Experimental Study On Fatigue Performance Of Prefabricated Reinforced Concrete Beams With Post-cast Sections After Fir

Prefabricated building are more and more widely used in the field of civil construction.The high temperature of a fire can greatly weaken the performance of materials such as concrete and steel bars,thereby seriously deteriorating the load-bearing performance of building structures.In practical engineering,in addition to bearing static loads,building structures often face situations of bearing cyclic loads,such as multi-story parking buildings,bridge structures,high-speed railway station buildings,single-story and multi-story industrial factories,etc.,often bearing cyclic loads from vehicles,cranes,mechanical vibrations,and other factors.Therefore,conducting research on the deterioration mechanism and evaluation method of building structure bearing capacity under the coupling effect of fire high temperature and cyclic load has important theoretical and practical significance for promoting the improvement of disaster resistance of prefabricated reinforced concrete structures.This article designs a full-scale prefabricated reinforced concrete simply supported beam specimen with prefabricated sections at both ends,mid span steel bars connected by grouting sleeves,and post poured concrete to simulate the performance of the prefabricated connection nodes of reinforced concrete beams.It also conducts fatigue performance comparison tests under static and cyclic loading after fire and high temperature,Analyzed the degradation law of static bearing performance and fatigue performance of prefabricated reinforced concrete beams under cyclic load under high fire temperatures.On this basis,a theoretical analysis of the fatigue performance of prefabricated reinforced concrete beams subjected to fire was conducted.The main research content and achievements are as follows:(1)With the help of a large-scale fire simulation experimental system,a fire action test was conducted on a prefabricated reinforced concrete test beam under the control of a standard temperature rise curve.The temperature field changes in the entire process of the test beam section during the fire action and temperature rise and fall stages were observed.The measured results show that the temperature distribution of each measuring point in the beam section is uneven,and there is an obvious temperature gradient;When the furnace temperature curve reaches its peak,the time when the temperature at the measuring point in the cross-section of the test beam reaches its peak will be relatively delayed;The temperature and heating rate of the measuring points within the beam cross-section gradually decrease as the distance from the fire surface increases;Compared with the three side fire condition,the beam section temperature distribution under the four side fire condition is more uniform and symmetrical.(2)The static loading mechanical properties of prefabricated reinforced concrete test beams after Free cooling due to fire heating were tested,and the crack development,deflection deformation,failure characteristics and ultimate bending bearing capacity of the test beams were observed and analyzed.Compared with the room temperature comparison test beam,the flexural stiffness of the prefabricated reinforced concrete test beam after 60 minutes and 120 minutes of fire and high temperature action decreased by 36% and 45%,the cracking load decreased by 12.5%and 50%,and the ultimate load decreased by 9% and 14%,respectively.(3)The fatigue performance test of prefabricated reinforced concrete test beam after Free cooling due to fire heating was carried out under cyclic loading.The test results show that the failure of the fatigue test has obvious brittle failure characteristics,and some test beams have experienced fatigue fracture of the bottom longitudinal reinforcement.(4)The fatigue test results show that the fire action time has a significant adverse effect on the fatigue life of prefabricated reinforced concrete test beams.Compared with the room temperature comparison test beam,the fatigue life of the test beam after 60 minutes and 120 minutes of fire and high temperature action decreased by 41.27% and 97.63%,respectively.(5)The fatigue test results show that the static bearing capacity of prefabricated reinforced concrete test beams after undergoing cyclic loading decreases.The more cycles they undergo,the higher the degree of fatigue damage of the test beam,and the smaller the remaining static bearing capacity.Compared with the comparative test beam that did not experience cyclic loading,the static bearing capacity of the test beam after experiencing 33% and 66% fatigue cycles decreased by 6.02% and 17.67%,respectively.(6)The fatigue test results show that different fire conditions have a significant impact on the fatigue life of prefabricated reinforced concrete test beams.As the number of exposed surfaces increases,the fatigue life of the test beam significantly decreases.Compared with the test beam subjected to three sided fire conditions,the fatigue life of the test beam subjected to four sided fire conditions decreased by10.54%.(7)Based on a comprehensive analysis of the experimental results,the development law of crack width and degradation law of flexural stiffness of prefabricated reinforced concrete beams under cyclic load after high temperature fire were explored;A calculation formula for the maximum crack width of prefabricated reinforced concrete beams under cyclic load after high temperature fire has been established;Based on the residual stiffness model,a prediction method for the residual static bearing capacity of prefabricated reinforced concrete beams under cyclic load after high temperature fire was established.