Study On The Effect And Mechanism Of Steel Fiber On Mechanical Properties Of Concrete After High Temperature

Fire is one of the major damages to the safety of human life and property.After a fire,in order to ensure the evacuation of personnel,transfer of property,and safety of fire and rescue personnel,the building needs to maintain stability and safety in sufficient rescue time.In the high temperature,when the concrete building fire fighting rescue,due to the fire water brought by the impact force,temperature stress and cold water damage,will make the overall structure instability or loss of safety,leading to the occurrence of secondary disasters.At the same time,water spray pressure,fire rescue water spray time,fire duration,post-disaster resting time and so on will have a certain impact on the building structure.Therefore,it is very important to study the degradation effects of different cooling methods on the properties of ordinary concrete and steel fiber reinforced concrete.The main research contents and results of this paper are as follows:（1）In this paper,the ordinary concrete and steel fiber concrete are taken as the research object,and the high temperature treatment of 200℃～800℃is carried out,and the influence of natural cooling and water cooling on the performance deterioration of ordinary concrete and steel fiber concrete after high temperature is studied.The mass loss,surface color change,resistance,relative dynamic elastic modulus,compressive strength and splitting tensile strength of ordinary concrete and steel fiber concrete after high temperature were investigated.The results show that after high temperature,the mass loss rate and resistance of concrete gradually decrease with the increase of steel fiber content,while the relative dynamic elastic modulus increases.The compressive strength of concrete reaches the maximum when the steel fiber content is 1.0%,and the splitting tensile strength of concrete reaches the maximum when the steel fiber content is 1.5%.（2）Low magnetic resonance technology was used to study the microscopic aspects of the concrete.The microscopic pore structure analysis included T₂ relaxation value,T₂spectral area and porosity of concrete.The change of microstructure greatly affects the macroscopic performance of concrete.Through quantitative analysis of the change of porosity,pore size distribution and pore proportion of concrete after high temperature.With the increase of temperature,the T₂ spectral area of concrete in each group showed an increasing trend after natural cooling and water-immersion cooling,indicating that the porosity of concrete increased continuously after the two cooling methods,and the number of middle pores and large pores after natural cooling was less than that after water-immersion cooling.（3）The acoustic emission ringing count and energy count are related to the generation of micro cracks in concrete.The sharp increase of acoustic emission signal indicates the serious damage inside concrete.The larger the temperature gradient,the larger the energy value and cumulative energy value.The cumulative energy value of concrete mixed with steel fiber is higher than that of ordinary concrete.