Experimental Study On The Bonding Performance Of Polypropylene Fiber Concrete After High Temperature

High-strength concrete has the advantages of high strength,high density,and strong resistance to deformation compared with traditional concrete.The use of this concrete in high-rise,super high-rise,and large-span structures can improve the structural load-bearing capacity,reduce the self-weight,reduce the number of materials,and lower the cost.Building fire is the most serious kind of fire,it is directly related to people’s lives and property,especially high-rise,super high-rise buildings,its good fire resistance can leave enough time for people to escape.Polypropylene fiber as an organic fiber,mixed into high-strength concrete,at room temperature can improve its crack resistance,and enhance its durability,high temperature due to its low melting point and disorderly distribution in the concrete,melting the formation of micro-crack channels can make the concrete internal high-temperature steam escape,improve the fire resistance of high-strength concrete prone to bursting defects,improve the fire resistance of buildings.In this paper,the bonding performance of high-strength concrete mixed with polypropylene fibers and deformed steel reinforcement after high temperature is studied experimentally,taking into account the effects of temperature,fiber admixture,and diameter of steel reinforcement on the bonding performance.The test data such as internal temperature field,bond-slip curve,ultimate bond strength,and damage pattern of the specimen were obtained,and the ultimate bond strength equation and bond-slip instantiation model after high temperature considering the effects of both temperature and fiber admixture were analyzed to provide some data reference for post-disaster damage assessment and repair of polypropylene fiber concrete structures.The main research work and the conclusions obtained in this paper are as follows:(1)Experimental studies of compressive strength and splitting tensile strength at room temperature were conducted on concrete specimens with four-volume admixtures of polypropylene fibers(0,0.1%,0.2%,and 0.3%)to test their basic mechanical properties.It was found that the addition of polypropylene fibers could improve the splitting tensile strength of concrete,and the strength showed a trend of increasing and then decreasing with the increase of admixture.(2)Concrete specimens with four types of fiber doping were tested at different temperatures,and their internal temperature fields were measured by pre-buried K-type thermocouples to observe the appearance of the specimens after natural cooling at high temperatures.It was found that the specimens were kept at a constant temperature of200℃,400℃ and 600℃ for more than 5 hours,and the higher the target temperature,the longer the time required for the specimens to be kept at a constant temperature;with the increase of temperature,the apparent color of the specimens gradually changed from dark to light,and no cracks appeared on the specimens when the fire temperature was below 200℃;when the temperature exceeded 200℃,cracks appeared on the surface of the specimens,and the cracks on the specimens not mixed with fibers More cracks were found in the specimens not mixed with fiber than in the specimens mixed with fiber.(3)The central pull-out test was conducted on 96 concrete specimens,and the force mechanism analysis was carried out according to the loading process and damage phenomena,and the bond-slip curve was drawn to analyze the effects of high temperature,reinforcement diameter,and fiber admixture on the bonding performance.It was found that: the damage form of the specimen can be divided into splitting,splitting-pulling,and pulling type damage,and the force damage process can be divided into four stages: slip,pulling,falling,and residual;the higher the fire temperature of the specimen,the lower the ultimate bond strength and the larger the peak slip;the ultimate bond strength and peak slip of the specimen gradually decrease with the increase of the diameter of the reinforcement,and the smaller diameter of the reinforcement has a relatively The ultimate bond strength and the peak slip of the specimens are increased with the increase of the fiber,and the ultimate bond strength of the specimens increases and then decreases with the increase of the fiber,and the damage rate of the bond strength of polypropylene fiber concrete is lower and the high-temperature resistance is better than that of the specimens without fiber.(4)In practical engineering,there are many factors affecting the bond stress between reinforced concrete.In this paper,based on the European model code,an empirical formula for the bond strength of post-temperature deformed steel and polypropylene fiber concrete containing the degradation coefficient of the strength of temperature damage and the coefficient of bond strength affected by polypropylene fiber is established based on the analysis of test results and bonding mechanism.According to the characteristics of the test curve,a two-stage principal structure model is selected,and Origin software is used to fit regression analysis to the test data.The fitting parameters vary significantly with temperature and polypropylene fiber admixture,and the model of bond-slip principal structure relationship of post-high temperature polypropylene fiber concrete is established by modifying the parameters of the principal structure model,and a parameter calculation formula considering temperature and polypropylene fiber admixture is proposed.