Study On Heat Transfer Mechanism Of Steel Fiber Reinforced Concrete After High Temperature

Steel fiber reinforced concrete is widely used in civil engineering structures because of its better mechanical properties than normal concrete,and the high-temperature thermal conductivity of steel fiber reinforced concrete and its structures is very important for the fire behavior of buildings.Therefore,this paper conducts high temperature residual mechanical and thermal tests on ordinary concrete and steel fiber concrete with different content,analyzes the influence of temperature and steel fiber content on the thermodynamic performance of concrete materials,and discusses the heat conduction mechanism of steel fiber concrete.Based on the analysis and discussion of the test results,a method for calculating the thermal conductivity of steel fiber reinforced concrete after high temperature is proposed,which is more convenient and accurate to obtain the thermal conductivity of SFRC.The main research contents are as follows:(1)The concrete specimens with different steel fiber content of mortar,normal concrete and volume ratio of 1%and 2%were prepared and heated to temperatures of 150℃,300℃,450℃ and 600℃.The compressive strength,three-point bending strength,fracture energy and air permeability coefficient of the specimens before and after high temperature were tested.Through the different content of steel fiber reinforced concrete and ordinary concrete contrast,analysis after high temperature compressive strength of concrete as temperatures rise after the fall of first,when the temperature reached 300℃,the maximum compressive strength,and three point bending of the increase of peak load with the temperature falling,when the temperature rises to 150℃,decline significantly increased;Under the same heating temperature,the mechanical properties of steel fiber are obviously greater than that of ordinary concrete,and increase with the increase of fiber content.The results show that Steel fiber can improve the mechanical properties and permeability of concrete at high temperature,but the cracking degree of steel fiber concrete at high temperature is greater than that of ordinary concrete.It also provides some experimental basis for the establishment of numerical simulation of rigid fiber reinforced concrete.(2)The thermal properties of mortar,normal concrete and steel fiber concrete samples before and after high temperature were tested,and the apparent density,porosity,thermal conductivity,temperature conductivity coefficient and specific heat capacity of different samples varied with the content and temperature of steel fiber.The relationship between porosity and thermal conductivity was explored based on the variation of meso-concrete structure.The results show the apparent density and thermal conductivity of steel fiber concrete.The temperature conductivity coefficient and specific heat capacity decrease with the rise of temperature,while the temperature conductivity coefficient of steel fiber concrete appears an inflection point at 300℃ with a slight increase,and the porosity increases with the rise of temperature.Among them,about 1-3%of the porosity is caused by the internal heat cracking of concrete.Based on the experimental results,a simple two-dimensional steel fiber reinforced concrete mesoscopic numerical model is established.The simulation results show that the crack thermal resistance formed by high temperature thermal cracking is also an important factor affecting the heat conduction of concrete.(3)Based on steel fiber reinforced concrete before and after high temperature thermal performance test results,presents a mesoscopic multi-scale method to calculate the thermal conductivity of steel fiber reinforced concrete,concrete mesoscopic heat cracking behavior by considering the heat thermal resistance effect(crack),rather than using the coefficient of thermal conductivity of the mesoscopic composition and the changes of temperature is more rapid,convenient and accurate to obtain the coefficient of thermal conductivity of concrete after high temperature materials.The results show that the model is in good agreement with the experimental results when the interface thermal resistance coefficient increases linearly with the increase of temperature.It is found that,the ITR effect makes approximately 50%、36%and7%-12%contributions to the overall thermal conductivity reductions in mortar,high strength concrete,and SFRC withe 1-2%fiber volume fraction up to 600℃ respectively.When the interfacial thermal resistance coefficient of normal concrete is 0.4 and,the interfacial thermal resistance coefficient of steel fiber reinforced concrete is 1,the addition of particles with good thermal conductivity has no help to increase the effective thermal conductivity of composite materials.