Research On Fire Resistance Of Precast Concrete Columns With Efficient Reinforcement

In order to improve installation efficiency,construction quality and reducing the cost of monolithic precast concrete frame structures,precast concrete columns reinforced by efficient reinforcement have been proposed in recent years.Their static and seismic behaviors generally meet design expectations,while,their fire resistance has not been properly studied.In order to evaluate fire safety and put forward fire protection design method of the new precast columns,the fire resistance of two types of precast concrete columns reinforced by efficient reinforcement and the mechanical properties of their corresponding connections were experimentally studied.Combined with the finite element analysis,the fire resistance performance and failure mechanism of the columns were further analyzed.Finally,the calculation method of fire resistance bearing capacity and design recommendations of the precast concrete columns with efficient reinforcement were given by theoretical analysis.The main research work and results are as follows:(1)The mechanical properties of 24 constrained grout-anchor connection and 36 sleeve grouting connection under different temperatures-load modes were carried out,and the effects of temperature and connection structure on the failure mode,bearing capacity and deformation performance of the connection subjected to high temperature were analyzed.The results show that the high-temperature mechanical performances of constrained grout-anchor connection and sleeve grouting connection are good.The failure of the constrainted grout-anchor connection is mainly caused by concrete crushing outside the core-restrained grouting connection zone.The tensile and compressive bearing capacity and stiffness of the sleeve grouting connection are greater than the corresponding continuous reinforcement.Finally,the calculation method of the tensile and compressive bearing capacity and stiffness of the sleeve grouting connection subjected to high temperature is proposed.(2)The fire resistance tests of 14 full-scale precast concrete columns with efficient reinforcement and 4 ordinary reinforced cast-in-situ concrete columns for comparison in ISO-834 standard heating process were tested,and the effects of reinforcement form,load ratio,fire position and concrete burst on the failure mode,axial deformation and fire resistance of precast concrete columns with efficient reinforcement were analyzed.The test results show that the fire resistance of the precast concrete column connection area is superior to non-connecting zone,and the failure always occurs in non-connection area.Under the same load ratio,the fire-resistance of the bunch reinforced column are larger than that of the large diameter reinforced column.The fire-resistance of precast columns with serious concrete spalling is generally difficult to meet the requirements of class I fire resistance rating(3 hours),while the fire resistance of precast columns without concrete spalling can meet the requirements of the specification.(3)The three-dimensional finite element models for temperature field calculation and fire resistance calculation of precast concrete columns with efficient reinforcement under fire were established and verified.The temperature field and working mechanism of the whole fire process of the two kinds of precast concrete columns with efficient reinforcement were compared and analyzed.The results show that the forms of the connection and reinforcement have little effect on the temperature distribution of concrete,and the average temperature of the longitudinal reinforcement of the column with cluster reinforcement is lower,which is conducive to the fire resistance of the member.The assembled connection modes have a certain effect on the high-temperature deformation of the column but have a smaller impact on the fire resistance.Different types of reinforcement have obvious effects on the deformation and fire resistance of concrete columns.(4)The relationship between fire resistance bearing capacity and fire resistance limit of fabricated columns was compared and analyzed,and the equivalent conversion method was given.On this basis,the influence laws of concrete burst,section size,slenderness ratio,thickness of protective layer,reinforcement ratio,concrete strength and yield strength of longitudinal reinforcement on highly reinforced fabricated concrete columns under standard fire were analyzed.The results showed that concrete burst,section size,slenderness ratio,and concrete strength on the fire resistance bearing capacity of the high-efficiency reinforcement precast column were relatively small.When considering concrete bursting,the fire bearing capacity of large-diameter reinforced columns decreases more,and that of cluster reinforced columns decreases less.Under the same fire time,the variation law of stability coefficient with slenderness ratio between high-efficiency reinforced fabricated column and ordinary reinforced cast-insitu column is basically the same.(5)Based on the fire resistance calculation methods of cast-in-situ reinforced concrete were used to calculate precast concrete column with efficient reinforcement.The results show that these methods can not be simply applied to precast concrete columns and need to make the necessary corrections.For the precast concrete column with efficient reinforcement,the calculation method of fire bearing capacity based on homogenization temperature was proposed,and the simplified calculation formulas of average temperature and equivalent reduction coefficient of concrete section were given by numerical regression and theoretical analysis of temperature field;The effective length correction factor was introduced to consider the influence of the connection area on the stable bearing capacity of the column;Based on the principle of limit analysis,the formula of axial compressive bearing capacity at room temperature was extended to fire,and the calculation formula of bearing capacity of precase concrete columns at any time under standard fire was obtained.The effectiveness of the formula is verified by the test data.This research work provides a scientific basis for the application of precast concrete columns with efficient reinforcement in engineering and the compilation of relevant specifications.At the same time,the relevant test data,simulation methods and theoretical research results can also provide a reference for the fire resistance research of similar precast columns.This research work has important scientific significance and engineering value.