Study On Bond-slip Behavior Of Steel Tube Filled Concrete With Fire Spray After High Temperature

In recent years,fire accidents occur frequently.Through the investigation of fire accidents at home and abroad,it is found that building fire accidents are one of the main forms.The adhesion and slip behavior of concrete-filled steel tube interface under high temperature fire spray cooling mode was studied.Considering the change parameters of steel tube size and shape,temperature,strength of concrete and length of anchoring,46 specimens of concrete filled with square steel tube were designed,and 24 groups of standard cube concrete test pieces were designed with temperature and concrete strength as variable parameters.After high temperature,the fire spray was used to cool the temperature,the apparent changes before and after spraying water were observed,the mass loss was recorded,and the static launch test of concrete filled steel tube specimens was carried out,and the failure process and morphology of the specimens were investigated.The load-slip(p-s)curves at the loading end and the free end of the specimen are recorded.The stress-strain law along the height of the steel pipe is recorded.The influence of various parameters on the bond slip performance is analyzed,and the failure mechanism of the interface bond between the steel tube and the core concrete is discussed.A formula for calculating the ultimate bond strength of concrete filled steel tube specimens,the ultimate load corresponding to slip,the residual bond strength and the residual load corresponding to the slip is proposed,according to the variation of load-slip curve(p-s)at the loading end.The constitutive relation of interfacial adhesion and slip of concrete-filled steel tube was established,and the variation of concrete strength under the fire spray cooling mode was tested.The results showed that the color of the specimen changed from light brown to black gradually with the increase of the target temperature,and the loss rate of fire was proportional to the temperature.The shape of the load-slip(p-s)curve of the fire spray cooling specimen is similar to that of the natural cooling sample,and it generally goes through three stages:the straight rise section,the curve descending section and the flat section,and the loading end begins to slip before the free end.The slip value of loading end is higher than that of free end,and the strain of steel pipe is exponentially distributed along the direction of height,and the strain value increases exponentially with the increase of height from loading end.The bonding strength of the specimen increases first and then decreases with the increase of temperature,and the bonding strength reaches the maximum at the target temperature of 400?,and the Anchorage length is inversely proportional to the average bonding strength.The average ultimate bonding strength and the average residual bonding strength of the fire spray cooling specimen are obviously lower than those of the natural cooling sample,and the strength of the fire water cooling concrete decreases gradually with the increase of temperature.Compared with natural cooling and cooling,the strength of concrete recovered.Through the study of the slip behavior of the interface between the steel tube and the core concrete under the fire spray cooling mode,the paper provides an important reference for the evaluation,reinforcement and repair of the concrete filled steel tube structure after the fire.