Study On Seismic Performance Of RC Columns Strengthened With Fiber Grid ECC After Fir

Fire can reduce the mechanical properties of concrete structures and make them unable to meet the seismic code requirements.Reinforced concrete columns,as important load-bearing members of structures,can be strengthened to restore their seismic performance after fire.In this thesis,we propose to reinforce the fire-damaged concrete columns by Engineered Cementitious Composite(ECC)with fiber grids to study the seismic performance of the reinforced concrete columns.The reinforcement method can improve the seismic performance of fire damaged reinforced concrete columns by taking advantage of the tensile hardening,multiple crack development,good deformation capacity and energy dissipation capacity of ECC,as well as the light weight,high strength and durability of fiber grids.The main research contents and conclusions of this thesis are as follows:(1)In order to study the interfacial properties of ECC and post-temperature concrete,ECC-post-temperature concrete bimaterial specimens were designed and fabricated,and the effect of the maximum temperature experienced by the concrete on the fracture properties of the bimaterial interface was investigated by wedge splitting test.It was found that when the maximum temperature of concrete was 400 ℃,the damage mode of ECC-high temperature concrete specimens changed from interfacial damage to mixed damage;the fracture toughness showed a trend of increasing and then decreasing with the maximum temperature of concrete and reached the maximum value at 400 ℃;the fracture energy also reached the maximum value when the maximum temperature of concrete was 400 ℃,and decreased rapidly when the temperature was higher than 400 ℃.The fracture energy also reaches its maximum at the maximum temperature of 400 ℃ and decreases rapidly when the temperature is higher than400 ℃.(2)Ten reinforced concrete columns were designed and fabricated,and fire tests were conducted on nine of them.It was found that mortar spalling occurred on the surface of each specimen after the fire.The temperature field of each specimen section was measured and it was found that the corner of the specimen warmed up faster;the further away from the specimen surface,the more obvious the phenomenon of time lag in the maximum concrete temperature appeared;the temperature plateau appeared when the temperature of the measurement point was about 100 ℃.Temperature field simulation based on ABAQUS finite element analysis software,the temperature of each measurement point of the test matches the temperature of the corresponding position of the simulation,in preparation for the subsequent simulation of the mechanical properties of the reinforced column after the fire.(3)The fire damaged specimens were reinforced with carbon fiber grid reinforced ECC and then subjected to the proposed static test.The hysteresis curve and skeleton curve of the specimens were obtained through the proposed static test,and the influence of fire time,thickness of reinforcement layer and number of layers of fiber grids on the bearing capacity,ductility,stiffness and energy dissipation capacity of the specimens were analyzed.The following conclusions were obtained: all specimens were bending damage;with the increase of fire time,the bearing capacity,ductility,stiffness and energy dissipation capacity of reinforced concrete columns decreased;when other conditions were the same,the mechanical performance of reinforced columns with long fire time was better than that of reinforced columns with short fire time;comparing the bearing capacity,ductility,stiffness degradation and energy dissipation capacity of specimens with different reinforcement layer thicknesses,10 mm reinforcement layer thickness was the best in terms of economic and performance repair.The test specimens with axial compression ratio of 0.3 have higher bearing capacity,lower ductility,faster stiffness degradation and poorer energy consumption capacity than those with axial compression ratio of 0.15.(4)Based on ABAQUS finite element software,the one-way pushover simulation of the test specimens was carried out by the thermal-force coupling method.It was found that the bearing capacity test values of the comparison column at room temperature and the unreinforced comparison column after fire were less than 10% of the simulated value of bearing capacity under one-way pushover simulation.The error between the simulated value of bearing capacity of reinforced column after fire and the test value of bearing capacity of reinforced column is larger,but the error is not more than 15%.