Study On Fire Resistance Of Bridge Strengthened By Prestressed CFRP Tendons Under Fire

In recent years,the construction of traffic facilities is continuously carried out,and the number of bridges is increasing with the rapid development of the global economy.With the increasing traffic pressure,the fire risk of bridges is also increasing,and Bridge fire accidents happen frequently.This poses a great threat to the safety of the bridge structure.Serious bridge fire accidents will lead to traffic paralysis,a large number of casualties and permanent damage to the bridge structure.Carbon fiber reinforced polymer(CFRP)has the advantages of light weight,high strength,corrosion resistance and fatigue resistance.It has a good prospect in the field of engineering structure reinforcement.However,the fire resistance of CFRP is very poor.In the presence of oxygen,when the temperature is higher than 400℃,obvious oxidation will occur,which seriously affects the mechanical properties of CFRP.Therefore,it is necessary to study the fire resistance and design of CFRP materials if it is to be widely used in the bridge and building structure reinforcement which need fire protection design.The following are the main research contents and achievements of this paper:(1)The study of the mechanical properties temperature relationship model of CFRP tendons is the premise of carrying out the fire protection design of the bridge strengthened by prestressed CFRP tendons.The mechanical properties of CFRP tendons without fire-proof material protection at room temperature and high temperature were studied.Use the self-developed clip type anchorage to anchor the CFRP tendons and test its mechanical properties at room temperature;The mechanical properties of CFRP tendons under high temperature were tested by universal servo testing machine and its supporting heating furnace,and the relationship model between tensile strength temperature and elastic modulus temperature was established.The test results show that the tensile strength and elastic modulus of CFRP tendons decrease with the increase of temperature in the range of 250℃～400℃,and the trend is basically linear.The tensile strength and modulus of elasticity of CFRP bars with 700 MPa load decreased by 21.1% and 18% respectively after being exposed to 300℃ for 2 hours.At this time,the tensile strength of CFRP bars can ensure that they can continue to work normally after cooling,so 300℃ is taken as the safe critical temperature for fire protection design.(2)Using high silica needled felt and ceramic fiber felt as fire-proof material to protect CFRP tendons,the fire-proof performance tests of different thickness of high silica needled felt and ceramic fiber felt were carried out,and the relationship model between the thickness of fire-proof material and the safety critical fire-resistant temperature of CFRP tendons was established.The results show that for the same kind of fireproof material,the fireproof performance increases with the increase of the fireproof layer’s thickness;When the same thickness of high silicon needled felt and ceramic fiber felt protect the CFRP tendons,the fire resistance performance of high silicon needled felt is better than that of ceramic fiber felt.The safety critical fire resistance time of CFRP tendons protected by 24 mm thick high silicon needled felt is 38 minutes,and the safety critical fire resistance time of CFRP tendons protected by 24 mm thick ceramic fiber felt is 33.6 minutes;According to the test results,the relationship model of thickness safety critical fire resistance time between high silica needled felt and ceramic fiber felt is established.The safety critical fire resistance time increases linearly with the increase of the thickness of fire-resistant material.(3)The numerical simulation calculation of the fire performance test of high silica needled felt and ceramic fiber felt is carried out,and the calculation results are in good agreement with the test results.The numerical simulation calculation method of the fire protection system of CFRP tendons is established,which provides the method support for the subsequent numerical simulation calculation of the fire resistance performance of the bridge strengthened by prestressed CFRP tendons.(4)The surface temperature distribution of Prestressed CFRP tendons under the protection of different thickness of high silicon oxygen needled felt and ceramic fiber felt was obtained by numerical simulation calculation under the heating mode of hydrocarbon(HC)and ISO-834 standard,so as to obtain the safety critical fire resistance time of CFRP tendons under different fire heating modes.The results show that the fire protection effect of high silica needled felt is better than that of ceramic fiber felt.In the ISO-834 heating mode,the thickness of high silicon oxygen needled felt and ceramic fiber felt needed for 2 hours of safe fire resistance is 40 mm and 45 mm.In the HC heating mode,the thickness of high silicon oxygen needled felt and ceramic fiber felt needed for 2 hours of safe fire resistance is 45 mm and 50 mm;The heating rate of HC mode is faster than that of ISO-834 mode in the early stage,which requires higher performance of of CFRP tendons’ Fire resistant material;The thickness safety critical fire resistance time model of high silicon oxygen needled felt and ceramic fiber felt under ISO-834 and HC temperature rising mode is established,which can provide reference for the required thickness of fire protection layer of components with different fire rating requirements.The research results provide data support for the fire protection design of the bridge strengthened with CFRP tendons.