Study On Flexural Behavior Of Reinforced Concrete Beams Strengthened With Steel Plates At Elevated Temperatures

Reinforced concrete beams strengthened with steel plates have been widely used in engineering practice in recent years due to the advantages of excellent reinforcement effect,fast construction speed,no on-site wet work,strong adaptability and high economic benefit.A lot of scholars at home and abroad have studied the mechanical properties of RC beams strengthened with steel plates at room temperature,but there is very little research on the mechanical response of the beams at elevated temperatures.The structural cement,gradually exiting from the work with the increase of temperature,shows poor fire resistance at elevated temperatures,which leads to the loss of bond between steel plates and concrete,which leads to potential safety hazard.The fire test is complished to explore the mechanical response of the beams at elevated temperatures,and parameter analysis are conducted in this paper to study the mechanism of the RC beams strengthend with steel plates,considering the lack of the research on the mechanical response of the beams at elevated temperatures.The main research contents and achievements include:(1)The fire test is complished to explore the mechanical response of the beams at elevated temperatures and the test results are analysised,which provides reliable data support for the numerical analysis.(2)The FE models of RC beams strengthened with steel plates are established to study the influence of thermal parameters of concrete and steel,fire insulation,emissivity coefficient,film coefficient and thermal contact of steel-concrete interfaces on the temperature field of the beams.The predicted results of the FE modles are compared with the test results to verify the validity of the FE models,which show the extremely uneven temperature distribution of the beams in the longitudinal direction because of some structural measures and the differences of thermal properties of materials.The predicted results also show a slower warming trend of concrete because of its themarl inertia when compared with steel plate in the same section of the beams.The temperature development of the beams is more related to the emissivity coefficient and film coefficient than the thermal contact at the steel-concrete interfaces and the moisture content of concrete.The specific heat of concrete recommended by the European code(EN1992-1-2)can better predicte the development of temperature of the beams and the fire insulation can effectively protect the beams from high temperature.(3)The time-deflection curve of the mid span under high temperature is calculated from the sequentially coupled thermo-mechanical analysis,considering the damage plastic under compression(CDP)of concrete and the reduction of strength and stiffness of materials at elevated temperatures.In order to get the ideal analysis results,a plenty of parameters analysis are conducted in this paper,which shows that the bond-slip relationship between the reinforced steel plate and concrete can be simulated well via the Coulomb friction model.The mechanical response of the beams at elevated temperatures can be well predicted when the thermal expansion coefficient of concrete given in ASCE and that of steel given in EN1993-1-2 are considered.For the beams without fire insulation,the dilation angle of concrete has little influence on the mechanical response of the beams in the early stage of fire,but has a great influence on the mechanical response of the beams in the later stage of fire.For the beams with fire insulation,the influence of the dilation angle of concrete on the mechanical response of the beams under high temperature can be ignored.The moisture content of concrete has a little influence on the temperature field and mechanical response of the beams.(4)26 numerical analysis models are established in this paper and parameter analysis are conducted on mechanism of RC beams strengthened with steel plates under high temperature.The results show: for the appropriate reinforced RC beams,although the deformation of the beams decreases with the increase of the area of tensile or compressive bars,the effect is very limited.The load ratio has less influence on the mechanical response of the beams with fire insulation,but has greater influence on the beams without fire insulation.The thickness of steel plates has little influence on the bearing capacity of the beams under high temperature when the same load ratio applied.The location and thickness of fire insulation have vital influence on the temperature development and mechanical response of the beams at elevated temperatures.The capacity of fire resistance of the beams can be effectively improved by applying fire insulation and increasing the thickness of the fire insulation.The fire resistance of the beams without fire insulation is more related to the strength of concrete than the beams with fire insulation when the same load ratios is applied.