| In our country,with the development of economy and the pursuit of green environmental protection,the proportion of assembled building structure system in the residential industrial system is increasing year by year.Compared with traditional castin-situ concrete buildings,assembled buildings have obvious advantages.They can not only improve the quality and speed of the project,but also help to protect the green ecological environment,which is in line with the green environmental protection concept of energy conservation and emission reduction in China.The technology of grout sleeve splicing of rebar is one of the key technologies for vertical connection of precast concrete structures.Its mechanical properties and high temperature properties have gradually become the focus of research by scholars at home and abroad.There are few studies on the failure mechanism and bonding properties of reinforced grouting sleeve specimens after fire.The existing studies mainly focus on the high-temperature properties of reinforced grouting sleeve under stress-free,simple and ideal conditions.In actual engineering,the grout sleeve splicing of rebar is under a certain stress state when subjected to fire.Based on this,this paper designed three groups of steel semi-grouting sleeve connection specimens at normal temperature,high temperature and after high temperature,and carried out uniaxial tensile tests under different initial stress states.The bond slip behavior between steel bar and cementitious grout in reinforced semi-grouting sleeve joint specimen under thermodynamic coupling is studied,and the bond slip curve is obtained.According to the characteristics of the curves,the constitutive equation of bond slip at high temperature and after high temperature is summarized,and the formula of bond slip varying with the initial stress level at different temperatures is fitted.It was applied to ABAQUS to simulate the bonding properties between the steel bar and the cementitious grout in the steel bar semi-grouting sleeve at high temperature to verify the applicability of the formula.The main research contents of this paper include the following aspects:(1)In order to study the influence of initial stress level on the bond slip behavior between the rebar and the cementitious grout in the semi-grouting sleeve connection of steel bar,a total of 56 specimens of the semi-grouting sleeve connection of steel bar were designed and made in three groups at room temperature,high temperature and after high temperature.The design and manufacture process of the specimen,the equipment used in the test and the loading process of the test are determined.At the same time,seven groups of cementitious grout block and steel bars with the same number of groups as the test pieces were designed and made for material property test.(2)The material property test of the cementitious grout and steel bar used in the specimen was carried out to study the variation law of the flexural and compressive strength of the cementitious grout with temperature at high temperature and after high temperature,and to explore the influence law of different initial stress level on the failure pattern and bearing capacity of steel bar under different temperature states.(3)During the high temperature test of the semi-grouting sleeve connection of steel bar under and after the high temperature,the thermocouple is tied in the middle of the anchorage section of the steel bar,and the temperature of the steel bar surface in the bonding section is measured,and the change rule of the temperature with time is obtained.(4)Through the analysis of bond slip curve,the variation law of bond slip curve of steel semi-grouting sleeve connection at different stages under different initial stress levels at high temperature and after high temperature,as well as the variation law of peak bond stress and ultimate slip value with temperature and initial stress levels are obtained.According to the existing formula of bond strength,the limit rang of the possible failure of plough-scraping type in the semi-grouting sleeve connection of steel bar is given.(5)In this paper,a bond-slip constitutive model was obtained according to the variation characteristics of bond-slip curves at high temperatures and after high temperatures,and a bond-slip formula was fitted with the change of initial stress level at different temperatures.The accuracy of the fitting formula was verified by comparing the bond-slip curve with the test.(6)The finite element software ABAQUS was used to perform the finite element analysis on the specimen at high temperature.The bonding effect between the reinforcement and the cementitious grout in the nonlinear spring element was defined by the fitting formula,and the stress nephogram and the bond slip curve of the reinforcement and the cementitious grout at the bonding part were obtained.The applicability of the proposed formula is verified by comparing the bond-slip curves obtained from experiments and simulations.The results show that the fitting formula can be used as the bonding constitutive relationship between the steel bar and the cementitious grout when the semi-grouting sleeve connection of steel bar with initial stress level is simulated at high temperature.It lays the foundation theory for the subsequent research on the performance of reinforced semi-grouting sleeve connection and provides the basis for the design and practical engineering application of reinforced semi-grouting sleeve connection. |
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