Study On Square Steel Stubs Of Mechanical Properties Filled With Section Steel And Precast Segments Containing Demolished Concrete Lumps And Fresh Concrete

To improve the field construction efficiency of vertical structural members casted with demolished concrete lumps(DCLs)and fresh concrete(FC),factory prefabrication of compound concrete made of DCLs and FC is an effective strategy.The square steel tube filled with steel and precast segments containing DCLs has been proposed in this dissertation.Furtherly,the preliminary study on mechanical behaviors of this member under axial loading at normal temperature and after fire has been carried on.The main research works are as follows:1.Tests on fifteen specimens under axial loading were conducted,and the influences of some parameters(i.e.construction method,replacement ratio of DCLs,thickness of steel tube,and strength of section steel)on the specimens’ axial loading behaviors have been discussed.According to the existing formulas for steel tubes filled with steel-reinforced concrete,the axial strength of the specimens were calculated and compared with the test results.It is found that:(1)compared with site construction,factory prefabrication of segments containing DCLs has been not harmful to the axial strength and initial stiffness of such columns;(2)The number and the surface roughness of precast segments have limited influences on the specimens’ axial behaviors.(3)The specimens built in high-strength steel has more advantages than those built in plain steel according to axial strength,initial stiffness and ductility coefficient of such columns.This advantage is most obvious for ductility coefficient,followed by axial strength and initial stiffness.(4)If the stress of section steel reaches the yield strength before the stress of core concrete reaches the peak value,or two of them reach at the same time,Compared with those built in plain steel,the columns built in high-strength(Q690)steel can save about 30% of the amount of steel and reduce the cost of steel by about 20% in the case of the same axial strength.2.Tests on compressive strength of 25 prisms and 25 cubes made of grouting material without coarse aggregates or fine aggregates after exposure to elevated temperatures,and the effects of temperatures on the grouting material’s cubic compressive strength,prismatic compressive strength,elastic modulus,Poisson’s ratio and stress-strain curves have been discussed.At the same time,the corresponding results of other grouting materials were compared.It is found that:(1)prismatic compressive strength and elastic modulus of the grouting material will decrease with increasing temperature.The reduction rate of the latter is faster than that of the former,but the decline of the two is relatively limited before 300℃.(2)When the grouting material is subjected to high temperature within 300℃,its Poisson’s ratio has a relatively narrow variation range(0.15 to 0.30)with the change of stress ratio.However the temperature is over 500℃,the Poisson’s ratio increases significantly with the increase of stress ratio.(3)The quantitative relationship between properties(i.e.mass loss rate,volume contraction rate,prismatic compressive strength,elastic modulus and ascending segment of the dimensionless stress-strain curve)of the grouting materials after exposure to elevated temperatures and the target heating temperature were established,which agreed well with test results.(4)Compared with cement mortar,geopolymer paste and bean stone grouting material,the grouting material has better mechanical properties after high temperature.3.Tests of nine specimens after exposure to ISO 834 standard fire under axially loading were conducted,and the influences of some parameters(i.e.construction method,replacement ratio of DCLs,and time of fire exposure)on the specimens’ residual axial loading behaviors have been discussed.It is found that:(1)There were different degrees of bulging deformation at the half height of the square steel tube after fire.The longer the time of fire exposure,the greater the bulging deformation.Enduring the same time of fire exposure,bulging deformation of prefabricated specimens is greater than that of the cast-in-situ specimens.(2)The use of DCLs has little effect on the temperature field of column section.The internal temperature of prefabricated specimens is lower than that of cast-in-situ specimens.(3)Compared with cast-in-situ specimens after fire,the initial stiffness degenerate amplitude of prefabricated specimens after fire is smaller than that of cast-in-situ specimens,but the axial strength degenerate amplitude is much larger.(4)For a long time before the load reaches the peak load,the longitudinal and lateral strain growth of the steel tube after fire are almost stagnant.Only when the load reaches peak load,they will increase rapidly.