Multi Physical Field Coupling Experiment And Finite Element Simulation Of Reinforced Concrete Columns

High performance concrete(HPC)is widely used in construction engineering for its high strength,high construction performance and high durability.However,in the fire,the physical and mechanical properties of HPC decline sharply,resulting in different degrees of damage to the building structure.Reinforced concrete column is the main load-bearing component of the building structure.The damage in high temperature environment is directly related to the safety and service life of the building.It is of great significance to study the deterioration damage of reinforced concrete columns under high temperature and the high temperature damage mechanism of high performance concrete to improve the fire resistance of components and buildings.Based on the National Natural Science Foundation of China(51478290),this paper studies the influence of different strength grades and PP fibers(plain concrete: C60,C80,polypropylene volume content: 0.2% fiber concrete: ppc60,ppc80)on the high temperature damage of reinforced concrete columns by the temperature load vapor pressure thermal strain multi physical field coupling test and finite element simulation analysis.The main research contents are as follows.Ⅰ.Experimental study on temperature load vapor pressure coupling of reinforced concrete columns.The deterioration damage law of reinforced concrete columns with different buried depth(25 mm,50 mm,75 mm)and different strength grades(C60,C80)under the coupling of temperature load steam pressure was tested by self-assembled steam pressure device.The results show that there are temperature gradient and vapor pressure gradient in concrete.C60 peak steam pressure range: 0.126mpa-4.02 mpa,C80 peak steam pressure range: 0.176mpa-1.68 mpa.PP fiber has a positive effect on restraining the peak steam pressure of concrete and delaying its high temperature damage.With the increase of fire temperature 20 ℃,100 ℃ and 200 ℃,the water pressure of concrete decreases.Under the same fire temperature,compared with plain concrete,the water pressure value of fiber reinforced concrete is larger.Ⅱ.Experimental study on temperature load thermal strain coupling of reinforced concrete columns.The damage law of reinforced concrete columns under the coupling of temperature load thermal strain multi physical field is studied by embedding transverse and longitudinal vibrating string strain gauge in reinforced concrete columns.The results show that the longitudinal thermal strain of C60,C80 and ppc80 keeps tensile strain all the time,and the longitudinal thermal strain of ppc60 changes from compressive strain to tensile strain at 33.8 ℃.At 39.7 ℃ ～ 56 ℃,the transverse thermal strain of C60 and C80 changes from tensile strain to compressive strain,while the transverse thermal strain of ppc60 and ppc80 keeps compressive strain all the time.Ⅲ.Simulation analysis of high performance concrete under temperature load steam pressure coupling.With the help of classical energy equation and mass equation,the law of relative humidity and vapor pressure of C60 and C80 concrete under the coupling of multiple physical fields is explored.The results show that the maximum error between the simulated value of vapor pressure and the experimental value is less than 28%.With the extension of the fire time,the relative humidity and vapor pressure in the concrete increased first and then decreased from the surface to the inside,and the saturated water layer gradually moved to the inside of the concrete.The relative humidity of PP fiber concrete is higher than that of plain concrete.Ⅳ.Simulation analysis of reinforced concrete columns under temperature load thermal strain coupling.The temperature,thermal stress,strain and displacement along the direction of gravity of reinforced concrete columns under the coupling of temperature,load and thermal strain are analyzed by using finite element software such as abuqus.The results of thermal strain experiment and simulation are compared and analyzed,and the following conclusions are obtained.Under the coupling of temperature and load,the simulation results of thermal strain and stress of reinforced concrete columns are basically consistent with the experimental results.The longitudinal thermal strain of reinforced concrete columns is mainly tensile strain,while the transverse thermal strain is mainly compressive strain.There are temperature gradient and stress gradient in concrete.The maximum value of thermal stress appears at the corner of column bottom.The displacement at the top of the column is the largest,and that at the bottom of the column is the smallest.Under the same fire temperature,the closer to the fire surface,the faster the change rate of displacement,and the farther away from the fire surface,the slower the change rate of displacement.In conclusion,the high-temperature burst damage of HPC is the result of the coupling of temperature load steam pressure and thermal strain,and the steam pressure is an important factor leading to the high-temperature burst of HPC.PP fiber plays an important role in reducing high temperature damage of HPC.