Study On Bond Behavior Between Concrete And Deformed Steel Bar After Exposure To High Temperatures

China construction market has seen a development by leaps and bounds in the past more than ten years,with a sharp increase in the number of high-rise buildings and a high degree of building intensity.On the other hand,the urban population density is increasing,so is fire risk.High temperature in a fire can damage reinforced concrete(RC)structures to different degrees and may even lead to building collapse,thus posing threat on human society.In order to improve the damage assessment method for reinforced concrete structures after exposure to high temperature.This paper,by adopting experimental means along with related theoretical analysis,studies the bond-slip performance of RC under different cooling types different cooling types after exposure to high temperatures as well as the bond-slip performance of RC under uniaxial lateral pressure after exposure to high temperatures,and analyzes the effect of such parameters as high temperatures,cooling types,stirrup constraints and lateral pressure ratio on the bond-slip performance between concrete and deformed steel bar after exposure to high temperature.Completed specific works are as follows:Firstly,completing compressive strength tests for 78 cubes and prisms after exposure to high temperature,and analyzing the effect of high temperatures and cooling types on the compressive strength of cubes and axial compressive strength of prisms.The research results suggest:the compressive strength of cubes and axial compressive strength of prisms under different cooling types both decrease with the rise of high temperatures.The relations between the compressive strength of cubes/the axial compressive strength of prisms and temperature under different cooling types were obtained by fitting respectively.The compressive strength of the cubes decreased more slowly under water spray cooling type than that under natural cooling type,and the two water spray cooling types in this paper had little effect on the compressive strength of the cubes.Below 300℃,the axial compressive strength of the specimens under natural cooling type was greater than that under water spray cooling type;above 400℃,the axial compressive strength of the specimens under water spray cooling type was greater than that under natural cooling type.Secondly,through the central pull-out test of 57 reinforced concrete specimens after exposure to high temperature,studying the bond-slip behavior of reinforced concrete specimens under different cooling types,and analyzing the effect of such parameters as high temperatures,cooling types and stirrup constraint on the bond-slip behavior of reinforced concrete specimens.The research results suggest:under the same cooling types,the influence laws of high temperature on peak bond stress and on peak slip were different.With the increase of temperature,peak bond stress presents a law of almost linear decrease,while peak slip first decreases and then increases.The peak bond stress of the specimens treated with natural cooling type was greater than the corresponding value of the specimens treated with water spray cooling type.The peak slip value of the specimens treated with natural cooling type below 200℃ was greater than the corresponding value of the specimens treated with water spray cooling type,while the law above 200℃ was the opposite.Based on the regression analysis of the experimental data,the relational expressions between the peak bond stress/the peak slip and the temperature under different cooling types were established.The peak bond stress and peak slip values of the specimens with stirrup constraint were both greater than the corresponding characteristic values of the specimens without stirrups.Different from the splitting failure of the specimens without stirrups,the failure patterns of the specimens with stirrup constraint under various cooling types were all pull-out failure,indicating that the stirrup constraint can effectively improve the ductility performance and the failure patterns of the specimensThirdly,this paper conducts theoretical study to the center pull-out test for 57 reinforced concrete specimens after exposure to high temperature,analyzing the complete curvilinear equation of bond-slip,the initial damage of specimens without stirrups,the interface damage of specimens with stirrup constraint,the total energy of the bond failure process of specimens and so on.The research results suggest:specimens without stirrups and specimens with stirrup constraint had similar bond slip curve shapes.Combined with existing research results,this paper establishes the complete curvilinear equation of bond-slip after exposure to high temperature.The parameter a for the ascent stage of the curve changed a little with temperature,cooling type and stirrup ratio;the parameter b for the descent stage of specimens without stirrups showed a decrease with the rise of high temperature.The development of interfacial bond damage is process from fast to slow.The stirrup constraint can effectively slow down the damage development speed of specimens.The total energy of bond failure of specimens without stirrups presented an obvious decreasing trend with the rise of temperature.The total energy of the specimens with stirrup constraint was over 4.35 times higher than that of specimens without stirrups.Based on the theoretical calculation model of Veen,the bond strength of reinforced concrete specimens after exposure to high temperature was calculated.The theoretical calculation results were compared with the experiment results,indicating good conformity.Finally,finishing center pull-out tests for 48 reinforced concrete specimens under uniaxial lateral pressure after exposure to high temperature,studying the bond-slip behavior of reinforced concrete under uniaxial lateral pressure after exposure to high temperature,and analyzing the effect of such factors as high temperature and lateral pressure ratio on the bond-slip behavior.The research results suggest:after exposure to high temperature,lateral pressure can effectively increase the peak bond stress,peak slip,relative residual bond stress,and the total energy required for bond failure of specimens.Different from the splitting failure of the specimens without lateral pressure,the failure patterns of the specimens under uniaxial lateral pressure is mainly split-pull failure,indicating that the lateral pressure can effectively improve the failure patterns and ductility performance of the specimens.When the specimen was damaged,the main crack was perpendicular to the acting surface of lateral pressure.With the rise of high temperature experienced by the specimens,the total energy required for the bond failure process of the specimens gradually decreased.Uniaxial lateral pressure kept both the residual bond stress and residual slip value at a high level,and the descent stage of the bond slip curve became more gentle.