Study On Law Of Heat Transfer And Mechanical Properties On Concrete Suffered Elevated Temperature During Water Cooling

The concrete structure is the most widely used structural form nowadays,with good fire resistance,slow degradation of bearing capacity in the high-temperature environment of fire,and can avoid the continuous collapse of the structure.In the fire rescue process,water spraying is often used to extinguish fires,and water cooling causes a huge temperature gradient inside the concrete structure at high temperatures,and the loadbearing capacity of the concrete structure degrades dramatically,which seriously threatens people’s lives.At present,domestic and foreign research focuses on the mechanical properties of concrete at high temperatures,and after natural cooling,water spray cooling on high-temperature concrete heat transfer and mechanical properties degradation law research is relatively scarce.In this paper,a combination of experimental research and numerical simulation is used to study the strength evolution and heat transfer of high-temperature concrete under different cooling methods,and the main work and results include the following parts.(1)The thermal conductivity of concrete in the process of water cooling was calculated by designing a concrete round cake spray water cooling test using the inverse method of thermal parameters,and it was found that there is a sudden change of thermal conductivity of concrete in the process of cooling,and it was concluded that the thermal conductivity changes with temperature and the thermal conductivity increases abruptly when the temperature of concrete is below 100℃ in the process of water cooling,and the thermal conductivity is affected by high-temperature damage in the early stage of cooling with small value.The law of change of thermal conductivity with water spraying time shows that the thermal conductivity increases abruptly when the internal cooling of concrete reaches the temperature platform of 100℃,reflecting that the temperature reaches dynamic equilibrium at this time,the internal heat exchange between concrete and external cooling fluid is accelerated and the heat transfer increases.(2)Forty groups of cubic standard specimens were designed to conduct hightemperature cooling tests and compressive strength tests with fire temperature,cooling mode,and cooling time as variables,respectively,to investigate the heat transfer and strength change laws of concrete under three cooling conditions of natural cooling,water immersion cooling,and water spray cooling after high temperature.The following are the contents of the study.The apparent damage of the specimens under water spray cooling is the most serious,and the higher the fire temperature,the more serious the compressive damage form;the relationship of mass loss of concrete after the high temperature is shown as natural cooling > water spray cooling > water immersion cooling.Under the water cooling condition,the cooling curve appears 100℃ temperature plateau,and the higher the fire temperature the more obvious its temperature plateau,the temperature drops to 100℃,the cooling rate of immersion cooling and water jet cooling slows down significantly.The residual compressive strength of concrete under different cooling conditions decreases with increasing fire temperature,and the relationship between the residual compressive strength of concrete cooled to room temperature after the high temperature is natural cooling > water jet cooling > water immersion cooling.In the water jet cooling conditions,the fire temperature of 400 ℃ and 600 ℃,the concrete compressive strength with the cooling time first decreased and then increased,after 30 min of water jet time,the residual compressive strength was significantly restored.The relative strength difference in the process of water spray cooling of concrete is proportional to its relative temperature difference between inside and outside;the proposed formula for calculating the steady-state strength of concrete after the high temperature is in good agreement with the comparison of existing experimental studies,which can provide a theoretical basis for the safety assessment of concrete structures under fire rescue.(3)In this paper,we use the available finite element software ABAQUS to build a model according to the existing thermal parameters and boundary conditions of hightemperature concrete,numerically analyze the cooling temperature field of concrete specimens treated with different fire temperatures and derive the temperature field distribution clouds of concrete sections under different water cooling conditions and different temperatures and the cooling curves at different locations.The numerical simulation results agree with the experimental results,and the thermal parameters and heat exchange boundary conditions of the concrete water cooling process are optimized through simulation.(4)The thermal conductivity of concrete obtained by the inversion theory of thermal conductivity is close to the value of the thermal conductivity of concrete in a water cooling process optimized in finite element simulation,which proves the feasibility of using ABAQUS finite element software to simulate the temperature field in water cooling process and provides a reference for further research on thermal parameters such as thermal conductivity of concrete.