Study On Three-Dimensional Numerical Simulation Analysis Method For Concrete Spalling Explosive Subjected To High-Temperature Based On Thermal Stress Theory

Concrete structure is one of the most common forms of building structures.when concrete structures are exposed to high temperature for a long time,the hightemperature mechanical properties of concrete materials will deteriorate significantly with the increase of temperature,and concrete spalling explosive subjected to hightemperature may occur at the same time.It leads to a significant reduction in the high temperature bearing capacity of structures or members,and may even lead to structural collapse,which brings a serious threat to the safety of buildings.In the study of early concrete spalling explosive subjected to high-temperature,the two-dimensional simplified model is often used to reconstruct the high temperature response of concrete meso-structure.However,in practical engineering,except for the three-dimensional problems which can be simplified to plane stress problems(such as thin plates)and plane strain problems(such as gravity dams),many problems are difficult to be simplified to two-dimensional problems.In particular,when subjected to high temperature,the burst mechanism of heterogeneous concrete is very complex,and the micro-spatial structure dominates concrete spalling explosive.On the other hand,the type,gradation and content of coarse aggregate play an important role in the microspatial structure of concrete,and also determine the distribution characteristics of nonuniform temperature field of concrete,making the mechanism of concrete spalling explosive show the characteristics of thermal stress failure.However,at present,the action mechanism of coarse aggregate on concrete spalling explosive subjected to hightemperature is still unclear.In this thesis,considering the influence of coarse aggregate on concrete micro-spatial structure,a three-dimensional random aggregate model of concrete is established by using general finite element analysis software,and the burst process of concrete in high temperature is simulated and analyzed.the influence mechanism of coarse aggregate on concrete spalling explosive subjected to hightemperature is discussed.The main research work is as follows:(1)According to the Monte Carlo theory,a three-dimensional model of random spherical aggregate of concrete is established,and the aggregate generated is uniform and dense,which satisfies the characteristics of random distribution of aggregate in space.(2)The rationality of the three-dimensional model of random spherical aggregate and the validity of material mechanical parameters and thermal parameters are verified by comparing the numerical simulation results with the results of uniaxial compression test and high temperature test.(3)A method for simulating the burst of concrete structures at high temperature is proposed.At high temperature,the closer the concrete test block is to the fire surface,the deeper the bursting degree is,the more obvious the burst phenomenon is,and with the continuous increase of temperature,cracks of different sizes and shapes are derived from the area where peeling off is just beginning and extending around.Compared with the mortar part,the aggregate part is difficult to destroy at high temperature because of its high strength,and the shape maintains a certain integrity.As the weakest part of concrete,the interface transition zone is damaged to a certain extent.(4)The effects of aggregate size,aggregate content and aggregate properties on concrete spalling explosive subjected to high-temperature are compared and analyzed.The larger the aggregate size is,the faster the internal temperature transfer of concrete is.When the maximum particle size of aggregate is between 30mm-40 mm,the influence of aggregate size on the temperature field distribution of aggregate can be ignored.The larger the maximum particle size of aggregate is,the earlier the initial and secondary bursting occurs,the lower the critical burst temperature is,and the shorter the interval between primary burst and secondary burst is;when the aggregate content in concrete is less than 35%,the more aggregate content,the better the temperature transmission effect of concrete.When the aggregate content in concrete is 35%-40%,the proportion of aggregate in concrete has little effect on the distribution of internal temperature field of concrete.The aggregate content in concrete is 30% 40%,the critical temperature of the first burst is 172.8 ℃-197.9 ℃,the temperature of the second burst is 900 s,and the temperature of the second burst is 223.6 ℃-226.3 ℃.The proportion of aggregate has little effect on the secondary burst time of the concrete test block;the higher the thermal conductivity of the basalt concrete test block is,the better its macro thermal conductivity is.The type of aggregate has no effect on the first burst time of concrete test block.With the increase of temperature,the higher the thermal conductivity of aggregate and the better the thermal conductivity of concrete,the higher the critical temperature of initial burst.The research results of this thesis have good theoretical significance for revealing the burst mechanism of concrete at high temperature,and provide technical reference for further study of high temperature burst of concrete.