed.Starting from the initiation conditions of micro cracks produced by sulfate attack,the damage evolution model of concrete with different disturbed conditions under sulfate attack was established.Based on damage mechanics theory and acoustic emission principle,the damage evolution equation and constitutive equation of disturbed corroded concrete under uniaxial compression were established.The influence of early disturbance on the reliability index of concrete was analyzed.The results show that the disturbance can cause the internal structure of the concrete to bear the tensile stress and reduce the bonding force between the cement and the aggregate,which is the essence of the influence of the disturbance on the strength of concrete.Using micro crack propagation to characterize the mechanical behavior of concrete after corrosion damage,the damage model curve could reflect the damage evolution law of disturbed concrete sulfate attack.The deduced constitutive relation of concrete agrees well with the experimental results by using the number of AE events to characterize the damage degree of concrete.The bigger the initial damage is,the more significant the reliability index decreases.The change in the macroscopic mechanical properties of concrete is the result of the changes in the microstructure of the concrete.Based on the above findings,the conclusions of the study on the impact of disturbance on the mechanical properties and durability of concrete are more systematic than previous research results,which have certain academic significance and application value for the quality control of cast-in-place concrete construction.The damage evolution model considering the influence of disturbance can provide a reference for the damage assessment and reliability evaluation of early disturbed concrete in the service period.